Hi Steve,
> I've got a question. Sometimes when you measure the shack end of a
coax
> run with an antenna analyzer, you get a different reading than when you
plug
> it into the rig. What is the simple reason for this?
Simple(?) reason, or several reasons:
1.) Calibration of a meter is off. (My stock IC-751A likes to say a 65 ohm
load is 1:1 SWR)
2.) RF in the analyzer. (They use broadband detectors).
3.) 50 ohm cables are virtually never 50 ohms, and cable length then
affects SWR readings. (The meter Zo and line Zo must be the same for SWR
reading to not change as the meter is moved along the line unless the line
SWR is EXACTLY 1:1. If line SWR is exactly 1:1, the meter reading won't
change even if the meter reading does disagree with the true SWR. For
example, if I have a matched lossless or very low loss 75 ohm cable and a
50 ohm meter it will read 1.5:1 anywhere along the 75 ohm line. If the 75
ohm line is terminated in 50 ohms the 50 ohm meter will change from 1:1 to
2:1 depending on where I place it. Since a 50 ohm line is almost never 50
ohms some SWR change is likely.)
4.) The extra stuff you add in line when the rig is hooked up
5.) Common mode current on the feeder (not a very likely cause for SWR
shift with long lines dressed near other stuff).
Those are the only likely causes I can think of.
73 Tom
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