tactical and emergency communications by nvis effect · by nvis horizontal polarization,...

OCTOBER 2013 – LISBON - PORTUGAL

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Abstract—The objective of this master thesis, is to design,

simulate, build and test an antenna operating in the high

frequency band, and to explore the propagation in the ionosphere

by NVIS effect, to establish short distance communications, in

tactical and emergency situations. This project may be a valuable

tool for the Portuguese Army, in operational theaters where the

terrain is very rugged, such as Afghanistan or Kosovo. The

designed antenna is composed by two crossed dipoles with an

inverted V topology allowing short distance communications in

the high frequency band, with emission angles from 60º to 89º,

for distances of the order of dozens of kilometers. The design was

performed by doing the theoretical analysis and simulation of the

antenna, which was then built and tested with satisfactory

results.

Index Terms— Ionospheric reflection, half wavelength dipole,

HF communication, NVIS propagation.

I. INTRODUCTION

N operational theatres, where the terrain is very rugged it is

difficult to establish communications by ground wave. For

large distances, of the order of thousands of kilometers, high

frequency can be used, with emission angles near 30º, but for

distances of the order of dozens of kilometers, it is necessary to use an higher emission angle, between 60º and 89ºdegrees.

From a tactical point of view, this concept of radio

communications, enables a tactical force to, establish

immediately a communication without depending on existing

infra-structures, like re-transmitters, satellites or other means

of transmission.

This concept can also be used in case of emergency

situations, caused for example, by natural phenomena or

terrorism, where the communication systems get inoperable,

and there is a need to reestablish the communications by HF

(High Frequency) in short time and with the reduced means

available in those situations. The reasons explained above were a great motivation for

this project which is in line with my future activities as a

transmission officer of the Portuguese Army.

Outubro 2013, Instituto Superior Técnico/ Academia Militar.

Renato Gonçalves Rocha, associated to Instituto Superior Técnico, and

simultaneous to Academia Militar, in Lisbon, Portugal (email:

[email protected]).

A. Overview

This work’s main goal is to develop an antenna with

horizontal polarization, strategically targeted to explore the

NVIS concept.

The several antenna types used presently by the Portuguese

Army have some problems on high frequency communication

by reflection in the ionosphere with high emission angles,

between 60º and 89º. The point is to explore this type of

communication with tactical and emergency purposes,

immediately above of the LOS (Line Of Sight) in HF.

Initially the theoretical design of a half-wave dipole antenna

was performed and the radiation patterns and radiation

parameters were obtained. Then another model featuring two half-wave dipoles with a single resonant frequency was

studied and then the final case of a crossed dipole antenna

with two frequencies of resonance (4 MHz and 6 MHz) was

studied. The respective radiation patterns were obtained using

the MMANA-GAL software environment.

This antenna was later built and tested with good results.

II. IONOSPHERIC PLASMA

The ionosphere can be modeled in some cases as a cold

plasma which is ionized mainly by the ultraviolet radiation

from the sun, and extends from an altitude of 60 km to about

600 km [1]. This plasma can be divided into several layers

where the electron density varies with the amount of

electromagnetic radiation received from the sun, thus

presenting variation with the hour of the day, season and solar

cycles. The maximum value of ionization is around noon, as

the sun has extreme influence on the variation of values of

ionization of the ionosphere. In the lower layers, the electron

density is low, and the frequency of collisions is high, so a

wave undergoing reflection from a higher layer suffers

attenuation upon traversing lower layers. [2].

A. Electron Densities

During periods of high solar activity, ultraviolet light and x-

rays have a high influence. Through an atmospheric model,

knowing the solar flux, absorption and ionization efficiency of

the various constituents, it is possible to compute the densities

of ions and electrons in the ionosphere [3]. The electron

density varies with the time of day with the seasons and with

distance from the surface.

Tactical and Emergency Communications by

NVIS effect

Renato Gonçalves Rocha, IST/Academia Militar

I


2

Fig.1. Example of electron density in the various layers of the ionosphere.

B. Plasma frequency

The plasma frequency, is related to the frequency use,

due to if the operating frequency is above fp, the wave enters

on a layer, and if the frequency in use is below fp, the wave is

reflected by that layer, and is given by the following

expression:

0e

2

pm

Nq

2

1f

(1)

where q is the charge of on electron, N is the density of the

electrons per volume, me is the mass of the electron and ԑ0, the

dielectric constant of vacuum.

