design of a uhf circularly polarized patch antenna as a

Design of a UHF Circularly Polarized Patch Antenna as a feed for a 9.1 metre Parabolic Reflector

Michel Clénet

Defence R&D Canada √ Ottawa TECHNICAL MEMORANDUM

DRDC Ottawa TM 2004-139 June 2004

Design of a UHF Circularly Polarized Patch Antenna as a feed for a 9.1 metre Parabolic Reflector

Michel Clénet

Defence R&D Canada – Ottawa TECHNICAL MEMORANDUM DRDC Ottawa TM 2004-139 June 2004

DRDC Ottawa TM 2004-139 i

Abstract

This document reports on the design of a circularly polarized feed at 436 MHz for a 9.1-metre reflector antenna. This feed is used with a reflector to establish a communication link with a Canadian picosatellite, CanX-1. The planar technology is chosen to achieve a simple, easy to fabricate and ‘inexpensive’ feed. The feed consists of a patch antenna mounted on a 2 cm thick dielectric material over a finite ground plane. The circular polarization is obtained with two ports excited 90o out of phase. The realised feed exhibits the expected results in terms of input impedance. Some picosatellites have been successfully tracked with this feed mounted on the reflector antenna.

Résumé

Ce document décrit la conception d’une source à polarisation circulaire opérant à 436 MHz pour une antenne à réflecteur de 9.1 mètres de diamètre. Cette source est utilisée avec un réflecteur afin d’établir une communication avec un picosatellite canadien, CanX-1. La technologie imprimée est choisie pour réaliser une source simple, à faible coût et facile à fabriquer. La source consiste en une antenne de type patch placée sur un substrat diélectrique de 2 cm d’épaisseur au-dessus d’un plan de masse fini. La polarisation circulaire est générée à l’aide de deux ports alimentés avec une différence de phase de 90o. La source réalisée possède les caractéristiques escomptées en terme d’impédance d’entrée. Les signaux provenants de plusieurs picosatellites ont été captés avec cette source montée sur l’antenne à réflecteur.

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DRDC Ottawa TM 2004-139 iii

Executive summary

The Space Flight Laboratory at the University of Toronto Institute for Aerospace studies initiated the CanX (Canadian Advanced Nanospace eXperiment) program. The primary goal of this project is to provide students with the opportunity to develop complete satellite systems and perform space-based experiments using relatively small and inexpensive satellites. The CanX-1 satellite was built and it is 10x10x10cm3 in size and 1kg in mass. It was launched on June 30th, 2003 as a secondary payload with a set of other Cubesats built by other groups.

DRDC Ottawa is interested in this project, and has offered to provide support in using their satellite ground station to communicate with CanX-1. The Gregorian 9.1-metre reflector antenna located in the Shirleys Bay campus was chosen for this task. However, since no feed exists for this reflector at the CanX-1 frequency of 436 MHz, a specific feed had to be fabricated. The antenna feed had to be simple, easy to fabricate, inexpensive, and able to be fabricated within a few days. A patch antenna design was chosen for the feed. This is a low profile radiating element, which provides circular polarisation using two ports fed with a 90o phase shift. The beamwidth can be shaped by varying the ground plane dimensions or by using dielectric materials.

This document reports on the design and the analysis of a circularly polarized patch antenna operating at 436 MHz. The different steps leading to the final design are detailed. The effect of the ground plane size and the conductor thickness were carefully investigated. The results obtained from simulation have shown that the radiation characteristics meet the requirements. The measurement of the realised antenna indicated a good isolation between the two ports and a good impedance matching throughout the bandwidth of interest. A good far field radiation pattern should be obtained with the reflector antenna, without reflecting significant power to the source.

Due to unknown circumstances, the satellite failed to activate its communication system after deployment. All subsequent attempts to communicate with the satellite have been unsuccessful. However, other picosatellite tracking has been successfully realised with this feed mounted on the reflector.

Clénet, Michel. 2004. Design of a UHF circularly polarized patch Antenna as a feed for a 9.1-metre parabolic reflector. DRDC Ottawa TM 2004-139. Defence R&D Canada - Ottawa.

iv DRDC Ottawa TM 2004-139

Sommaire

Le laboratoire des vols spatiaux de l’Institut des études aérospatiales de l’Université de Toronto a initié le programme CanX (Canadian Advanced Nanospace eXperiment). Le but premier de ce projet est de permettre aux étudiants de développer des systèmes satellites complets et de réaliser des expérimentations dans l’espace à l’aide de satellites de petites tailles et de faible coût. Le satellite CanX-1, de taille 10 x 10 x 10 cm3 et de poids 1 Kg, fut construit. Il fut lancé le 30 juin 2003 comme charge utile secondaire avec un jeu de satellites similaires fabriqués par d’autres groupes de recherche.

RDDC Ottawa est intéressé par ce projet, et a offert son aide en permettant l’utilisation de leur station de base pour communiquer avec CanX-1. L’antenne grégorienne à réflecteur de 9.1 mètres de diamètre située sur le site de Shirleys Bay fut choisi pour ce projet. Néanmoins, comme aucune source électromagnétique pour opérer à 436 MHz existe pour cette antenne à réflecteur, une source spécifique devait être fabriquée. Cette antenne source devait être simple, facile à fabriquer, à faible coût, et délivré en quelques jours.

