Why is the dipole resonant at 3.8? Are you using it there or only on CW?
Wes N7WS
On 6/24/2020 11:20 AM, Steve London 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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