underwater communications system with focus on antenna design

Institutionen för systemteknikDepartment of Electrical Engineering

Examensarbete

Underwater communications system with focus onantenna design

Examensarbete utfört i Elektroniska kretsar och systemvid Tekniska högskolan i Linköping

av

Erik Carlsson

LiTH-ISY-EX-ET–15/0444–SE

Linköping 2015

Department of Electrical Engineering Linköpings tekniska högskolaLinköpings universitet Linköpings universitetSE-581 83 Linköping, Sweden 581 83 Linköping

Underwater communications system with focus onantenna design

Examensarbete utfört i Elektroniska kretsar och systemvid Tekniska högskolan i Linköping

av

Erik Carlsson

LiTH-ISY-EX-ET–15/0444–SE

Handledare: J Jacob Wiknerisy, Linköpings universitet

Per HagströmCombitech

Examinator: J Jacob Wiknerisy, Linköpings universitet

Linköping, 06 September, 2015

Avdelning, InstitutionDivision, Department

Division of Communication SystemsDepartment of Electrical EngineeringLinköpings universitetSE-581 83 Linköping, Sweden

DatumDate

2015-009-006

SpråkLanguage

Svenska/Swedish Engelska/English

RapporttypReport category

Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport

URL för elektronisk versionhttp://www.commsys.isy.liu.se

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-ZZZZ

ISBN—

ISRNLiTH-ISY-EX-ET–15/0444–SE

Serietitel och serienummerTitle of series, numbering

ISSN—

TitelTitle

Svensk titelUnderwater communications system with focus on antenna design

FörfattareAuthor

Erik Carlsson

SammanfattningAbstract

In this thesis the possibility of building an underwater communication system usingelectromagnetic waves has been explored. The focus became designing and testingan antenna even if the entire system has been outlined as well. The conclusion isthat using magnetically linked antennas in the near field it is a very real possibilitybut for long EM waves in the far field more testing needs to be done. This isbecause a lack of equipment and facilitates which made it hard to do the real-world testing for this implementation even if it should work in theory.

NyckelordKeywords Underwater, Antenna, NFC, Wireless, Communications

AbstractIn this thesis the possibility of building an underwater communication system usingelectromagnetic waves has been explored. The focus became designing and testingan antenna even if the entire system has been outlined as well. The conclusion isthat using magnetically linked antennas in the near field it is a very real possibilitybut for long EM waves in the far field more testing needs to be done. This isbecause a lack of equipment and facilitates which made it hard to do the real-world testing for this implementation even if it should work in theory.

SammanfattningI avhandlingen har möjligheten för att bygga ett undervattenskommunikationssy-stem som använder elektromagnetiska vågor undersökts. Fokus blev konstruktionoch provning av antenner, även om det hela systemet också har beskrivits. Slutsat-sen är att en användning av magnetiskt kopplade antenner i närområdet är mycketmöjligt, men för långa EM vågor i fjärrfältet måste fler tester göras. Detta är pågrund av brist på utrustning och lokaler vilket gjorde det svårt att göra tester påden verkliga världen även om det skulle fungera i teorin

v

Acknowledgments

I would like to thank the following:

• Per Hagström for being my adviser at Combitech and giving me feedback onthe thesis.

• J Jacob Wikner for being my examiner and setting my on the right trackwhen I was getting sidetracked by something.

• Magnus Karlsson for helping me with the antenna measurements.

• Jon Staffeldt for aiding with ideas on what and how to measure with theantennas.

• Martin Nielsen Lönn for always being helpful when I came asking for accessor help with something.

• The departments of ISY and IFM for allowing me accesses to laboratoriesand measuring equipment.

vii

Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Goals of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 General idea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theory 32.1 Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Maxwell equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3.1 Radio waves . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3.2 Acoustic waves . . . . . . . . . . . . . . . . . . . . . . . . . 62.3.3 Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3.4 Refraction loss . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.5 EM-signals in seawater . . . . . . . . . . . . . . . . . . . . . 8

2.4 Antenna types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4.1 Loop antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4.2 Dipole antenna . . . . . . . . . . . . . . . . . . . . . . . . . 102.4.3 J-pole antenna . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . 112.5.1 Impedance matching a loop antenna . . . . . . . . . . . . . 12

2.6 Digital modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6.1 OOK modulation . . . . . . . . . . . . . . . . . . . . . . . . 132.6.2 BPSK modulation . . . . . . . . . . . . . . . . . . . . . . . 132.6.3 ASK modulation . . . . . . . . . . . . . . . . . . . . . . . . 132.6.4 PSK modulation . . . . . . . . . . . . . . . . . . . . . . . . 14

2.7 Gaussian white noise . . . . . . . . . . . . . . . . . . . . . . . . . . 142.8 Commercial systems . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.8.1 Near field communication . . . . . . . . . . . . . . . . . . . 152.8.2 Bluetooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.8.3 Oceanreef . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3 Related Research 17

ix

x Contents

4 Method 194.1 The process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.2 The system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.2.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.2.2 Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.2.3 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.3 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.3.1 Low pass filter . . . . . . . . . . . . . . . . . . . . . . . . . 214.3.2 Analogue digital converter . . . . . . . . . . . . . . . . . . . 214.3.3 Micro processing unit . . . . . . . . . . . . . . . . . . . . . 214.3.4 Signal generator . . . . . . . . . . . . . . . . . . . . . . . . 214.3.5 RF filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.3.6 RF amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . 224.3.7 Low noise amplifier . . . . . . . . . . . . . . . . . . . . . . . 224.3.8 Power detector . . . . . . . . . . . . . . . . . . . . . . . . . 234.3.9 Digital analogue converter . . . . . . . . . . . . . . . . . . . 23

4.4 Antenna/transfer system choice . . . . . . . . . . . . . . . . . . . . 234.4.1 Discarded choices . . . . . . . . . . . . . . . . . . . . . . . . 23

4.5 Loop antenna construction . . . . . . . . . . . . . . . . . . . . . . . 244.6 Impedance matching . . . . . . . . . . . . . . . . . . . . . . . . . . 244.7 Measuring the signals . . . . . . . . . . . . . . . . . . . . . . . . . 254.8 A second system . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5 Result 275.1 First trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.2 Second trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285.3 Third trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295.4 Conclusion of the first three trials . . . . . . . . . . . . . . . . . . . 295.5 Fourth trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.6 Fifth trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325.7 Impedance measuring . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.7.1 3-coil loop antenna . . . . . . . . . . . . . . . . . . . . . . . 345.7.2 2-coil loop antenna . . . . . . . . . . . . . . . . . . . . . . . 345.7.3 Caveats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.8 Longer range trials . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.8.1 Sixth trial . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355.8.2 Seventh trial . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.9 Trials in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375.9.1 Tap water ranged trial 1 . . . . . . . . . . . . . . . . . . . . 385.9.2 Tap water ranged trial 2 . . . . . . . . . . . . . . . . . . . . 385.9.3 Salt water ranged trial 1 . . . . . . . . . . . . . . . . . . . . 385.9.4 Salt water ranged trial 2 . . . . . . . . . . . . . . . . . . . . 395.9.5 Tap water short trial 1 . . . . . . . . . . . . . . . . . . . . . 395.9.6 Tap water short trial 2 . . . . . . . . . . . . . . . . . . . . . 405.9.7 Salt water short trial 1 . . . . . . . . . . . . . . . . . . . . . 405.9.8 Salt water short trial 2 . . . . . . . . . . . . . . . . . . . . . 40

Contents xi

5.9.9 Brackish water short trial . . . . . . . . . . . . . . . . . . . 425.9.10 Seawater short trial . . . . . . . . . . . . . . . . . . . . . . 42

6 Discussion 456.1 Conclusion of the air measurements . . . . . . . . . . . . . . . . . . 456.2 Conclusion of water measurements . . . . . . . . . . . . . . . . . . 466.3 Conclusion of impedance measuring . . . . . . . . . . . . . . . . . 486.4 Security and integrity . . . . . . . . . . . . . . . . . . . . . . . . . 496.5 Issues with the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . 506.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7 Future Work 517.1 Building the system . . . . . . . . . . . . . . . . . . . . . . . . . . 517.2 Perfecting the antenna . . . . . . . . . . . . . . . . . . . . . . . . . 517.3 Outdoor testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

1 | Introduction

1.1 BackgroundThe idea for this thesis was a videoclip [1] using a cellphone while submerged inwater but it was still transmitting. This raised the question if you could develop awireless communication system based on radio waves that could transmit throughwater and other kinds of medium that is not air. Our prior knowledge is that theiris no such system on a broad and commercial level.

1.2 Goals of the thesisThe goal of this thesis was to develop a system that will provides wireless voicecommunication in different kinds of water, examples are: seawater, brackish waterand river water. Different systems on the market, like Bluetooth and NFC as wellas own solutions will be theoretically tested, where one of them will be chosen toget a real world implementation. The voice communication should be lossless forthe operator and have a high enough quality to motivate the change from a wiredcommunication line. The specifications for the system were as follows:

• Have a range of at least 1 m.

• Be able to transfer data speech sampled at 12-bit, 11,025 KHz in a monoconstellation of the sound.

• The operator will be able to interpret what is said over the system withoutany difficulties.

1.3 LimitationsThis thesis was focused on making a working model of a system and not a fullyfleshed out one, it did however take a turn towards building and testing antennaswhich was all that was done due to time limitations. Other limitations were:

• Both the company and the university lacks a proper testing facility, thismeans that theory and practice might differ a fair bit since all testing willbe done in none ideal environments.

1

2 Introduction

• There was no possibility of doing long range underwater testing, the longestrange was in the [dm] span made in a 100 L container.

• Only one of the systems theorized was to have a real world implementationbecause of time limitations.

1.4 General ideaThe idea of the system was that it will be consist of an analogue radio signal, whichis digitally sampled and modulated by a microprocessor and then sent through anantenna (Tx). The signal will then be received in the other end by an antenna(Rx) then demodulated, reconstructed by a microprocessor and is played for theoperator through a some kind of audio device, most likely a headset.

Figure 1.1. General sketch of the system

2 | Theory

In this chapter there will be descriptions of the theory used to deliver on thesubjects and goals stated in Chapter 1. It will also explain the notations andconcepts used throughout the thesis.

2.1 Notation• EM = Electro Magnetic.

• ε = Permittivity.

• εr = Dielectric constant.

• µ = Permeability.

• σ = Conductivity [S/m].

• α = Attenuation constant [dB/m].