Une antenne de type patch (antenne imprimée) fut choisi comme source. C’est un élément rayonnant de faible épaisseur, qui peut générer une onde à polarisation circulaire en utilisant deux ports alimentés avec une différence de phase de 90o. L’ouverture à mi-puissance peut-être ajustée en changeant la dimension du plan de masse ou en utilisant un substrat diélectrique.

Ce document décrit l’analyse et la conception d’une antenne de type patch à polarisation circulaire fonctionnant à 436 MHz. Les différentes étapes menant au modèle final sont détaillées. L’effet des dimensions du plan de masse et de l’épaisseur des plans métalliques fut particulièrement analysé. Les résultats obtenus à partir de simulations ont montré que les caractéristiques de rayonnement satisfaisaient au cahier des charges. Les mesures de l’antenne fabriquée indiquent une bonne isolation entre les deux ports et une impédance d’entrée adaptée sur toute la bande de fréquence considérée. Un diagramme de rayonnement de bonne qualité devrait donc être obtenu avec l’antenne à réflecteur, en retournant une puissance négligeable à la source.

A cause de circonstances inconnues, le satellite CanX-1 n’a pu activer son système de communication après son déploiement. Aucun signal provenant de CanX-1 n’a été capté. Néanmoins, des signaux provenant d’autres picosatellites ont été captés avec succès.

Clénet, Michel. 2004. Design of a UHF circularly polarized patch Antenna as a feed for a 9.1-metre parabolic reflector. DRDC Ottawa TM 2004-139. R & D pour la défense Canada - Ottawa.

DRDC Ottawa TM 2004-139 v

Table of contents

Abstract .......................................................................................................................................................... i

Résumé .......................................................................................................................................................... i

Executive summary...................................................................................................................................... iii

Sommaire ..................................................................................................................................................... iv

Table of contents........................................................................................................................................... v

List of figures............................................................................................................................................... vi

List of tables................................................................................................................................................vii

1. Introduction...................................................................................................................................... 1 1.1. Goal..................................................................................................................................... 1 1.2. Antenna considerations....................................................................................................... 1

2. Design on infinite substrate and ground .......................................................................................... 2

3. Influence of the dielectric and metallic size and thickness .............................................................. 6 3.1. Patch antenna over air......................................................................................................... 6 3.2. Patch antenna on dielectric substrate ................................................................................ 10

4. Fabrication and measurement ........................................................................................................ 20

5. Concluding remarks ....................................................................................................................... 22

6. References...................................................................................................................................... 23

7. Appendix........................................................................................................................................ 24 7.1. Satellite information ......................................................................................................... 24 7.2. Drawing for fabrication .................................................................................................... 25 7.3. The 9.1-metre reflector antenna with its UHF feed .......................................................... 29

vi DRDC Ottawa TM 2004-139

List of figures

Figure 1 : Geometry of a microstrip patch antenna........................................................................................ 2

Figure 2 : S-parameters of the patch antenna on air over an infinite ground plane........................................ 3

Figure 3 : Input impedance of the patch antenna on air over an infinite ground plane.................................. 4

Figure 4 : Radiation patterns of the patch antenna on air over an infinite ground plane ............................... 5

Figure 5 : Axial ratio of the patch antenna on air over an infinite ground plane ........................................... 5

Figure 6 : a) S-parameters and b) Input impedance of the patch antenna on air 3.5 cm over a ground plane of different sizes .............................................................................................................. 6

Figure 7 : Radiation patterns of the patch antenna on air 3.5 cm over a) a 500 x 500 mm2 ground plane and b) 400 x 400 mm2 ground plane ......................................................................................... 7

Figure 8 : Axial ratio of the patch antenna on air 3.5 cm over a) a 500 x 500 mm2 ground plane and b) a 400 x 400 mm2 ground plane.................................................................................................. 7

Figure 9 : a) S-parameters and b) Input impedance of the patch antenna on air 2.5 cm over ground plane of different sizes .............................................................................................................. 8

Figure 10 : Radiation patterns of the patch antenna on air 2.5 cm over ground plane of different sizes – a) infinite ground plane, b) 500 x 500 mm2, c) 400 x 400 mm2 and d) 350 x 350 mm2 finite ground planes ................................................................................................................... 9

Figure 11 : Axial ratio of the patch antenna on air 3.5 cm over 2.5 cm over ground plane of different sizes – a) infinite ground plane, b) 500 x 500 mm2, c) 400 x 400 mm2 and d) 350 x 350 mm2 finite ground planes......................................................................................................... 10

Figure 12 : a) S-parameters and b) Input Impedance of the patch antenna on a 2 cm-thick Rexolite substrate over substrate and ground plane of different sizes ................................................... 11

Figure 13 : Radiation Patterns of the patch antenna on a 2cm-thick Rexolite substrate over substrate and ground plane of different size: a) infinite substrate and ground plane, b) 600 x 600 mm2 substrate and ground plane, c) 300 x 300 mm2 substrate and 600 x 600 mm2 ground plane, and d) 300 x 300 mm2 substrate and ground plane ....................................................... 12

