In message , Cecil Moore
writes
I have seen this before, but somehow I would have to be able to
connect and
unconnect the stub.

No you don't. The stub is a high impedance on 80m in
parallel with a low feedpoint impedance and therefore
has a negligible effect on 80m. It can be left in the
circuit.

The stub is a low impedance on 40m in parallel with
a high feedpoint impedance which lowers the SWR and
makes your transmitter happy. On 40m, the stub acts
as a low impedance load that eventually delivers most
of its stored energy to the high impedance antenna
(assuming the stub is made from low-loss transmission
line).

I don't understand that, Cecil. Yes, the stub will be a halfwave on 40m,
and therefore add a low impedance in parallel with the antenna
feedpoint. Depending on the exact resonant length of the stub, the
impedance will be either a very low value resistor, a low value
inductor, or a large value capacitor. None of these is going to help to
convert the resistive part of the input impedance of the antenna to
something around 50 ohms.

However, I suspect that what the matching arrangement actually consists
of a stub which is connected across the feeder AT SOME DISTANCE from the
antenna feedpoint. If the antenna feedpoint presents a high impedance on
40m, there will be a point at some distance (less than a quarterwave)
back along the feeder (towards the TX) where the resistive part of the
impedance (at that point) is 50 ohm, in parallel with a considerable
capacitive reactance. By choosing the correct length of stub and the
correct distance between the connection of the stub and the antenna
feedpoint, you can probably get a 'reasonable' match for both 80m and
40m.

If this IS the actual physical arrangement, it's time to get the Smith
Charts out!

Ian.
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