Synthesis of dual offset Gregorian reflector antennas with very low cross polarization under practical constraints for mass production

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Virginia Tech


This dissertation starts with a complete survey of the depolarization characteristics of axisymmetric and offset reflector antennas. Single and dual reflector configurations are considered. Cross polarization (XPOL) and beam squint are examined in detail. Although many of the phenomena are known, they are interpreted and explained in new ways and presented in a single coherent treatment.

It is shown that single offset reflector antennas are limited in performance by high XPOL. A cost effective way to improve the performance of an existing offset prime-focus reflector is to add a subreflector, forming a dual offset reflector system with very low cross polarization (e.g., -35 dB). The motivation to use a specified main reflector often arises from a desire to use an existing mold which is usually very expensive. Within this context, procedures to upgrade existing reflector configurations are developed and presented.

In addition, the influence of XPOL effects caused by low-cost, conventional feeds is analyzed in detail. A model for predicting the total system XPOL due to the reflectors and feed is discussed. Various techniques to reduce feed XPOL effects are introduced. Also, practical manufacturing constraints for large scale production are imposed on low-cost dual offset Gregorian reflector antennas. In particular, a design for adequate clearance between the bottom of the main reflector and feed axis is addressed. These constraints are not taken into account by other design procedures and are not addressed in the open literature.

All innovative design algorithms developed in this dissertation were implemented as numerical codes referred to as DORA (Dual Offset Reflector Antenna Synthesis Package). DORA is a complete suite of codes for the synthesis of nonconventional, low-cost dual offset Gregorian reflector antennas with very low cross polarization. Several practical examples are discussed, including a performance assessment of the largest steerable reflector in the world, the Green Bank Radio Telescope located in Green Bank, West Virginia.

Finally, an overview of the various analytical and numerical methods employed in the analysis of reflector antennas is presented in the appendices. The philosophical differences between the methods are highlighted. In particular, the physical optics approach and the Jacobi-Bessel series expansion method are described in detail. The combination of these two formulations results in one of the most accurate and efficient numerical tools in the analysis of reflector antennas. This is shown with the developed code PRAC, Parabolic Reflector Analysis Code. The effectiveness of PRAC is confirmed through extensive comparisons with measured data and results obtained from the literature and with the commercial code GRASP7. Most of the reflector antenna configurations obtained from the procedures developed in this dissertation are analyzed with PRAC and/or GRASP7.