In the latest DUBUS magazine (2/2019), I found a description of the dual-band feed for an offset reflector antenna, published by Mike Willis – G0MJW, Remco den Besten – PA3FYM and Paul Marsh-M0EYT. Since I am generally interested in reflector antenna and authors have not published more information about the performance of their antenna design, I performed a computer analysis of this feed. Maybe some members of this forum are also interested in a performance of this feed, so I am sharing my calculated results here.

Thanks to RF spin s.r.o. company, I was able to use CST MW Studio software for modeling and calculation. For feed performance calculation, I used time domain solver approach. For the whole antenna performance calculation, I- solver was applied. The feed radiation patterns were calculated and generated in 5 deg. increments of angles Phi and Theta and inserted into a focus of the parabolic reflector as a far field, shifted with amount of the phase center position (23.6 mm). Subsequently, next calculation was done in I-solver For directivity calculation of whole antenna assembly, I used a model of prime focus parabolic reflector, since geometry of offset reflectors are not unified. The expected error with this simplification should be smaller than few percent.

Conclusion for X band

Pros:

- Simple design

- Symmetrical radiation patterns for both E and H planes - good efficiency for dishes with f/D ratio of about 0.5 – 0.65

- Good impedance match

Cons:

- Worsening of LNB noise figure (0.2 – 0.3 dB), due to use of lossy dielectric material (Nylon 6, dissipation factor 0.2)

- None UV resistance

- Difficult adjustment in offset parabolic reflectors for the best performance