A Low Cost Design for Double Offset Antenna with Spherical Main Reflector

Luis Cliiudio P. Pereira CPqD - Telebrh

e-mail: lclaudio@cpqd.br Rodovia Campinas - Mogi Mirim km118 - Campinas 1300818-060 - SP - Brazil

J. R. Bergmann CETUC - Ceriter for Telecommunication Studies - Catholic University of Rio de Janeiro

e-mail: bergmann @cetuc.puc-rio.br Rua Marquh de S5o Vicente 225 - Rio de Janeiro 22453-900 - RJ - Brazil

Abstract. This paper presents the double offset antenna with a symmetrical spherical main reflector, that is a spherical cap, as a low cost configuration for antennas to be used in fixed satellite service ground stations. Electrical performance results for a 3.6 m diameter equivalent aperture antenna designed to work in the F'SS Ku band are presented.

1. Introduction. Double offset antennas with spherical main reflectors have recently been considped as an interesting alternative for satellite earth station applications subjected to the newest ITU-R radiation p k " recommendatons [l] (29- 251og0). The great appeal of this configuration being the reduction of main refl@tor manufactllring costs, which contrary to shaped and classical double offset reflector antennas can be realized by fitting panels of the same radius of anvature. h the range of larger diameters (above 1OOh) this amfiguration is cansidered to have a setious competitor in the symmetrical double reflector antenna, due to W t y of ~mauufauure of both main and subreflector. The symmetrical configuration also offers the possibility of shaping reflectors in order to obtain the desired aperture illumination, while the double offset spherical main reneaolr configuration presents limited aperture power control, since the subrefleaor is basically designed as a phase currector to provide an apeamre constant phase wayefront. However the symmetrical codguration presents spne significants drawbacks due mainly to subrefleuar aperture blockage and strut scattenn - g which results in sidelobe levels degradation, particularly in the p h e s of struts. As shown in reference 2, by designing a dpble oBet with spherical main reflector with an appropriately chosen elliptical aperture, a superior radiation paqern performance can be achieved when comparing to a symmetrical shaped double reflector antenna of the same aperture efficiency. In this paper a further possibility of reducing m a " e cats is explored by employing a symhetrical spherical main refleuor (spherical cap), which can be easily obtained by Spinning machines, instead of the optimum elliptical rim spherical reflector-

2. Comparison of spherical cap reflector and elliptical rim spherical reflqctor. Initially an optimum elliptical rim Gregorian double reflector with main spheqical refleuor was designed. The antenna aperture was chosen to currespond to a 3.6m (1281 at 10.7GHz) diameter circular aperture and the kequency band to be provided for was the Ku FSS band (10.7 - 12.75GHz and 131.75: 14.8GHz). The subrefleuor synthesis procedure utilized a general formulafm proposed by Westcott and lbrickel[3] such as applied and implemented by Beagmann et al[4] for this particular configuration. The method is based on gemmetrical optics and uses a complex number notation parametrizing real space directions of i" and reflected rays, resulting in a compact formulation for the difFerencial equations system derived ftom Snell's law direct application to both reflectors. Feeding antenna parameters such as aperture dimensions and its centex, spherical main refleuor ClnvaMe radius, subrefleum illumination cone and feed horn radiation &uact+istics, the implemented method yields main reflector cu~aape center position, subrenector surface definition and the primary feed pointing direction for maxi" gain. These parameters were chosen in order to obtain marhum gain compatible with low aosspol peak levels and a compact antenna geometry. The primary feed used in the design was a low amp01 m g a t e d horn. The design also incinporated a subrefleaor rim extension, as shown in fig-la, m order to prevent excessive feed horn spillover levels. Chosen aperture d i m d o n s for the elliptical rim antenna were Rx=l.6m and Ry=2m, resulting in 78% &ciency for the entire band. Po radiation patterns at 11.7 GHz [5] and ITU-R envelop for elevalicm and azjmuth planes are shown m figurea 2a and 2.b. Thos for 1425 GHz are presented in figures 4a and 4b. Once this design in which "mn gain compatible with limited spillo~ver and aosspol levels had been obtained, and in or& to achieve the desired cost reduction, the main reflector das replaced by a 3.66m diameter spherical cap manmining the same suMector and aperture center @tion. This implied changing aperture

0-78034 165- U976 10.00 0 1997 IEEE. 567 SBMO/IEEIE MTT-S IMOC'97 Proceedings

dimensions to Rx=1.77m and Ry=1.83m (fig. lb), resulting 75% efficiency for the entire band. Radiation azimuth and elevation patterns for this configuration at 11.7 GHz are shown in figures 3.a and 3.b respectively. Those for 14.25 GHz are presented in figures 5a and 5b. Antenna theoretical equivalent noise temperature (Kelvin) calculated at 11.7 GHz is also shown as a function of boresight elevation angle in figure 6 as determined by the method described in reference 6. Calculations did not take into account ohmic losses, but included a 20 dB feed horn return loss.

3. Conclusion. In this paper it has been shown that by designing an optimum performance elliptical rim double offset antenna with spherical main reflector, it is possible to replace the main reflector by a spherid cap of the same mature radius. Although the antenna obtained presents a slight efficiency decrease (3%), it stiU has an excellent sidelobe performance when compared to ITU-R envelop pattern recommendations. The design incorporates all double offset electrical performance advantages, such as elimination of losses and scattering due to struts and subreflector blockages, plus main reflector cost “tion due to the possiblity of main reflector manufacture by spinning machjnes. Thus, the conliguration presented can be regard& as a se.ri~us competitor to the symmetrical double reflector configuration, sti l l widely used m satellite systems ground stations.

4. Acknowledgements. Authors would like to thank &liton Cruvinel and FrAsio de S o w for helping with illustrations.

5. References. [l] - Rec. 580-1 Radiation Diagrams for Use as Design Objectives for Antennas or Earth Stations Operating with Geostationary Satellite; ITU-R 1990 Plenary Assembly. [2] - PEREIRA, LufS CLkJDIO P. - A Prospective Theoretical Study of Double Offset with Spherical Main Reflector Application to Ku Band Ground Stations; Proceedings of EEE AP International Symposium 1995, L.A., USA. [3] - WESTCOTT, B. S . / BRICXEU, F. - G.O. Synthesis of Reflector Antenna Using Complex Coordinates; proceedings of European Microwave Conference 1977, Copenhagen, Denmark- [4] ~ BERG-, J. R. / DA COSTA, J. C. W. - Dual Reflector Antenna with SphericaI Main Reflector; proceedings of IEEE AP International Symposium 1990, Houston, USA. [5] - PONTOPPIDAN, IC - Technical Description of GRASIW; TICRA Engineering Consultants, Copenhagen, Denmark. [6] - IEEE Antennas ans Propagation Society News Letters; pp 8-9,1984.

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