Analysis and reduction of scattering from the feed of a cheese antenna

The far field of a cheese antenna can be described in terms of three components: a) one due to the unobstructed aperture field, b) one due to that portion of the primary feed energy not intercepted by the reflector, and c) a scattered component due to the fact that the feed acts as an obstacle in the path of energy emerging from the reflector. This scattered component is usually calculated by considering a perturbation in the aperture field, in the form of an out-of-phase component sufficient to produce zero field in the geometrical shadow region behind the feed. A more exact analysis of a reflector excited by a longitudinally-slotted circular cylinder shows this engineering approach to be satisfactory, provided the out-of-phase component is assumed to exist over an area of width 1.5 times the projected width of the feed. An empirical investigation shows a similar result for a slotted rectangular waveguide feed and for a horn feed. An antenna is described in which the scattered field and the backlobe of the primary feed are made to partially cancel. An additional control over these field components is provided by a series of vanes or waveguides located either side of the feed.