• VLF = Very low frequency [3 to 30 KHz].

• ELF = Extremely low frequency [3 to 30 Hz].

2.2 Maxwell equationThe Maxwell equations outline the fundamentals of how electric and magneticfields interact. If one have a polarized, linear plane and an EM wave propagatingin the Z direction (random straight line), this can be described in terms of fieldswhere the following holds:

Ex = E0 · ejωt−γz, (2.1)Hx = H0 · ejωt−γz, (2.2)

γ = jω

√εµ− j σµ

ω= α+ jβ and (2.3)

β = 2πλ

(2.4)

3

4 Theory

where E and H represent the electric and the magnetic field, while γ is the prop-agation constant. The propagation constant is dependent on permittivity (ε),permeability (µ) and conductivity (σ). It can be represented as a sum of the at-tenuation constant (α) and the phase constant (β).

If an example with σ = 4 and f = 10MHz is used the sum of α+ jβ will be110 + j0.5 dB. The real part is so much larger then the imaginary that it can beignored and then say that the propagation constant is equal to the attenuationconstant.

The Maxwell equation gives a negative (kx+m) line based around the attenuationconstant over distance (Figure 2.1) for seawater. What has been found by earlierresearch [2] is that in the near field region the real world follows the equation butwhen it enters the far field region, the signal decrease will level out to only a fewdecibels per meter, see the graph (Figure 2.1).

Figure 2.1. Signal propagation over 90 m using a RF transmitter with the power of5 W at the Liverpool marina [2]

In this thesis there will be use of the Maxwell equations to speculate and calculatea theoretical minimum distance. For the real world application, tests must beperformed to have any real idea where the maximum effective range of the systemis.

2.3 PropagationThis thesis will be handling wave propagation, which is how waves travel throughany medium. Electronic/light waves can also traverse a non medium which is anvacuum, no other type of wave can do this [3].

Propagation of radio signals in the High Frequency (HF) spectrum can be verylimited in an underwater environment. This has to do with absorption in the

2.3 Propagation 5

medium and reflection from its surface. If the medium is a conductor, like a metalor seawater (if a poor one), the signal will generally be absorbed into the materialwhile some of it is reflected. If one instead have a dielectric medium, for exampleporcelain or wood, which are none conductors, some of the signal will pass rightthrough while some of it will be reflected back.

In seawater the attenuation [4] shows that most of the signal power will be lostover just one meter, this means that all this energy is either absorbed into themedium, reflected from the surface or distorted due to scattering, for examplewhen it leaves the antenna and passes into the water which is a different medium.

An example of what have been mentioned above can done by looking at the pen-etration depth of the different mediums, using the skin depth. This is a mea-surement how far the signal penetrates before it has about 37% or 1

e of its energyremaining [3]. When travelling through a conductor which has the permeability forvacuum (u0) and the conductivity for the medium (σ). In the example used thereis a 10 MHz signal, a rounded value for µ0 and three different kinds of medium:

δ = 1√πfµσ

, (2.5)

δair = 1√π · 107 · µ0 · 2.95 · 10−15

= 2930291 m, (2.6)

δtap water = 1√π · 107 · µ0 · 0.02

= 1.12539 . . . ≈ 1.13 m and (2.7)

δseawater = 1√π · 107 · µ0 · 4

= 0.07957 . . . ≈ 0.08 m. (2.8)

As seen, there is almost no loss of effect due to conductivity when in air, sinceit is a resistive medium. When using water on the other hand and especiallyseawater the signal gains a huge propagation loss just to the fact that the wateris a conductor and not a resistor. Seawater is a lossy medium, meaning it isn’t agreat conductor but still a decent one. If one were to take a material like copper,which is a great conductor, the penetration would be 20 µm using a 10 MHz signallike in the example above.

2.3.1 Radio wavesRadio Waves was the focus of this thesis. They are electromagnetic waves andare defined in the frequency spectrum of Hz to GHz. Radio waves do not travelvery far in water and especially not in seawater due to it being a conductor or a’lossy medium’ [5]. The higher the frequency of the signal the faster it attenuatesor ’dies out’ (Section 2.3.3). This posses an interesting challenge when designingthe system, the trade-off between distance and data rate.

One important part about radio waves are that they are divided into 3 differentfields or ranges when they are leaving the antenna. The near field, transition zone

6 Theory

and the far field [6]. Most models and equations about the behaviour of radiowaves are based on them reaching into the far field. This is when electric field(E) and the magnetic field (H) have (Section 2.2) become completely planar andintersect. Before this we have the near field and an undefined area, usually referredto as the transition zone. In this regions there are very few models of the signalsbehaviour since they are ’rounded’ close to the antenna and every case becomesunique depending on the type and structure. For example, in the very close nearfield a loop antenna will work as a transformer because of the strong magneticfield it generates [6].

2.3.2 Acoustic wavesAcoustic waves are a type of mechanical waves and when it comes to underwateruse the frequency spectrum is in the ELF to VLF range, from Hz to KHz. Thisleads to very low data rate but the signals can be very far reaching in seawater.Acoustic waves have a propagation speed of 1500 m/s, this can lead to time delayin communication systems since it is rather slow compared to radio waves, whichtravel at 3.3 · 106 m/s in the same medium (Section 2.3.5). They also suffer frommultipath propagation in shallow waters due to reflection and refraction since theylack the ability to penetrate through objects, often creating shadow zones with nosignal behind the object.

2.3.3 AttenuationAttenuation is a physical phenomena in which a flux (EM-wave for this thesis) thatpasses through a medium that is non-vacuum have some of its power removed alongthe way, when it interacts with objects in the non vacuum. This could be downto a lot of thing, like backscattering, reflection, absorption and so on. In physicsthere is often a pre-measured value of different materials, often denoted α and iscalled the attenuation factor [7]. Which is then multiplied with the distance andfrequency to learn how much of the signal will be lost, for example:

Attenuation = α [db/(MHz · cm)] · l [cm] · f [MHz]. (2.9)Butler [7] believed that to calculate the attenuation factor in seawater with aconductivity that is denoted σ = 4 S/m and frequency which is f = 10 MHz. Heapplied the formula:

α = 0.0173 ·√f · σ ≈ 110 [dB/m]. (2.10)

This have been expanded upon by a later scientific paper [5]. They instead listthat the signal attenuation has to be split into near field and far field. In the nearfield the attenuation follows Maxwell’s equations (Section 2.2):

E = E0 · e−σz · ej(ωt−βz) and (2.11)

α =√ωµσ

2 dB/m. (2.12)

2.3 Propagation 7

This will create a huge near field attenuation loss, especially at high frequencies.This can be seen in Figure 2.1. This is where Equation 2.10 comes out to prettymuch the same result as Lucas. There then follows the far field attenuation losswhich is much lesser and come out as:

E

E0= e−9.29·106·f2z ↔ 20 · log10

(E

E0

)= Attenuation loss in dB. (2.13)

At this point the attenuation will less sever, even for frequencies in the MHz band.For example a signal at 25 MHz will have a far field attenuation at just 5 dB over adistance of 100 m. This is line with current theory and papers which are outlinedin the related work (Chapter 3).

As seen in Equation 2.11, the attenuation for this thesis will be based around thefrequency chosen, the medium the signal is sent through as well as the distance itis to propagate. Research has also show that the total path loss will be larger atdeeper depths (Figure 2.3.3). This has not a clear explanation in any mathematicalmodel but is something that has been measured [8]. This needs to be taken intoconsideration when designing the system.

Figure 2.2. Total path loss as a function of frequency and depth [8]

Attenuation can give us a decent estimation off how much signal power there willbe after Z amount of distance. There will be more information about other factorsthat will affect the signal power in the receiver, Section 2.3.4 and 2.5.

8 Theory

2.3.4 Refraction lossThis subject will only be touched upon lightly in this thesis since the focus ismainly on pure underwater communication. Refraction loss is the signal powerlost when a flux passes between two mediums with different refraction index, forexample air and seawater. According to Lloyd Butler [7] this is calculated as:

Refraction loss [dB] = −20 · log10

(7.4586

106 ·√f

σ

). (2.14)

So using the example from Section 2.3.3 there would be a refraction loss of≈ 39 dB.This can in many applications be negated by having the signal pass between thetwo mediums using an antenna in one medium connected by a cable to the equip-ment in the other. To calculate general refraction there is a need to know how thesignal bends when passing between mediums, the equation used is:

n = c

v(2.15)

where n is the refraction index (RI), c is the speed of light and v is the speedthrough the medium. For air RI ≈ 1 which means that v ≈ c.

2.3.5 EM-signals in seawaterElectromagnetic signals in seawater have a few different properties from whenthey pass through air. They attenuate faster, have a shorter wavelength andslower travel time [5]. This is because the speed of light changes when it passesthrough a medium such as water, on account of its dielectric constant (or rela-tive permittivity) (εr) and relative permeability (µr). While µr = 1 when talkingabout seawater, the εr is a function of frequency and is calculated as a the differ-ence between permittivity for the medium divided by permittivity of a vacuum,following:

εr(ω) = ε(ω)ε0

. (2.16)

It is known that the εr of seawater varies between 72-81 depending on frequencyand for the MHz range of signals this is ≈ 81 [9]. When these values are used andput into an equation, the result is:

cwater = 1√εr · µr

≈ 33.3 · 106 m/s. (2.17)

This combined with the higher refraction index also gives the signal a much shorterwavelength. The equation for calculating wavelength in conductive mediums is thefollowing according to peer reviewed reports [7, 2]:

λ [m] = 1000 ∗√

10f ∗ σ

≈ 1.56 ∗

√4 ∗ 106

f ∗ σ(2.18)

2.4 Antenna types 9

which can create the example of using the frequency(f) 10 MHz and the conduc-tivity (σ) being 4, giving a wavelength that will be λ = 0.5 m. This is about60 times shorter than what was had in air. So when designing an antenna forunderwater use, it will shrink to a fraction of the size that is needed for resonancein air. This could have some very interesting properties if one wants to be ableto move between mediums, since the only variable that is changeable will be thefrequency input to the antenna.

2.4 Antenna typesOne of the first parts for the implementation of the system is to choose the hard-ware and antenna needed so it is possible to dimension the system. There aremultiple antenna types that can be used in for signal propagation underwater. Inthis section the pros and cons of each will be discussed.