C. Ionosondes

The ionosondes measure the critical frequency fc, to which

the wave is reflected with normal incidence. Through an

emitter that emits a carrier, with a vertical angle of incidence,

scanning a range of frequencies from 1 MHz to 20 MHz. [4].

The emitted signal is received in a receiver near the

transmitter and computes the time of return. This is how the

ionosphere is characterized in different layers [5].

Fig.2. Real time ionogram.

D. Layer model of the ionosphere

The International Reference Ionosphere (IRI), established

an empirical model composed of different ionospheric layers

used by the international community as a reference model that describes the average behavior of the ionospheric plasma.

These layers are identified by the electron density and the

frequency of collisions [6]. The D layer starts at 50 km above

the ground. The E layer is located above the D layer and starts

at an altitude of 100 km. The F layer, can be subdivided into

the F1 and F2 layers, which start at about 140 km and 200 km,

respectively. Fig. 3 illustrates the layer model.

Fig.3. Layer model of the ionosphere.

E. Chapman’s Model

Chapman was the first physicist to establish a theoretical

model for the distribution of the particle density of an

ionospheric layer. This model is the theoretical model of the

ionosphere which gives the variation of electron density with

height. It is assumed that there is only one type of gas, the

stratification is considered to be planar and that there is a

parallel beam of , monochromatic ionizing radiation, c that

comes from the sun, and the atmosphere is considered isothermal [1].

The electron density is given by the following expressions:

)2/))exp(*))X(sec(2/2/1exp(*NN m (2)

H/)hh( m (3)

Mg/RTH (4)

Where Nm is the maximum electron density, X is the angle

which the sun makes with the vertical, R is the universal gas

constant; T(h) a function of temperature with time in ºK; M is

the mass of one kilogram-moles and g is the gravitational

acceleration.

Later, simpler models have been considered, in order to

obtain well-known differential equations, such as the linear

model, parabolic model, exponential model, among others.


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The following figure contains the values of the frequencies

due to different time of the day and respective altitude:

Fig.4. Plasma frequency, for several altitudes based on the time of day.

F. Maximum Usable Frequency (MUF)

The International Telecommunications Union (ITU) created

the recommendation ITU-R p.373-7 [7] which defines the

meaning of MUF:

“operational MUF, is the highest frequency that

would permit acceptable performance of a radio

circuit by signal propagation via the ionosphere

between given terminals at a given time under

specified working conditions(…);

basic MUF is the highest frequency by which a radio

wave can propagate between given terminals, on a

specified occasion, by ionospheric refraction alone(…)”

The maximum plasma frequency, determines which waves,

emitted with vertical incidence can penetrate a certain layer,

and which are reflected. The maximum plasma frequency is

called the critical frequency, fc or fo

The MUF can be calculated by the following expression:

cos/fMUF c (5)

Where fc is the critical frequency and θ corresponds to the

angle between the radius and the vertical to the ground wave.

G. The calculation of the propagation distances

In this sub-section the ranges of the communication link are

computed as a function of the incidence angle and the altitude

at which the reflection is performed. The Matlab software was

used in this calculation.

The F2 layer is located approximately between 200 km and

400 km, and setting the minimum and maximum range

between 20 km and 120 km, respectively, the following figure

is obtained:

Fig.5. Ratio range and elevation angle for the F2 layer, according to various

heights.

From Fig. 5 the values presented in the next table were

obtained:

TABLE I

ELEVATION ANGLES FOR REFLECTION IN THE F2 LAYER

Virtual height (km) Range (km) Emission angle

200 20 87º

200 120 73º

250 20 88º

250 120 77º

300 20 88º

300 120 78º

350 20 88º

350 120 80º

400 20 89º

400 120 81º

III. THEORETICAL STUDY OF THE ANTENNA

In this chapter the theoretical study for the different types of

antennas mentioned earlier were performed and the radiated

fields were obtained.

A. Half-wave dipole

From the vector and scalar potentials,, the following

expressions for the fields of a half-wave dipole were

obtained:

sin

)2

klcos()cos

2

klcos(

r2

IZjE e

jkrM0

(6)

sin

)2

klcos()cos

2

klcos(

r2

Ij

Z

EH e

jkrM

0

(7)


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Where Zo is the characteristic impedance, IM is the

maximum current, l is the length of the antenna, r is the

distance of the radiation field, and k is the wavenumber.

