An electrically small cassegrain antenna with optically shaped reflectors

The efficiency of an electrically small (diameter less than
30 \lambda
) Cassegrain antenna was increased by specially shaping the reflectors using ray optics. Diffraction effects were reduced by modification of the reflector designs in the central region and at the edges. Special attention was given to the reduction of gain loss caused by blockage. Although the antenna was very small, the technique resulted in an overall efficiency varying from 60-65 percent over a 1.16:1 frequency band. An "unshaped" Cassegrain antenna was also built and tested for comparison. Measured data shows an increase in gain of 0.5 dB over the conventional Cassegrain. About 0.35 dB of this difference is attributed to reflector shaping, and the remainder is the result of replacing the subreflector quadripod legs with a dielectric cone. The conclusion is that the efficiency of electrically small Cassegrain antennas can be significantly increased by specially shaping the reflectors according to ray optics with allowances for diffraction effects.