Some pretty woolly thinking on measuring Thevenin equivalent source impedance of a ham transmitter

A ham seeking to optimise his station based on some measurements with a VNA and some modelling of a matching network posted the results of a test in the process.

The radio is an Icom IC-7300. I bypassed the built in tuner, transmitted a tone into my external tuner, adjusted it for SWR=1. I then disconnected the tuner from the radio, and measured the impedance looking into the tuner with a VNA. Surprisingly, (to me anyway) the result was a pretty good 53-j3 Ohms at 14 MHz.

What should we / have expected? It is an interesting case to study.


  1. the VSWR measurement mentioned was made using the IC-7300 internal VSWR meter;
  2. if that internal VSWR meter nulled at exactly VSWR(50)=1;
  3. the second measurement was made by moving the cable connector from the IC-7300 output jack to the VNA; and
  4. the VNA was OSL calibrated at that jack;

we should expect that the VSWR observed by the VNA was exactly 1.0.

The VNA measurement translated to impedance is reported as 53-j3Ω which implies VSWR(50)=1.09.

What we can say, subject to the numbered conditions listed above, at the power level tested, and allowing for measurement error is that the VSWR meter in the IC-7300 does not seem perfectly accurate, but it is not very far out. Weakness in any of the conditions above might well lead to a conclusion that the test does not show the IC-7300 VSWR has significant error.

What has this to do with optimal matching and the Jacobi maximum power transfer theorem?


It is wrong to interpret this (as the posted seemed to do) as a means of measuring the Thevenin equivalent source impedance of the transmitter. It is an evaluation of the reference (or null) impedance of the IC-7300 internal VSWR meter. An alternative simple test is to connect a high grade 50Ω load and observe the indicated VSWR (which might be power dependent, one of the weakness of the IC-7300) which you might hope to be less than say 1.1… if only you could read it to that resolution (which plays into the accuracy of the reported test).

The IC-7300 VSWR meter is not too easy to read to high resolution, not that it needs to be, but hair splitting tests fail on the uncertainty of such measurements.

None of this addresses whether such a transmitter is well represented by a Thevenin equivalent circuit.