Following is my understanding of some theory and the questions are at the end.
Is this right ? ...
A standing wave is an AC waveform oscillating at a particular frequency which has a varying amplitude as time progresses (only in a sinusoidal fashion if the AC source is emitting a sine wave, which in our case it hardly ever is), but has a fixed amplitude profile in space (or length) along say in this case the inside of some coax, and is the interference pattern created when two AC wave forms traveling in opposite directions cross paths (and the superposition theorem explains this).
And remembering that the AC wave forms in question are in reality just the movement of electrons (which is current which is coulombs per sec) in sympathy with the potential difference of the output of the AC source, which hardly ever is a sine wave, away from the antenna during the periods of source output which have a positive voltage with respect to the AC ground, towards the antenna during the negative periods, the whole thing moving in space away from the source at the speed of light x the velocity factor.
At any one particular point along a transmission line,
VSWR is the the ratio (one divided by the other) of two things :
The potential difference between the maximum value in volts of an existing oscillating standing wave on the outside of the inner conductor and the inside of the coax shield at that point.
The potential difference between the minimum value in volts of an existing oscillating standing wave on the outside of the inner conductor and the inside of the coax shield at that point.
I've noticed that there are a few different kinds of SWR meters, some use a Bridge and some use a Directional Coupler. The different kinds probably have advantages and disadvantages.
However i want to point out that no SWR meters measure the Standing Wave Ratio directly right ? They either measure the ratio of peak voltage for two wave forms traveling in opposite directions (directional coupler) or measure the apparent change in impedance inflicted upon the source by the presence of the standing waves (bridge) ... is that right ? So they indirectly measure SWR and even more indirectly measure the impedance match of the coax to the antenna. That's like a 3rd hand me downed measurement.
Also it's probably worth noting here that the VSWR on a transmission line, excluding I2R losses, is the same at every point along the transmission line.
If there is an error current flowing on the outside of the coax due to incorrect balancing of the coax to the antenna because the AC current flowing on the inside of the shield splits up between the antenna and the outside of the shield because the impedance that the outside of the coax presents to the AC signal is not negligible compared to the antenna impedance, then apparently according to various sources such as the ARRL Antenna Handbook this can affect a VSWR measurement because it results in a "Common Mode Impedance" which affects the measurement made by the SWR meter.
Can someone tell me EXACTLY how the presence of current on the outside of the coax changes the VSWR reading ?
Or, what are the other causes if any of a SWR meter changing it's reading as the length of coax is changed ?
Oh and btw the answer i'm looking for is not : SWR meters are designed to work with a specific feed line impedance and if that impedance isn't the correct value the SWR meter doesn't measure correctly.