A multiple aperture feed system for Ground Moving Target Indication (GMTI) radar

applications

Captain R.B.P. Bourassa, CD, rmc, B Eng, P Eng Canadian Armed Forces

A thesis submitted to:

Department of Electrical and Computer Engineering Royal Military College of Canada

Kingston, Ontario

In partial fulflment of the requirements for the degree of:

Master of Engineering

April 1999

@ Copyright 1999 by R. Bourassa, Kingston, Ontario This thesis may be used within the Department of National Defence But copyright for open publication remains the property of the author.

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Abstract

Recently, the Air Force has been required to participate more actively in

Peacekeeping and Joint operations with h y and Navy elements as well as with Allied

Forces. Many of these Operations requin! strategic/tacticai radar based information,

which is currently provided by the CP- MO. Studies are now being done to try to expand

the aircraft's capability by adding a Ground Moving Target Indicator (GMTI) mode to

the radar in order to support ground operations.

The present AN-APG 506 radar on board the CP-140 aircraft uses an elliptical

rim offset single refiector antenna fed by a rectangular waveguide (WR-90) and a

pyramidal horn. In order to be able to detect moving targets on the ground, clutter

becomes a big issue, especially ground clutter. To get rid of this clutter, there's a

requirement to acquire radar information for the same position on the ground, but at

different times. A multiple aperture system based on three different phase centres would

enable the radar to relocate the target and to reject the clutter.

This thesis investigates different types of feeds with the aim to corne up with a

multiple aperture feed system to provide Ground Moving Target Indication capability tu

the CP-140. Examination of different antennas and possible feed architectures are first

investigated. A new feed scheme that has three displaced phase centres antema and

which would conform to the present antenna system is proposed and analysed. In view of

the analyticai complexity of the antenna system, a state of the art extensive numerical

code for reflector antennas is adopted for the study. The simulations are based on the

physical optics analpis rnethod provided with GRASPI W. Software validation is

performed using representative examples available fkom the literature and also some .* U

available experimentd resuits previously obtained for the present AN-APG 506 radar

with the original feed. The specific parameters examined are the antema and feed gain,

radiation patterns, sidelobe levels, azirnuth and elevation beamwidth, cross-polarkation

level, spillover and feed blockage effects.

An extensive study of three different feed configurations: the open-ended

waveguide, the sectoral hom and the pyramidal hom, was performed. During this study,

a special attention was given to the effects of dielectrically loading the aperture on the

radiation charactenstics. From this study, an optimum feed design was obtained. This

feed design comprises three pyramidal hom loaded with a dielectric materiai which yields

good overall radiation results that will enable the radar to accomplish GMTI.

Dédicace

A mon épouse Dany et

rnonjils Thomas

Acknowledgements

This thesis was written with the help and assistance of a wide variety of

people and sources. 1 would like to take this opportunity to acknowledge the tremendous

help they have given me while 1 was completing my thesis.

First 1 would like to thank Dr Yahia Antar, for his counselling, technical help

and guidance which helped m e stay in the right direction.

1 would also like to thank Mr George Haslam and Mr Anthony Darnini fiom

DREO, from whom 1 gathered usefid information with regards to starting this research.

i would also like to convey my greatest thanks to Capt Jean Hurtubise fiom

the Aurora project in Ottawa who provided me with tremendous help and tirnely

information on the CP-140. 1 can Say with no hesitation that his help was paramount to

rny completing this research.

1 would also like to convey my greatest thanks to the Canadian Space Agency

(CSA) in St-Hubert and especially to Mr Tony Pellerin and Dr. Guy Séguin of the Space

Technology Division, who provided me with the main tool of my research, the simulation

software fiom TICRA, GRASP8 W.

1 wish to convey my thanks to Mr Stig Busk Sorensen of TICRA in Denmark,

who has given me great technical assistance on the phone and over the intemet in order to

solve many of my simulation problems, Thanks.

1 would like to thank Capt Eric Laiiier, a fellow student whose fneadship 1

have corne to rely on during my pst-graduate work at W C .

1 would address my final thanks to rny wife Dany, who has helped me

overcome many hurdles over the span ofthis research. Dany without you none of this

would have been possible, Merci beaucoup mon amour!

vii

Vita

Name: René Bourassa

Place and year of Birth: Sept-lles, Québec, 1969

Educatiou: Bachelor of Engineering in Electrical Engineering - Royal Military College of Canada, 199 1

Experience: 1 997-Present: Post Graduate Studies, Royal Military College of Canada, Kingston, Ontario

Canadian Contingent Maintenance Officer, Port-au-Prince, Haïti

Maintenance Oficer, 1'' Bataillon du Royal 22' Régiment, Valcartier, Québec

Training Oficer and Log Ops Staff Officer, 5e Bataillon des Services du Canada, Valcartier, Québec

Maintenance Officer, Canadian Contingent Support Group, Daruvar, Croatia

Assistant Adjudant and Material Platoon Commander, 5e BataiIlon des Services du Canada, Vaicartier, Québec

Canadian Forces School of Electrical and Mechanicd Enginee~g, Borden,Ontario

René Bowassa is married to Dany Giguère and they have one son: Thomas.

viii

Table of contents

Abstract ................................................................................................................. ii

Dédicace .............................................................................................................. iv

Acknowledgements ............................................................................................. vi

... Vita ..................................................................................................................... VIII

Table of contents ................................................................................................... x

List of Abbreviations and Symbols .................................................................... xiv

Index of Tables and Figures ............................................................................... xvi

.............,*....... ..................--...*.*..**...**..~*............,....*. CHAPTER 1 : Introduction .,. 1 1 . 1. Background information ............................................................................. 1 1.2. Thesis Objective ........................................................................................ 2 1.3. GMTl implementation ................................................................................ 2 1.4. Thesis Methodology ............................................................................... 3 1.5. Thesis organization .............................................................................. 4

CHAPTER 2: General Background and Reflector design considerations ............. 7 ..................................................................................... 2.1. Basic GMTl theory 7

2.2. Fielded GMTl System .............................................................................. 11 2.3. Equivalence between planar array and reflector antenna approach to GMTI 13 2.4. Reflector antennas .................................................................................. 14

2.4.1 . Reflector antenna configurations ................................................. 1 5 2.4.2. Symmetrically cut paraboloids ........................ .... ........................... 1 6

2.5. Reflector antenna design - considerations ............................................ 1 9 2.5.1 . Feed blockage .............................................................................. 20 2.5.2. Cross polarkation .............. ... ....................................................... 20 2.5.3. Sidelobes .......................................................................................... 21 2.5.4. Spillover ....................................................................................... 22 2.5.5. Beamwidth ........................................................................................ 22 2.5.6. Mutual coupling .............................................................................. 23

2.6. Rectangular Waveguides ................................................................. 24 2.7. Homs and feeds .................................................................................... 2 6

..... ................... CHAPTER 3:Antenna Modelling and Software validation ... 29

3.1 . Analysis Methods ................................................................................. 29 3.1 . 1 . Geometrical optics ....................................................................... 29 3.1.2. Geornetrical theory of diffraction (GTD) ............................................ 30

................................... ............................ 3.1.3. Physical optics (PO) .... 31 3.1.4. Physical theory of diffraction (PTD) .................................................. 32

................................................................................. 3.2. Simulation software 33 ................................... 3.2.1 . Simulation sohare exploitation architecture 35

............................................................. 3.2.2. Antenna and feed modelling 37 ........ .......................................... . 3.2.2.1 Refiector antenna geometry ...... 37

............................................................................. 3.2.2.2. Feed geometry 4 0 .................................................................................. 3.3. Software validation 40

............................................................... 3.3.1 . Radiation pattern analysis 42 .................................................................... 3.3.i . 1 . Feed design analysis 42

3.3.1.2.Antenna design analysis ............................................................... 45

CHAPTER 4: Multiple Feed Design ................................................................. 51 ............................................................................................. 4.1 . F eed array 51

................................................................................ 4.1.1. Phase tenter...+ 51 4.1.2. Feed requirements ...................................................................... 52 4.1.3. Effects of Lateral feed displacement .............................................. 52

........................................................................... 4.2. Initial feed array design 54 ...................................................................... 4.2.1 . Initial simulation study 55

...................................... 4.2.2. Optimum distance between phase centres 56 ........................................................................................... 4.3. Types of feed 59

