[Elecraft] Using XG1 to measure feedline losses

[Ron D'Eau Claire]

Vic wrote:
Right now I'm using an end fire array antenna that I also can use as a 52
ft. circumference loop.  Given the very high SWR on the feedline, especially
when running the loop on 40M and 80M, I recently swapped out the 18 feet of
450 ohm "window" ladder line between my tuner and the loop, and replaced it
with a homemade ladder line made from 3/8" outside diameter copper
refrigerator tubing... I immediately noticed much stronger signals after
doing the feedline swap.  That had me wondering just how much loss I was
experiencing with the ladder line, in comparison to my current feedline. 

 I was thinking about how I could measure this... using the XG1, attaching
it to a 10 ft. dipole made of insulated wire, then taping the dipole to the
center of the loop opposite the feedline, using the XG1 to generate a weak
signal onto the antenna.  Then I would measure the signal strength at the K2
speaker output, going through the tuner,(after having first tuned up the
antenna to 7.040 without the XG1
attached).   Then I would repeat the same experiment with the different
feedline and then compare the difference.  One of my concerns would be the
impact on the XG1 by hooking it up to a 10 ft. or so dipole, and the fact
that it would not be transmitting into a 50 ohm load.  

Is there an easier way to do this?

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That sounds like a good idea, Vic. 

One good preliminary test might be to disconnect the loop at the feedline,
tie the ends of the feeders together and see if you can detect any signal
from the XG1 after it is in place. That's to be sure you aren't picking up
anything by any means other than the loop. 

Turn off the K2's AGC and make sure you aren't overloading the K2 at any
point. If you have one, a step attenuator at the antenna connection to the
K2 would give you actual dB measurements, based on comparisons, but if you
aren't overloading anything and the K2's AGC is off, you can calculate the
dB difference by comparing the voltages. 

I'm a long-time user of open wire line, and I've slipped and called it
"lossless" more than once. Compared to coax, it seems so, but it really
isn't. And ladder line has a much higher potential for losses than true open
wire line because of its tiny conductors. My open wire line feeding my
doublet is made of #12 copper wire, and I've occasionally thought of
switching to 1/8" or 1/4" copper tubing instead. The issue is, of course,
ohmic losses at the high current points. The currents can get really high -
many, many amperes of current flowing in the wire - and that leads to high
resistive losses in the wires. With true open wire line (mine is spaced with
dog-bone insulators at about 3-foot intervals) the insulation losses are
miniscule, but the resistive losses can still cause a lot of trouble with
high SWR's. 

One real advantage of open wire (or ladder) line is the relatively high
impedance. When 50 ohm coax is looking into, say a 2,000 ohm impedance the
SWR is 40:1. A 450 ohm open wire line looking into the same 2000 ohms has an
SWR of only 4.5:1. In the other direction if the antenna presents an
impedance of, say, 10 ohms, the 50 ohm line shows 4:1 while the 450 ohm line
shows 9:1. In either case the 450 ohm line isn't going to show anything like
the SWR, or RF current extremes 50 ohm line can experience. 

So, depending upon the SWRs you have on that feedline, I can readily imagine
that you will see an improvement of several dB by switching to a more
efficient line. 

Ron AC7AC