Design of a dichroic Cassegrain subreflector

The design of a dichroic subreflector for a dual-frequency reflector antenna is described. This antenna consists of a
Ku
-band Cassegrain feed requiring the subreflector surface to be highly reflective at 13-15 GHz and a primary focus
S
-band feed requiring the subreflector to be transparent at 2.0-2.3 GHz. Such a performance is achieved by a surface of crossed dipoles printed on a dielectric sheet. The influence of parameters, dipole length, width and spacing, and the dielectric constant and thickness of the sheet on the reflection and transmission coefficients is experimentally evaluated. An analysis based upon the Floquent mode theory is shown to correctly predict the experimental results. The construction of a hyperbolic subreflector using the selected surface parameters is briefly described. As compared to a solid subreflector of identical shape, this dichroic subreflector produced a negligible loss (less than 0.1 dB) over a 13-15 GHz band. At the
S
band the loss was less than 0.2 dB over narrow selected bands and the axial ratio deterioration was also no more than 0.2 dB.