[ELECRAFT] Using XG1 to measure feedline losses

[Vic Rosenthal]

kg4htt at juno.com wrote:

> I tested the idea tonight, and got some surprising results.  I attached
> the XG1 to a 12 foot dipole, which I taped to the center of my 52 ft.
> loop (16 X 10 ft.) opposite the feedpoint. 

I'm not sure why you got the result that you did, but it's possible that the 
feedline picked up some xg1 signal in common mode.  If you have an antenna 
analyzer like the MFJ-259B there's a simpler way to measure the total system loss:

1) Tune your loop for as close to 1:1 as you can get, using the antenna analyzer 
connected to the tuner input.

2) Then either short the end of the feedline or disconnect it from the loop. 
The SWR should now be Very High.

3) Switch the antenna analyzer to Coax Loss mode, if it has one!  It will tell 
you what the loss is in db.

If it doesn't have a loss mode (like the Autek VA-1), note that the measured 
impedance of the shorted or open line varies as you change frequency.  Measure 
the MINIMUM impedance at the impedance dip closest to the frequency at which you 
want to measure the loss, and multiply it by 0.17 to get the loss in db.

What happens is that if the feedline and tuner had zero loss, then you would get 
100% of the forward power reflected from the short or open.  But you don't, 
because some of the power is eaten up by losses in the feedline and the tuner. 
So the analyzer can compute the loss.

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco