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.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|