[TenTec] OT: Openwire/Window Line and Bad Wx

[Steve Hunt]

I found time today to do some experimenting on this issue of how 
window-line losses increase when the line gets wet.

I took 60ft of 300 Ohm window (JSC 20AWG conductors) and suspended it 
about 4ft above ground. At one end I terminated it with a precision 50 
Ohm load; at the other end I fed it from an 
OmniVI-LP100Wattmeter-tuner-balun combination. I drove it with 50W from 
the radio at 28.5MHz, having adjusted the tuner for a 50 Ohm match; then 
I measured the current into the 50 Ohm load with a clip-on current 
meter. Next I thoroughly wetted the line using a hand sprayer filled 
with water from a rain butt; the SWR at the input of the tuner rose to 
2:1. I next adjusted the tuner for a match, and noted the change in load 
current under the new conditions.

There was no large change in the power delivered to the load between dry 
and wet. However, at this point I realised that the changes in Zo and Vp 
that had occurred on wetting the line had changed the tuner and balun 
load impedances, and so it was possible that changes in the tuner and 
balun losses could be masking changes in the line loss. So I looked for 
an alternative method.

I used my VNA2180 to make S11 measurements at the input of the 300 Ohm 
line when dry and when wet. By looking at the impedance maxima and 
minima it was possible estimate the change in Zo, Vp and loss. I'm still 
crunching the numbers, but here are some early results when going from 
dry to wet:

Zo 303.8 --> 305.4 (+0.5%)
Vp 0.796 --> 0.783 (-1.6%)
Matched Loss/100ft 0.56dB --> 0.77dB (+0.21dB)

All at 26MHz

These loss increases are much smaller than those reported by Wes Stewart 
and by the various on-line calculators; they are much more in line with 
the ARRL measurements.

Steve G3TXQ