2.4.1 Loop antennaA basic loop antenna is a metal ring where the diameter is specified to a certainfrequency. Electrical loop antennas are usually classified into small and large [10].There was a focus on the small variant since it is the mostly widely implementedand a large antenna would be extremely cumbersome to move around since itscircumference is ≈ λ when a small loop would have its circumference < λ

3 [11]. Anexample of electrically smaller loop antenna is a half-wave loop, where N is thenumber of turns of the loop. This is calculated with:

λ

2 = Nπd↔ d = λ

N · 2π (2.19)

giving the loop antenna the advantage of being able to be built smaller then mostof its counterparts because it only gets a fraction of the wavelength as the diame-ter. As the λ in water is already so much shorter (Section 2.3.5) than in air, thiscan create a very manageable diameter for the antenna.

A loop antenna typically has a very low radiation resistance and as a result of thisa very low efficiency. It creates a magnetic field with a current and this is done bypushing a voltage through a resistance, what is seen is that:

Rr =√uo/ε06π β4(NA)2 =

(√uo/ε0

) 83π

3(NA

λ2

)2(2.20)

where β = 2πλ , A = Area m2 and N = number of turns of the loop antenna. So

to use an example that has a λ = 3.3 m, N = 2, r = 13.5 cm and is lowered inseawater there will be a radiation resistance off:

Rr = 4283π

3(

2(13.5 ∗ 10−2)2π

3.32

)2

= 0.3839 . . . ≈ 0.38 Ω. (2.21)

10 Theory

According to Ohm’s law for time varying currents the radiated power from a loopantenna can be calculated. Based on the radiation resistance and the current feedinto the antenna:

Rr = 2PradiatedI2

0and (2.22)

Pradiated = 112π

√uo/ε0 ∗ β4(I0A)2. (2.23)

Look at Equation 2.22 and one can clearly see that having a low radiation resis-tance will directly affect the output power, making a loop antenna very inefficient.Another drawback of the loop antenna is that it needs almost perfect impedancematching since any effect lost due to reflections will give it an even worse efficiency.

2.4.2 Dipole antenna

A dipole antenna is the oldest and most basic antenna type of them all. Thissection will focus on a centre-fed half-wave dipole. It simply has two parts, apositive and negative half that forms a rod capable of receiving both the positiveand negative part of the electromagnetic wave or in layman terms, the "radiosignal".

Length of a half-wave dipole [m] : l = 12 ·Aλ. (2.24)

The equation above is for the most widely used type of dipole, where l = lengthof the antenna in meters. It is rather straightforward with the exception of theA. That is an adjustment factor to cancel the reactants in the antenna, thatappears because the signal isn’t propagating in free space but in a conductor (likea copper wire) before being transmitted. A is dependant on the diameter (d) ofthe conductor and the wavelength of the signal (λ), see Figure 2.3.

2.4.3 J-pole antenna

A J-pole antenna has its name from the shape of the design. It is relatively easyto build and takes a lot of its design from the dipole. It’s built around having asmaller and larger dipole configured like J-type in Figure 2.4.

The advantage of a J-type antenna is that is have a greater underwater reach thena loop antenna [13]. The drawbacks is that is has a lower SNR then the loop andas the dipole, suffers from being bulky. Reaching upwards 3 m when the signal isin the 10 MHz range even when submerged in seawater.

2.5 Impedance matching 11

Figure 2.3. Graph of A depending on the λdratio [12]

Figure 2.4. J-pole Antenna and variations of same [14]

2.5 Impedance matchingImpedance matching is the technique to minimize reflection of the signal in theantenna or to put it blunt, to send the same amount energy as you input and nothave anything go backwards.

γ = ZL − Z0

ZL + Z0(2.25)

where ZL and Z0 are the impedance values at the load and transmission line re-spectively. γ is a measurement of how much of the signal is reflect, that goesbetween −1 ≤ 0 ≤ 1. If the load and transmission impedance matches up per-fectly there is no reflection according to Equation 2.25. But if they are mismatched

12 Theory

or one of them is zero it feeds back a portion of the out energy into the system. Ifthe γ = −1 there is a short-circuit case and full reflection with a negative phase,if γ = 1 there is a open-circuit case and full reflection but with a positive phasethis time. The implication of this is to take great care when designing the systemand always keep the impedance in mind to minimize transfer loss.

Since impedance often can be imaginary, Equation 2.26 says that if the "StandingWave Reflection" is equal to one there is zero reflections between the source andthe load. For example, between a signal generator and an antenna.

|γ| = SWR− 1SWR+ 1 (2.26)

where SWR or VSWR (Voltage Standing Wave Ratio) is the value mostly com-monly referred to in data sheets and alike. This is the ratio between the highestand lowest voltage along the transmission line. This means if the V SWR = 1,there is zero reflection and the voltage is equal at all points along the transmissionline.

2.5.1 Impedance matching a loop antennaAs was mentioned in the end of Section 2.4.1, there is always want for the perfectimpedance matching to minimize VSWR. For an electrically small loop antennathe impedance (Z) is:

Z = Rr + Zi + jωLe = Rr +Ri + jω(Le + Li) (2.27)

Where Rr is radiation resistance, Zi is the internal impedance, Le is the reactanceof the external inductance. The terms of Ri and Li are parts of the internalimpedance, Zi = Ri + jωLi. A equivalent circuit for the impedance is picturedin Figure 2.5.1 with a theoretical C added. This is most often omitted since avariable capacitance is usually placed in parallel with the loop to tune out itsinductance; the capacitance of the loop simply decreases the value of the parallelcapacitance needed [10, p.5-3].

2.6 Digital modulationModulation is technique in which data is transferred from one unit to another.It’s mostly used when talking about wireless communication but most be done forany system where to units are going to be ’talking’ to each other. In this sectionI’ll go over the basics of the more used modulation techniques.

Modulation can in many ways be seen as the language between to systems. Insimpler systems 1 and 0 might be represented as having energy present or not.With more advanced modulation techniques it will usually be a combination ofenergy, phase and frequency that decides which string of bits that are being sent.

2.6 Digital modulation 13

Figure 2.5. Equivalent circuit of the input impedance Z

2.6.1 OOK modulationOn-Off keying is one type of binary modulation and is the simplest form of alldigital modulations. It simply works of there being signal energy present or not.If there is a signal into the receiver over t amount of time that is a 1 and no signalis a 0. This is represented as [15]:

s0(t) = 0 and s1(t) =√

4EavgT· cos (2πfct) (2.28)

where the signal goes over 0 ≤ t ≤ T with a carrier frequency such as 2fcT > 0.

2.6.2 BPSK modulationBinary Phase Shift Keying is another type of binary modulation. With this tech-nique the 1 and 0 are represented by using a shift in phase, normally π. Thesystem will interpret the binary bits like:

s0(t) =√

2EavgT· cos (2πfct) and s1(t) =

√2EavgT· cos (2πfct+ π) (2.29)

which gives that a system checks for a signal with a certain energy level (EAvg)over T amount of time. When this is present, the phase of the signal is measuredto detect if the system should interpret it as a 1 or 0. For this type of modulation,the receiver always has to know the exact phase of the transmitted signal else therecan be a phase error and 1 turns to 0 and so on. This problem just gets worse theless phase there is between the bits, see Section 2.6.4.

2.6.3 ASK modulationAmplitude Shift Keying is a straightforward extension of OOK with more powerlevels than on and off. Usually there is an even amount of signals and they are inturn evenly distributed in a signal space. An example is when there is 4 signals orM = 4.

14 Theory

si(t) = siφ(t) for i = 1, 2, 3, 4, (2.30)

φ(t) =√

2T

cos(2πf0t) for 0 ≤ t ≤ T (2.31)

and referring to Figure 2.6.3 it can clearly be seen that a clear knowledge ofwhich power input generates which output is needed to be able to demodulate thedifferent strings of bits.

Figure 2.6. Signal space diagram ASK with 4 signals

2.6.4 PSK modulationPhase shift keying is an extension to BPSK, where there is more signals in thesignal constellation than 2. SinceM = 2k or an even number, larger then two thatis a base of two is preferred. The example used M = 4.

si(t) =√

2EavgT· cos(2πfct+ (2i− 1) π

max(i) ) 0 ≤ t ≤ T (2.32)

where a constellations of four points is gained, that are all√E from origo. They

are phase shifted with π4 , so to demodulate the signal the receiver needs to know

the original phase from the transmitter know much it shifted.

2.7 Gaussian white noiseWhite Gaussian noise is special case of gaussian noise. gaussian noise is a sta-tistical noise where PDF (Probability Density Function) is equal to the normaldistribution. The special case for white gaussian noise is that the values at anypair of times are identically distributed and statistically independent (and henceuncorrelated).

In telecommunication and communication channels in general, the use white gaus-sian noise is prevalent to create additive white gaussian noise (AWGN) on a chan-nel. This is used to mimic the random process and noise that can occur in nature

2.8 Commercial systems 15

while still having a uniform power over a frequency band and a normal distri-bution, making it rather easy to create a mathematical model. The stochasticvariable for the Gaussian PDF, can said to be X and that is given by:

fX(x) = 1√2πσ

e−(x−m)2

2σ2 (2.33)

where E(x) = σ is the variance of the PDF, V (x) = m is the mean. For an idealreceiver (i.e. perfect components) there will be white noise that is equal for anyand every frequency. This not possible in the real world since such a signal wouldhave infinite energy, so it starts to fall off in the [THz] region but for the purposeof this thesis it is treated as:

N0 = kT0 (2.34)

where k = 1.38 ·10−23 joule/kelvin and T0 is the local temperature in kelvin. Thisgives a noise level of Sn(f) = N0

2 for any frequency since the noise level is splitbetween the positive and negative part of the frequency spectrum.

-

6Sn(f)

N02

f

Figure 2.7. White Gaussian Noise

2.8 Commercial systems

2.8.1 Near field communicationNear field communication is a version of Radio Frequency Identification (RFID).Developed and classified in 2002 by Sony and Philips (today NXP Semiconduc-tors). NFC is used to communicate between small electronic devices and just asRFID, has the ability for the transmitter to power up the receiver through wirelesspower transfer. The radius for the system is in the maximum range of 10 cm inair, works with the frequency of 13.56 MHz and with a maximum bandwidth of424 Kbit/s [16].

The possibility of using both RFID and NFC for underwater applications wasstudied in 2013 by Benelli and Pozzebon [17]. While they confirmed that lowfrequency RFID has a space to fill for identification of fish and rocks under water,high frequency RFID as well as NFC had an effective range of 3 cm. For sucha low power interface with the near field attenuation present in seawater, they

16 Theory

would in my opinion require to be in physical contact to guarantee a stable dataconnection. This is the reason that NFC was ruled out for this thesis.

