Hi all, I've been thinking about what happens when VSWR meters don't agree, and why VSWR seems to change cyclically as transmission line length is changed. I say "seems to" because, whilst measured VSWR reduces as a lossy line is lengthened, it doesn't change cyclically (despite some information on the web!). I've read that common-mode currents can cause odd VSWR reading behaviour, and it certainly makes sense to say that if the feedline has become part of the antenna then changing its length is bound to affect the (now extended) antenna impedance and hence VSWR - however I've seen no theoretical treatment of this and I wonder how significant the effects can be; significant enough to perpetuate stories of VSWR varying cyclically with transmission line length? Seems a stretch but maybe. What I've discovered after a day of experimenting is that VSWR meters can be brought into alignment, so that they agree across a range of frequencies and complex load impedance, and read 1.0:1, i.e. correctly, into a dummy load where they didn't before, by adjusting the coupler balance. In the case of the Bruene coupler there is often a variable capacitor on the input voltage divider to allow this adjustment. This prompted me to create a little numerical model of the Bruene coupler, which confirmed as I suspected that the indicated VSWR varies (and, cyclically) with the phase of the reflection coefficient if (and only if) the coupler balance is incorrect by some percentage. So - if your coupler is a little off balance, then changing the line length will give a change in VSWR reading. I've put some results from my model below & this shows that the effect can be significant - e.g. with the coupler 30% out of balance, adding a 1/4 wave of line(*) would show an indicated VSWR change from just under 4:1 to about 2.3:1, where in reality the VSWR is constant at 3:1 (this example is in the second picture). Also, perhaps obviously, if the coupler is out of balance, a VSWR of 1.0:1 will not be seen even into a matched load, and - tbc - can never be seen into any load (proving this might be a challenge!). Anyway - I hope this provides food for thought and sheds some light on some of the tales that float around. I've not seen any analysis like this elsewhere? The maths is scrappy hand-written notes currently but ends up with: where Z is the load impedance (normalised here i.e. assuming Zo = 1.0) and K a constant which rolls up all of the usual voltage divider, inductance turns ratio & load resistance values etc into one constant that characterises coupler balance and is 1.0 for perfect balance). So the graphs below are produced from that equation above by making a complex reflection coefficient suitable for some chosen values of *actual* VSWR across 360 degrees of reflection coefficient phase, working out the Z load that must be associated with it, and plugging that Z in above. The legend on each graph indicates the *actual* VSWR. (*) adding a 1/4 wave of lossless line has the same effect as changing the load impedance such that the reflection coefficient magnitude stays the same but the phase increases by 180 degrees.