| 摘要 |
A cassegrain-type feed for a (parabolic) antenna is a dualband fed and employs a waveguide (40) feeding a dieletric cone (23) feeding a subreflector (24). The waveguide has an end-portion (49) adjacent the narrow end of the cone, the impedance of an inner wall (48) of which is modified by the inclusion of, in one embodiment, a dielectric sleeve (47) of thickness between/6 and/4 relative to propagation in the sleeve at a mean value of the upper of the two bands concerned. The sleeve helps to provide a rationally substantially symmetric illumination of the subreflector in said upper frequency band and, when used used with a parabolic main reflector, a similarly symmetric illumination of the main reflector also. The sleeve may be replaced by a series of grooves formed in the inner wall of the waveguide end-portion, these grooves being nominally/4 deep.
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