[AMPS] Amp Pi Network Behavior

[Tom Rauch]

Hi Pete,

> 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?

It could be that the antenna's feedline presents some equivalent 
parallel capacitive reactive, or the resistance is higher than 50 
ohms, or both of those cases at the amplifier. 

It also could mean the dummy load presents a parallel inductance 
or is slightly lower than 50 ohms resistive or both at the amplifier.

Most likely it is a combination of all of the above, a bunch of little 
things adding up in opposite directions. 

Your SWR meter may not be able to resolve the small mismatches 
that are in opposite directions. The SWR meter might be adjusted 
for an impedance slightly off of 50 ohms. Both are pretty common 
in SWR meters.

What kind of meter is it?  Are your feedlines all close to 50 ohms?
73, Tom W8JI
w8ji@contesting.com

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