.................................................. 4.3.1 . Pure mode horns and waveguides 60 .............................................................................. 4.3.1 . 1. Pyramidal horn 61 ............................................................................... 4.3.1.2. Sectoral homs 63

................................................................ 4.3.1.3. Open-ended waveguide 63 .................................................................................. 4.3.1.4. Conical hom 64 ............................................................................... 4.3.2. Multimode homs 65 ............................................................................... 4.3.3. Corrugated horn 66

..................................... 4.3.4. Dielectricaly loaded horns and waveguides 67 .................................................................................. 4.3.5. Microstrip feed 69

4.4. Feed array design ................................................................................ 69 ................................................................... . 4.4.1 Open-ended waveguide 70

4.4.2. H-plane sectoral hom ....................... .. .......................................... 79 .................................................... 4.4.3. Pyramidal horn (srnaller aperture) 83

....................... 4.5. Detrimental effects of dieledrically loading an aperture ... 87 .................................................. .................. 4.6. Microwave feed networû .. 87

............................................................... 4.7. Mutual coupling considerations 88

................................... . . . CHAPTER 5: Conclusion and Recommendations ..... 93 Conclusions ........ .. .............. ... ... 93 lmplementation concems ....................................................................... 95 Future work ................................ ... ...................................................... 96

............................................................... Summary .................... .... 97

References ........................ .. ............................................................................ 99

Annex A: Radar Characteristics ....................................................................... 103

Annex B: Reflector's rim tabulated points ................................................... 105

................................................... Annex C: Example of GRASPBW output file 109

xii

List of Abbreviations and Svmbols -

2Dl3D Two or Three Dimensions

AMTI Airbome Moving Target Indication

Az HPBWAzimuth Half-power Beamwidth

CP-140 Aiuora aircraft

CSA Canadian Space Agency

dB Deci bels

dBi Decibels above isotropie

DPCA Displaced Phase centre antenna

DREO Defence Research Establishment Ottawa

El HPBW Elevation Half-power Beamwidth

FT

GHz

GMTI

GO

GTD

HPBW

PO

PRF

PTD

RMC

SAR

SLL

STAP

TE

TEM

TM

X-Pol

Feed taper

Giga Hertz

Ground Moving Target Indication

Geometrical Optics

Geometrical Theory of Diffraction

Half-power Beamwidth

Physicaf Optics

Pulse Repetition Frequency

Physical Theory of Diffbction

Royal Military College of Canada

Synthetic Aperture Radar

S ide10 be level

Space-the adaptive processing

Transverse Electric

Transverse electromagnetic

Transverse Magnetic

Cross-polarization

h Wavelengîh

xiv

Cutoff frequency

Reflector coordinate axis

Feed coordinate axis

radian fiequency

waveguide f i lhg material's permeability

cutoff wavenumber

mode indices

waveguide's width

waveguide's height

propagation constant

arbitrary amplitude constant

waveguide fiIling material's permittivity

distance fiom the centre of the refiector's r im in cylindrical coordinates

angle from the centre of the reflector's nm in cylindncai coordinates

Index of Tables and Figures

Table 1 : Cutoff frequencies for the WR-90 waveguide ..................................................... 26

Table 2: Pyramidal horn parameters definition ................................................................. 43

....................................... Table 3 : Pyramidal hom radiation characteristics d

Table 4: Reflector radiation characteristics ....................................................................... 47

......*................... ....................... Table 5: Assessrnent of different types of feed [32] ... 59

.................................................................. Table 6: Open-ended waveguide parameters 70

Table 7: Reflector radiation characteristic with an open-ended waveguide array

(d= 1.895~1~1) .............................................................................................................. 72

............. Table 8: Reflector radiation characteristic with an open-ended waveguide array 73

Table 9: Reflector radiation characteristics for Beams 1 & 3 (Open-ended waveguide) .. 74

......... Table 1 0: Reflector radiation characteristics for Beam 2 (Open-ended waveguide) 76

................. Table 11 : Reflector radiation characteristics for Beams 1 & 3 (Sectoral hom) 80

........................ Table 12: Reflector radiation characteristics for Beam 2 (Sectoral hom) 82

Table 1 3 : E-plane sectoml hom design parameters ................... .. .................................. 83 ............. Table 14: Reflector radiation characteristics for Beams 1 & 3 (Pyramidal hom) 85

.................... Table 15: Reflector radiation characteristics for Beams 2 (Pyramidal hom) 86

.................................*...............*........*..... 9 Fime 1 : Three-beam sideways looking antenna

.............................. .........*..*..*....*.*. Fimue 2: Three-beam forward looking antenna .......... 1 O

..~....................~~....*~**~~~.~~ 1 2 Figure 3 : Multi-aperture antenaa of the AN/APG-76 radar [3 11

Fieure 4: Example of an offset single reflector antema [4 11 ............................................ 14

F i m e 5: Offset feeding technique [4 11 ........................................................................ 1 6

Fime 6: Symmetrically cut paraboloid using equi-intensity contour cut . [4 11 ................ 17

Fimire 7 : Example of constant intensity contours in paraboloid aperture [41] ................. 18

Fimire 8: Aumra's reflector rim ..................................................................................... 19

Fimre 9: Spillover region [43] .......................................................................................... 22

Figure 10: Rectangular waveguide geometry [32] ............................................................. 24

F i w e 1 1 : Reflection and diffraction fiom an impenetrable surface (GTD) [2 11 ............. 30

F i m e 12: Geometry of an obstacle illuminated by an antenna (PO) [2 1 ] ........................ 32

Figure 13: Simulation software exploitation architecture .................................................. 36

Figure 14: Geometry of the reflector (yz plane) .........................................*... ................... 38

Figure 15: Reflector coordinate system used for reflector's rim definition [(p, 4) values] 39

Figure 16: View of the reflector's rim geometry (xy plane) ........+..................................... 39

Fi- 17: Pyramidal hom geometry ...+............................................................................. 40

....................................... F i w e 18: Simulated far-field radiation pattern of a symmetric 41

........................................................ Figure 19: Far-field radiation pattern of a symmetric 41

................................. Fimue 20: Pyramidal hom Copolar radiation pattern for f=9.5GHz 44

................................. F i w e 21 : Contour display of the pyramidal hom radiation pattern 45

............................................ Fieure 22: Reflector antenna radiation pattern for F9.5GHz 46

............................... Fime 23 : Sunulated antenna radiation pattern for f=9 SGHZ (+=O> 48

Fiwe 25: Contour plot of the reflector radiation pattern ............................................. 49

......................................................... Fime 26: Feed horn phase centre representation 51

.......... F i w 27: Example of the effects of Off-axis feeds on the radiation pattern [43] 3 3

Fime 28: Initial pyramidal horn array design ............................................................. 54

Figure 29: Radiation pattern with a 3 horn array as seen in Figure 28 .............................. 55

Fiwe 30: Amy radiation pattern with feed's phase centres 2.921 cm apart ................... 57

Fieure 3 1 : A m y radiation pattern with feed's phase centres 1.27 cm apart ..................... 58

Figure 32: A m y radiation pattern with feed's phase centres 1.89 cm apart ..................... 58

Fiwe 34: Pyramidal and sectoral hom [32] .................................................................... 61

F i m e 35: Pyramidal hom geometry [32] ....................... ... ......................................... 62

F i m e 36: Smooth-wall conical horn geometry ................................................................ 64

. Fime 37 Transition fiom rectangular to circular waveguide ....................................... 64

F i w e 38: Design regions of a comgated hom (321 ......................................................... 67

Figure 39: Two different designs of dielectric-loaded homs ............................................. 68

Fieure 40: Feed radiation patterns of 4 different feed configurations (e9.5 GHz) .......... 71

Fime 42: Reflector radiation pattem with the open-ended waveguide array (e9.5 GHz)

for E= 1 and ~ = 6 ........................... .... ...................................................................... 75

............ Fi m e 43 : Electric fields inside the dielectrically loaded open-ended waveguide 77

Figure 44: Impact of dielectrically loading an open-ended waveguide with different

.......................................... dielectric materid on the Antema gain. SLL and X-POL 78

Fime 45: Impact of dielectrically loading an open-ended waveguide with different

.............................................................. dielectric material on the Az and El HPBW 79

F i m 46: Reflector radiation pattern with the sectoral hom array (e9.5 GHz) .............. 81

