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
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.
John EI7BA http://www.iol.ie/~bravo/
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
a monobander, sized according to the standard formulas, and imported
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
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
optimizing it, I imported it into the six bander. The pattern really
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
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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