In 1993, when this project started, Australia was in the early stages of developing Direct Broadcast Satellite Television (DBS-TV) services. Optus had just launched B3, a satellite carrying high power linearly polarised Ku-Band transponders to be used for DBS-TV. To access DBS-TV services in urban areas of Australia, an antenna equivalent in performance to a 60cm diameter parabolic reflector is required. Such an antenna requires linear polarisation, a gain of around 30dBi, a beamwidth of less than 3°, sidelobe suppression greater than 11 dB and a 500MHz bandwidth around a centre frequency of 12.5GHz. An offset fed parabolic reflector is one suitable option for such a requirement. Despite its well-proven electrical performance, this antenna creates visual pollution when installed in large quantities and hence is not welcome in many instances. The demand for more aesthetically pleasing antennas created an opportunity in this thesis project to search for a suitable replacement for the parabolic reflector. One possible choice is a Radial Line Slot Array antenna. This antenna initially introduced in the 1960s by Kelly et al for radar applications exhibits low insertion loss and low profile. It comprises a conductor-coated dielectric-disc body with a surface of radiating slots on one side and a coaxial feed on the other and hence is easy to manufacture.
In the 1980s, researchers, mostly from the Tokyo Institute of Technology showed RLSA antenna configurations suitable for DBS-TV reception. Their main designs concerned a circularly polarised (CP)-RLSA, required to receive DBS-TV in Japan. Australian DBS-TV requires a linearly polarised subscriber antenna, creating an extra challenge to the designer because the distribution of radiating slots to achieve a linearly polarised (LP)-RLSA antenna creates a poor feed return loss. The Japanese researchers have shown methods to overcome the problem of poor return loss in linearly polarised RLSA antennas, however the scattered collection of these research contributions is not complete and hence has not been of full benefit to the antenna designer.
The present thesis describes a unified approach to the design of single-layer RLSA antennas of both linear and circular polarisation. This approach, termed herein as the Rapid Prototyping Method utilises theoretical design and modelling components as a starting basis for the design, and couples them with efficient computer algorithms and experimental methods to enable the quick prototyping and optimisation of this type of antenna. The methods are developed using, as an example, the design of an antenna suitable for DBS-TV reception in Australia.
In the approach undertaken in this thesis, the design and development of the RLSA antenna is divided into two separate tasks, the design and development of the feeding sub-system for the radial guide and the design and development of the radiating surface. The feed design is assisted by suitably developed software algorithms for the analysis of coaxial to radial guide transitions. These include disk ended probes and recessed cavity resonators. The developed algorithms execute on a standard IBM compatible PC and result in the design of coaxial-to-radial guide transitions featuring return loss values in the order of 20dB across the entire frequency band used for Australian DBS-TV. To achieve suitable antenna gain and radiation patterns, theory detailing the layout of slots to produce linear or circular polarisation is developed. This theory eventuates in software algorithms that generate slot positioning data for surfaces carrying thousands of slots. Having obtained a given slot pattern, a Radiation Pattern Model based on field expressions for a single slot and the principle of superposition, is utilised to predict the radiation pattern.
To overcome the poor LP-RLSA return loss, two methods are investigated, being the addition of reflection cancelling slots for when a boresight main beam is required, or alternatively, squinting of the antenna main beam. Both methods lead to improvement in the return loss to greater than l0dB for all prototypes constructed in this work. A novel method of placing the additional reflection cancelling slots on the rear, non-radiating surface is also demonstrated, allowing potential slot overlap to be easily avoided. The alternate, preferred method of squinting the main beam allows the slot configuration causing the poor return loss to be avoided without the introduction of any additional slots. It is found that return loss improves to greater than 20dB for squint angles as small as 5° from boresight, and main beam stability has been achieved for squint angles up to 45°. Benefits of the beam squint design include an ability to mount the antenna on a dwelling wall or roof while pointing the beam towards the satellite.
The design methods for RLSA antennas presented in this thesis have resulted in one US patent and many technical papers in international journals and conference proceedings. The final product of this work is an LP-RLSA antenna prototype having 32dBi power gain, -3dB gain bandwidth of 580MHz, a 2.7° beamwidth and side-lobe suppression greater than 15dB, making it suitable for receiving Australian DBS-TV transmissions.