Through MMANA-GAL basic environment, the following

radiation diagrams for the half-wave dipole with a frequency

of 4 MHz, the ground height of 10 meters, and the physical length of the antenna of 18.75 meters were obtained:

Fig.6. Radiation patterns for the half-wave dipole, 10 feet above the ground.

B. Half-wave crossed dipoles - Antenna NVIS projected

In this chapter the new antenna with a wide frequency band

located in HF, between 4 MHz and 6 MHz and constituted by

two crossed dipoles, is studied.

The first dipole is tuned to =4 MHz and the second

dipole is tuned to the 6MHz Therefore, for the dipole 1,

the wavelength is and for the dipole 2 the

wavelength is , which leads to the following

physical lengths of the dipoles, and ,

respectively. The total electric field is given by:

21total EEE (8)

For the dipole 1, the expression is given by:

e)sen(cosecoselkcos1Z2r

jI E rjk

11o11

(9)

For the dipole 2, is given by:

e)cos(cosesenelkcos1Z2r

jI E rjk

22o21

(10)

The electric field corresponds to antenna with physical

length of and for the field where . In order to obtain a reference field, it will be considered the

following parameters. A current , a distance of

and impedance , which gives the

following expression:

)E E()E E( E E 2121

2

21

(11)

The radiation patterns for frequencies of 4 MHz, 5 MHz and 6

MHz, give the following result::

Fig.7. Radiation patterns for frequencies of 4 MHz, 5 MHz and 6 MHz.

IV. ANTENNA NVIS MR13

A. Construction of the antenna

The antenna t consists of two crossed inverted v dipoles, in

which the arms of the dipoles form between them an angle of

about 120º, reducing the characteristic impedance of both antennas (f1 and f2) from 75 ohm to 50 ohm.

This t antenna therefore designated by NVIS MR13 is

composed of two half-wave dipoles with different resonant

frequencies, where , corresponds to the dipole of

physical length 37,5 meters whereas for the

resonant frequency of the second dipole, has a physical length

of 25 meters.

Fig.8. Antenna NVIS MR13 built.

The construction of the antenna can be divided into two

parts, the part of the internal connections of the dipoles, from

the point of view of its power supply and the part of the dipole

arms.


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1) First part

The arms of the dipoles are symmetrically connected and

matched to the same transmission line (coaxial cable), where

each arm of dipole 1is connected to the arm of the dipole, 2 so

the dipoles are connected with each other in parallel, as it can be seen in the following figures:

Fig. 9. Top view of the connection of the dipoles.

Fig. 10. Bottom view of the connection of the dipoles.

These connections are made inside a PVC structure 10 cm

long, enclosed on top and bottom, by two caps. The structure

of PVC will provide protection for connections and will allow the arms of the dipoles to be fed through the plugs and

monopolar terminals connected to the ends of the arms of the

dipoles. At the bottom of the structure of PVC a metal base is

screwed, through a female plug PL259. The antenna is fed

through this plug, which is connected to a standard RG58

coaxial cable linked to the radio.

2) Second part

Regarding to the construction of the arms of the dipoles,

these are made with wire Ormiston Wire Limited reference

999-2011, provided by EID. The four antenna wire bonds, which are the arms of the dipoles are socked to the PVC

structure by monopole plugs (in black in Fig. 11).

Fig. 11. Antenna’s base metal.

The installation of the antenna relative to the ground is made

with a height equal to or less than 0.1λ so as to have a

compromise in the vertical radiation gain close to 90 ° in relation to the two wavelengths (λ) wherein the antenna

operates both in f1 and f2. Under these conditions the

impedance is less than 50 ohm, due to ground proximity

(approximately 15 to 30 ohm), which is matched by an

Automatic Tuning Unit (ATU).

3) Balum

This device which is necessary to match the antenna to the

cable was built using a coaxial cable RG316 (2.5 mm) a

toroidal balum (current) at a ratio of 1:1, which allows to pass from an unbalanced to balanced configuration, and assures the

symmetry for the antenna radiation characteristic in both arms

of the dipole. This balum has 22 turns, and adapts the antenna

for the working frequency range. The following figure

illustrates this balum:

Fig.12. Toroidal balum with 1:1 ratio.