2.8.2 BluetoothBluetooth is a wireless communications interface in the 2.45 GHz with a trans-fer rate of up to 54 Mb/s [18, p.168] , using a frequency hopping technology tominimize interference in the communication. It has a maximum specified powerof 100 dBm that gives it a range of about 100 m in air [18, p.346]. It works bypairing two devices with each other and having them follow the same jumpingpattern, this way they are essentially only talking with one another but can workin the same frequency range as quite a few other devices without any interferencebetween them.

Advantages of Bluetooth is that it is easy to use, have a built in encryption in formof frequency hopping and still has a relative low power consumption, using only afew mA. The disadvantages is that in the 2.45 GHz range it is rapidly approachingpenetration depths (Equation 2.5) of zero when submerged in seawater. There havebeen some trials with systems in this frequency range [19], though none of themhave been with Bluetooth. What was found was that they could not penetratebeyond 17 cm without suffering large packet loss and even when increasing thepower this range would not move. This makes Bluetooth unsuitable for the systemsince the goals stated (Section 1.2) was to have an effective distance of at least 1m.

2.8.3 OceanreefOceanreef is a commercial product used for underwater voice communication usingwireless ultrasonic technology [20, p.27]. This system works around the frequencyof 37.768 kHz and is specified to have a range of up to 200 m. This system fillsup most of the requirements that are specified in Section 1.2 but it is an acousticwave (Section 2.3.2) system and therefore not in the goals of this thesis. Since thisthesis focuses on the use of RF waves (Section 2.3.1). Because of the advantagesdetailed in the mentioned sections.

3 | Related Research

In this chapter, related research will be covered and their differentiated conclusionsare listed. Since part of the work builds on this information it is relevant to havea basic grasp of what they include for full understanding of the thesis.

A. Shaw and A.I. Al-Shamma’aThese Liverpool based researchers have been writing reports from 2004 on thissubject [4] and still are to this day. They have done a lot of research on real lifebehaviour of electromagnetic signals under water. They have mostly been usingloop antennas to explore on how an EM signal behaves in a test environment orin shallow seawater like a harbour.

Much of their work has laid the foundation for this thesis since they have proven[2] that a tone can be sent over a distance of at least 90 m when using a 5 MHzcarrier in DSB-LC modulation. This make it seem plausible that you should beable to transfer full audio using 10 MHz carrier with a yet to be decided modula-tion.

In their paper from 2009 [9] they also showed that 6.0 and 10.7 MHz could be idealfrequencies for transmitting data under water, see Figure 3.1. Using an easy tobuild system with loop antennas and OOK modulation (Section 2.6.1) they wereable to have a signal level of about -60 dB into the receiving antenna. A caveatwas that this experiment was done in a lab environment and not outside, in anactual harbour or equivalent.

J Lucas and Ck YipThese (also) Liverpool based researchers have written a very interesting paper onsignal attenuation in seawater [5]. Their focus is to find which frequency is suitablefor electromagnetic transmissions when the distance is in the +100 meters range.They also want to find a solution to the disparity in signal behaviour between thenear and far field range.

The report differentiates it self from a few others by having different εr valuesand going into more detail in behaviour between sea- and fresh water. Though it

17

18 Related Research

Figure 3.1. Attenuation graph from the concluding part of the paper [9]

comes the same conclusion that frequencies up to 10 MHz are suitable to propagatesignals up to 100 m in seawater.

Carlos Uribe and Walter GroteThe authors of this report [21] is out to try and define a propagation model forEM signals in seawater. Their equations make a decent bit of sense when readingthe whole thing and what they achieve is to sum up much of the research up until2009 and forward, as can be seen in Figure 3.2.

Figure 3.2. Summary from the concluding part of the paper [21]

4 | Method

4.1 The processWhen this thesis started the original goal was to create a fully functional systemfor underwater use. Due to complications in the process and time limitations onlya few antennas was created and tested, under non ideal environments. The processwent from starting to outline the system, then looking into antenna choices andcoming up short. This lead into design of own antennas for use which took moretime and effort then was anticipated at the start of it all. This lead into therealization that there was a lack of proper testing equipment for antenna usageand especially for underwater usage. Which meant that the testing was madewhich what equipment that was available, in suboptimal environment.

4.2 The systemIn this section there will be a general description of the system and layout. Howthe information will be transferred inside the system and how the signal shouldbe modulated to have greatest efficiency in water and for the systems design.

4.2.1 LayoutAs seen in Figure 4.1, the layout of the system would have been rather straightforward with a ADC sampling the signal, transferring it to the MPU which willbreak it down into individual bits, pass these through to a multiplier that adds thecarrier frequency from the signal generator (Figure 4.3.4), it then passes throughRF-filer and RF-amplifier before going up into the antenna.

Figure 4.1. Sketch of the system with all signals represented

19

20 Method

4.2.2 Signals

As seen in Figure 4.1 there will 3 stages of the signal. A sample with 12-bitresolution from the analogue signal that is transferred as square waves to thereceiver. What needs to be said about the square wave in Figure 4.1 is that itisn’t a true square wave that goes between a Vhigh and Vlow, it instead would haveonge between Vhigh and 0 . Since the system is using OOK keying it represents ifhave an energy over the channel or not.

4.2.3 Modulation

For the hardware implementation of the system it supposed to go with OOKmodulation (Section 2.6.1). Because of its ease to implement and since it hasno phase or anything of the sort, the receiver only has to measure Pin over aperiod of T time to see if there is a 1 or a 0 being sent. Another large advantageof using OOK modulation is the ease to change the frequency of the signal inboth the transmitter and receiver. Since the antenna properties will change in thedifferent mediums there is an advantage to be able to have different frequencies,the potential is that the system can use the same antenna for all mediums, it’s justthe frequency that changes. A problem with OOK modulation is that it requiresa larger bandwidth then most modulations. Since the thesis never reached a pointwhere implementation become a reality there is no way of knowing how well OOKmodulation had worked out in comparison to other modulations.

4.3 Hardware

For this system to work it needs to have an ADC with a sample depth of 12 bitsthat can at least handle more then 11025 samples, see Section 1.2. This equationthen becomes that:

Transfer Rate = 12 · 11025 = 132300 Hz = 132.3 Kb/s (4.1)

and for sending out the information it uses a RF-filter and a RF-amplifier toremove any distortions that could be applied to the signal and then amplified itwith enough transmission strength to penetrate even through a lossy medium, suchas seawater. A note here is that the decision of the amplifier will be dependant onhow sensitive the power detector for the system is. An advantage is that there willbe very little noise since it is working under water, so even if the system have alarge attenuation it will just need a sensitive enough power detector and possiblean LNA (Low Noise Amplifier) to get rid of any harmonics in the signal. The bitstransferred will then input and saved in the MPU, making it possible to rebuildthe signal by sending the 12-bits through a DAC, generating a sound to an outputdevice, like a headset.

4.3 Hardware 21

4.3.1 Low pass filter

The system would have used a simple low pass filter to remove any input frequen-cies outside of the human speech range before it enters the system. The humanrange of speech is between 50-3400 Hz, even though we can hear up to 20 kHz.The low pass filter could therefore have a cut off frequency at 3.4 kHz, this wouldlower the bandwidth in use by a great deal. If a simple RC filter is used, wherethere is a 50 Ω resistor present, it would have the function off:

fc = 12πRC ↔ C = 1

3.4 · 103 · 2π50 ≈ 936 nF. (4.2)

4.3.2 Analogue digital converter

This piece of electrical equipment samples an analogue signal and converts eachof them into a string of bits, where the resolution is decided on how many bits areused. 12-bits means that the value of the signal can be represented with a depthbetween 0-2048, since one bit is used to represent if the signal sample is negativeor positive. The systems sampling rate should be at least 11025 Hz (Section 1.2)which at least a bit depth of 12 for each sample, which can easily be achieved withoff the shelf hardware, for example [22].

4.3.3 Micro processing unit

Their will be two micro processing unit (MPU) in the circuit. The first ones maintask will be to take the bits incoming from the ADC, split them up and outputthem to the RF-amplifier and the transmitting antenna (Tx), at a pace where thepower detector will follow the timing of the OOK modulation. Where the secondrecreate the string of bits dependant on what the power detector detects as theincoming bits. The focus will be on keeping them small, power efficient and withgood timings so that the OOK modulation doesn’t drift off and get bit errors.

4.3.4 Signal generator

The signal generator can either be a stand alone apparatus or a smaller integratedpart, like an oscillator. In this system it would have been used to generate thecarrier frequency sine wave that is multiplied with the square waveforms comingout of the MPU. This will create a waveform that looks alike the line off Fig-ure 4.3.4. The sinus wave inside of each 1 for the power detector, no signal isequal to no power. This is an idealized description of the generator, the multipli-cations won’t be perfect and there will be noise in the spectrum, that might haveto be compensated for.

22 Method

Figure 4.2. OOK Modulation [9]. Energy over T time indicates a ’1’.

4.3.5 RF filterThe RF-filter used before the amplifier will usually be a bandpass filter. Thisshould be frequency matched or be even more narrow to avoid that any amplifyingof resonance or noise occurs.

4.3.6 RF amplifierThe RF amplifier should would have been tuned to the frequency used in theantenna, preferentially with a narrow amplification band that has a steep dropoff. This means that in a perfect world it would act as in Figure 4.3. In a noneperfect world the RF-filter will have a larger BW then wished for and that iswhy a RF-filter (4.3.5) is used beforehand. This removes as much of the addednoise as possible, preventing it from being lumped together and amplified in thetransmitted signal.

-

6' $High dB

Zero dB fc

Figure 4.3. Theory of a RF-amplifier

4.3.7 Low noise amplifierA low noise amplifier (LNA) is used to amplify very weak signals, for example,those captured by an antenna. The main advantages of the LNA is that it has avery low noise figure but a large gain. Then look at the Frii’s formula for noiseand see that the total noise factor F will be small since the gain suppresses it inevery stage.

4.4 Antenna/transfer system choice 23

Ftot = F1 + F2 − 1G1

+ F3 − 1G1G2

+ . . .+ Fn − 1G1G2 . . . Gn

. (4.3)

What is seen in Equation 4.3 is that each cascading stage adds less noise powerthen the one before it. Since nearly all the noise power is created in the first step,it can summed into one term Frest and then it becomes that the noise power forthe LNA as:

Freciver = FLNA + Frest − 1GLNA

, (4.4)

so when choosing the LNA one will have to take into account the gain and noisefigure in the first hand. As well as bandwidth, stability and input, output VSWR(Section 2.5).

