Dual-circularly polarized antennas usually consist of a radiating structure and a feed network capable of generating the �90� modes required for dual circular polarization. The problem is that these feed networks often take up a large amount of space. It was found that a compact dual-circularly polarized antenna could be made by placing a reduced-size hybrid coupler on top of the center conductor of a ring-slot antenna. Two cavity-backed ring-slot antennas are presented with unidirectional radiation properties. The first is a linearly polarized CPW-fed ring-slot antenna. A T-shaped CPW feed was used to capacitively excite the ring-slot structure. The bottom of the cavity is formed by an AMC reflector consisting of an array of rectangular patches, a substrate and an electric ground plane. Experimental results for the final antenna design with a size of 0.457_0 _ 0.457_0 _ 0.056_0 exhibits a 5.5% impedance bandwidth, maximum gain of approximately 7 dBi, a front-to-back ratio of 15 dB, and good cross-polarization. In the second design a reduced-size microstrip branch-line coupler was placed on top of the center conductor of the ring-slot in order to feed two T-shaped microstrip feedlines, in order to achieve dual circular polarization. Two coaxial cables were used to feed the branch-line coupler from the back of the structure. This resulted in a significantly better front-to-back ratio. Experimental results for the final antenna design with a size of 0.5_0 _ 0.5_0 _ 0.056_0, exhibits a 4% isolation bandwidth, 12% impedance bandwidth, maximum gain of approximately 6.8 dBic, and good cross-polarization and axial ratio results were achieved. If the branch-line coupler is removed, dual-linear polarization can be achieved. This makes the antenna polarization diverse. Future work will focus on improving the isolation bandwidth without significantly increasing the overall size of the antenna. These antennas are simple to manufacture using simple photolithography as no shorting posts or solid sidewalls are necessary. It can potentially be used as a flush-mounted single radiator or as an element for an array. This polarization-diverse small-footprint and lowprofile antenna with relatively high gain is a good candidate for 2.4 GHz WLAN applications.
Dissertation (MEng)--University of Pretoria, 2018.