[TowerTalk] Limit to Concentric Quads or is 2M the Bad Actor?
k2xx at swva.net
Sat Nov 13 13:38:56 EST 2004
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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