Abstract:
A method of designing circularly symmetric shaped dual reflector antennas is presented
in this thesis. The method is used to design shaped Cassegrain and shaped Gregorian
reflector antennas. The two step design procedure includes a proposed method to define
shaped dual reflector surfaces, and an optimization algorithm that determines the
optimum shaped surfaces. The proposed method successfully describes practical shaped
reflector surface using a small number of parameters compared to other commonly used
methods found in literature. The reduction of design parameters reduces computational
complexities. Differential evolution optimization algorithm is used to optimize the
parameters that define the shaped geometry. Two separate sets of optimizations have
been performed for each of the dual reflector geometries (Cassegrain and Gregorian).
The first optimization concentrates on reducing beamwidth by creating uniform
illumination of the main reflector while keeping main reflector size constant and
spillover losses to a minimum. The second optimization concentrates on maintaining a
small beamwidth by creating a uniform illumination over a smaller main reflector while
keeping spillover losses to a minimum. The design requirements are incorporated into
the definition of the cost function of the optimization algorithm.
As the optimization requires evaluation of the radiation characteristics of the shaped
dual reflectors, field analysis is inherent in the design procedure. Field characteristics of
the feed, the subreflector, and the main reflector are formulated. A corrugated horn
antenna is designed as the feed antenna and its field characteristics are evaluated using
standard equations. Uniform theory of diffraction is used for analysis of the field
scattered from the subreflector on to the main reflector. The field radiated from the main
reflector is calculated using physical optics method. The beamwidth is computed for the
shaped and unshaped dual reflectors from corresponding far-field patterns. It is found
that the shaped reflectors outperform the unshaped reflectors in terms of beamwidth.
Other performance characteristics of the designed shaped dual reflectors are also found
to be within satisfactory limits. The obtained values are found to be in good agreement
with numerical and experimental values reported in literature.
