In a recent long running thread on impedance matching on one of the online fora, one poster offered the Ten-tec 540 manual as a reference for clarity on the subject (which of course got murkier with every posting as contributors added their version to the discussion).

The Ten-tec 540 was made in the late 1970s, one of the early radios with a solid state PA, and their manual give the Technical facts of life

to guide new owners to successful exploitation of this new technology.

Amongst the technical facts of life

is this little gem:

The standing wave ratio is a direct measure of the ratio between two impedances, ie an SWR of 3 to 1 tells us that one impedance is three times the other. Therefore the unknown impedance can be three times as large or three times as small as the known one. If the desired impedance that the transceiver wants to see is 50 ohms, and SWR of 3 to 1 on the line may mean a load impedance of either 150 or 17 ohms. …

This says that the SWR wrt 50Ω implies just two possible impedances, he is very wrong… it implies an infinite set of possible impedances.

So, for example, SWR wrt 50Ω of 3 could be due to a load impedance of 150+j0Ω, 16.67+j0Ω, 75+j66.1Ω, 100+j64.5Ω, 125+j52Ω, 137+j40Ω, 112+j60Ω, and their conjugates to name just a few, and an infinite number of other combinations.

It is not unusual for hams to not properly account for the reactance component of impedance, and that failure often rolls up into some pretty specious statements about SWR like Ten-tec’s above.

The Ten-tec 540 manual betrays a lack of understanding on the part of the author, and perhaps the organisation that signed it off.

Their advice that depends on their flawed understanding may also be wrong. Their discussion of load reactance, load impedance, and PA efficiency are quite flawed… but cater to traditional ham myths propagated by those with less understanding… and it is interesting to see this flawed work cited by a poster some 40 years after it was written.

Simple explanations are appealing… but only insofar as they are are sufficiently accurate for the purpose at hand.

Last update: 14th December, 2016, 6:55 PM

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