For what it's worth, I modeled a resonant 160m dipole @ 10' below a generic
40m beam at 64'. The model predicted almost no interaction between the two
antennas on 40m. I modeled the 160m dipole as a 1/2 wave wire optimized
for resonance @ 1.9 mhz @ 10' (very low impedance).
Matt
KM5VI
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.
elease Date: 12/04/13
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