Figure 14 : Axial Ratio of the patch antenna on a 2cm-thick Rexolite substrate over substrate and ground plane of different sizes: a) infinite substrate and ground plane, b) 600 x 600 mm2 substrate and ground plane, c) 300 x 300 mm2 substrate and 600 x 600 mm2 ground plane, and d) 300 x 300 mm2 substrate and ground plane ....................................................... 13

Figure 15 : Influence of the metal thickness on a) the S-parameters and b) the Input Impedance............... 14

Figure 16 : Influence of the metal thickness on the radiation patterns – a) infinitely thin metal layers, and b) infinitely thin patch and 2 mm thick ground plane....................................................... 15

DRDC Ottawa TM 2004-139 vii

Figure 17 : Influence of the metal thickness on the axial ratio – a) infinitely thin metal layers, and b) infinitely thin patch and 2 mm thick ground plane.................................................................. 16

Figure 18 : S-parameters of the final design ................................................................................................ 17

Figure 19 : Input impedance of the final design........................................................................................... 17

Figure 20 : Radiation patterns of the final design at 436MHz ..................................................................... 18

Figure 21 : Axial Ratio of the final design at 436MHz................................................................................ 18

Figure 22 : 436MHz circularly polarized patch antenna (August 2003) ...................................................... 20

Figure 23 : Return loss – Comparison between measurement and simulation............................................ 21

Figure 24 : Coupling between the two ports – Comparison between measurement and simulation........... 21

Figure 25 : The 9.1-metre reflector antenna (December 2003).................................................................... 29

Figure 26 : The UHF circularly polarized patch antenna feed (December 2003) ........................................ 30

List of tables

Table 1 : Summary of the radiation characteristics of the patch antenna on air over an infinite ground plane.............................................................................................................................. 4

Table 2 : Summary of the radiation characteristics of the final design patch antenna............................. 19

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DRDC Ottawa TM 2004-139 1

1. Introduction

1.1. Goal The aim of this work was to build a circularly polarized 436 MHz antenna feed for a 9.1-metre reflector antenna. The design had to be simple, inexpensive and done quickly in order for DRDC Ottawa to be able to communicate with the first Canadian CubeSat picosatellite, CanX-1, in the UHF-Band, [1]. The primary goal of the CanX (Canadian Advanced Nanospace eXperiment) program of the Space Flight Laboratory at the University of Toronto Institute for Aerospace Studies is to provide students with the opportunity to develop complete satellite systems and perform space-based experiments using relatively small and inexpensive satellites [1]. The CanX-1 satellite was built at the University of Toronto. It is 10x10x10cm3 in size and 1Kg in mass. It was launched on June 30th, 2003 as a secondary payload with a set of other Cubesats built by other groups. The objective of this first satellite was to verify the functionality of several technologies in space. DRDC Ottawa is interested in this project, and has offered to provide support in using their satellite ground station to communicate with CanX-1. However, since no feed exists for this Gregorian reflector at the CanX-1 frequency of 436 MHz, a specific feed had to be fabricated. Because of its physical size at this frequency, the secondary reflector will be removed, and the feed will be placed at the focal point of the 9.1-metre parabolic reflector.

1.2. Antenna considerations The required antenna will be used as a feed for the 9.1-metre parabolic reflector with F/D = 0.36 (feed point to reflector diameter ratio) corresponding to an aperture illumination of 140o. The feed must then have a 140o beamwidth. According to [2], the power level could be around –7 dB to –12 dB at the extremities of the beamwidth without reducing the efficiency significantly. The picosatellite transmits a beacon signal at 437.88 MHz with linear polarization. However, as the satellite spins on axis with unknown speed and trajectory, a circularly polarized feed is necessary to receive signals at anytime when the satellite antenna is visible from our ground station. Several antennas can be considered as a reflector feed. Crossed dipoles fed 90o out of phase, and placed horizontally above a ground plane can be used. The required beamwidth can be adjusted by varying the distance between the ground plane and the dipoles. However, beamwidth discrepancy exists between E- and H-plane, which can deteriorate the axial ratio for large angles. From a mechanical perspective the dipoles must be maintained in the air at a specific distance from the ground plane, and must also be placed at the focal point of the reflector. The ground plane must be placed approximately at a quarter wavelength behind the dipoles. Unfortunately, the existing mounting system on the 9.1-metre parabolic reflector does not allow this placement. Another solution is to consider a patch antenna. This is a low profile radiating element, which can be designed to support circular polarization using two ports fed with a 90o phase shift. Also, the beamwidth can be shaped by varying the ground plane dimensions or by using dielectric materials. This radiating element was selected as a feed for the 9.1-metre parabolic reflector.

2 DRDC Ottawa TM 2004-139

2. Design on infinite substrate and ground

Figure 1 : Geometry of a microstrip patch antenna

The first approach is to estimate the antenna dimensions using approximate equations [3]. The microstrip patch antenna is a resonant antenna (Figure 1). Its length L is then roughly half a wavelength. Corrections must be made to take into account the fringing field, depending on the dielectric permittivity of the material, its thickness and the dimensions of the antenna. The physical length of the radiating patch is related to the different geometrical and electrical parameters by the following equations:

(1) LLLp ∆−= 2 with rerf

cLε2

=

where c is the velocity of the light, fr is the resonant frequency and εre is the effective dielectric permittivity, and

(2) 813.0264.0

258.03.0

412.0++

−+

=∆hWhWhL

re

reεε

where W is the patch width, and h is the substrate thickness.

