For illuminating a parabolic dish with small f/D (around 0.4, not critical) a ring feed can be used. Its advantages are a wide illumination in both axis' without a large shadowing of the dish. It offers linear polarisation. Such a feed was described by DJ9HO in his VHF papers, but there without dimensions.
The one that I built is just a full wavelength round loop, which is situated in front of a reflector with half wavelength diameter. The distance between reflector and loop is wavelength/eigth. The SWR is adjusted by changing the distance between reflector and loop. Values of SWR=1.1 can be achieved without problems.
The diagrams show the simulation of the feed with NEC4WIN95. These were done on the optimum resonance frequency of 1325MHz, but thats not very far away from the 1296MHz needed.
Paul Wade, W1GHZ, verfied my results with his experience and NEC2 and confirmed my results.