F&re 47: Effects of dielectricaly Ioading the fced hom on its radiation pattern ............. 84

F&ure 48: Reflector radiation patkm with the pyramidal horn array (69.5 GHz) for ~ = 1

........................................................ and r= ...................................*.......................... 85

Fime 49: Modified antema feed network ....................................................................... 88

Fieure 50: Relationship of methods for analysing mutual coupling (321 .......................... 91

Fimue 51 : Mutual coupling from TEio mode as a function of side length [6] .................. 92

CHAPTER 1 : Introduction

1 Background information

Over the paît couple of years, the Air Force has been required to participate

more actively in Peacekeeping and Joint operations with Army and Navy elements as

well as with Allied Forces. Many of these operations require strategic/tactical radar based

information, which is currentiy provided by the CP-140. The Aurora's main functions

are to cany out maritime surveillance and anti-submarine warfare. In addition, the

aircraft is used for fisheries patrols, the surveillance of the Canadian North, anti-surface

warfare, joint operations with Allied Forces, search and rescue, and surveillance

operations in support of other govemrnent agencies and departments. The radar, the

AN/APG-506, is used to detect and locate ships and submarines on the surface of the

ocean, to detect other aircraft in the area, to localise bad weather, and to ensure collision

avoidance with any object that could lie in the Aurora's flight path.

One impact of recent research has been the enhancement of the surveillance

capability with the addition of an imaging radar capability, the Spotlight SAR to the CP-

140. With this major project behind, DREO is now examining the feasibility of

expanding this capability by adding a Ground Moving Target Indicator (GMïï) mode to

support ground operations. One of the main constraints is to achieve this capability while

minimizing the impact on the present antenna and the existing radar mode of operations.

Although many air& in the world have GMTI capability and that the technology

already exists, no system has been developed to date that uses an offset single refîector

antenna, which is the antenna configuration presently used by the CP-140.

1

1 2 . Thesis Objective

The objective of this thesis is to Ulvestigate the optimum aperture using

multiple phase centres to obtain nearly identical beams that conform to the same

requirements as the present antenna configuration in order to add GMTI capability to the

detection system while rnaintaining the AN-APG 506 radar antenna's present capabilities.

This could involve a new feed scheme with multiple bearn capability or the use of the

present feed system but with elements that conform to the aperture limitation and at the

same time provide the required performance.

1.3. GMTI implementation

On an initial note, although the word used in this thesis is GMTI, it should be

noted by the reader that it is identical to what some scientist refer to as AMTI (Airborne

Mn). Although many modem systems in the world use hm technology, none were

found to use reflector antennas as the root of their installation. Almost dl systems

surveyed while doing this research use planar arrays, a type of technology though

expensive, but it is easier to implement on board today's aircraft Many M'Il radars used

for air-tranic control use reflector antennas but none of the systems require multiple feed

to be able to compensate for the movement of the clutter with respect to the airbome

platform while uskg difference Doppler to locate and fïnd the direction and velocity of

the targets. As stated earlier, one of the challenges in this work is to obtain GMTI

capability while maintaining the existing antenna with its curent radar modes and

operationai capabiiities.

1 4 Thesis Methodology

A thorough search of the literature was done to see what kind of GMTi system

existed today and what kind of antenna technology is being used. An important

consideration in this research was to try to find a system based on a reflector antenna.

However, it was found that most of today's systems use Plana anays which would not

totally fit with our objectives. Hence the main focus of the research activities was on

fmding an analysis or modelling tool for reflector antennas with suffiicient fiexibility to

invedgate al1 panmeters of the antenna and its feed structure. The only modelling

software powerful enough to do the type of extensive and accurate needed simulations

required by this research was the software GRAPS8W fiom TICRA in Denmark. This is

an extensive, state of the art antenna software that is used by major satellite and other

industries, however while this software was not available at RMC because of the

prohibitive cost, it was available at DREO and at the CSA, and use was made of the

Iatter.

The modelling process started by consideration of the existing antenna

configurations using data provided by the Aurora ce11 in Ottawa. This was done in

parallel with the requirement to do an in depth software familiarization. The next phase

of the research involved modelling of the present radar antenna. After completion of the

model, cornparison of simulated results with measwments taken in 1982 from the same

antenna was made. The mode1 was found to be very accurate and the good agreement

with the measurements provided confidence in the process.

At that stage a feed anay structure had to be modelled in order to get three

nearly identical beams as necessitated by the initial requirements. This was accomplished

by doing a survey of al1 existing possible feeds and their characteristics. M e r careful

examination, the potential types of feed were chosen and a specific analysis was

completed on al1 of them. The configuration which best met the requirements was chosen

and the investigation process was completed.

This chapter has laid out the background as to why this research was canied

out and outlined the expected goals and methodology. Chapter 2 details the general

background on reflector antenna., waveguides, homs and feed used with reflector antenna

and their design considerations Chapter 3 covea the background on some of the analysis

methods used for reflector antenna modelling, the simulation software utilized in this

research, the design considerations when modelling reflector antennas and also how the

software validation was accomplished. Chapter 4 outlines the approach used for

investigating various codigurations for the feed of the reflector in order to be able to

produce a multiple beam antenna used for GMTI applications. Some of the

considerations studied are the effect of the additional feed blockage when increasing the

number of feed horns, the effect of lateral feed displacement in the focal plane and

muhial coupling between feeds. Some of the most common and viable feed design have

been investigated and wiU te discussed. It aiso coven the implementation concems of

rnodifjhg the feed structure on the existing system and how it may affect the radar's

operation. Chapter 5 presents discussions and coaciuding remarks, concems about

mechanicd considerations like weight limitations, system balancing and unwanted

vibration as well as recommendations for fiirther research.

CHAPTER 2: Generaiil Backaround and Reflector design considerations

Ln this chapter, we will introduce the basic concepts of GMTI and how it

applies to this research. An overview of a GMTI system already implemented is aiso

introduced to see its main hct ions and design aspects. The main considerations in

reflector design: feed blockage effects, X-Pol, SLL, spillover, beamwidth and mutuai

coupling are defined. Finally a description of the modes for the rectangular waveguides,

currently used in the present feed is provided.

2.1. Basic GMTI theory

Airbome surveillance radars provide powerful tools for addressing a variety of

ground surveillance activities ranging fiom terrain height mapping to moving target

detection. This range of different activities could well be in principle perforrned by a

single radar system designed to operate in a variety of different modes. Two common

modes for ground surveillance are the SAR mode for static target detection and the GMTI

mode for detection of ground moving targets [49]. While the SAR mode design has been

completed and a demonstrator is currently in use by NRC's Convair aircraft, the GMTI

mode implementation is still being studied. The focus of this research is to investigate

the modeling of a multiple feed system for the radar antenna on board the CP-140 in

order to add GMTI capability to the aircraft. We will now briefly introduce the basics of

GMTI theory.

An Airbome Ground rnoving target indication radar utilizes the Doppler shift,

imparted on the reflected signal by a rnoving target, tu separate small, moving targets

fiom competing extraneou returns, called clutter, which typically result fkom reflections

obtained fiom the ground, sea, rain and other hydrorneters, chaff and birds. The M n

radar ideaily fiinctions to elirninate clutter through the use of a comb filter (filter

conceptualised as a comb which elirninate al1 the clutter returns while retaining al1 the

target rehims), the stopband of which is placed in regions of heavy clutter concentrations.

Moving targets are passed through those velocity regions not occupied by the clutter [39].

Because the Doppler retums fiom slow moving ground targets are masked by clutter

retums, the GMTI mode would have to cancel ground clutter using a displaced phase

centre antema (DPCA) technique or the space tirne adaptive processing (STAP)

technique [3 11. Ofien in the literature, STAP as been referred to as either generalised

DPCA or adaptive DPCA. The f'undamental difference between the two is that DPCA

attempts to achieve complete clutter rejection and STAP attempts to rnaximize the output

signal to noise ratio, but for the purpose of this research we will now refer to the two

techniques as DPCA techniques [37l. The DPCA techniques attempt to arrest the moving

aperture over two or more successive radar pulses [30]. in order to detect moving targets,

generally, we require at least 2 phase centres, and if it is needed to relocate that target, a

minimum of three is required, two of which are required for DPCA to allow us to reject

the clutter and the third one would be used for angle measurement of the detected moving

targets. A sirleways looking three-beam DPCA set-up based on three phase centre

antenna can be seen in Figure 1.

