Part 6 - Design, Calibration & Performance of Standing-Wave-Ratio Meters - Measuring ErrorsAuthor: R.J.Edwards G4FGQ © 28th October 2000
Program Notes (Continued)
after the Previous Calibration Procedure has been Performed
SWR meters function correctly only when used in transmission systems having a Zo for which they have been designed. Meters and Zo's are not interchangeable.
It is unnecessary for the whole system between transmitter and antenna to have a common Zo, only that the meter, the line between transmitter and meter, and transmitter internal resistance should all have that value for minimum error.
Under these conditions the only internal meter errors are due to tapping off a small fraction of line current and a small fraction of line voltage which are compared with each other and then used to drive a moving coil voltmeter. This upsets the line impedance by one or two percent and something less than one watt of Tx power may be dissipated inside the meter in the process.
When the source/generator resistance is not Zo - it seldom is - reflected and forward power and also Tx output power will still be correctly measured.
BUT the indicated reflection coefficient and SWR will be in error, especially when the load has a large impedance angle. However, if needed, the reflection coefficient can be calculated exactly from RC = Sqrt(Refl.Watts/Fwd Watts). If the SWR is needed then SWR = (1+RC)/(1-RC).
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