Stub matching loss can bite you

An article by K2PO in QST Feb 2020 entitled An SWR shifting T illustrates the pitfalls in naive design and implementation of transmission line matching systems. I say naive because the article does not address the matter of loss, yet QST publishes it as an example.

K2PO outlines an issue that looking into the end of the feed line at 3.58MHz he measures Z=135-j90Ω, and his solution using a single stub tuner of RG-8X right on the back of the power amplifier.

Let’s model that in Simsmith (though I must declare I have issues with Simsmith’s transmission line modeling).

The model above implements a near perfect match, and the source is set to supply 1500W. If we are to accept Simsmith’s loss model, the calculated power values as the signal flows right to left (huh!), 35W is dissipated in the open circuit stub, and 269W is dissipated in the 20m long series section. In all 20% of the 1500W transmitter output is converted to heat in the matching system.

Above is a pic of the series section that dissipates 269W, fairly tightly coiled foam dielectric coax may get quite hot risking migration of the centre conductor.

Of course this is less a problem for low power or low duty cycle modes that high duty cycle modes like RTTY or even CW.

By contrast, a good ATU should provide the same impedance transformation at perhaps a quarter of the power lost.

A purpose specific L match does even better.

Above, a model of an L match with a good capacitor and mediocre inductor yields a loss of less than a tenth of the power of the single stub tuner solution described.