The Phase Centre of a Pure Mode, Smooth Wall, Conical Horn

In this paper the phase centre of a pure mode, smooth wall, conical horn is studied. The method employed uses phase information obtained by aperture integration of the phase corrected mode field but differs from earlier presentations in first reducing the radiation integrals in terms of an identified family of functions prior to integration. The result is formulas for the principal plane radiation patterns that better exemplify operation of the antenna. The procedure uses these formulas together with least squares minimization to determine a best fit phase centre for each principal plane. It is shown that contrary to published results, even in its principal planes, there is no unique phase centre that is a property of the horn alone. The use of such a horn as the feed element of a radio-optical antenna system is then considered. It is shown that in situations where cost is paramount and the well known drawbacks of its unequal principal plane beamwidths and the diminished gain that results from lack of a uniquely defined phase centre can be tolerated, perhaps contrary to conventional wisdom, the pure mode, smooth wall, conical horn does have a useful role to play. This is illustrated by a design example in which such a horn was used to feed a 6.4 m. dish working in X-band with a Gregorian feed system. The result is confirmation of the usefulness of the method presented here for location of a best fit phase centre.