Compact circularly-polarized mode converter

A novel compact circularly-polarized(CP) mode converter that converts a TM01 (or TEM) mode into a CP TE11 mode was developed. It is composed of two turnstile junctions. The first turnstile junction transforms the input TM01 circular waveguide mode into TE10 modes in four rectangular waveguides with non-uniform lengths; the second then converts those four TE10 modes with different phase delays into CP TE11 circular waveguide mode. A mode converter at 1.75 GHz was designed. The conversion efficiency of TM01 to TE11 is 99% at center frequency with an axial ratio of 0.03 dB. Over the frequency range of 1.575~1.900 GHz, the conversion efficiency exceeds 90%, and the axial ratio is less than 2.5 dB with the corresponding bandwidth of 18.6%.