[Elecraft] SWR Readings:Differences

[Wes (N7WS)]

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
>
>