[1000mp] What is Wrong with SWR Meter?

[Tom Rauch]

> When there is some swr on the line the impedance will change with line
> length. Meters at different points in the line may read diferent  even
> though swr doesn't change with line length. Many meters will say that
> it does change with length.

The reason that happens is almost exclusively because the meter's 
impedance is not the same as the surge impedance of the transmission 
line. Both meters and transmissions lines are not perfect.

Most 50-ohm transmission lines are really between the 48-ohm and 56 
ohms. I have a heck of a time finding true 50-ohm cables for test 
bench work. manufacturers offer precision 75-ohm video cables, but 
very few offer 50-ohm tight impedance tolerance cables.

A second cause is common mode currents changing the SWR of the 
antenna as you disturb the line, if you move the ground length. But 
that has nothing to do with two meters reading differently. That is 
virtually always a meter/line Z0 problem.

Different readings almost always go back to the meter not being 
nulled for the line's characteristic impedance...and the meter 
disagreement has nothing directly to do with actual operating 
mismatch (SWR) of the load since SWR is virtually constant along a 
mismatched line (unless it has significant loss).73, Tom W8JI
W8JI@contesting.com