4.3.8 Power detectorA power detector circuit is most often used to sense what signal input is presentand then convert it into a voltage on a linear scale specified by the manufacturer[23]. It is needed to do specific readings with the antenna, amplifier and LNAmounted to know what the general output (dBm) is at. This is then used todecide where to draw the line between the 1 and 0 bit. This is of course donein the MPU (Section 4.3.3), where it will have a voltage level generated from thedetector and decide what it count that bit as.

4.3.9 Digital analogue converterA digital analogue converter (DAC) that takes digital data and converts into ananalogue signal on the other side, it’s the reversal of an ADC (Section 4.3.2) . Theform and accuracy of the output signal is dependent on the sample rate of theDAC as well as the bit depth received.

4.4 Antenna/transfer system choiceBased on the pros and cons of each antenna/system type (Section 2.4.1, 2.4.2,2.4.3 & 2.8) as well as what researchers have used beforehand (Chapter 3), it waschosen to go with the loop antenna. The first advantage is the size which shrinks toalmost a fraction of either the dipole or the J-pole which will be a huge advantagewhen it comes to implementation of the system. Even if the other antennas willgive us a slightly better received voltage [13]. There is also the fact that nearlyall research up to this point have been using loop antennas [4, 5, 9, 21, 24] whichlead to the belief that this had to be the more advantageous choice.

4.4.1 Discarded choicesAs mentioned in theory chapter (Section 2) there were multiple choices includingboth antenna constellations as well as complete commercial systems. The other

24 Method

choices of antenna was mostly deemed to bulky to be really feasible for use bya person moving underwater. The commercial system were all interesting butfailed in some aspect. The first one to fall was Blutooth due to it being in ato high a frequency spectrum. There have been tests with 2.4 GHz signals inseawater and the maximum distance achieved was 18 cm [19]. Oceanreef is a fullyfunctional underwater communication system used by diver but it uses ultrasonicwaves which are acoustic waves and not in the scope for the goals of this thesis.Lastly is the NFC communication system which was the hardest to rule out amongall of them. Tests have been done with RFID which is the predecessor to NFC[17], they confirmed that the power output and combined with the attenuation atthe frequency is to much to transmit any distances. Combine this with generalinformation found among enthusiasts that say NFC never reaches its peak potentialfor data transfer raised the question what would be the advantage over just a cablewith a magnetic clamp. This last part was taken from forums and no tests havebeen found that would verify or dismiss the information.

4.5 Loop antenna constructionThe loop antenna is built out of copper pipes with a diameter of 15 mm and thecopper itself is 1 mm wide. It was built with a diameter of 27 cm with 2 coilswhich should have been the resonance for a half wave antenna working at about90 MHz (Equation 2.19).

λ = 2 · 2π0.27 = 3.4m→ f = 88.4 MHz and (4.5)λ = 2 · 3π0.27 = 5.1m→ f = 58.9 MHz. (4.6)

The antennas had an air core, the reason being the skin effect (Section 2.3), whichmeans that 63% of the current will travel in a few µm of the cladding at thefrequencies that was to be used in the system, in MHz that is. A thing to noteabout the tests below is that they are done in a very noise environment that alsoincludes a lot of close by metal objects. This can very easily change the radiationpattern since the signals can be absorbed or reflected by nearby materials. Thetests were therefore only an indicator of how well the antennas would be workingin a real world environment but the resonance frequencies should match up wellenough towards the real world values.

4.6 Impedance matchingAs show in Section 2.5.1, the impedance match for the antenna one need to gaina zero sum reflection which means that γ = 0. Referring back to Equation 2.25it is stipulated that ZL = ZS for this to happen, this means there has to bemeasurements over the antenna after it is built to know how much is needed tocompensate with. This was supposed to be done using a impedance analyser wherethe antenna at first will be measured in air, then put into tap-, sea- and brackish

4.7 Measuring the signals 25

water, to make an estimation of how large capacitance is needed to match theimpedance in each medium. Because of the lack of access to a network analyserin the region the experiments had to be cut short and could only be achieved forthe results for air during this thesis. This means that calculating the impedancematch for water became speculation.

4.7 Measuring the signals

Measuring was started with sending just a pure sinus to test the connection, thefirst measurement test was with an oscilloscope that was connected between theTx and Rx of the system. This allowed for understanding how much of the powerwas being transferred and allowed one to see how the transmission medium dis-torts the signal.

There also needed to be measurement for any kind of EM propagation throughthe air down into the water [5]. This could either be done by having a separatespectrum analyser or having an antenna port on the oscilloscope used, the laterone would be the most effective since it is easier can compare all 3 signals at thesame time, in the same graph.

The goal is to have a signal level stronger then the noise level into the receivingantenna (Rx). This only required that the power detector (Figure 4.1) is sensitiveenough to pick up the very attenuated signal after filtering, since it will have anoise level in the range of -100 dBm or less.

4.8 A second system

During the thinking process about this system it came up another layout for thesystem. This layout is less cluttered and could be an interesting for further use.Ultimately it was chosen to not go with the second system since it’s (in my opinion)harder to find fitting hardware for it. It will also be more difficult to change thecarrier frequency for the system without a switch in hardware.

Figure 4.4. Layout of the second system.

The difference main of the system described in Figure 4.8 to the system in Fig-ure 4.1 is that signal is transmitted somewhat directly from the MPU to theantenna. This is a form of OOK modulation but now it’s sending pure square

26 Method

wave over the antenna, which changes the detection to a signal level and not justdetection of a signal being present.

5 | Result

5.1 First trial

The first pair of loop antennas are a 3 turn loop as the transmitter and a 5 turnloop as the receiver. They are made of coiled copper pipe with the two ends havingthe plastic removed to make room for the connector. The connector was made outof a peeled coax cable with a BNC connector in one end and in the other, theground and conductor has been split up, then each of them has been soldered toone end of the antenna. This makes the voltage travel through the entire lengthof the coil and creates the signal which is passed over to the receiving antenna.The transmitter is connected to a high bandwidth signal generator, generating asignal with an amplitude of -10 dBm and the receiver is connected to a spectrumanalyser with a DC-stop in place (Figure 5.1).

Figure 5.1. Layout of the first trial. Two antennas suspended by wires.

For the first trial it had the best SNR values at the signal level of 30 MHz. Thesignal was completely buried in the noise at 90 MHz (Figure 5.2). This was ratherexpected since the two loops had resonances frequencies at 60 MHz respectively 35MHz. What should also be noted is that the amplitude of 30 MHz didn’t changewhen even when bringing the antennas further together or having them almost 2meters apart. It was a constant -30 dBm.

27

28 Result

Figure 5.2. Signal levels in Rx for Trial 1

5.2 Second trialThe second trial has a change in antenna placement since it was determined thatthe wires suspending them might be affecting the higher frequencies (Figure 5.7).The signal configuration is the same as Sections 5.1 but with the antennas changedfor a 2 turn loop as the Tx and a 3 turn loop antenna as the Rx. The signal powerfrom the transmitter is still -10 dBM.

Figure 5.3. Layout of the second trial

What was found here is that it had a signal level at -30 dBm for the frequency of 38MHz but now it also had a better SNR for the frequency of 91.2 MHz (Figure 5.4).This is in close to the calculated results in (Section 2.4.1) for the Tx.

λ = 2 · 2π(0.27) ≈ 3.4 m↔ f = C

λ= 88 MHz. (5.1)

5.3 Third trial 29


5.3 Third trialThis experiment was very close to Trial 2 in its set up (Section 5.2). This timeonly the receiving antenna (Rx) was changed to a 2 loop antenna, which was tobe closer to an ideal.

Figure 5.5. Layout of the third trial

5.4 Conclusion of the first three trialsWhat was found was that all the antennas used have resonant frequencies aroundthe 30 MHz mark. It was found that one constellation (Section 5.2) has an evenstronger peak at 90 MHz which is around the theoretical resonant frequency of

30 Result


that system (Equation 5.1). It can also been seen, that at peak performance it hada drop of 20 dBm from the original signal, this can be noted as the near field lossesfor the antenna for air. This is important information going into the underwatermeasurements, to see if the equations in the theory chapter holds up in the realworld (Section 2.3.5 and 2.3).

The conclusion of this trial was full of caveats but still gives a general idea on whatantenna constellation to use, as well as which frequencies that are ideal. Somethingthat had to be noted is that these trials was done in a none ideal environment,with both large metal objects and electronic equipment in close range. It is alsoacknowledged that the wires suspending the antennas in the first trials might haveinterfered with the signal. This was only discovered after 5 turn loop had beendestroyed the to create the smaller loops used in trial 2 and 3.

5.5 Fourth trial 31

Figure 5.7. Measuring results for Trail 1,2 & 3

5.5 Fourth trial

Trial 4 and 5 were done after the impedance testing (Section 5.7). The reasonfor this is that it was discovered just how much the cable placement as well aspositioning of the antenna affected the VSWR and signal levels of the system. Thesetup was close to that off the first 3 with the same distance of 1 meter between theantennas and a signal level at -10 dBm. In trial 4, 2-coil loop antennas was usedas both the Tx and Rx. What was seen in Figure 5.9, is that there are now highersignal levels across the board then previous trials. The test was also measuringup to 200 MHz which made the falloff beyond 100 MHz much more visible thenin the previous trials. The reason for the change in frequency range was to see ifthere would be higher range resonance frequencies to go with the lower levels ofVSWR in that area (Section 5.7).

32 Result

5.6 Fifth trialThe fifth trial was done with a 2-coil loop as the Tx and a 3-coil loop as the Rx.As in the fourth trial, the distance between the two antennas was 1 meter, thesignal level into the Tx was -10 dBm and the frequency ranged swept was 10-200MHz. This setup gave a slightly lower levels of signal in the region of 50-100 MHz.It has been acknowledged it could be an error that depends on the environment.

