Design and performance improvement of electrically small feeds for prime focus reflectors
In this thesis, two new types of electrically small feed antennas, slotted circular waveguide feeds, and microstrip dipole feeds are designed and studied for prime focus reflectors. The existing electrically small feeds used in prime focus reflectors exhibit some limitations such as asymmetrical radiation patterns, complex integration with microwave devices, requirement of struts support, and high cost. The design of these two new kinds of feeds is of particular interest since they provide symmetrical radiation pattern, and are low profile, easy to fabricate, simpler to mount on the reflector system, and low cost. Circular waveguide feeds have been a good choice for prime focus reflector systems. The main drawback of these feeds is that the E- and H-plane radiation patterns are not equal in small diameter apertures, and hence the reflector efficiency is low. In this report, an attempt is made to equalize the radiation patterns of these feeds, by slotting their wall at the open-end. Even though radiation pattern equalization is possible with these feeds, the efficiency remains still low. Therefore, chokes are introduced with the slotted waveguide feeds to improve the radiation characteristics as well as the efficiency. Slotted circular waveguides with a cavity is also studied, where slots are used as additional parameters, to equalize their radiation patterns, and hence increase the efficiency. In the second part of the thesis, microstrip dipole feeds are studied because of their light weight, low aperture blockage and ease of assembly on the reflector system. These feeds are designed for backward radiation and thus eliminate strut support. Parametric studies are conducted to improve the input impedance, miniaturize the antenna, and design for dual frequency operation. All these feeds have unequal E- and H-plane radiation patterns. Therefore, they are candidates for elliptical reflector system. To improve the pattern equalization of microstrip dipole feeds, the dipole arms are modified. Two techniques are used for this purpose, bending microstrip dipole arms, or adding corrugations. In the bent microstrip dipole case, the design is suitable for narrow band operation. In order to improve the input impedance while equalizing the radiation patterns, a corrugated microstrip dipole is designed and investigated. Satisfactory performance is obtained. All feed examples studied in this thesis are fabricated and experimentally evaluated for the input impedance match and performance on a reflector. Satisfactory results are obtained.