TE11-to-HE11mode converters for small angle corrugated horns

A theoretical and experimental study of mode conversion from the transverse electric (TE
11
) mode in a circular waveguide to the HE
11
hybrid mode in a small angle conical corrugated horn is given. Mode conversion is achieved through a transition section where the slots are either ring-loaded or are conventional slots of varying depth. Using varying depth slots the optimum design gives a return loss exceeding 30 dB over a 1.45 bandwidth ratio, with the low frequency limit determined by the deterioration in return loss, and the upper frequency limit determined by the excitation of excessive levels of the unwanted EH
12
hybrid mode. Using ring-loaded slots a bandwidth ratio of 1.55 is possible, increasing to at least 2.0, if the small amount of of EH
12
mode generated (being an order of magnitude less than the case using varying depth slots) is acceptable.