Figure 5.8. Layout of the fourth and fifth trial

Figure 5.9. Signal levels in Rx for Trial 4 and 5

5.7 Impedance measuringThe impedance measuring was done at campus Norrköping, LIU using a VectorNetwork Analyzer of the brand Rhode and Schwarz. Since the antenna lab was

5.7 Impedance measuring 33

temporary de-constructed it was needed to do the measurements inside an office,this creates the scenario that was had in Section 4.5, with a lot of metal close byand any tug on the cable would change the VSWR. The measurements were doneover a range from 10-200 MHz with a 1601 points accuracy and the antenna placedon a cardboard box (Figure 5.7). More detailed explanation is be provided belowbut one can say it is possible to push the two measured antennas down to a VSWRof 2 even with all the metal and human bodies close by. Another thing that waslearned from the tests is that the coaxial cables used isn’t as well isolated as onewould think, since they be could wrap around the antenna and that would createanother coil, moving the frequency where the VSWR was at its smallest towardsa lower point in frequency. This has afterwards been replicated when doing thesignal transfers as in Section 4.5, making the measurements rather moot since allthat was needed is a small flip of the cable to change it.

Figure 5.10. VSWR results scaled for visibility. Lower is better.

The impedance matching was not in line with the current theory, showing theantennas having their lowest VSWR at completely different frequencies then pre-dicted. This will be expanded upon in the discussion (Section 6). The reasonthat only two antennas were impedance tested is that one of the 2-coil antenna’sconnector was damaged during transport and was not available for measurement.

34 Result

Figure 5.11. The setup used for impedance measuring

5.7.1 3-coil loop antennaThe 3-coil antenna was tested with a piece of the cable striped to the side of it.This was done through to trial and error, giving the ideal VSWR that can be seenin Figure 5.7. The lowest values of VSWR was found at the frequency of 146 MHzwith a value of 2.5. Then take Equation 2.26 and make it a function of γ.

V SWR = 1 + |γ|1− |γ| ↔ 2.5 = 1 + |γ|

1− |γ| , (5.2)

|γ| = 2.5− 12.5 + 1 = 0.4285 . . . ≈ 0.43 (5.3)

and good area for the VSWR was at the point off 34MHz (Figure 5.7) where theVSWR was about 3. So that gave:

V SWR = 1 + |γ|1− |γ| ↔ 3 = 1 + |γ|

1− |γ| and (5.4)

|γ| = 3− 13 + 1 = 0.5. (5.5)

5.7.2 2-coil loop antennaThe 2-coil loop antenna was tested with a piece of the cable laying under it. Withthis structure it gained the lowest VSWR at the frequency of 180 MHz with avalue of about 2, as can bee seen in Figure 5.7. Using yet again Equation 2.26, itgives:

V SWR = 1 + |γ|1− |γ| ↔ 2 = 1 + |γ|

1− |γ| and (5.6)

|γ| = 2− 12 + 1 = 0.33333 . . . ≈ 0.33. (5.7)

5.8 Longer range trials 35

This antenna also showed a dip in the VSWR around 36 MHz (Figure 5.7). Thevalues here was as for the 3-coil antenna around 3.

V SWR = 1 + |γ|1− |γ| ↔ 3 = 1 + |γ|

1− |γ| and (5.8)

|γ| = 3− 13 + 1 = 0.5. (5.9)

5.7.3 CaveatsAs mentioned in Section 5.7 there are quite a few things that affects these resultsfrom being conclusive. The antennas placement was imitated as best possiblebut with the signal leakage through the cable as well as being two human beingscombined with a lot of metal close by, it is rather impossible to say what the signalwas interacting with. This is something that needs to be accepted for this thesis.The thesis can and will use the results as a baseline for how the antennas areworking. New signal measurements was also done in a broader frequency range tosee if a better result can be gained then the ones found in Section 5.4.

5.8 Longer range trialsAfter consulting with one of Jon Staffeld from Combitech there was a realizationthat the first trials were stuck in the near field (Section 2.3.5), this should havebeen realized because the receiving antenna could be moved around the transmitterwithout any noticeable difference in signal. As in it could be held above it, movedbetween 1-4 m in distance and polarised in the other direction. This lead to newtrials being done with a longer distance and confirmed change in signal whenmoving the antennas. Both tests are done with a distance of 10 m between thetransmitter and receiver, a -10 dBm signal with a 10 MHz bandwidth and thenoise floor was -60 dBm.

5.8.1 Sixth trialWhat can be seen is that the signal level has dropped and now it’s clear wherethe antenna is resonant, it is in the frequency range of 40-44 MHz. There is alsoa slight spike in the region of 150-154 MHz. In the first frequency there top valueis -39 dBm which gives a SNR of 60− 39 = 21 dBm. This means a drop from thetransmitter to the receiver by 39− 10 = 29 dBm. This is a rather large differencefrom the third trial (Section 5.3) which can be attributed to leaving the near fieldand having an increased distance between the antennas.

5.8.2 Seventh trialIn this seventh trial there is a signal level drop equal to that of the sixth (Sec-tion 5.8.1) but in the 37-44 MHz region of frequencies. So the first peak for this

36 Result

trial has a slightly larger bandwidth in the lower region, while not reaching the fullpeak in the higher one. Admitted this could be down to a measuring error sincethe testing was done by hand with a cursor being moved over the spectrogram togather the data.

Figure 5.12. Layout of the sixth and seventh trial

Figure 5.13. Signal strength into Tx over 10m

5.9 Trials in water 37

5.9 Trials in water

Because of the limited space of the containers used, the long trials were doneby submerging each antenna in a container filled with water, then separating thecontainers with 10 meters. These trials can somewhat be compared to the onesdone in Section 5.8 since they are done in the same range and close to the sameenvironment. Differences are that the signal strength had been increased by 10dBm making it 0 dBm, the antenna polarization was also changed to vertical, tomake the fit inside the containers. A reference test was done in air, which can beseen in the figure.

The short ranged trials were done in a plastic container filled with 100 L of water.The signal level used was once again -10 dBm because of the short distance of 2dm between the antennas. The measurements were done in both tap and differentkinds of salt water. One thing to note about these trials is that they go from 1-200MHz. The spectrum analyzer is supposed to go down to 10 KHz but there mightbe some measuring errors closing into the lower end of the spectrum.

Figure 5.14. Layout of the long range water testing

Figure 5.15. Layout of the short range water testing

38 Result

5.9.1 Tap water ranged trial 1This trial was done with 2-loop antennas being used as the Tx and Rx, the signaloutput from the generator was 00 dBm and noise floor was around -58 dBm.What can be seen in the graph is that the energy of the spectrum have moveddown in frequency, which is in line with the theory from Section 2.3.5. The peaksare focused around 20-25 MHz as well as a few lesser ones in the 100 MHz and150 MHz range. The least signal lost is -37 dBm which still is a loss down to athousand part of the signal energy. Something else that needs to be considered isthe refraction loss (Section 2.3.4) from changing mediums two times, water to airto water.

5.9.2 Tap water ranged trial 2This trial was done with a 2-loop antenna as the Tx and a 3-loop antenna asthe Rx. The signal output from the generator to the Tx was 00 dBm and rangebetween the two containers with the antennas was around 10 m. Comparing thisto the first trial (Section 5.9.1), the signal energy has a slightly higher peak at 24MHz but also a sharper drop off after that. Using this the least signal lost is -34dBM which in is in the same range as the previous trial.

Figure 5.16. Long range tap water trials with reference

5.9.3 Salt water ranged trial 1In this trial we used 2-loop antennas as both the Tx and Rx. The tap water wasdiluted with 100 ml of table salt, this gave it a salinity at about 0.3% but as can


be seen in Figure 5.9.4 it still made a huge impact on the signal behaviour. Thereshould be larger losses according to the refraction loss (Section 2.3.4) since thesalinity has increased and with that the conductivity (σ). The peak value is nowat -39 dBm at 24 MHz with an extremely narrow drop off after that. The saltwater in combination with the environment works almost as a low pass filter.

5.9.4 Salt water ranged trial 2

This trial is works almost as the first one (Section 5.9.3). The Rx has been changedfrom a 2-loop to a 3-loop antenna. The energy peak for this is just slightly lowerin frequency at 22 MHz. This constellation also had a sharp drop off after that.In the higher frequencies there is no signal values above the noise floor.

Figure 5.17. Long range salt water trials with reference

5.9.5 Tap water short trial 1

This trial was done with both a 2-coil loop antenna as both the Tx and Rx. Assaid in the beginning of the section, the antennas were submerged in a big plasticbox filled with about 100 L of water. The result of the measured signals can beseen in Figure 5.9.6. There is a steep drop off in the range between 50-100 MHzand then again after 200 MHz. The peaks are focused around 22 MHz and higherup in the spectrum at 125 MHz.

40 Result

5.9.6 Tap water short trial 2The trial was done with a 2-coil loop antenna being the Tx and the Rx was an3-coil loop antenna. The signal level here are generally higher then in the previousshort range trial and with one more peak at 70 MHz. The 3-coil loop antennagenerally has a higher signal level across the spectrum with the exception of a fewpeaks. The signal level in was -10 dBm so the effect lost lies between 15-30 dBmin the areas where the interesting areas for usage might be.

Figure 5.18. Tap water signal levels over 2 dm distance

5.9.7 Salt water short trial 1The trials for short ranged salt water was done over a distance of 2 dm. This firsttrial was done with a 2-coil loop antennas being both the transmitter and receiver.As for the long range trials with salt water (Section 5.9.3 & 5.9.3) the plot couldalmost be mistaken for a low pass filter. With one or two bumps in the higherregion of frequency for this antenna constellation. The highest real peak is at 22MHz just as for the tap water trial (Section5.9.5) with the same signal level at -25dBm.

5.9.8 Salt water short trial 2The second trial was done with the 3-coil loop antenna as the receiver instead ofthe 2-coil loop. As seen in Figure 5.9.8 this curve looks extremely close to the firstone. With minor differences in how the curve looks at 1 MHz and a slightly earlierfall off then the 2-coil loops constellation. This could be done to the Tx antenna


being resonant at a lower frequency therefore moving the energy in the spectrumlower in frequency.

Figure 5.19. Salt water signal levels over 2 dm distance

42 Result

5.9.9 Brackish water short trialThese trials were done with a 2-coil loop antenna as the transmitter and differenti-ating the receiver between a 2-coil and 3-coil loop. The signal power was increasedwith 10 dBm to an output of 00 dBm to see if there was anything that had beensuppressed in the lower region of the spectrum and therefore didn’t show in pre-vious tests. About 100 L of tap water was diluted with 2 L of table salt, bringingit to a salinity level of 2%. As can be seen in Figure 5.9.9 there were some en-ergy levels in the lower region of the frequency spectrum that showed when usingthe 2-coil loop but when using the 3-coil the cut off was lower in the spectrum.This could be explained by saying that the 2-loop antenna has a higher resonancefrequency (Section 4.5)

Figure 5.20. Brackish water signal levels over 2 dm distance

5.9.10 Seawater short trialThese two trials were done with a 2-coil loop antenna as the transmitter anddifferentiating the receiver between a 2-coil and 3-coil loop. The signal power wasincreased with 10 dBm to an output of 00 dBm. The salinity level of these trialswere at 4% with an addition 2 L of salt being added from the trial in Section 5.9.9.The spectrum up to 40 MHz is almost a copy from the brackish trial and continuingon as well.


