Another possibility: It sounds like the stub is tuned for minimum
impedance on 40. That may not be necessary. If you tune the stub for
maximum impedance on 80, its impedance on 40 may still be low enough to
prevent the interaction. It doesn't have to be a dead short; just
something that will prevent the antenna from resonating on 40.
One could, I suppose, design an LC network that would be a high
impedance on 80 and low on 40. Whether it would be practical to build is
another matter.
73,
Scott K9MA
On 6/24/2020 21:38, john@kk9a.com wrote:
I am curious what is the interaction on 40m? Can you see this in a model?
When I lived near Chicago I had a 90+ft 80m dipole on the same boom as a
homebrew 2el full sized 40m Yagi and they seemed to get along well together.
John KK9A
Steve London n2ic wrote:
This is a little complicated, so follow along if you are interested....
I have a 40 meter beam, and a rotatable, loaded, 80 meter dipole on the same
mast, a few feet above the 40 meter beam. For years, I have positioned the
dipole perpendicular to the beam to avoid interaction. Works well, but is
inconvenient when working 80 and 40 simultaneously in a contest,
Now I am trying to position the dipole and beam parallel to each other. Big
interaction to the 40 meter beam. No effect on the 80 dipole. Neither is
surprising.
So, I thought, this could be easy to solve. Add a 40 meter 1/2 wavelength
shorted stub at the 80 dipole feedpoint. Indeed, that solved the 40 meter
problem, but at the expense of 80 meters. In theory, the perfect stub should
have an infinite impedance on 80 meters. The reality, using real coax, is
that
the stub impedance is around 1000 ohms on 80 meters. When you put 1000 ohms
in
parallel with a purely resistive load, such as 30+j0 ohms, the result very
close
to 30+j0. Good. However, in my case the 80 meter dipole is resonant around
3800
kHz, and at 3525 kHz, Z = 30-j140. When you put the 1000 ohm stub in
parallel
with 30-j140, the result is 47-j130. Bad !
I can't simply use a relay to switch the stub in/out - I want to work 80 and
40
simultaneously.
And, in case you are still reading, my matching network for CW operation is
series coils to cancel out some of the capacitive reactance, and a hairpin
to
bring the Z up to 50 ohms and cancel the rest of the capacitive reactance.
Is there another solution I am overlooking ?
73,
Steve, N2IC
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Scott K9MA
k9ma@sdellington.us
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