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RE: [TowerTalk] Funniest thing I've seen in weeks

To: "Joe Subich, K4IK" <>
Subject: RE: [TowerTalk] Funniest thing I've seen in weeks
From: Jim Lux <>
Date: Wed, 30 Jun 2004 13:44:07 -0700
List-post: <>
At 04:08 PM 6/30/2004 -0400, Joe Subich, K4IK wrote:

> From: Jim Lux
> You're right, though, they all reradiate because of mutual
> coupling, and there's no particular reason why you couldn't
> for instance, make an antenna  with 3 reflectors and a driven
> element.

Other than the empirical (and some computer) studies that seem
to show that there just isn't any return on the investment in
additional reflectors.  For some reason (and I'm not an expert
in EM) there is so little coupling from additional reflectors
after the first that they do not provide a real advantage.
Have you any references on this phenomenon that I could look for? From an EM standpoint, the coupling from one element to another is essentially independent of whether or not there's another element in between (otherwise there would be no advantage for multiple directors). I have a hard time believing there's any inherent electromagnetic reason for preferring directors over reflectors. Maybe it has to do with element losses or something?

I just ran a couple quick models with a 3 element 20m yagi, putting the feed point on each of the three elements (keeping element lengths and spacings, hence the mutual coupling,the same). The general shape of the pattern was identical, with the gain changing slightly (0.5 dB out of 12dBi), probably attributable to the less ideal current distribution when feeding the front or back element as opposed to the middle.

I should think that if you optimized the antenna, there wouldn't be a significant difference between feeding at the back or the front. Feeding in the middle is always going to be better. In fact, doing a quick optimization, I get F/B ratios within 0.5 dB and gains within 0.5dB for feeding either the front or the back element

One aspect is that the feedpoint impedance may be "nicer" with one reflector and many directors than with one director and many reflectors, although I have no basis for the surmise.

The fall-off in incremental performance is even more (expo-
nentially) after a second reflector.  On the other hand, the
performance increase per additional director seems to be fairly
linear with the percentage increase in boom length.

This would be true for ALL arrays.. the performance is proportional to the aperture. Yagi-Udas just happen to be end fire arrays where the aperture is in the same direction as the preferred propagation direction. There are some subtleties in superdirective arrays, but, as you say, directivity/gain is proportional to length to the first order.


... Joe, K4IK


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