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.
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