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From: David Jefferies
Date: 17 Apr 2002
Time: 14:09:03
Remote Name: 195.92.168.166
Dan,
Many thanks for the reference.
Actually, for the 0.5 wavelength wide patch, the voltages at the opposing edges must be in antiphase (half a wavelength apart, not a whole wavelength) but the sense of the fields is reversed also by symmetry which is why we get constructive radiation with E field polarisation direction parallel to the 0.5wl width. On the other hand, because the length is 1 wl the voltages are in phase, and oppositely directed by symmetry, and so the other edges radiate destructively.
I think I twig.
Now, how does a circularly symmetric patch, fed in the middle, radiate (if it does)? One might argue that it needs to be 0.5 wl diameter for the opposite ends of any diameter to radiate constructively (by the same argument), but when one sums up all the possible diameters, the E field resultant must be zero at broadside.
I hadn't appreciated the importance of the fringing fields. Of course, the edge of the patch forces a current node (null) and therefore a viltage antinode (maximum) and so it is the E fields in the gap which provide radiation. We might regard this as a "preexisting em wave" and therefore our requirement for current is relaxed.
That was very helpful. Tell Jack Stone that the Forum has come up trumps for me at any rate. Thanks.
David. G6GPR.