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B. Antenna caracterization

To perform measurement of antenna parameters the

network analyzer, HP 8752C Network Analyzer (300KHz-

1.3GHz).was used to measure the resonance frequency and

Standing Wave Ratio (SWR). The antenna is designed to work

between the frequencies of 4 MHz and 6 MHz. The resonance

frequencies measured in the network analyzer were

respectively , with a SWR of 1.14 and , with a SWR of 1.11.

The discrepancy between the theoretical values and the experimental values of the frequencies may have several

causes related not only to the final configuration of the

antenna, but also the structure of the wire used in the

construction of the dipole arms. The SWR values are

satisfactory since they are very close to unity which would be

the optimal adaptation.

Fig.13 Resonant frequency of 3.527 MHz.

Fig.14. Resonant frequency of 5.607 MHz.

The characterization of the balum using the network

analyzer gave a reflection coefficient value (between 2 MHz

and 50 MHz) of approximately -32 dB, and SWR about

1,055:1. The insertion loss in this frequency band is between -

0.15 dB and -0.3 dB.

The introduction of the balum leads to an improved signal

level at the receiver of about 10 to 15 dB, and also improves the symmetry of the lobes of radiation.

C. Terrain profiles

The profile intended for communication NVIS, is a profile

with one or more obstacles and distance between emitter and

receiver ranging between 20 km to 120 km,. The obstacle

must have a minimum altitude of 300 meters, in order to

ensure that there is only NVIS communications. Several types

of profiles, near Setubal, Lisbon and Sintra were studied. The

chosen profile is the profile between Barcarena (Oeiras) and

Cheleiros (Mafra).

Fig.15. Profile link Barcarena - Cheleiros.

The profile presented, contains many natural obstacles, from

the order of 500 meters, and the distant between sender and

receiver, is approximately 18 km.

TABLE II

CHARACTERISTICS OF BARCARENA-CHELEIROS LINK.

Barcarena (Oeiras) Cheleiros (Mafra)

Latitude 38.731858º Latitude 38.887091º

Longitude -9.280915º Longitude -9.335461º

Antenna Height (m)

4 Antenna Height

(m) 4

Altitude above

sea level (m) 52

Altitude above

sea level (m) 57.6

Distance (km) 17.897

Emission angle 88.4º

This profile was chosen also taking into account the logistic

factor as Barcarena is where the antenna was built, and

according to the means that are available, and by the fact that

one end of the link locates in Barcarena and facilitates the

implementation of tests.

V. ANTENNA TESTS

A. Test implementation

To perform the tests for NVIS communications a reference

antenna, model RF-1936/38 Harris, assigned by Corpo de

Fuzileiros da Marinha was used. Another one of this antenna

was used for the reception. The antennas are portable, with an

easy assembly, and consist of two crossed dipoles, that use

NVIS effect, and can communicate between 10 km and 400

km. The mast consists of several coaxial sections, constituting the transmission line feeding the antenna..

B. Experimental results

The tests were performed during the morning of September

24, 2013, between the period of 11h and 14h. As noted above,

the connection made between Barcarena (Lisbon) and

Cheleiros (Mafra). In order to establish a reference two antennas were used in the transmitter section: MR13 and one

of the Harris antennas provided by Corpo de Fuzileiros. The

antennas used in this connection, were fed with 20 W of

power and connected to the radio ICOM IC-706MKIIG of the

antennas located in Barcarena (MR13 and Harris). The

antenna located in Cheleiros, was connected to the PRC 525

HF/VHF.


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On this day, the absorption in the D layer, in terms of

attenuation, was 'clean', thus showing null values for the

attenuation of this layer for the several critical frequencies of

reflection, as shown in the following image:

Fig.16. Layer D ionospheric absorption, withdrawal from http://nvis-

tuga.blogspot.pt/2011/01/absorcao-na-camada-d.html, accessed on September

24, 13.

The following results were obtained:

TABLE III

POWER SIGNAL FUNCTION OF FREQUENCY FOR BARCARENA-

CHELEIROS LINK.

