[TowerTalk] Limit to Concentric Quads or is 2M the Bad Actor?

[Joe Giacobello]

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.

73, Joe