Figure 5.21. Seawater signal levels over 2 dm distance

6 | Discussion

6.1 Conclusion of the air measurementsA lesson learned is that all the antennas have a resonance in the 34-36 MHzfrequency range. This was not the expectation going into the measuring processsince the thought was that they would be having the resonance in 60-90 MHz(Section 4.5). This means that for all 3 antennas with a diameter of 27 cm theideal wavelength is in the range of 10-8.3 m using the normal equation of c

fc. Using

Equation 2.19 the number of coils in the antenna would need to be:

N = λ

d · 2π = 100.27 · 2π ≈ 6. (6.1)

The experiments was done with mostly 2-coil antennas and there is undoubtedly asstrong point around 34 MHz, so what changed? First the though was there couldbe a wrong use of the equation but looking at peer review rapports this seems tobe the standard equation to use when dealing with loop antennas. Instead lookingat the pure theory there could be an issue with this equation, since it is very clearcut and the most papers on the subject isn’t. If the equation is rewritten:

N · d · 2π = λ (6.2)

it became easy to see that the formula simply states you only need to bend adipole N number of turns and it still keeps the same resonance frequency. Is thisa reasonable line of thinking? According to peer reviewed papers it is but in theseexperiments we don’t reach the same conclusions as the authors, since the antennagain is focused in a much lower band. Taking the equation and adding on the extra3 to it while aiming for 90 MHz would instead give a diameter off:

d = 3.333332 · 6π = 0.08831 . . . ≈ 9 cm. (6.3)

Would this have been a more accurate diameter for the antenna? It could possiblebe, this would lower the radiation resistance (Equation 2.20) of the antenna and inan extension lower the power radiated. It could also be that for these trials therewere no real impedance match in place, this could give about the same match foreach antenna, which is in each case wrong. This would mean that the informationgathered from both the trials one to five would be essentially useless as well as

45

46 Discussion

the VSWR testing. Since there was no match in case for either of the systems.If this is the case, the test data is still valid in its current from but might not beconsidered useful for onward building of the system.

The other part of the air testing is the power transfer in the different constellations.When transmitting a -10 dBm signal between the two antennas in the fourth trial(Section 5.5) there is a loss of about 15 dBm into the next antenna. What hasbeen found after five first trials where done is that we are still in the near fieldof the antennas which makes it very difficult to extract any real information fromthe tests since there aren’t any standardized models for this region (Section 2.3.1).What was needed was to either gain a greater distance for testing the antennas orreplacing the air with a conductive medium. The second option would effectivelycut down the wavelength by the dielectric constant (Equation 2.17) and thereforedecreasing the near field by the same amount. There was more testing done with arange of 10 m between the antennas. This should at least put them outside of thenear field (Section 2.3.5) which should give a much clearer picture of how they willreact when submerged in a conductive medium. The tests were done in the samelab as previously but with a longer range between the antennas (Figure 5.12). Theynow showed different signal levels then in the five first trials but in the somewhatsame regions of frequency (Figure 5.12). The find is that the antennas are resonantin both the near and far field in the range of 35-45 MHz with slight differencesbetween them. One thing that needs to be noted was that in the near field thereceiving antenna could be moved almost anywhere around the transmitter andstill keep the same signal level in the mentioned frequency region. Since the nearfield is none linear and a loop antenna act as a transformer at close range [6] theremight was a magnetic link between them that is maintained until enough distanceis achieved to break it or more accurately, it leaves the near field. This fact wasconfirmed by Jon Staffeldt from Combitech. There was also here it was reallyhammered home that one could have used a smaller loop with more coils and havethe antennas just communicate via the near field underwater instead of going intothe far field.

6.2 Conclusion of water measurementsThe first thing that is obvious to notice is that the signal levels are dampenedacross the board, in both the tap and salt water trials. This is first the nature ofthe long range testing, that the medium is changed two times. The signal passesfrom water into air and back into water. Using Equation 2.14 for a frequency of24 MHz and an estimated conductivity (σ) at 0.02 S/m.

Refraction loss [dB] = −20 · log10

(7.4586

106 ·√

24 ∗ 106

0.02

)= 11.7 dBm. (6.4)

If the equation is taken at face value, there should be a loss at both containersbringing the signal down by an extra 23.4 dB. This is not line with what was

6.2 Conclusion of water measurements 47

observed when doing the real world testing, where there was a loss of about -39dBm for the long range salt water as well as -35 dBm for tap water. For seawaterthis effect should be even greater since the conductivity will increase and with thatthe reflection loss should be larger. The missing losses at 39− 23 = 16 dBm couldbe attributed to the antenna impedance miss matching as well as attenuation lossesin the near field for having both antennas submerged in a conductive medium. Thereflection have some affect in these tests as predicted by Butler [7]. But the effectfor salt water is that it also works like a low pass filter with a cut off around 25MHz. What could be showing is high frequency attenuation that increases withhigher values according to Equation 2.10.

α = 0.0173 ·√

25 · 106 · 1 = 86.5 dB/m. (6.5)

and giving that the antennas where only a few cm from the edge of the containerin the long range tests this near field attenuation will account some of the signallost but not the full value that would be at 1 m. The signal lost due to the an-tennas being submerged in salt water is close to -15 dBm, this is can attributedto a combination of refraction loss and near field attenuation loss in a conductivemedium. The same value for tap water is -10 dBm from the reference, a differenceof 5 dBm between the two mediums. This change is most likely dependent onthe salinity level of the medium and since all equations for ’signal loss’ containsconductivity (σ) it becomes hard to pinpoint exactly where.

What can be seen from the long range tests were that submerging the antennas inmediums that are more conductive then air changes the behaviour of the antenna.Even when air makes up most the transmission medium the conductance of thewater changes how the antenna interacts with the signal. This can be seen as theenergy peak has moved down in frequency from around 42 MHz to 22-24 MHz forboth the tap and salt water constellation. As mentioned the salt water trial canalmost be seen as a low pass filter, which was seen in both the ranged and closetrials, 10 m and 2 dm in distance. This effect can most likely be traced back to ahigh near field attenuation because of the higher conductivity of the medium. Theantenna should be resonant at a lower frequency but the reason for this extremecut off is a bit unclear. What is known is that a loop antenna is lowered ina conductive medium, this means that the wavelength will decrease and so theresonance frequency will move down, this can be seen with the peak between airand water tests changing from around 42 MHz to 22 MHz. The cut off between saltand tap water is dependant on an increased (σ), which means that attenuation,refraction and propagation is affected. What can be determined from all watertrials is that there is a drop off after 24 MHz. The higher frequencies will havea larger attenuation with increased conductivity. This was what was happening,the signal levels are under the noise floor of the spectrum analyzer. In hindsightit would have been better to do all the measurements with a higher output energyto see exactly what was going on in the lower region on the spectrum. For thelast trials in brackish and seawater the output was increased to 00 dBm whereit was noted that frequencies up to 130 MHz could still be noted but that the

48 Discussion

cut off still was around 24 MHz. This seems to be the largest frequency which ismagnetically linked and what is seen after that is the electric field being in heavilyattenuated by the conductive water. A distance of one meter could very well beachieved in a better testing environment considering that the noise floor of the seais around -130 to 140 dBm (Figure 2.1). The reason no conclusion can be madeis Equation 2.18 which puts 24 MHz at a wavelength of 3 dm where as at 2 MHzwould be needed to reach a wavelength of over 1 m which would put the antennasin near field range in seawater.

6.3 Conclusion of impedance measuring

What we can see in Section 5.7 is that we have a decent VSWR at 34-36 MHz com-bined with a high and tested signal level in the same region (Figure 5.9 and 5.7).This testing was done without any form of antenna matching since that wasn’tcompleted at the time. This creates the predicament that the impedance values ofthe antenna became known but there wasn’t a proper chance to test an impedancematch afterwards. This creates a scenario where there might be better to use theunmatched antenna since the specifications are known and can be built aroundrather then having it fitted and only know the theoretical specifications of thesystem.

There is also the factor of how much moving the cable to twisting it around like anextra coil would affect the system VSWR and this also affected the signal transfer.This meant that very small changes to the antennas would change the propertiesto such an extent that it become trial and error on how to create the best setup,even if the theory is solid. The question then becomes if the measurements shouldhave been done after an impedance match was created, this would have reversedthe problems but might have been a better way to go about it since the antennascould have been matched to their theoretical resonance frequency 90 MHz insteadof what was gained now at 34-36 MHz. The large problem with this was todimension the impedance match, there were ruff estimates out there but afterconclusion of these first test there was a verdict that either the measuring wascompletely wrong or the antenna was being conductive and not inductive in itsmost efficient regions. This led to some calculations what inductance should beused to match it for 50 Ω. The interpretation is that there is conductivity in serieswith a resistor creating ZL. There needs to be a inductance matched in parallelto create the match.

sL(sC +R)sL+ sC +R

= 50↔ L = 50(sC +R)s(sC +R− 50) (6.6)

where the s is substituted for jw when doing the calculation and for 36 MHz the|ZL| = 33 where RL = 21.2 Ω and C = 166 pF, using Equation 6.6 the matchbecomes:

6.4 Security and integrity 49

L36MHz = −4.9346 · 10−10 + 1.6007 · 10−7i and (6.7)|L36MHz| = 160 nH. (6.8)

Implementing this back in the Equation 6.6 there is almost a perfect match to 50. Ifthere then is a need to create a match for 96 MHz the values are |ZL| = 92.4, RL =10.5 and C = 19.2. Using the Equation 6.6 the result is L90MHz = 22 nH. Thesecalculations could be used for the whole frequency spectrum using the measuredvalues from Norrköping (Section 5.7). Since there was no possibility of impedancemeasuring the antenna in water there are no values to account for the impedancematch in that medium.