Fiare 1: Three-bearn sideways looking antenna

Using this set-up, a pulse is transmined and received using the leading phase centre at

time t, at a spatial location (x,y,z). As the aircraft moves almg track at velocity V, the

second phase centre will arrive at location (x,y,z) at a t h e t+At. At this point, a second

pulse is transmitted using this second phase centre. Again the aircraft moves dong its

track, the third phase centre will arrive at that same location. At that point, a third pulse

is tnuismitted using the trailing phase centre. Three consecutive pulses have been

transrnitted and received fiom the same location in space but separated in tirne by At. If

the beam pattern are identical then the three looks at identicai patches of the gound have

been received. The second received puise can be subtracted Eom the fim and dl retums

fiom static scatterers will cancel leaving only rems due to moving targets allowing for

slow target detection. The third received pulse can also be subtracted fiom the second

one thus allowing the rate of change of phase to be measured giving a direct measure of

the targets intrinsic radial velocity and its position in the radar beam.

There exist two types of airborne radars used for GMTI, the sideways and

the forward looking radars. The sideways looking radar is based on the DPCA property

discussed previously, but it is obviously not the case for the forward-looking antenna.

The radar antenna on board the CP- 140, is precisely a forward looking configuration type

of antenna. A three-beam forward-looking antenna c m be seen in Figure 2. and £tom that

figure it is easy to see that no spatial coincidence between any of the phase centres

occurs. Therefore, at a first glace, low correlation and, hence poor clutter rejection can

be expected. It has been shown [17] that even though there appears to be little correlation

between the phase centres, there exists an approximate linear dependence between the

main lobe clutter Doppler and spatial fkequency that provide good clutter suppression.

Look direction A I I

time t + 2Dt

time t + Dt

time t

m r e 2: The-beam forward looking antema

10

Based on the previous descriptions, we can now establish what would be the

generd antenna design requirements. We now know that the antema design will need to

provide three fairly identical beams with sidelobe levels as low as possible. These basic

requirements will now be compared with present system currentiy in use.

2.2. Fielded GMTl System

As mentioned in the previous chapter, almost ail systems now being

implemented in today's aircraft use planar anays, a type of technology easier to

implement on board an aircraft. A good example that was found during the literature

survey is the AN-APG-76 radar built by Northorp Grumman Norden System. The

antenna design for this radar i s a wide-band, Bat-plate array for which the upper two

thirds of the array area is occupied by an AdEl monopulse array and the lower third by a

three-port, receive-ody, azimuth interferorneter. Figure 3 presents a view of the mdti-

aperture antenna of the AN-APG-76 radar. The triple-port interferorneter is present on

the antema solely to support the GMTI mode. Since the mode is sirnultaneuus with radar

mapping, a total of four channels are processed in parallel.

MONOPULSE Guard antcslaa is used for ECCM.

Fimire 3: Multi-aperture antenna of the ANIAPG-76 radar [3 11

Because the Doppler returns from slowly moving ground targets are masked

by clutter rems, the GMTI mode gains sensitivity by cancelling ground clutter using a

DPCA technique. Two of the interferorneter ports are required for DPCA and the

presence of a third port enables interferometric angle measurement of detected moving

targets.

Although, the system under consideration doesn't make use of planar arrays,

the methodology utilised in this design justifies orïenting the present research towards a

multiple aperture antenna. While an important consideration in deciding not to go for a

planar array configuration was the financial aspect, the lack of additional room on the

ahcrafi to install this ncw antenna and the technology cornplexity were aiso a factor.

Addiag another aatenna on board the CP-140 wouid be too expensive compared to

modifying the existing reflector to add the GMTI mode to the other k e e modes already

in operation with the Aurora's radar.

2.3. Equhralence between planar array and reflector antenna approach to GMTI

Although the pnnciple by which a planar anay conf@g.mtion obtains relevant

GMTI information has been stated previously, it is not the antenna configuration under

consideration in the present work. It is thus important to state how a reflector a n t e ~ a

configuration is able to achieve the same performance in obtaining GMTI information.

The same DPCA technique is utilized but since we are in the presence cf an amplitude

cornparison monopuise system, we need to synthesize the two outside bearns using the

s u m and difference pattems. By manipuiating the weighting of the sum and difference

pattern, the desired patterns for the outside two bearns (leA and right beams) can be

achieved. Although the general concept of obtaining GMTI information using a reflector

antenna configuration has been discussed during a private communication with Mr

Georges Haslarn and Mr Anthony Damini fkom DREO, the exact concept is still under

consideration and for this reason, this issue won't be dealt with in this work. We will

now study in more details the antenna configuration researched, the reflector antenna.

2.4. Reflector antennas

Reflector antennas, in one fom or another, have been in use since the

discovery of electromagnetic wave propagation in 1888 by Hertz However, the fine art

of analysing and designing reflectors of many various geometry and shapes did not

appear until the days of World War II [3]. Reflector antennas do take many geometrical

shapes, but some of the most popular ones are the plane, corner and curved reflectors. In

this thesis we are concerned oniy with curved reflector in particular the paraboloid

reflector. An example of an offset single reflector antenna can be seen in Figure 4.

Fimre 4: Example of an offset single reflector antenna [41]

2.4.1. Reflector antenna configurations

There exists four main configurations to a paraboloid reflector and these are

the following:

a Axially - Symmetric Single Reflector;

b. Offset Single Refiector;

c. Axidly - Symmetric Dual-Reflector; and

d. Offset Dual Reflector.

The AN-APG 506 radar antema on board the CP-140 is an elliptical rim

offset single reflector. A demonstration of how this design was accomplished will pave

the way for the antenna modelling in the simulation software and M e r research in the

feed design.

Reflector surfaces are commonly generated either by translation of a conic

section dong the y-axis, or by its rotation about the focal axis z. An Offset reflector c m

be constructed by carving out a portion of the rotationaily syrnmetric reflector. This is

typically achieved by either intersecting the reflector with a circuiar or elliptical cylinder

with its axis parailel to the reflector axis, or by intersecting the reflector with a circular or

elliptical cone with its tip at the focal point [41].

By offsetting the feed, the problem of feed blockage is reduced tremendously

as the feed is not in the path of the most illuminated area of the reflected rays. The centre

of the feed is stiU placed at the focus of the paraboloid but the horn is tipped so that the

peak of the primary pattern makes an angle y0 with the paraboloid axis (Figure 5). At

that point, the major portion of the lower section of the paraboloid is discarded. By

removing the hom fiom the most Uuminated area of the aperture, it results in

improvement in both gain and sidelobe characteristics.

Fieure 5: Offset feeding technique [4 11

One of the most important drawbacks of the offset feeding technique is that

the elimination of one of the planes of s p e t r y destroys the symrnetry of the cross-

polarization component of the aperture field leading to cross-polarkation in the plane of

the large dimension of the aperture. The cross-polarization pattern has lobes on either

side of the main lobe in the plane of the narrower beamwidth, which has serious impacts

on the operational use of this type of design.

2.4.2. Symmetrically cut panboloids

The reflector studied is a symmetrically cut paraboloid using equi-intensity

contour cut as shown in Figure 6.

Fimire 6: Symmetrically cut paraboloid using equi-intensity contour cut. [41]

Normaily a feed with a circular primary pattern wouid be well suited for a

paraboloid but the equi-intensity symmetncdly cut paraboloid would not be illuminated

efficiently by such a feed as a large fraction of the energy would be wasted in spillover.

To optimize design efficiency, the primary feed must be shaped to the same symmeûy as

the reflector. Homs with rectangular apertures lend themselves particularly weli to the

design of suitable feeds, since the beamwidth in the two principal planes cm be

controlled wtually independently of one another by choice of the principal plane

dimensions. The primary pattern of the horn (designed to meet the principal plane

requirements) has an elliptical cross section. Consequently the equi-intensity

illumination contours on the reflector are also elliptical in shape.

