[Elecraft] SWR Readings:Differences

[Wes (N7WS)]

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.

Wes  N7WS

On 9/3/2014 1:19 PM, Jeffrey Otterson wrote:
> Unless your antenna is exactly the same impedance as your feedline at the
> desired frequency (pretty unlikely)  then the feedline is going to
> transform the antenna impedance based on distance from the antenna.  The
> exception to this is feedline lengths that are perfect multiples of a half
> wave, electrically (that is to say, accounting for the velocity factor)
>
> Any other length will result in a transformed impedance, and corresponding
> different VSWR.
>
> You can demonstrate this by changing the feedline length and watching the
> VSWR change.  Try adding some small fraction of a electrical wavelength of
> coax at 40M and see what your meter shows.  You might be surprised.
>
> TLDR; try adjusting the length of your coax and see if the readings change.
>
> Jeff n1kdo
>
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