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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