The design procedure for such a reflector is fairly simple, first the dimensions

di and d2 are chosen in accordance with the beamwidth requirements. The focal length

and the dimensions of the hom aperture are chosen as though the reflector is to be cut

syrnmetrically, the horn is then constnicted. Then for the chosen value as seen in

Figure 5, the primary feed pattern is transformed into equi-intensity illumination contours

on the surface of the paraboloid. Figure 7 gives a view of an example of such constant

intensity contours based on the feed pattern used for designing the reflector aperture

periphery. The paraboloid is then cut to follow an equi-intensity contour, generally

chosen as the 14 dB contour.

x/f length of the reflector dong x divided by the focal length of the refl ector

ylf length of the reflector along y divided by the focal length of the reflector

Fieure 7 : Example of constant intensity contours in paraboloid aperture [41]

There are several reasons for cutting the reflector along such a contour as seen

in Figure 6. The main reason being that in general the gain factor increases and the

general features of the pattem improve when the sidelobes in the principal plane are

reduced

The ovoid shape of the Aurora's reflector seen in Figure 8 has the dual

advantage of low wind resistance and smaller moments of area and inertia which are of

considerable importance when considering the mechanical problems of support and

rotation of the antenna Since the antenna considered in this paper is placed inside the

nose of the aircraft and is required to rotate at high velocities to gather relevant radar

information, an ovoid shaped reflector is most suited for the required operationai

environment.

Fimire 8: Aurora's reflector rim

2.5. Reflector antenna design - considerations

When designing an antenna for a specific purpose, rnany factors must be taken

into account, but fim it is essential to clearly understand the purpose and operational

environment of the system. This will serve to establish the kind of antenna configuration

that will be needed. The effects of feed blockage m u t be considered, dong with the

evaluation of the sidelobe level requirements, the cross-polmiulzation level and if designing

a feed array, mutuai coupling rnust also be considered. These aspects will now be

discussed in detail.

2.S.l. Feed blockage

Most reflector systems s a e r fiom feed blockage to some extent if the

reflecting area includes the vertex of the parabola. The magnitude of the resulting

sidelobes depends on the square of the blocked area.

In this particular problem, even though an offset single reflector is used, some

minor feed blockage is still present. As stated in many references [44] [45] , to a fUst

approximation, the feed scattering can be treated as if it created a hole in the aperture

field of the main reflector, which has the shape of the largest projected area of the feed.

This is the method that will be used in the next chapter to account for the feed blockage

in simulations. Blocking then causes an increase in the near-in sidelobe levels as the

unblocked radiation pattern lobes are superimposed on the blocked lobes [44].

2.5.2. Cross polarization

In many antenna designs, cross-polarkation is of a major concern and cannot

be overlooked. It is defined as the polarization orthogonal to a reference polarization.

Ludwig hm identified three possible definitions for the case of a linearly polarked

source. nie third one is the preferred one by all and it States that:

"The reference polarization is that of a Huygens source (electric and magnetic dipoles with orthogod axes lying in the aperture plane and radiating equai fields in phase dong the z-axis). The cross-polarization is then the polarization of a similar source rotated 90' in the aperture plane." ~ 4 1

In the problem under consideration here, we have an offset single reflector and

usually these types of reflectors are very susceptible to cross-polarization. This is due to

the fact that the two principal linear polarizations cotate in the same direction and the

cross-polarization is anti-symmeûicai with respect to ody one principal axis. Because of

that the principal linear polarkations are added up usuaily giving us a high levei of cross

polarization. In this design, cross-polarization though is important, but it is not the major

concem since the angle (va) at which the feed is transrnitting its energy on the reflector is

quite low but its impact will still be considered.

2.5.3. Sidelobes

The lobe structure of the antenna radiation pattern outside the main-bearn

region usually consists of a large number of minor lobes, typically referred to as

sidelobes, an important source of concern when designing a radar system. When

ûansmitting, they represent wasted radiated power illuminating directions other than the

desired main-beam direction. In the receive mode, they allow energy from undesired

directions to enter the system. A GMTI system is particularly succeptible to antenna

sidelobes since it is used to detect moving targets on the ground. The radar cm receive

strong ground echoes or clutter through the sidelobes, which could mask weaker echoes

coming fiom low radar cross-section targets through the main beam. As a resuit, to be

able to reject as much clutter as possible in our GMTI system environment, we should

look for sidelobes as low as possible [43].

2.5.4. Spillover

To ensure proper system optimization, the energy radiated fiom the feed must

be distributed over the aperture with a reasonable degree of uniformity. With most

primary feeds this results in a sigrilncant arnount of energy radiating outside of the

reflector. Spillover (Figure 9) is one of the most significant factors in feed efficiency and

is somewhat dificult to evaiuate due to edge discontinuities.

Fimre 9: Spillover region [43]

In the present research, this factor is calculated using physical optics through

cornputer analysis of the resdting primary radiation pattern.

One of the rnost important features of an antenna radiation pattern is the

beamwidth of the main beam. But since the main lobe is a continuous hction, its width

22

varies from the peak to the nuiis. The most fiequently expressed width is the half-power

beamwidth (HPBW) also often referred to as the 3dB beamwidth. This half-power width

is also used as a measure of the resolution of an antenna, so that two identical targets at

the same range are said to be resolved in angle if separated by the half-power beamwidth.

The beamwidth of an antenna depends on the size of the antenna aperture as

well as on the amplitude and phase distribution across the aperture. The bearnwidth in a

particular plane is inversel j proportional to the size of the aperture in that plane. In the

case of the reflector shown in Figure 8, the size of the aperture in the azimuth plane is

different than the one in the elevation plane so we will have to consider a different

HPB W in each of those planes. The HPB W will be a dnving factor in the reflector

analysis presented at chapter 4 [43].

2.5.6. Mutual coupling

When antenna elements, like in a feed anay, are in close proximity, they

interact in a complicated manner. This interaction is called mutuai coupling and the

effect is to change the current on an antenna element fiom that which it wouid have if it

were isoolated in fiee space [46]. The relative spacing between the different feed forming

an array could very weff lead to mutual coupiing. Large mutuai coupling levels can

degrade the sidelobe levels and alter the main beam shape. Mutual coupling n o d l y

causes dctrimentai effects. Characterishg the effect of mutuai coupling between feeds is

23

a compIicated task. A mutual coupling assessrnent study based on known previous work

found in the literature will be accomplished in Chapter 4.

2.6. Rectangular Waveg uides

In our specific problem, the system already built in the aircraft is fed by a

WR-90 waveguide. The WR-90 waveguide is specially designed for fiequencies ranging

from 7.5 GHz to 13.2 GHz. Ahost dl-standard rectangular waveguides have a=2b. The

exception is the common X-band waveguide WR-90, which has a=2.25b. Due to a

different convention with our design, k2.25a. Figure 10 offers you a view of the

geometry of a rectangular waveguide.

Fim te 10: Rectangular waveguide geometry [32]

A rectanguiar waveguide can propagate TE and TM modes but not the TEM

mode, since ody one conductor is present. . The fundamental mode of a rectangular

waveguide is the TEio mode when the dimension a > b. In our specific application, it

happens that a c b, which means that the fundamentai mode of this rectangular

waveguide configuration is TEoi. The TE modes are characterised by fields with Ez=O.

The transverse field components of the TE,, mode can be found using the following

equations.

jwpnn m m Ex = A,, COS - nv e - ~ B sin - k:a a b ( 1)

- j a p m r E = m m nls,e-,p

Y A, sin - COS -

k:b a b (2)

the Electric field component in the dy direction

radian fiequency

waveguide filling material's permeability

cutoff wavenumber

mode indices

waveguide' s width

waveguide's height

propagation constant

arbitrary amplitude constant

But the only electric-field component of the TEoi mode is the Ex component

and is given by equation (3).

The cutsff fiequencies of the different modes propagating in our rectangular

waveguide are given by equation (4).

Computing the f, for the first values of m and n gives:

1 Mode 1 m 1 n 1 f-(Gb) 1

Table 1: Cutoff tiequencies for the WR-90 waveguide

Since the operating fiequencies of ouï- radar, as stated in Annex A, are

between 9.5 to 10.0 GHz, the only propagating mode in our waveguide at those

fiequencies is the TEol mode.

2.7. Homs and feeds

It is well known that paraboloid reflectoa couvert incornkg plane waves into

sphericai phase hnts centred at the focus. For this reason, feeds must be point-source

radiators (Le. they m u t radiate spherical phase fronts if the desired antenna pattern is to

be achieved) [43]. The main problern we were faced with while designing this system,

was the feed design. Since it is an important part of th is research, we have devoted a

whole chapter to the feed design. Many factors determine the type of feed, which should

be used for a specific application and for that reason an extensive study was done and is

presented in Chapter 4.

