Dual-mode coaxial feed with low crosspolarisation

Design data for optimised TE11-TM11 dual-mode coaxial prime focus feeds is presented. The optimised feeds have a crosspolarisation below ¿¿30 dB, an input reflection coefficient below 0.1 and very little phase error in their radiation patterns, over a 5% bandwidth. Experimental patterns are also given that show that the theoretically derived design data can be relied upon. The optimised feeds are useful for paraboloidal reflectors with focal-length/diameter ratios in the range 0.3 to 0.5, and will give overall efficiencies as high as 70% or more with sidelobes below ¿¿30dB.