At the time this study began, a centre frequency of 436 MHz was considered. As the operating frequency was approximately known, a relatively large impedance bandwidth was needed. An air-medium was first chosen to obtain the largest bandwidth. As the bandwidth depends on the substrate height h, amongst other parameters, a large separation between the patch and the ground plane was chosen according to [4]. A distance of 35 mm, corresponding to 0.05 wavelength, was considered. This leads to a theoretical bandwidth of 14%. Using Equations (1) and (2), the physical length of the patch antenna was calculated to be 296mm. This dimension was used as a starting point for optimizing the patch geometry with full-wave simulation tools.


Figure 2 : S-parameters of the patch antenna on air over an infinite ground plane

Using IE3D, a simulation software package from Zeland [5] that is based on the method of moments, the antenna dimensions are optimized to obtain the best impedance matching at 436 MHz. The patch antenna, placed over an infinite ground plane, is fed first with a 50 Ω coaxial cable of 7 mm inner diameter. The optimization was carried out with one port only to save computation time. Finally, a 305 x 305 mm2 patch placed at 35 mm above an infinite ground plane and fed with a 7 mm inner diameter 50 Ω coaxial cable located at 90 mm from the patch centre was chosen to obtain the best impedance matching. Results of the input impedance versus frequency are given in Figure 2 and Figure 31 when the patch is fed with one and two ports. Note that the metal plates are infinitely thin. The impedance bandwidth for S11 < -10 dB for one and two ports is similar, and reaches 6.6% around a centre frequency of 437.5 MHz. The bandwidth is narrower than expected, mainly due to the reactance introduced by the long feeding probe. The coupling between the two ports, depicted by S12 in Figure 3, is below –15 dB across the bandwidth.

1 Figure 3 represents the normalized complex input impedance on the Smith Chart. The centre of the Smith Chart is the 50 Ω normalization impedance point. The closer the input impedance is from this point, the better the matching is. The circle centred on the 50 Ω point and crossing the real impedance axis at the values of 0.5 and 2.0 indicates the area the antenna is matched (the return loss is better than 10 dB)


Figure 3 : Input impedance of the patch antenna on air over an infinite

ground plane

The radiation patterns for circular polarization are given in four different planes in Figure 4. The patterns are not symmetric, especially in the ϕ = 0o plane, due to the radiation of the feeding probes. The half-power beamwidth (HPBW) and the power levels at +/-70o are mentioned in Table 1. As the power level at +/-70o is below the –12 dB limit specified in Section 1.2, this antenna would not be suitable as a feed for the 9.1-metre parabolic reflector.

Plane φ = 0o φ = 45o φ = 90o φ = 135o HPBW (deg.) 66 66 64 65

Power Level at –70o (dB) -15.0 -20.5 -25.9 -17.9

Power Level at +70o (dB) -15.4 -13.7 -13.7 -13.5

3 dB Axial Ratio (deg.) 54 [-30;24]

50 [-26;24]

106 [-37;69]

89 [-34;55]

Table 1 : Summary of the radiation characteristics of the patch antenna on air over an infinite ground plane

The axial ratio (AR) in the four different planes is shown in Figure 5. In the bore-sight direction, the axial ratio equals 1.87 dB. Again, the axial ratio is not symmetric due to the unwanted radiation from the probes.


Figure 4 : Radiation patterns of the patch antenna on air over an infinite ground plane

Figure 5 : Axial ratio of the patch antenna on air over an infinite ground plane

The resulting radiation characteristics are not good enough to use this antenna as a feed for the 9.1-metre reflector. However, the size of the ground plane, which will be finite in reality, affects the radiation characteristics. An air medium was then considered to keep the antenna simple to realise and low cost.


3. Influence of the dielectric and metallic size and thickness This section describes the study of a patch over an air-medium, with ground planes of different size and thickness. The same study is also presented with a dielectric substrate instead of an air-medium.

3.1. Patch antenna over air The size of the ground plane is reduced, while keeping the same patch dimensions and probe location. Even though the radiation characteristics are more of interest for this study, the input impedance is presented in Figure 6 for infinite, 500 x 500 mm2 and 400 x 400 mm2 ground planes. The impedance is decreased when the ground plane size is reduced, but it can be adjusted by moving the probe toward the edge of the patch.

a) b)

Figure 6 : a) S-parameters and b) Input impedance of the patch antenna on air 3.5 cm over a ground plane of different sizes

Reducing the ground plane size does not improve the radiation characteristics significantly. With a 400 x 400 mm2 ground plane, the HPBW is only increased by 2o (69o in the φ = 0o Plane, and 67o otherwise) compared to the case of an infinite ground plane, and the back radiation increases to –10 dB, as shown in Figure 7. Results with a 500 x 500 mm2 ground plane are in-between. Similar comments can be made for the axial ratio, presented in Figure 8: no significant improvement can be noted, except in the ϕ = 0o plane (72 o versus 54 o for the case of an infinite ground plane with an axial ratio lower than 3 dB).


a)

b)

Figure 7 : Radiation patterns of the patch antenna on air 3.5 cm over a) a 500 x 500 mm2 ground plane and b) 400 x 400 mm2 ground plane

a) b)

