A dual circularly polarized single element microstrip patch antenna

[摘要] ENGLISH ABSTRACT:This thesis discusses the design of a dual circularly polarized single element antenna with specialemphasis on achieving a wide bandwidth and high polarization isolation. The aim of the study is toproduce an antenna for transmission of colour video signals between a ground station and a lowearth orbit satellite.Microstrip patch antennas are suitable for satellite applications because they are light weight,conformal and have a thin profile. However, the antennas' inherent naITOW bandwidth is one oftheir major drawbacks. The aperture coupled microstrip patch antenna boasts of a much-improvedbandwidth over the traditional single layer microstrip antenna and the freedom of using separatesubstrates for the patch and the feeding network. Hence the designed single element antenna isaperture coupled. To achieve dual circular polarization with good polarization purity, the antennahad a crossed slot aperture with a balanced feed. The feed network was designed on a single layer.The design was executed in two steps using IE3D moment of methods simulation software: designof the radiating part, and design of the feed part. Each part was simulated and optimized on its ownbefore the two were combined, simulated and optimized again. Parameters used in the antennaoptimization were: the substrates' thicknesses and dielectric constants, patch and aperturedimensions, and the feed line width and offset.The designed antenna was built and measured. Initial measurements of S-parameters yieldedunacceptable results, especially for S22 and S21, and so an investigation into the matter wasconducted. That led to the sizes of the feed and reflector planes being extended. Absorbing materialwas used between the parallel feed and reflector planes because some power was, to a lesser extent,still escaping, due to the fact that the planes were fmite.Measurement results demonstrated reasonable agreement with the simulation. The final antenna hada reflection coefficient of less than -10 dB for the entire operating band of 400 MHz centred on 3GHz, an isolation of approximately -15 dB in the operating band and a gain of 2 dBi and I dBi forPorts 1 and 2, respectively, at 3 GHz. These results, especially the gain, are influenced by the backradiation and the finite sizes of the feed and reflector planes. It is therefore recommended that anenclosed cavity be used at the back of the antenna and a 'dogbone' aperture be used to minimizeback radiation.

[发布机构] Stellenbosch University