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
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