Figure 8 : Axial ratio of the patch antenna on air 3.5 cm over a) a 500 x 500 mm2 ground plane and b) a 400 x 400 mm2 ground plane

The radiation characteristics of a patch placed at 25 mm above a ground plane with air as the dielectric are also analysed. This height reduction will shorten the probe length, and reduce the probe radiation. The patch dimensions have to be adjusted to keep the minimum return loss around 436 MHz, and the probe location has to be modified as well. Then, for an operating frequency of 436 MHz, placed 25 mm above an infinite ground plane, the patch becomes 312 mm wide and long, and the probe is located at 95 mm from the patch centre. For this case, the probe diameter is reduced to 1.27 mm, which is a more realistic number.


a) b)

Figure 9 : a) S-parameters and b) Input impedance of the patch antenna on air 2.5 cm over ground plane of different sizes

The input impedance is presented in Figure 9. Different ground plane sizes are considered: infinite, and 500 x 500 mm2, 400 x 400 mm2 and 350 x 350 mm2 finite ground planes. As mentioned for the previous results (patch placed 35 mm above the ground), the impedance is decreased when the ground plane size is reduced. The impedance can be increased by moving the probe toward the edge of the patch antenna. The coupling between the two ports is about 2 to 3dB lower than the coupling obtained with the previous configuration. The radiation patterns are shown for the four configurations in Figure 10. Considering all four cases, the half-power beamwidth varies between 64o and 69o in all the planes. The power level at +/-70o is decreased when the ground plane size is reduced, and fluctuates between –16.5 dB and –11.3 dB, for the four planes of the three finite-ground-plane configurations. The back radiation increases when the ground plane decreases, reaching –5 dB for the 350 x 350 mm2 finite ground plane. The cross-polarization is reduced by about 2 dB. The best results are obtained with a 400 x 400 mm2 finite ground plane. For this case, the half-power beamwidth in the four planes is 68o, and the power level at +/-70 o varies between –15.4 dB and –11.4 dB. Overall, the radiation patterns are similar to those of the previous case. The axial ratio for the four different configurations is shown in four cuts in Figure 11. In the case of the infinite ground plane, the beamwidth for a 3 dB axial ratio is a few degrees larger with a patch placed 25mm than with the patch placed 35 mm above the ground. Note that similar comments can be made comparing the axial ratio of the patch placed above finite ground planes. In the boresight direction, the axial ratio is 1.85 dB for the three first configurations, and decreases to 1.15 dB with the 350 x 350 mm2 finite ground plane.


a) b)

c) d)

Figure 10 : Radiation patterns of the patch antenna on air 2.5 cm over ground plane of different sizes – a) infinite ground plane, b) 500 x 500 mm2, c) 400 x 400 mm2 and d) 350 x 350 mm2 finite ground planes

Reducing the distance between the microstrip patch and the ground plane does not improve the radiation characteristics significantly. Therefore, we now consider a dielectric material between the patch and the ground plane. This solution, which is more expensive and requires extra work in terms of machining, is needed to achieve better radiation characteristics. First, as the dielectric constant is greater than unity, the patch will be reduced in size while operating at the same frequency. As the radiating surface decreases, the half-power beamwidth will increase along with the power level at +/-70o as well. Also, as the probe feed will be buried in the dielectric material, its radiation will not significantly perturb the antenna radiation patterns.


a) b)

c) d)

Figure 11 : Axial ratio of the patch antenna on air 3.5 cm over 2.5 cm over ground plane of different sizes – a) infinite ground plane, b) 500 x 500 mm2, c) 400 x 400 mm2 and d) 350 x 350 mm2 finite ground planes

3.2. Patch antenna on dielectric substrate Rexolite dielectric substrate material (εr = 2.53) will be used for this project. This material is usually used for realising electromagnetic lenses [6] and is available onsite. The substrate thickness, h, is first chosen thick enough to obtain a large impedance bandwidth without exciting appreciable surface wave modes. Using equation 1.61 on page 46 of [3], it follows that oh λ03.0≤ , or a maximum value h = 20 mm. Using equations (1) and (2), the patch dimension is determined to be 214 mm. The patch size and probe location have been optimized with the help of IE3D for minimum return loss at 436 MHz, when the antenna is on an infinite ground plane and substrate. Finally, a 204 mm square patch, fed 50 mm from its centre, exhibits a 3.2% bandwidth around a centre frequency of 436 MHz


(BW = 429-443 MHz), as shown in Figure 12. This result is obtained with 2mm thick metal layers (patch and ground). The probe diameter equals 1.27 mm. The bandwidth is narrower than the one achieved for the patch on air, due to the presence of the dielectric substrate. The input impedance and radiation characteristics of this patch antenna are reported in the next section for different configurations. First, the effect of the ground plane and substrate sizes is described, and second, the variations due to different metal layer thickness are investigated.

