The size of the flare determines the relative amplitude of each mode and the length of the larger section is designed to cancel the two modes at the outer edge of the horn. Because the phase centres of both planes are almost identical the horn is easier to position at the correct focal point of the dish. Focal point accuracy Feedhorn sidelobes Blockage by the feedhorn and support struts Inaccuracies in the parabola's geometry Feeder loss Focal point accuracy is often overlooked. Feeds with asymmetrical E and H planes are harder to position accurately. The popular rectangular horn feed, for example, has difficulty in producing a common phase centre and equal radiation on both planes.
Computer designs can help here though.
The offset, circular horn dish feed seems ideal in view of the above criteria. This was in excess of the more appropriate 30 degrees taper required for 10GHz. As a result, a ring of 3mm diameter soft copper pipe had to be fitted inside the reducer at the wider end of the taper. This seemed an unnecessary complication to the author of the second article in Microwave Update '91 which showed a modification made by WA5VJB, of 10GHz EME fame. Here a reducer of longer taper length was used.