CHAPTER 3: Antenna Modellina and Software validation

3.1. Anaiysis Methods

In order to be able to perform an efficient analysis of electrornagnetic

radiating system that are large in tenns of wavelength, such as a reflector antenna, high

frequency mathematical techniques based on ray optical fields must be used. There exist

four main techniques and we will now go through an overview of their basic concepts.

3.1.1. Geometrical optics

The Geometrical optics (GO) technique is a high-fiequency approximation

which employs rays to describe the fields that are directiy incident from the source, and

to describe the fields which are reflected at an interface between two different medium,

According to classicai GO, the high fkequency electromagnetic field is assumed to

propagate dong ray paths which satisw Fermat's principle ("rays travel dong the path of

least tirne"), and al1 the rays are everywhere orthogonal to the wavefionts in an isotropie

media. The rays paths are straight lines in a homogeneous medium, but they can change

directions at an interface between two different medium according to Snell's Law of

reflection and rehction.

A problem arises with this method when in the evaluation, the incident GO

field strikes an impenetrable surface, the field is blocked by that surface (Le feed).

Therefore, the d a c e creates a shadow region behind it where the incident rays cannot

exist, and consequently, GO predicts a zero field in the shadow zone [2 11.

Although GO fails to account for a proper non-zero field within the shadow

region behind an impenetrable obstacle, it can be overcome through the Geometrical

theory of ciBiaction (GTD) technique, which we will discuss next [2 11.

3.1.2. Geometrical theory of diffraction (GTD)

As rnentioned above, the GTD technique is a systematic extension of the ideas

of GO in which dihcted rays are introduced via a generalization of Fermat's p ~ c i p l e .

While the GO rays exist only in the lit zone, the difiacted rays, in general, enter in both

the shadow as well as the lit zones.

' RSR (Rcflection shodow boundsry) 1 I I I cgB (Incident shadow boundary)

R e m RAY sse (Surfacc Shadow bound;iry)

Ficmre Il: Reflection and ciiffiaction nom an impenetrable surface (GTD) [21]

These rays may be classifïed as the geometricd and edge-difhcted rays emanating fkom

the d a c e reflection and edge-diffiacted points in accordance with Snell's Iaw [21]. The

total GTD field consists of a superposition of the GO (incident and reflected) field and

the field of al1 diffkacted rays that c m reach the observation point [2 11. Figure 1 1

illustrates very well this principle.

3.1 -3. Physical optics (PO)

The Physical Optics field is based on the fact that the total electromagnetic

field of a source, the reflector in this particula. application, which radiates in the presence

of a perfectiy eonducting surface as c m be seen in Figure 12, may be expressed as a

superposition of the incident field (Ei, Hi) and the fields (Es, Hs) which are scattered by

the surface S (the feed) [2 11. The incidents fields referred above denote the eiectric and

magnetic fields of the source that exist everywhere. These fields exist as if the scatterer

was not present; this is different fiom the GO incident field as stated previously.

The foundation of the PO technique is based on the assumption that the

induced current on the reflector surface is given by equation (5).

2R X H' lighted region J = { O

othenvise

Where n is the unit normal to the surface and Hi is the incident magnetic field.

Although PO current is an approximation for the m e current on the reflector surface, it

nevertheless gives very accurate results for far-field of reflectors as small as

approximately 5k in diameter.

Figure 12: Geometry of an obstacle illurninated by an antema (PO) [2 11

This method is far more powerfùl and gives the closest approximation

possible to that of real life measurements. This technique foms the basis of the

numericai tool used to cornpiete the snidy of the radiation patterns of the reflector

antenna researched in here.

3.1.4. Physical theory of diffraction (PTD)

The physical theory of difiaction technique, which constitutes an extension of

PO was developed originally for analyzhg the high f'requency scattering fkom conducting

surfaces. In the PO technique, the currents induced on the surface of a scatterer are

approximated according to the GO technique. However, it is clear that the GO

32

approximation for the currents would be accurate only on the portion of the scatterer that

is strongly illuminated by the source. Whereas it wouid be totaily inaccurate in the

shadowed portion of a smooth convex surface, where the GO yields a zero value for the

surface currents. In the PTD approach, the GO current approximation is irnproved by

including a correction which is referred to as a "non-uniform" component of the current.

The GTD technique consthtes a far more efficient and physically appealing

solution than the PTD, which in general requires integration over the GO currents and

over the Ufmtsev-based Iine currents. Nevertheless, the PTD is very useM for

estimating the fields in regions where there is a confluence of transition regions

associated with shadow boundaries [2 11.

3.2. Simulation software

There exist many simulation software packages for reflector antennas but only

a few that give enough flexibility to mode1 reflectors in 3D. Moa of the simulation

softwares available on the market today are very limited for the type of geometry the user

c m design. GRASP8W Version 8.1.3 fiom TICRA was selected because it is recognized

as the most powemil modelling and simulation software for reflector antennas available.

The program is written in standard FORTRAN 90. GRASP8W is a 3D-simulation

software that calculates the electromagnetic scattering fiom reflector antennas. The

scattered fields can be cdculated by PO combined with the PTD or GO combined with

the GTD, the standard analysis methods for reflector antenna

GRASPIW program is composed of a pre-processor and post-processor. The

first one is used for assisting the user in defining the scattering structures and the analysis

tasks to be carried out. The pre-processor c m also display the geometry in 3D. The post-

processor is available for displaying the calculated field in cuts and also provides various

other types of useful displays.

The simulation software is based on an object structure in which the

geometricai data is arranged and used for simulations. Every aspect of the reflector

design is considered as an object: the feed coordinate systern, the reflector coordinate

system, the reflector surface, the reflector rim tabulated points, for example, are al1

separate objects. When the objects have been defined, the sequence in which the analysis

should be carried out is specified. The analysis normally starts with one or more feeds

which act as the primary source of the electromagnetic field. The field from this source

illuminates a scatterer (the reflector), for which the scattered field can be computed.

In GRASPIW, the reflector surface can be defined numerically in a 2D grid or

dong radial Iength assuming that it is rotationally symmetric. Other types of reflector

surfaces cm also be defmed, but the most common ones being the paraboloid, the

hyperboloid and the ellipsoid surfaces. The scatterers are assumed to be perfectly

conducting, but special suface materials rnay be defined. These materials may be

combined in a layered structure and attached to the surface of the scatterer. A variety of

feed models can be defined both andytically and numerically. The most important

anaiytical rnodels are the circuiar and the rectangular hom and the gaussian beam. The

numericd types hclude a feed defhed by tabulated far-field cuts and a feed defined by

sphencal wave coefficients.

3.2.1. Simulation software exploitation architecture

Although the simulation software user's manuai is thorough and provides the

different class hierarchy, it fails to ident* a flow chart that explains the process 60m

the program's object creation to the output of the results. Figure 13 provides a view of

the software exploitation architecture. This flow chart is essential in understanding the

complete process utilized by the software to compute the scattered fields from the

reflector. The black triangle is set as a marker and will be used later in the feed array

design chapter We will now consider the modelling of the antema and the feed of the

Aurora' s radar.

Crcatc gcneial rcflcctor de finition 1

surface

Dcfinc thc clcctricnl propcrties of the rtflcctor

I Dcfinc the fccd configurau'an 1 Dcfinc scaamr's murcc 1

I Choosc the type of anaiysis to pcrform I

l C m types of cuts Ki k pcrformed I

I Output muits bascd on specific cuts I- Wre 13: Simulation software exploitation architecture

36

3.2.2. Antenna and feed modelling

The initial step, before looking at the multiple feed design, was to produce an

antema and feed mode1 that represented well the radar of the CP-140. In order to design

this rnodel, original drawings were obtained fiom the Aurora project office in Ottawa.

When the design was completed, wide ranges of simulations were perfomed in order to

validate the antenna and feed design. We will now go in the details of the geometry of

the reflector and the feed and we will establish the coordinate system convention.

