I'm talking about what goes on in the transmission line.
You're bringing up a change in the "antenna". That's another subject :-)
Wes N7WS
On 9/3/2014 2:41 PM, Alan wrote:
On 09/03/2014 02:12 PM, Wes (N7WS) wrote:
Oh dear me!
If I take a lossless 50-ohm line and terminate it in 100 ohm and measure the
VSWR using an ideal bridge/coupler/VNA/etc that is calibrated for a 50-ohm
system, I will measure 2:1 SWR no matter how long the line is, from zero to
infinity. The transformed Z will change with length, but the SWR will not.
That's why one can draw a circle of constant SWR on a Smith Chart. Any point
on the circle will have a different Z from another, but they all have the
same SWR.
If you change line length and the SWR reading changes, then: 1) the line has
loss, 2) the line Z and the SWR meter Z are different, 3) the source match is
poor, 4) the bridge/coupler directivity is poor, or 5) all of the foregoing.
With most ham stuff, it's 5.
Or 6) there is common-mode current on the feedline. In other words, the
feedline is part of the antenna so when you change its length you change the SWR.
Alan N1AL
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