High-Q resonances due to surface waves and their effects on the performances of corrugated horns

We present a study of high-Q resonance phenomena that occur in corrugated horn structures. It is found that high-Q resonances occur within certain frequency bands where two types of modes, i.e., space-wave and surface-wave modes, simultaneously propagate. A simple model is presented to explain the resonance mechanism. The model yields the conditions for resonance, explains the Q of the resonance, and provides the associated resonant frequency. Approximate formulas for the upper and lower bound frequencies of a resonant band are obtained. It is found that the high-Q resonances deteriorate the gain and crosspolar performances of a horn even if the return losses are acceptable at some resonant frequencies. Rectangular corrugated horns are more susceptible to these resonances than circular corrugated horns, and the reasons for such a difference are explained.