3.2.1. Effect of the dielectric and ground plane size This section presents the results of the characteristics of a patch antenna for different configurations in terms of ground plane and substrate size. As the size of the different parts affects the characteristics of the radiating element, a preliminary study is necessary. Four different configurations are considered:

a) infinite substrate and ground plane, b) 600 x 600 mm2 substrate and ground plane, c) 300 x 300 mm2 substrate and 600 x 600 mm2 ground plane, and d) 300 x 300 mm2 substrate and ground plane.

a) b)

Figure 12 : a) S-parameters and b) input impedance of the patch antenna on a 2 cm-thick Rexolite substrate over substrate and ground plane of different sizes

The results of the input impedance are shown in Figure 12. Similar bandwidths are obtained for the different configurations. The coupling between the two ports is also equivalent, being lower than –20 dB throughout the impedance bandwidth. Note that the coupling is 5 dB lower than the coupling obtained when the patch is placed in an air medium 35 mm above the ground plane.


a) b)

c) d)

Figure 13 : Radiation patterns of the patch antenna on a 2cm-thick Rexolite substrate over substrate and ground plane of different size: a) infinite substrate and ground plane, b) 600 x 600 mm2 substrate and ground plane, c) 300 x 300 mm2

substrate and 600 x 600 mm2 ground plane, and d) 300 x 300 mm2 substrate and ground plane

The radiation patterns for the four configurations are shown in several planes in Figure 13. When placed over an infinite substrate and ground plane (Figure 13a), the patch antenna exhibits a half-power beamwidth of 90o in all planes. The power level at +/-70o varies between –8.2 dB and –6.6 dB below the maximum radiated power. The cross-polarization level is lower than –26 dB in the boresight direction in all planes, and reaches a maximum of –13 dB at about θ = 80o in the φ = 90o plane. At +/-70o, the cross-polarization level is below –15 dB. The 3 dB axial ratio beamwidth, shown in Figure 14a, is considerably larger compared to the configuration where the patch is placed in an air medium 35 mm above the ground plane. This beamwidth equals 119o in the φ = 0o plane, 124o in the φ = 45o plane, 109o in the φ = 90o plane, and 114o in the φ = 135o plane. At +/-70o, the axial ratio fluctuates between 4.3 dB and 7.2 dB, and equals 0.8 dB in the boresight direction. Overall, the radiation characteristics are much closer to the specifications when a dielectric substrate is used.


a) b)

c) d)

Figure 14 : Axial ratio of the patch antenna on a 2cm-thick Rexolite substrate over substrate and ground plane of different sizes: a) infinite substrate and ground plane, b) 600 x 600 mm2 substrate and ground plane, c) 300 x 300 mm2

substrate and 600 x 600 mm2 ground plane, and d) 300 x 300 mm2 substrate and ground plane

Figure 13b shows the radiation patterns of the patch when it is placed on a 600 x 600mm2 substrate and ground plane. This dimension corresponds to 0.87λo, or 1.39 λg, where λo is the wavelength in the air medium and λg is the wavelength in the dielectric. The ground and substrate edges are only 0.29λo (0.46λg) away from the patch edges. The radiation patterns in the plane φ = 0o, 45o, 90o and 135o exhibit narrower half-power beamwidth (about 75o) compared to the previous configuration. Also, the power level at +/-70o drops a little bit and varies between –10.5 dB and –8.3 dB. The cross-polarization level is lower than –25 dB in the boresight direction, and stays below –20 dB except in the φ = 45o plane, where it reaches –15 dB around θ = 60o. As shown in Figure 14b, the axial ratio is also improved compared to the previous configuration. Without considering the result in the φ = 45o plane, the axial ratio varies between 1.6 dB and 5.9 dB. In the plane φ = 45o, the axial ratio is larger, reaching 4.7 dB at –70o and 8.8 dB at +70o.


Figure 13c presents the radiation patterns of the patch on a 300 x 300 mm2 substrate and 600 x 600 mm2 ground plane, and Figure 14c presents the corresponding axial ratio. In this case, the distance between the patch edges and the substrate edges is reduced to 0.07λo,or 0.11λg. This configuration does not show significant change compared to the previous configuration. The radiation patterns of the patch placed on a 300 x 300 mm2 substrate and ground plane are given in Figure 13d. The half-power beamwidth is larger compared to the other finite-ground-and-substrate configurations. It reaches the value of the half-power beamwidth obtained when the patch is placed on infinite substrate and ground plane (88o -90o). At θ = +/-70o, the power level varies between –7.9 dB and -6.2 dB, depending on the planes. The cross-polarization drops to –29.6 dB in the boresight direction, but rises to –15 dB in the back direction. Between –70o and +70o, the cross-polarization level is lower than -20 dB except in the φ = 45o plane, where it is a couple of decibels higher. The axial ratio, presented in Figure 14d, is considerably lower over the 140o beamwidth, reaching a maximum of only 4.8 dB at θ = +70o in the ϕ = 45o plane. The axial ratio in the boresight direction equals 0.6 dB. This configuration, a 204 x 204 mm2 patch fed at 50 mm from its centre and placed on a 300 x 300 mm2 substrate and ground plane, gives the best results in terms of radiation characteristics. It also shows the wider impedance bandwidth (426-441 MHz, or 3.46% around a 433.5 MHz centre frequency).

