Frequency compensating reflector antenna

A reflector antenna adapted to avoid defocussing with frequency of an antenna beam being emitted or received by the antenna, said antenna comprising a reflector system, a broadband feed system for illuminating said reflector system to produce a focussed antenna beam, said feed system being operative over a continuous frequency band with the phase center of said feed system moving along an axis towards the reflector system as the frequency increases, the reflector system comprising a structure having a plurality of parts which are dimensioned respectively in relation to the different wavelengths in said frequency band of said feed system to provide a plurality of effective surfaces of reflection that are located at different depths within said structure which depths increase respectively with increasing frequency over said frequency band, whereby radiation in a given frequency region of said band is reflected at an effective surface of reflection located at a given depth within said structure while radiation of higher frequency penetrates beyond it, said structure being dimensioned depthwise in relation to the axial movement of the phase center of said feed system so that changes in the axial position of the phase center of said feed system are matched by changes in the depth of the effective surface of reflection in said structure thereby to maintain the position of the focus of the reflector system substantially coincident with the position of the phase center of said feed system as the frequency changes over said frequency band.