Frequencies (MHz)

Received signal power (dBm)

Antenna NVIS MR13 built Antenna Harris RF

With balum Without balum

3,550 -102 -103 N/D

4,150 -102 -103 N/D

5,495 -96 -99 -102

6,290 -80/-91 -91/-99,5 -99,5

7,245 -103 N/D N/D

Situations in which it was not possible to obtain a value of

measurement are represented by N/D (not defined). As seen in

Table III, the NVIS communications, starts at about 3,550 MHz and extends to about 7 MHz. At the last measurement at

7,245 MHz there is a loss due to the link that is above the

critical frequency for reflection (foF2) for these emission

angles (approximately 89°).

After the insertion of the balum, improvements of 8 dB to 10

dB were noted in the received signal, due to the balancement

of the antenna and improved lobe’s symmetry.. From the data

listed in the same table, it is possible to conclude that there

was transmission NVIS effect as intended.

The antenna Harris performance was worse than expected.

The causes may have several origins and are still being

investigated, but there is a suspicion of a bad contact particularly in the connections of the various elements that

constitute the coaxial mast.

Table III, also proves the link is made only through NVIS

effect (absence of ground wave), because below 3,550 MHz

and above 6,300 MHz, the communication was not possible,

or was very scarce and insufficient to be carried out. If there

was ground wave it communication would be possible below

3,550 MHz and above 6,300 MHz and continuously across the frequency range shown in the same table.

1) Alternative links

Besides the previous link, there were two more alternative

links, which are among Barcarena (Oeiras) - Santa Cruz

(Torres Vedras) and between Barcarena (Oeiras) - Alpiarça

(Santarém).

a) Barcarena (Oeiras) – Santa Cruz (Torres Vedras)

This link is located around 45 km from the antennas, and

was made using the frequency of 7,065 MHz. The received

signal was -80 dBm. The antenna was located in Santa Cruz,

belonged to a radio amateur service station, operating in the

frequency range close to the same band of the previous tests.

This connection allowed to prove that the NVIS propagation

for distances over 20 km, with emission angles bellow 89º,

corresponding to frequencies reflection above the critical

frequency of 6,290 MHz.is possible

TABLE IV

CHARACTERISTICS OF BARCARENA – SANTA CRUZ LINK.

Barcarena (Oeiras) Santa Cruz (Torres Vedras)



Antenna Height

(m) 4

Antenna Height

(m) 4

Height above sea level (m)

57 Height above sea

level (m) 39.3


Emission angle 86º

b) Barcarena (Oeiras) – Alpiarça (Santarém)

Another link was established with a distance of about 84 km, between Alpiarça, a place in the region of Santarém and

Barcarena This antenna also belongs to a radio amateur

service station, operating in the frequency range close to the

same band that were the previous tests made. It was used the

same frequency, which for Santa Cruz, the 7,065 MHz which

received a signal from -91 dBm.

This link is quite important and interesting for the tests

because it proves s determined that propagation by NVIS

effect, assured a communication of 84 km, which proves that

it is possible to ensure coverage of links between distances

ranging from 20 km to about 120 km, as defined previously.

http://nvis-tuga.blogspot.pt/2011/01/absorcao-na-camada-d.html

http://nvis-tuga.blogspot.pt/2011/01/absorcao-na-camada-d.html


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TABLE V

CHARACTERISTICS OF BARCARENA – ALPIARÇA LINK

Barcarena (Oeiras) Alpiarça (Santarém)



Antenna Height (m)

4 Antenna Height

(m) 4

Height above sea level (m)

57 Height above sea

level (m) 22.5


Emission angle 83º

Finally, for 84 km link, a filar antenna developed by EID

for the military radio PRC 525, was used. This antenna

consists of a stranded conductor ,15 meters long, directly

coupled to the ATU and ending in a counterweight connected

to an insulated metal which gives tension to the wire, so that

this antenna can be throw up a tree or bush at a given ground

height. In the test performed, the antenna was place in less

favorable conditions, close to the ground. The results are

shown in the following table:

TABLE VI

POWERS OF A FUNCTION OF FREQUENCY SIGNAL FOR THE

LINKS TO BARCARENA NVIS OF SANTA CRUZ AND ALPIARÇA.

