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

[Joe Giacobello]

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  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
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