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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
