[Elecraft] SWR Readings:Differences

[Alan]

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