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

[John Tait]

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  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 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