[TowerTalk] Change in SWR

[Jim Lux]

On 12/4/13 6:57 AM, Rick Stealey wrote:
> Doug, I'd like to suggest something to try.  I haven't actually done
> this but it might bring results, and won't cost you a dime, or even
> get your feet or fingers cold, no matter how cold it is at your QTH.
>
> Your low dipole, MAY be coupling to the yagi, that's the concern,
> right?  And if so, if you detune it, the amount of interference would
> change.  So my idea is to NOT go out to the antenna feedpoint and do
> anything but rather to put variable impedance across the feedpoint of
> the low dipole in the shack.  Use an antenna tuner, and tune it
> through every impedance you can, all the way from a short to an open
> and everything in between.  These impedances will be transformed to
> some other impedance at the feedpoint of course.  The objective is
> just to see if ANY impedance will cause any change in SWR.  If you
> see any effect whatsoever you know there is some amount of coupling
> and you can take it from there.  But right now I believe your concern
> is whether the new dipole is involved in the SWR changes, and this
> technique should answer that question for you.
>

This is a pretty standard way to look for interactions and pretty easy 
to do.  Another similar approach is to measure the coupling.  You might 
see a big peak or dip as you vary the frequency. If you have one of 
those two port VNAs or antenna analyzers, it might be a good way to look 
for it.

One thing that this won't be so good at is if you have a high gain Yagi, 
and you're detuning one of the parasitic elements.  So the match of the 
Yagi might not change very much, but the pattern gets spoiled (a deep 
null becomes not so deep or becomes a sidelobe).  Again, if you have 
something where you can sweep the SWR of the yagi, and do Rick's thing 
of changing the apparent Z of the dipole, you might see something 
change.  the challenge is in looking for tiny changes in the curve.