Dichroic Antenna Reflector for Space Applications CCITT ...

ANDERS DERNERYDHANS WILHELMSSONEricsson Radar ElectronicsOne antenna serves as twoThe metallic subreflector in a dual-reflectorsystem is replaced by a frequency-selectivesurface, consisting of a metallic patternon a carrier made of dielectric material. Thefrequency-selective subreflector gives accessto both the primary and the secondaryfocus, Fig. 2. Near the resonance frequencyof the periodic metal pattern the surfaceis reflecting as if it were made of homogeneousmetal. Signals from the feed horns inthe secondary focus are reflected.At the same time the surface is almost completelytransparent to the lower frequenciesfrom the feed horns in the primary focus.This means that the antenna system canbe used for two different frequency bandssimultaneously. Fig. 3 shows the frequencycharacteristics of the manufactured frequency-selectivesubreflector. Its performancehas been optimised primarily for thefrequencies 18.1-20.2 GHz and 10.7-11.7GHz, and secondarily for 17.7-18.1 GHzand 12.5-12.85 GHz. Also, two linear polarisationmodes can be used simultaneously,thereby increasing transmission capacityfurther.Box 1TERMS AND DEFINITIONSCTECoefficient of thermal expansionDiplexerDevice that permits parallelfeeding of one antenna fromtwo transmitters at the sameor different frequencies withoutthe transmitters interferingwith each otherGrating lobes Secondary main lobesCross polarisation Polarisation perpendicular toa reference polarisationmodePlyOne layer in a compositeLaminateThe whole structure in themould, consisting of severalpliesMatrixThe encompassing materialin the composite (i.e. the plastic)Offset geometry Shadow-free geometry. Thefeeder in the primary focus ismounted so that the effect ofits shadow on the secondaryradiation is negligibleRadomeProtective antenna cover thathas little effect on the radiationpattern of the antennaSkinSurface layer in a sandwichstructureFig. 2Reflector system with a frequency-selectivesubreflector