6.4 Security and integrity

As we can note from the experimental results in Figure 2.1, the reach of the systemcan many times be farther than we think when only looking into the mathematicalside of things. This has the implication that our system could have a much largerreach then first expected, making it an interesting proposition for experiment butalso a security risk since it could lead to it being detected by electronic warfaresystems or alike. There is also the fact that a goal of (Section 1.2) almost 130 KHz(Section 4.3) was chosen as a bit rate for the system. The knowledge that this ishi-fi quality of sound, which is far larger then ever needed for this kind of system.It is fully possible that a system could have reached this level of bandwidth butfrom a security standpoint it shouldn’t. If one can minimize the data being sent,one should do it, why? Because more data over the air means more bandwidth,more bandwidth means higher energy levels and more energy means that it will beeasier to detect over a greater distance. So if the thesis is restarted, focus shouldhave been on minimizing the data sent while still maintaining the goal of losslessaudio.

Earlier research and experimental results have shown that the signal can be de-tected as far as 90 m (Figure 2.1) from an underwater system, since the noise levelis so low. During this thesis there have not been trials at this range but since itis peer reviewed research we will base this discussion of that. If a signal pans outat longer distances but still remains detectable there is an even larger incentiveto keep the signal levels as low as possible. As mentioned above there might havebeen a better way to simply link the signal in the near field and being able tokeep a low signal level even with the operator moving around, as mentioned withthe transmitter and receiver in Section 6.1. If it is in the near field and magnet-ically linked one can move the antennas quite a fair bait around each other withundiminished signal strength.

50 Discussion

6.5 Issues with the thesisThe goals of this thesis was to create a fully functional system for transferring sig-nals under water. What can be seen is that only the antennas have been designedfor the time being, why you might ask? The answer is that designing the antennaitself took more time and learning that was ever thought needed. In retrospect thesystem should have been designed and built at first, some kind of ’off the shelf’antenna could have been used to test it. This is because the thesis was done bystudents in electronics engineering, specialising in on chip design and electro con-struction. The work done was what time allowed with the information possible totake in. In retrospect the hardware design of the thesis should have started a lotsooner since some of the problems that arose could have been dealt with faster,like buying components, measuring for impedance and ordering materials to beable to test in water.

6.6 ConclusionThe verdict reached for this thesis is that is a lot easier for the magnetic fieldthen an electric field to penetrate in a conductive medium. The lowest frequencypossible for this thesis antenna construction is 24 MHz in the high energy partof the spectrum. At peak values only -15 dBm is lost over 2 dm in water, thisvalue did not change as the as the antennas was moved closer to each other. Sincethere was no possibility of moving the antennas further apart and still maintainingthe connection with just water as the transfer medium the assumption had to bethat the antennas are magnetically linked at this point. The argument that couldbe raised is that the long range testing also showed peak values around 24 MHz,which means that this is always the antennas resonance frequency both in shortand long range testing. If this is the case the most ideal frequencies to be usedfor the system is around 22-24 MHz in water of any conductivity and in the rangeof 40-44 MHz when using the system in air. Such a slight change in frequencywill hopefully lead to small difference when implementing the impedance match,making it a real possibility to use one smaller inductance to match the antenna.A range of 1 m should be fully possible to achieve in this or a lower frequencyregion, (even considering the added attenuation) outdoor or facility testing wouldbe needed to confirm this thesis trials with a spectrum analyzer that has a muchlower internal noise then -60 dBm. The goal of lossless audio should also be fullyachievable and possibly the full 134 KHz that was set at the beginning of thisthesis. This would need to be tested but with a bandwidth at least 2 MHz shouldbe more then enough to make this feasible.

7 | Future Work

7.1 Building the system

This thesis was supposed to be focused on this part but due to complexity ofcreating an antenna the circuits generating the signals behind it fell behind. Withthe antennas in a useful condition one could focus on building the system behindit. The first focus should be on easy to use components described in Section 4.3and trying a few frequencies and modulations to see what detects the best resultsin the receiver and power detector. The system could be built on an integratedcircuit (IC) after the exact details of each component have been chosen. Thiswould minimize the circuit size and could be standardized for mass production,this would also give the ability to perfectly impedance match the loop antennasince it will be a standardized source impedance. During this thesis, the sourceimpedance has been 50 Ω because of the oscilloscope in use.

7.2 Perfecting the antenna

After the thesis there are a lot of things that could have been done differently ifthe theoretical background had been there from the start and not learned alongthe way. There might be a case for using a magnetic antenna working in thenear field, that would mean changing one of the thesis goals but could lead to lesschance of detection (Section 6.4) and might also give a more stable connection invery close ranges. The reason for the might is that inside the near field we don’thave a proper model and when also submerged in a conductive medium, tests willhave to be performed but as mentioned in Section 2.8.1 the range of such a systemrisks to be ridiculously low, that it might be required to be fitted with a magnetbetween the transmitter and receiver, essentially making it a replacement for theconnector on the end of a cable but not for the cable as a whole.

If the focus is kept on the far field signal range there is always the case of impedancematching and trying out different build types for the antenna. If a standardizedsystem is created as in Section 7.1 one can create an antenna with almost a perfectimpedance match for that system, getting the VSWR closer to 1 then has beenachieved so far.

51

52 Future Work

7.3 Outdoor testingThe antennas would need to be tested outside, in the "real world". This wouldallow for different levels of depths when testing the antennas as well as seeing howthe signal reacts to being close to a bottom, which is made of earth, rocks andother segments of material. The problem with doing these tests have been gettingthe hands of signal testing systems (Spectrum Analyser, Function generator) thatis allowed to come close to a large amount of water. Another thing has been howto power them when close to water, one train of thought is to go down to a harbouror equivalent to have access to power and the possibility for measuring on deeperdepth by standing at the edge of a pier. As seen in Figure 2.3.3 attenuation willchange with depth so it would have been very interesting to see how it changes asthe depth in differentiated for the system in experiments.

Going back to Section 7.1, tests could be performed on a fully complete and watersealed system. They could for example be placed on two divers and then havethem move X distance until the lose connection. There might even the chance ofconnecting the divers equipment with the surface and transfer real time data backup to the researchers to see how the system is doing. A recommendation herewould be a fiber cable as a connection since this would minimize any interferenceusing a coax cable or the like over a longer distance.

Bibliography

[1] Inam Ghafoor. SONY XPERIA Z Underwater NFC Speaker Test!https://www.youtube.com/watch?v=RiIo-DzVfmA, April 2013. Accessed:2015-08-31.

[2] A Shaw, AI Al-Shamma’a, SRWylie, and D Toal. Experimental investigationsof electromagnetic wave propagation in seawater. In Microwave Conference,2006. 36th European, pages 572–575. IEEE, 2006.

[3] John S Seybold. Introduction to RF Propagation. Wiley, Newark, NJ, 2005.

[4] Ahmed I Al-Shamma’a, Andrew Shaw, and Saher Saman. Propagation ofelectromagnetic waves at mhz frequencies through seawater. Antennas andPropagation, IEEE Transactions on, 52(11):2843–2849, 2004.

[5] J Lucas and CK Yip. A determination of the propagation of electromagneticwaves through seawater. Underwater Technology, 27(1):1–9, 2007.

[6] Lou Frenzel. What is the difference between em near field and farfield? http://electronicdesign.com/energy/what-s-difference-between-em-near-field-and-far-field, 2012. Accessed:2015-05-25.

[7] Lloyd Butler Vkbr. Underwater Radio Communication. Amateur Radio,(April):1–8, 1987.

[8] Ghaith Hattab, Mohamed El-Tarhuni, Moutaz Al-Ali, Tarek Joudeh, andNasser Qaddoumi. An underwater wireless sensor network with realistic ra-dio frequency path loss model. International Journal of Distributed SensorNetworks, 2013, 2013.

[9] JH Goh, A Shaw, and AI Al-Shamma’a. Underwater wireless communicationsystem. In Journal of Physics: Conference Series, volume 178, page 012029.IOP Publishing, 2009.

[10] Henry Jasik and Richard C. Johnson. Antenna Engineering Handbook, ThirdEdition. McGrac-Hill, inc, 1993.

[11] Lecture 11: Loop antennas. http://www.antentop.org/004/files/tr004.pdf,2004. Accessed: 2015-05-27.

53

54 BIBLIOGRAPHY

[12] Dipole length | antenna length calculation & formula. http://www.radio-electronics.com/info/antennas/dipole/length-calculation-formula.php. Ac-cessed: 2015-04-27.

[13] Hao Zhang, Dawei Geng, Guoping Zhang, and T. Aaron Gulliver. The im-pact of antenna design and frequency on underwater wireless communications.IEEE Pacific RIM Conference on Communications, Computers, and SignalProcessing - Proceedings, pages 868–872, 2011.

[14] J-pole antenna. https://en.wikipedia.org/wiki/J-pole_antenna, September2015.

[15] Mikael Olofsson. Telecommunication Methods. Compendium from LinkopingUniversity, Linkoping, June 2011.

[16] Roland Minihold. Near field communication (nfc) technology and measure-ments white paper. page 18, 2011. https://goo.gl/HWwh4G.

[17] Giuliano Benelli and Alessandro Pozzebon. Rfid under water: Technical is-sues and applications. Radio Frequency Identification from System to Appli-cations, pages 379–395, 2013.

[18] Specification of the blutetooth system. https://www.bluetooth.org/en-us/specification/adopted-specifications, April 2014. Accessed:2015-09-07.

[19] Jaime Lloret, Sandra Sendra, Miguel Ardid, and Joel J P C Rodrigues. Un-derwater wireless sensor communications in the 2.4 GHz ISM frequency band.Sensors, 12(4):4237–4264, 2012.

[20] Oceanreef 2015 Catalogue. http://www.oceanreefgroup.com/pdf/2015catpro_eng.pdf,2015.

[21] Carlos Uribe and Walter Grote. Radio communication model for underwaterwsn. In New Technologies, Mobility and Security (NTMS), 2009 3rd Interna-tional Conference on, pages 1–5. IEEE, 2009.

[22] Analog to digital converter ads7823. http://www.ti.com/lit/ds/symlink/ads7823.pdf,2003. Accessed: 2015-05-04.

[23] Low noise amplifier lt5538. http://cds.linear.com/docs/en/datasheet/5538f.pdf,2008. Accessed: 2015-05-05.

[24] AA Abdou, A Shaw, A Mason, A Al-Shamma’a, J Cullen, and S Wylie.Electromagnetic (em) wave propagation for the development of an underwaterwireless sensor network (wsn). In Sensors, 2011 IEEE, pages 1571–1574. IEEE,2011.

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underwater communications system with focus on antenna design

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