3.2.2. Effect of metal plate thickness The following study examines the effect of the thickness of the conductive material (patch and ground plane). Three configurations are considered:

a) infinitely thin patch and ground plane, b) infinitely thin patch and 2 mm thick ground plane, c) 2 mm thick patch and ground plane.

a) b)

Figure 15 : Influence of the metal thickness on a) the S-parameters and b) the Input Impedance

The study and the comparison of these three cases help us to determine the effect of each metal surface on the electromagnetic characteristics of the antenna. The size of the ground plane and the dielectric substrate is 300 x 300 mm2. The results of the input impedance for the three configurations are given in


Figure 15. The influence of the thickness on the input impedance is small. However, note that the impedance increases slightly when the ground thickness increases. Also, a downward frequency shift is observed when the patch thickness increases.

a) b)

Figure 16 : Influence of the metal thickness on the radiation patterns – a) infinitely thin metal layers, and b) infinitely thin patch and 2 mm thick ground plane

Figure 16a and Figure 16b show the radiation patterns of the patch with infinitely thin metal plates, and an infinitely thin patch associated with a 2 mm thick ground plane, respectively. These radiation patterns are to be compared with those shown in Figure 12d, when the patch and the ground plane are 2 mm thick. The results for the half-power beamwidth are similar (only 2o to 3o difference for an average of 90o HPBW), as well as for the power levels at +/- 70o. The axial ratio characteristics, shown in Figure 14d and in Figure 17 exhibit similar behaviour. Even though the responses are similar, the configuration with a 2 mm thick patch and ground plane shows the best results. This is interesting knowing that metal plates cannot be infinitely thin.


a) b)

Figure 17 : Influence of the metal thickness on the axial ratio – a) infinitely thin metal layers, and b) infinitely thin patch and 2 mm thick ground plane

3.2.3. The final design From these parametric analyses, the antenna design is finalised. The patch dimensions are adjusted to tune the resonance frequency. The ground plane and the dielectric material sizes of 300 x 300mm2 are chosen. The ground plane is realised with a 10 mm thick aluminium plate. Its thickness is larger than the one used for the analysis, to have some flexibility for mounting the patch antenna at its feeding position on the reflector support. The patch is realised with a 2 mm copper plate. Its optimum size is 202 mm x 202 mm. The patch is fed by two 1.27 mm diameter probes to obtain circular polarization. For simulation purposes, the probe diameters are increased to 3 mm through the ground plane, and they are surrounded by a cylindrical hole of 6.9 mm in diameter. These dimensions correspond to those of a type-N connector, which will be used to connect the antenna to the RF circuitry. The results of the simulation of the final design are presented in Figure 18 to Figure 21. The graph representing the return loss (Figure 18) shows a minimum value at 435 MHz. It also indicates an impedance bandwidth from 428 MHz to 442 MHz (3.2% around a 435 MHz centre frequency). The coupling between the two ports is lower than –24 dB throughout the bandwidth.


Figure 18 : S-parameters of the final design

Figure 19 : Input impedance of the final design


Figure 20 : Radiation patterns of the final design at 436MHz

Figure 21 : Axial Ratio of the final design at 436MHz

The radiation patterns in the φ = 0o, 45o, 90o and 135o planes (Figure 20) exhibit similar characteristics as the previous studied cases. The radiation characteristics are summarised in Table 2. The half-power


beamwidth is about 90o in all the planes. The power level at +/-70o fluctuates between –7.8 dB and -6.4 dB. The cross-polarization is lower than –27 dB in the boresight direction, and stays below –20 dB in the [-70o; +70o] beamwidth. The graph representing the axial ratio (Figure 21) also shows good characteristics in all planes (Table 2). The axial ratio in the boresight direction is 0.77 dB. The 3 dB axial ratio is larger than 150o in the φ = 0o, 90o and 135o planes, and reaches 136o in the φ = 45o plane. The axial ratio is lower than 4.1 dB in the interval [-70o; +70o].

Plane φ = 0o φ = 45o φ = 90o φ = 135o HPBW (deg.) 88 89 89 89

Power Level at –70o (dB) -7.0 -7.3 -7.8 -7.6

Power Level at +70o (dB) -7.5 -6.9 -6.6 -6.4

3 dB Axial Ratio (deg.) 155 [-90; 65]

136 [-78; 58]

156 [-81; 75]

158 [-68; 90]

AR at -70o (dB) 0.17 2.64 1.89 3.17

AR at -70o (dB) 3.55 4.07 2.58 0.83

Table 2 : Summary of the radiation characteristics of the final design patch antenna


4. Fabrication and measurement Basically, the patch antenna is made of four different parts: the radiating patch, the dielectric material, the ground plane and the probes. The radiating patch (202 x 202 mm2) is realised from a 2 mm thick copper plate. The dielectric substrate of dimension 300 x 300 x20 mm3 is manufactured from a thick block of Rexolite material. The ground plane of dimension 300 x 300 x 10 mm3 is machined from a thick aluminium plate. The 1.27 mm diameter probes are made of brass. The drawings are presented in Appendix, section 7.2. The three first parts are assembled using Teflon bolts and nuts. The probes are soldered to the N-type connectors used to feed the antenna. They go through the ground plane, the dielectric material and the radiating patch. Nuts are then used to realise an electrical contact between the probe and the patch. This solution is preferred to soldering to ease the assembly and disassembly of the antenna, even though it allows deterioration with time due to oxidation at contact points.

