Pete Smith <n4zr@contesting.com> wrote:
>When I operate my SB-220 into a dummy load on 40M, the loading capacitor is
>at approximately mid-scale. The dummy, a cantenna, measures about 1.04 SWR
>on my VFD, so I assume it is ~50 ohms+/- j0. When I attach my real yagi to
>it, at the same frequency, where its SWR is virtually the same, the loading
>cap wants to be full clockwise for maximum output.
>
>My question is, what is this telling me? Does more capacitance in the
>output of the pi (than the dummy load value) indicate inductive reactance
>on the feedline? Would a requirement for less capacitance indicate that
>the feedline was capacitative?
>
>Reason for asking is that I replaced my top guys with Phillystran yesterday
>and to my surprise, the tuning of my 40m yagi seems to have changed
>drastically. Tune capacitor setting is somewhat different than before, as
>well. Lowest-SWR frequency moved up about 50 kHz, but the SWR bandwidth
>and approximate values vs. frequency haven't changed.
It could be (as you suggest) inductive reactance seen looking into the
feedline, but (at least in principle) it could also be a smaller resistive
component of the impedance seen looking into the feedline. To achieve the
same tube loading, you'd need a larger cap. at the output of the pi
network, and a smaller cap. at the input (to keep the resonant freq. the
same.)
Chances are that both the resistive and the reactive parts of your load
impedance changed.
-Chuck W1HIS
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