page 10&11 show the issue with sweeping the antenna looking through a piece of coax which eddie was talking about recently and the falacy of swr changing significantly with coax length due to transformer action of mismatched coax,
using the conversion charts i posted
loss in 500ft of rg213 = 5db
2way loss because your signal travels up and back down the coax =10db
return loss @ vswr of 1.2:1 @ the radio = 20.8278537031645dB
subtract your two way loss = 10.8278537031645dB
convert your new return loss of 10.8278537031645dB back to vswr
vswr=1.80693791462797:1 or 1.8:1 @ the antenna
i used 500ft of rg213 as an extreme example,
clearly if you are dealing with small changes lets say going from 9ft to 12ft of coax you would see virtually no change in vswr yet people with perfectly good meters do see significant changes with small changes in coax length such as swapping a 3ft jumper for a 6ft jumper,
that is an indication you have common mode currents on the coax braid.