Figure 22 : 436MHz circularly polarized patch antenna (August 2003)

Only the input impedance of the antenna has been measured. The results of the return loss and the coupling between the two ports are shown in Figure 23 and Figure 24, respectively. Good agreement is observed between the simulation and measurement results for the input impedance. The prototype antenna exhibits an impedance bandwidth from 427 MHz to 444 MHz (3.9% around 435.5 MHz) for the first port,


and 428 MHz to 441 MHz (3.0% around 434.5 MHz) for the second port. The discrepancy is due to the fabrication and mounting of the connectors. The measured coupling indicates a maximum level of –25 dB. A deep null occurs around 435 MHz, contrary to the expected simulation results.

Figure 23 : Return loss – Comparison between measurement and simulation

Figure 24 : Coupling between the two ports – Comparison between measurement and simulation


5. Concluding remarks A circularly polarized patch antenna has been optimized at 436 MHz for an application as a feed for a 9.1-metre parabolic reflector. The results obtained from simulation have shown that the radiation characteristics meet the requirements: The power level at +/-70o is between –8 dB and –6 dB from its maximum, and the axial ratio is lower than 4.1 dB over the same beamwidth. The measurement of the input impedance indicates good isolation between the two ports and an impedance bandwidth from 428 MHz to 441 MHz. A good far field radiation pattern should be obtained with the reflector antenna, without reflecting significant power to the source. To date, no signals have been received from CanX-1. However, other picosatellite tracking have been successfully realised with this feed mounted on the reflector.


6. References [1] G. J. Wells, L. Stras and T. Jeans, “Canada’s Smallest Satellite: The Canadian Advanced

Nanospace experiment (CanX-1)”. [2] Y. T. Lo and S. W.Lee, “Antenna Handbook”, Chapman and Hall, 1993. [3] R. Garg, P. Bhartia, I. Bahl and A. Ittipiboon, “Microstrip Antenna Design Handbook”,

Artech House, 2001. [4] J. R. James, A. Henderson and P. S. Hall, “Microstrip Antenna Performance is determined

by Substrate Constraints, MSN, August 1982, pp73-82. [5] Zeland Software Inc., www.zeland.com. [6] R. Poirier, J. Moffat, G. A. Morin and Y. Y. Antar, “Millimetre wave antennas using an

array of lenses”, DREO TR 2000-081, Dec. 2000.


7. Appendix

7.1. Satellite information CanX-1 is a Canadian (University of Toronto) students-built, photo-imaging nanosatellite that was launched by a Rokot rocket from Plesetsk at 14:15 UT on June, 30th 2003. Initial orbital parameters were:

- period 101.4 min, - apogee 830 km, - perigee 817 km, and - inclination 98.7°.

Due to unknown circumstances, the satellite failed to activate its communication system after deployment. All subsequent attempts to communicate with the satellite have been unsuccessful.


7.2. Drawing for fabrication


7.3. The 9.1-metre reflector antenna with its UHF feed

Figure 25 : The 9.1-metre reflector antenna (December 2003)


Figure 26 : The UHF circularly polarized patch antenna feed (December 2003)

UNCLASSIFIED SECURITY CLASSIFICATION OF FORM

(highest classification of Title, Abstract, Keywords)

DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified)

1. ORIGINATOR (the name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Establishment sponsoring a contractor’s report, or tasking agency, are entered in section 8.)

Defence R&D Canada – Ottawa 3701 Carling Avenue Ottawa, On, K1A 0Z4

2. SECURITY CLASSIFICATION (overall security classification of the document,

including special warning terms if applicable) UNCLASSIFIED

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C or U) in parentheses after the title.)

DESIGN OF A UHF CIRCULARLY POLARIZED PATCH ANTENNA AS FEED FOR A 9.1-METRE PARABOLIC

REFLECTOR (U)

4. AUTHORS (Last name, first name, middle initial)

Clénet, Michel

5. DATE OF PUBLICATION (month and year of publication of document)

June 2004

6a. NO. OF PAGES (total containing information. Include Annexes, Appendices, etc.)

30

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6

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Technical Memorandum

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15cl01

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DRDC Ottawa TM 2004-139

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11. DOCUMENT AVAILABILITY (any limitations on further dissemination of the document, other than those imposed by security classification) ( X ) Unlimited distribution ( ) Distribution limited to defence departments and defence contractors; further distribution only as approved ( ) Distribution limited to defence departments and Canadian defence contractors; further distribution only as approved ( ) Distribution limited to government departments and agencies; further distribution only as approved ( ) Distribution limited to defence departments; further distribution only as approved ( ) Other (please specify):

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UNCLASSIFIED SECURITY CLASSIFICATION OF FORM

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This document reports the design of a circularly polarized feed at 436 MHz for a 9.1 metre reflector antenna. This feed is used with a reflector to establish a communication link with a Canadian picosatellite, CANX-1. The planar technology is chosen to achieve a simple, easy to fabricate and ‘inexpensive’ feed. The feed consists of a patch antenna mounted on a 2 cm thick dielectric material over a finite ground plane. The circular polarization is obtained with two ports excited 90o out of phase. The realised feed exhibits the expected results in terms of input impedance. Some picosatellite have been successfully tracked with this feed mounted on the reflector antenna.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus-identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

Antenna Microstrip Patch Circular Polarisation

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design of a uhf circularly polarized patch antenna as a

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