3.2.2.1. Reflector antenna geometry

As explained previously the radar is formed of an offset single reflector

antenna. A reflector is defined by its surface and its rim. The Front face contour of the

reflector is a paraboloid of revolution whose axis is the z mis. The surface of a

paraboloid is defined by the parabofic equation (al1 dimensions are in inches):

Where xo, y0 and are the vertex coordinates

The surface of the reflector conforms io the following equation:

Equation (7) was obtained tiom the onginai drawings of the reflector supplied by the

Aurora ceil in Ottawa The paraboloid is defined by its focal length and the location of

37

the vertex in the reflector coordinate system. From the above equations, the following

orm mat ion was extracted:

Xo = O vertex coordinate s =

zo = O

Using the above-mentioned information, the reflector was modelled. Figure 14 gives a

view of the reflector's geometry (In Figure 14, the r stands for reflector and f for feed).

Figure 14: Geometry of the reflector (yz plane)

As mentioned previously, a reflector is defined by its surface and rim. Since

the refiector surface has now been modelled, the rim is still to be defined. This part of

the work turned out to be a very demanding task The r h had to be defhed point by

point using the original dtawiogs. The rim data was specified by tabuiated (p,4) values in

38

the reflector coordinate system. The p value refers to the distance fiom the centre of the

reflector to the rim for the given 4 angle. Figure 15 explains the coordinate system used

to define the reflector's nm. The rim was defined by 360 diffierent points, a list of which

can be found in Annex C.

Figure 15: Refiector cwrdinate system used for reflector's rim definition [(p,~)) values]

Figure 16 gives a view of the rim (xy plane) of the reflector. The reflector as seen in

Figure 16 is 1 .O lm wide and 0.66m hi*. The feed is off the centre of the reflector hence

the name offset single reflector antenna. Y, , Feed

Fiwre 16: View of the refl ector's rim geometry (xy plane) 39

* 3.2.2.2. Feed geometry

With an elliptical reflector aperture as seen above, homs with rectangular

apertures lend themselves particularly well to the design of suitable feeds, since the

beamwidth in the two principal planes cm be controlled vimially independently of one

another by choice of the horn's principal plane dimensions [41]. The horn used to

illuminate the reflector is a pyramidal hom and can be viewed in Figure 17.

Fimn 17: Pyramidal horn geometry

3.3. Software validation

The choice of an off the shelf available simulation software was made for

purposes of the insights to be gauied as well as expediency. A ngorous development

using numerical methods could have been done but it was judged to be more useful to

examine and study reflector antennas and different types of feed design rather then

spending the entire research time Wtiting software code.

40

The validation process was done in order to confirm that the results provided

by the software agreed with the actual measured results. An initiai validation was

completed using a symmetnc reflector antenna with f/D=0.4 and a Feed Taper =-IO dB,

data which was obtained fiom [21].

30

rai

n

16

8

-10

a

Fimre 18: S imulated far-field radiation pattern of a syrnmetric reflector antenna (FT= -10 dB)

i ' I 1 - . - - - 1 I '

L

I I

-ta 5 O 5 ai 19

Fimire - 19: Far-field radiation pattern of a symmetric refiector antema (FT= 1 O dB) [2 11

The simdated results showed a very close agreement with these values if you consider

Figure 18 and 19. Then to m e r vaiidate the software package, the radiation pattern of

the antenna, which were measured in 1982 by the antenna designers, were compmed to

the results obtained fiom the so &are simulations.

3.3.1. Radiation pattern analysis

The radiation patterns sets, which were measured in 1982, provide sufficient

information to be able to compare many parameters but not ail of them. It should be

noted that no previous validation of these experimental results was provided. The

c o m p ~ s o n will be based on the gain, sidelobe levels and beamwidth in both azimuth and

elevation. No information was available on antenna spillover and cross-polariuition to

M e r confim the validity of the design and the software.

3.3.1 .l . Feed design analysis

The pyramidal hom feed was designed using GRASP8W facilities. It is

defined by rnany parameten, some of which were obtained from drawings of the original

feed design which are given in Table 2. A 2-D view of 2 different planes of the

pyramidal horn with theu dimensions is given in Annex A.

Table 2: Pyramidal horn parameters definition

Aperture width Aperture height Flare length in the xz Flare length in the yz Phase displacement Dielectric constant Reflectioncoefficient Ground plane Modes

When the design was completed, a study of the feed radiation pattem was

done. The simulation results were found to be very accurate. In Figure 20 you cm see

the simulated far-field radiation pattem of the pymmidal horn designed based on

parameters given in Table 2. The Azimuth (#=O) Copolar radiation pattem is in black

and the Elevation (+=90°) in blue. Find listed in Table 3, the radiation characteristics of

the pyramidal horn. In Figure 21, a contour display of the pyramidal hom simulated far-

field radiation pattem can be seen. As expected the primary pattem is designed to meet

the principal plane requirements of the reflector, thus it has an eliiptical cross-section.

2.92 1 cm 5.08 cm 5.3 1 8 1 cm 7.6708 cm O 1 O Off TEci

20 ~ . . . . l . . . . l . . . . l . . . , l . . . . l . . . a P . . 4 1 I I l I I

F i a r e 2 0 Pyramidal hom Copolar radiation pattern for F9.5GHz

/ Gain (ciBi) 1 12.03 1 12.03 1 12.23 1 12.23 1 12.45 1 12.45 1

F=9.5 GHz

1 HPBW (Degrees) 1 52.82 1 41.6 1 51.88 1 40.66 1 50.48 1 39.73 1

Az

SLL (dB)

X-Pol (Bi)

Tabk 3 : Pyramidal hom radiation characteristics -

EI

W.75 GHz H O GHz

Az

22.68

Iow

Az El El

30.14

low

23.18

low

30.59

low

23.68

low

3 1.1

Low

. --. - , - , .. . . ---. 1 -m. - I

-120. -Y. -38. 1. Y. Y. dl. 110. oz CdeqJ

F i ~ u re 2 1 : Contour dispiay of the pyramidal hom radiation pattern

3.3.1.2. Antenna design analysis

The reflector was designed using GRASP8W facilities. As mentioned in

section 3.2-2.1, the most demanding task tumed out to be the reflector rirn definition

which had to be done point by point using the original drawings. The rim data was

specified by tabulated (p,4) values in the reflector coordinate system. The p value refers

to the distance fiom the centre of the reflector to the rim for the given 4 angle. The rim

was defined using 360 different points. A table containhg the tabulated (p&) values for

our reflector rim definition can be found in Annex B. Figure 16 gives a view of the Nn

(xy plane) of the reflector. The simulated far-field radiation pattern of the reflector

designed based on parameters obtained in section 3.2.2.1. can be seen in Figure 22. The

Azllnuth (w) Copolar radiation pattern is in black and the Elevation (@+O@) in blue.

-15 -10 5 O 5 10 dcg I S

F i ~ u re 22: Reflector antenna radiation pattern for f=9 5GHz

For the reflector, we were able to obtain measured radiation pattern and data from the

antenna characteristic technical document in order to do a cornparison between them and

our simulation resulu. As expected, the gain increases as the frequency increases and its

level fdls well in the requirements (stated in Annex A) of 34 dB Min. The sidelobe level

requirement is for - 20 dB below the peak in both the azimuth and the elevation plane and the results meet those requirernents for both planes. As for the beamwidth, in the

&uth plane, the requinment is for 2.4" Max and the sirnulateci resuits meet that

repuirement. In the elevation plane, the requirement is for 4' Max and again the results

are well beyond the requirernents. Which confinns that the designed reflector represents

well the real reflector in ail its characteristics. Table 4 lists the radiation characteristics of

the reflector antema for different fiequencies.

F=9.5 GHz

Gain (ciBi)

SLL (dB)

Table 4: Reflector radiation characteristics

F-9.75 GHz

Az El I Az

X-Pol (dBi)

HPB W (Degrees)

A graphitai cornparison of the measured data fiom 1982 with the simulated results for the

El 35.68

22.42

azimuth plane for e9.5GHz cm be seen fiom Figures 23 and 24. A cornparison between

the two shows a 1.6% difference for the gain, a 2% difference for the HPBW and a 5%

difference for the SLL. These nsults and a look at Figures 23 and 24, clearly confïrm

e l 0 GHz

1 1 .58

2.3

that our design represents weli the reaiity, thus providing confidence in the software.

Az

35.68

20.07

El

35.84

22.58 I

45 .8

3 ,3

45.5

3.4

35.84

20.23

1 1.61

2.26

1 1.62

2.23

35.98

22.65

46.1 5

3.24

3 5.98

20.37

4 5 -tO 4 O 5 IO deg 15

Fimire 23: Simulated antenna radiation pattern for F-9.5GHz (+=O0)

Fiare 24: Measured (1982) antenna radiation pattern for H.5GHz (+=O?

