[TowerTalk] Stubs and Reactances in parallel

[Wes]

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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