Phase-correcting zone plates may be used with off-axis feeds. For angles up to about 20Ã‚Â° off-set, the performance is well-behaved, with minor degradation. The design is given for a transmission type and an equivalent reflecting version having a 17Ã‚Â° feed offset. An investigation has been carried out for angles larger than 20Ã‚Â°, apd the results indicate that the annular zones on the zone plate should not be circular. A ray- tracing analysis shows that the zones need to be egg-shaped, and these results will be described. The analysis has been completed for both transmission or reflection-type zone plates.
James C. Wiltse, James C. Wiltse,
"Off-axis-fed fresnel zone plate antenna", Proc. SPIE 1929, 17th International Conference on Infrared and Millimeter Waves, 19293W (14 December 1992); doi: 10.1117/12.2298257; https://doi.org/10.1117/12.2298257