[TowerTalk] tower resonances

[jimlux]

Rudy Severns wrote:
> Wow,  I didn't realize this was such a hot topic!
> 
> My interest is driven by a recent practical problem.   I was helping a 
> friend with a new 80m 4-square.  He had done a wonderful job mechanically. 
> The array was very symmetrical and looked great.  I then proceeded to do the 
> standard measurements: i.e. element self impedances, coupling impedances, 
> etc.  It soon became very clear that while the array as mechanically 
> symmetrical it was very asymmetric electrically.  The readings were really 
> crazy!  It was obvious that one or more of the not too distant towers was 
> interacting strongly with the array. 


Brings that whole "near field/far field" thing into importance, eh? 
When you say "not too distant" do you mean 100 meters away? 20 meters? 1km?


  The whole project came to a screeching
> halt until we could sort out which tower(s) was the culprit(s) and then 
> properly detune it.  I should have checked each of the towers ahead of time, 
> identified a possible bad actor and fixed the problem in advance so that the 
> array tuning could have proceeded without interruption.  I should have known 
> better but I've learned my lesson this time.


Hmm.. maybe an easier way is, rather than trying to measure towers, 
figure a fast way to look for unwanted interactions.  Off the cuff, I'm 
thinking that if you were to put up just a short vertical at each future 
element location, I wonder if measuring field strength/phase from a 
centrally located vertical radiator would tell you anything.

Some sort of "survey" technique is really what you're looking for. You 
might be able to do it with a untuned radiator and a VNA and an active 
whip as a probe that you drag around.

Easy to answer with a model. Maybe I'll give it a shot tomorrow morning.

Of course, even if you have the survey method, once you've identified a 
problem, you still have to figure out how to fix it.