The Letter describes preliminary theoretical results for a new type of
feed which offers the potential for low crosspolarisation over a broa
dband. The canonical structure is a corrugated waveguide partially fil
led with a dielectric rod. The new structure radiates like a corrugate
d horn when the corrugation depth is near lambda/4 and like a dielectr
ic loaded smooth wall horn when the corrugation depth is near lambda/2
.