Frequency

(MHz)

Received signal power (dBm)

Santa Cruz Alpiarça

Antenna NVIS

MR13 Filar

antenna

Antenna NVIS

MR13 Filar

antenna without balum without balum

7,065 -80 -101 -85 -102

With this antenna, NVIS communications were established

with Santa Cruz and Alpiarça, but with values of received

power, much lower than those obtained by the link with the

MR13 NVIS antenna.

Fig.17. Filar antenna built by EID for the PRC 525.

VI. CONCLUSIONS AND FUTURE PERSPECTIVES

A. Conclusions

The results obtained, allow us to conclude through the

various graphs relating to the variation of the plasma

frequency with the hour of the day, that in Summer it is

possible to maintain a NVIS communication during the entire

day. By contrast, in Winter the time interval in which NVIS

propagation exists is much shorter and in some cases only

lasts a few hours.

The tests were satisfactory as it was verified successfully

from the standpoint of practice and theory, the operation of a

radio link using NVIS propagation, for the proposed distances between 20 km and 120 km.

The MR13 NVIS antenna was designed to be resonant at

frequencies of 4 MHz and 6 MHz, which are close to the

extreme values where reflection occurs in the Summer period,

at the latitude of Portugal. The resonance frequencies showed

a deviation of 500 kHz compared with the values calculated.

These differences may be due to several factors, including the

interaction between the dipoles, due to the configuration of the

antenna and also ground influence, as well as the mesh

characteristics of the antenna wire used in it. With respect to

the curve of resonance frequencies to provide greater bandwidth and improve the adaptation of the frequencies

between 3,5 MHz and 5,6 MHz, a RC matching circuit could

be used. However, this device would insert losses for the

resonance frequencies and for the whole band, reducing the

antenna gain.

In connections with the Harris antennas, the results obtained

were worse than expected because the values of the received

signal in relation to the MR13 NVIS antenna were much

lower. This difference may result from bad contacts in the

connections between the various elements of the mast coaxial

of these antennas as well as a possible oxidation of the

corresponding connections. In conclusion, it can be said that the antenna MR13, which

was n designed and built in this project, has shown a good

performance, better than the Harris antennas. The MR13 is a

low-cost antenna, easy to assemble and which will certainly be

very useful in tactical and emergency communications such as

those that occur often in operational theaters, where the

Portuguese Army is involved.

B. Future perspectives

This work provides several interesting perspectives for

future developments, such as:

Building an antenna with more than two elements

with different resonance frequencies, which would

allow, an, increase in the bandwidth of the;

Using other antenna configurations, for example, one

or more coils with different resonance frequencies.is

also an interesting perspective for future work.


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REFERENCES

[1] A. C. Mateus, Soluções W.K.B. para o cálculo da

intensidade de campo na baixa ionosfera, 2007.

[2] V. G. Pillat, Estudo da inosfera em baixas latitudes

através do modelo computacional Lion e comparação

com parâmetros inonosféricos observados, São José dos

Campos, SP, 2006.

[3] K. Davies, Ionospheric Radio Propagation, 1965.

[4] M. J. A. Faro, Introdução ao Estudo das Ondas

Electromagnéticas, AEIST, 1961-62.

[5] A. Government, “Introduction to HF Radio Propagation,”

IPS Radio and Space Services.

[6] D. Bilitza, International Reference Ionosphere, Radio

Science, 2001.

[7] ITU-Radiocommunication, Definitions of Maximum and

Minimum Transmission Frequencies.

[8] J. Figanier, Aspectos de Propagação na Atmosfera.

[9] J. K. Hargreaves, The Solar-Terrestrial Environment,

Cambridge University Press, 1995.

[10] C. A. Balanis, Antenna Theory, Analysis and Design,

second edition, John Wiley & Sons, Inc, 1997.

[11] J. S. Belrose, “Fessenden and Marconi: Their Differing

Technologies and Transatlantic Experiments During the

First Decade of this Century,” 1995.

[12] J. J. Carr, Practical Antenna Handbook, Fourth Edition,

McGraw-Hill, 2001.

Renato Gonçalves Rocha was

born January 28, 1988 in Caldas da

Rainha. It is natural from Peniche.

Completed high school in June

2006. In October 2006 he joined

Academia Militar, the Engineering

course Transmissions.

In September 2011, he enrolled in

the Master Thesis of

Electrotechnical and Computer Engineering in the field of Telecommunications in Instituto Superior Técnico, in Lisbon,

and is currently finishing the master's degree.

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