[Antennas] Re: [TowerTalk] Limit to Concentric Quads or is 2M theBad Actor?

[John Tait]

And thank you Joe..  I was wondering what I had done wrong, so it's nice to 
know that you had the same result. Like you, I'll be putting the VHF/UHF 
stuff on a second tower.
    73
   John EI7BA   http://www.iol.ie/~bravo/


> 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.
>>>