In reply to:
I do not think there is a "true" SWR meter which can work by measuring
things at only one point on a transmission line.
SWR is a ratio and can only be indicated by making a set up which responds
to it and even then only by empirical calibration - I think.
With all due respect, there's nothing magical about SWR. If the voltage is uniform along the length of a line (ratio 1:1), it is because the line is terminated with a load that matches its characteristic impedance. If not, then reflections exist because of a mismatch. The extent of that mismatch will be reflected-- no pun intended-- in the ratio of the voltage at a location where it is maximum along the line (constructive interference between the forward and reflected wave), versus a location where it is at a minimum (destructive interference).
The degree of cancellation at the minima is dependent upon the percentage of power being reflected, and the phase relationship between the two waves. Thus, "true" SWR can only exist where a transmission line is involved. But even so, it is still only a tool for measurement, an *effect* arising from precisely the same phenomena that are measured by a well constructed reflectometer setup or the power and phase detectors in a network analyzer.
When measuring VSWR on a physical transmission line, you need only read the voltage at one of the maxima and the voltage at one of the minima along the line; whereas, with a reflectometer, the sampler (be it terminated loops or bridge) must match the intrinsic impedance of the line to discriminate the direction of power flow accurately, and you must reference the reflected power as a percentage of the forward power (PSWR, if you will) then convert the power ratio to a voltage ratio (VSWR) before you can arrive at a result expressed in the same units.
Rigorously speaking, without a transmission line as in Peter's example, it shouldn't be expressed as "SWR" at all; it should be expressed in terms of "return loss." And yet one is measuring the same fundamental condition either way, so the common nomenclature from the days of measuring "true" VSWR has stuck.
John
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