Figure 25 gives a contour plot of the reflector radiation pattern with the azimuth angles

on the x-axis and the elevation angles on the y-axis. As expected, since the reflector has

an eliiptical rim, the patîem has an elliptical cross section, but rotated by 90' cornpared to

the ellipticai cross section pattern of the feed radiation pattern. That was obviously

expected since that for a given distni.ution, the pattern beamwidth in a piirticuiar planar

cut is invenely proportional to the size of the aperture in that plane.

Fimre 25: Contour plot of the reflector radiation pattern

These conclusive results now lead us to the design of the feed anay and confirms that the

antenna design is correct and that we should now proceed on to the main part of this

thesis; the investigation of a new feed design for the GMTI mode.

4.1. Feedamy

This chapter is concemed with the implementation of more than one element

to form the feed for the researched reflector antenna. Arrays, as antennas, have a long

history extending back to the early years of radio broadcasting. Their use at Mcrowave

fiequencies emerged during the Second World War when they were deployed in radar

systems, sometimes in conjunction with reflectors. When designing a feed array, lirnits

are imposed by the element patterns, the number of element and their spacing, the size

and the total mass of the array. The other important factor that complicates the whole

design study is mutual coupling, which is considered later on in this chapter.

4.1.1. Phase center

Often the terminology multiple phase centres is used to denote a muitiple feed

system, basically an array with more than one feed element. Al1 horns and feeds have a

Finure 26: Feed hom phase centre representation

'Phase centre'. It is the theoretical point dong the axis of the hom (Figure 26) which is

the centre of curvature of the phase fronts of the spherical waves radiated fiom the hom

[321*

4.1.2. Feed requirementr,

It must be remembered that the objective of this thesis is to investigate the

possibility of modifjhg the feed structure of the AN-MG 506 radar on board the CP-

140 to give GMTI capability to the a i r c d in order to support ground operations. As

stated in the GMTI theory section, in order to detect moving targets, generaily, there is a

requirement for at l e s t 3 phase centres (feeds) in the array. The design and the analysis

are thus based on an array of 3 feeds radiating the reflector.

4.1.3. Effects of Lateral feed displacement

The main advantage of the offset paraboloidal reflector lies in the capability to

mount a large feed array without blocking the most illuminated portion of the antenna

aperture. Obviously not al1 the elements of the array cm lie in the reflector focus, thus

making it important to understand the effects of lateral feed displacement on the radiation

pattern [32].

A feed at the focus of a paraboloidal reflector forms a beam parallel to the

focal axis. Additionai feeds displaced h m the focus form additionai beams at angles

from the axis. We know that a parabola reflects a spherical wave into a plane wave only

when the source is at the focus. With the source OR the focus, a phase distortion result

52

that increases with angular displacement. Figure 27 shows an example of the effects of

this phase distortion on the reflector radiation pattern. In the example portrayed in Figure

27, it cm be seen that as the beam squint increases to SO, the gain decreases a little but

generates SLL much higher (10 dB) than with a feed situated at the focus @=O0). This

detrimentai effect generated by the movement of the feed away from the focus of the

parabola increases as the beam squint increases, thus generating lower gain and higher

sidelo bes.

-10 O 10 20 30

ANGLE IDEGREES)

Fimire 27: Example of the effects of Off-axis feeds on the radiation pattern [43]

This phase distortion or error produces many unwanted effects such as coma

(image of a point source produces a cornet-shaped blur) and astigmatism (when rays fiom

a point fail to meet in a focal point resulting in a blurred and imperfect image)

aberrations, which result in pattern distortion in t e m of gain loss and sidelobes

degradation. These perturbations are mainly the cause of a poorly illuminated reflector as

the feed is laterally displaced h m the focus. The phase error is norxndy expressed in

53

terms of path length Merence between a generai ray and the reference central ray as a

function of the exit point through the aperture. One way to decrease the effects of such

perturbations is to tilt the feed towards the reflector's centre as the feed is laterally

displaced fiom the reflector focus, but this is not usually possible in an array. In the

design anaiysis part of this chapter, we will investigate how the lateral displacement of

the feed affects the radiation pattern.

4.2. Initial feed anay design

The obvious first step in the anay design, since we have already established

the reqkement for three fairly identical beams, was to simdate a three hom structure

which could provide those 3 beams. The horn utilised for this design was the horn

aiready used to feed the reflector and simulated in the previous chapter. The 3 pyramidal

horns were collocated as it cm be seen in Figure 28.

Wre 28: Initiai pyramidal hom anay design

The 3 hom are tilted in the yz plane by 20' to radiate towards the reflector centre (see

Figure 14), but the outside two homs are not tilted in the xz plane to radiate towards the

reflector centre.

4.2.1. Initial simulation study

The first simulation resdts uçing the m y configuration shown in Figure 28,

for H . 5 GHz is shown in Figure 29. If we look at those results for W S G H z , it cm be

seen that 3 fairly identical beams are obtained with fairly identical beamwidth. A small

decrease in gain is noted from 35.57 dB for the centre beam to 3 5.43 dB, which was

expected if considering Figure 27. The beams inteaect at 27.2 dB, which corresponds to

8.2 dB below the peak, which is very high.

-1 5 -1 a d 0 5 10 4.1 1s

Figure 29: Radiation pattern with a 3 hom m y as seen in Figure 28.

These results show that it's not possible to consider the existing pyramidal horn to

produce 3 beams that meet the requirernents stated in Annex A. To get better results, the

distance between the phase centres has to be decreased which will have for effect to

decrease the lateral feed displacement and its associated detrimental effects (higher

sidelobes and higher intersection of the beams). This is due to the fact that, as it can be

seen in Figure 28, the 3 homs are already as close as they cm be. In order to decrease the

distance between the phase centres the aperture width will have to be decreased, thus

decreasing the illumination of the reflector, which will probably affiect the bearnwidth in

the elevation plane.

4.2.2. Optimum distance between phase centres

The initial step before doing anymore simulations of the array design, would

be first to establish at what level the three beams m u t intersect in order to obtain proper

target information. It is well know that the HPBW is usually a rneasure of the resolution

of an antenna, so that two identical targets at the sarne range are said to be resolved in

angle if separated by the HPBW. If the beams intersect at their 3 dB, it is known that

good probability of detection and good relocation capability can be achieved. Based on

the results fiom Figure 29, it is known that using the three existing pyramidal homs side-

by-side, the phase centres are separated by 2.92 1 cm. Using that array design, the 3

beams intersect at 8.2 dB below the peak, which is fa. too great for the set requirements.

It is thus obvious that the phase centres have to be closer together but the question is how

close? The software enables the user to artificiaily move the phase centres closer

together ushg the same pyramidal horn configuration aithough the physical size of the

56

horn wouidn't permit to do so in real life. The software treats the anay on an element-

byelement basis; the computed field thus contains the same number of beams, as there

are elements in the army. In order to establish the ophum distance between the phase

centres, a large set of simulations was completed moving the hom closer and closer

together. The 3 simulated bearns using different phase centres separation on be seen in

Figure 30,3 1 and 32. The result for a distance of 2.921 cm (closest allowable distance if

we use the pyramidal hom) can be seen in Figure 30, distance for which the bearns

intersect at 8.2 dB below the peak. By looking at that pattern closely, it can be seen that

poor probability of detection would be achieved, but if the target is detected it would be

easier to relocate. The simulated radiation pattern for a distance of 1.27 cm (distance

equal to the width of a WR90 waveguide) cm be seen in Figure 3 1, a distance for which

the beams intersect at 1.34 dB below the peak. If the patterns are again looked at ciosely,

it c m be seen that good probability of detection wodd be achieved, but it would be more

difficult to relocate the target.

3s 1

I 5 * 18

Fimire 30: A m y radiation pattern with feed's phase centres 2-92 1 cm apovt

. tO d O 6 *B I b

Figure 3 1 : Anay radiation pattern with feed's phase centres 1.27 cm apart

Fipure 32: A m y radiation pattern with feed's phase centres 1.89 cm apart

The simulated radiation pattern for a distance of 1.89 cm (distance obtained so that the

beams intenect at their HPBW point) can be seen in Figure 32, which is the distance