[BC] "VSWR" Metering

[Richard Fry]

Poster 1:
>See, most VSWR detectors read voltage. By adding to
>(or taking away) line, you are moving the voltage peak
>away from the voltage reading detector (think sinewave here).
>The VSWR does not change, only the meter's reading of it.

A "VSWR meter" that samples only RF voltage or only RF current at a single 
point in the transmission line is not very useful for that purpose.  I 
think the belief that a modern VSWR meter measures only voltage might be 
traced to the "V"  (for voltage) in the term VSWR, and the term itself 
probably relates to slotted line measurements that DO use a voltage probe. 
But that probe moves along the transmission line during the measurement.

For modern SWR meters (Bird, etc), the common practice is to sample both 
current and voltage, which, when properly scaled will be a measure of the 
relative _power_ in the transmission line.  The pickup probe is made to be 
directional, so that forward and reflected power sampling can be done 
independently.   Load SWR is a mathematical relationship of the two 
readings.

Here is a quote from a paper by Jim Norton of Bird Electronics.  He is 
writing about the directivities of their coupler elements, but it is clear 
that the coupler samples both voltage and current (the magnetic field).

[Quote]Directivity is a measure, or figure of merit, as to the ability of a 
directional coupler to discern between the forward and reflected traveling 
wave in the transmission system. Directivity in the directional coupler is 
established by balancing the contribution in the coupler circuit from the 
electric field component and the magnetic field component. When the 
contribution from these two sources are balanced the coupler directivity is 
optimized. The electric and magnetic field contributions are determined by 
the coupling capacitor and inductor on the coupling plate.[End Quote]

Poster 2:
>On a matched line, the voltage, current, and VSWR is the same
>at any point along its entire length.  This is not the case on a
>mismatched line.

Voltage and current vary separately along the length of a line having SWR 
on it, but the power contained in the forward and reflected waves stays 
constant along the line, therefore, so does SWR.  The forward and reflected 
voltages and currents each combine as vector sums at every point along the 
line, which produces the standing wave pattern with voltage and current 
maxima that can damage system hardware.

To clarify with an example: if the load impedance at the far end of a 
perfect 50 ohm transmission line provides a 1.5:1 mismatch to the line, the 
SWR at every point along that transmission line also will be 1.5:1, and 
that is what a properly calibrated "Bird" meter should read if installed at 
the near end, and at any point along the line.

RF