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Re: [Antennas] Re: [TowerTalk] Limit to Concentric Quads or is 2MtheBad

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Subject: Re: [Antennas] Re: [TowerTalk] Limit to Concentric Quads or is 2MtheBad Actor?
From: "John Tait" <>
Date: Tue, 16 Nov 2004 18:08:19 -0000
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And thank you Joe.. I was wondering what I had done wrong, so it's nice to know that you had the same result. Like you, I'll be putting the VHF/UHF stuff on a second tower.
John EI7BA

Thanks, John. Nice to hear from you again. You're the only one to get back to me on the subject. I was pretty sure that my guess about resonances was accurate, but wanted to hear the opinions and experiences of others also. I guess I'm going to have to convince my XYL to relent on her objections to my putting up a second tower so I have place for the V/UHFarrays.

73, Joe

John Tait wrote:

I have a multiband Quad as you describe Joe. I built (but did not model)
concentric multielement Quads for two metres and 70cms. These behaved fb
until I nested them into the existing HF Quad. The results are as you
describe..lobes are all over the place.
  Vy 73
   John EI7BA

I have been using six band (6-20M) concentric quads for 15 years now and
tribanders for double that. My current quad is a six bander which I
modeled with EZNEC before building it. The actual characteristics
conformed quite well to the model. (BTW, I originally modeled each band as
a monobander, sized according to the standard formulas, and imported them
to make the six bander. There was about a 0.2 dB reduction in gain in
going from monoband to six band. I should add that I did not try to
optimize the monobanders, while the six bander was optimized for maximum
forward gain.) The bottom line was that there was very little compromise in
performance in going from monoband to six band.

Always eager to press the envelope, I had planned on adding a 10 element
2M quagi to the six bander. After modeling the quagi as a monobander and
optimizing it, I imported it into the six bander. The pattern really went
to pot: major lobes developed on the rear and the pattern was really
brutalized. Surprisingly, there was not that much change in feed
impedance. I then replaced the quagi with an optimized 2M quad, and the
same thing happened. I played with the model and found that if I slid the
2M quad outside of the cage formed by the six band quad (in other words,
so that the reflector end of the 2M boom was at the last director end of
the six band boom), the original monoband pattern returned.

My guess is that given the high frequency of the 2M quad, there are just
too many potential harmonic resonances in the six bander's elements to
interact and disrupt the performance and pattern of the 2M antenna.  If
anyone has any thoughts or a similar experience, I'd be interested in
hearing about it.


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