Calculations using EZNEC with NEC5 show the same thing as your
measurements. You can model any wire with insulation on it to simulate
the rain water ... Er about 80 and loss tangent about 0.015 at 20 MHz.
The change in VF can be seen but the effect on loss isn't noticeable.
I do think that old, cracked, dirty window line will show additional
loss when wet, though.
Dave AB7E
On 7/21/2025 9:55 AM, Bob McGraw via Elecraft wrote:
I agree, window line changes when wet. But my measurements indicate
it is the velocity factor that changes. Thus the electrical length of
the line changes from dry to wet, etc. In a 100 ft length of new
window line, from dry to wet and then with a wetting agent, with the
line correctly terminated into its characteristic impedance, the loss
measurement or changes were almost not perceivable. ( Less than 0.1
dB, 2 - 30 MHz).
In the case of the original G5RV, the design specified 34 ft of OPEN
WIRE line as a matching section on 20M. The open wire line is much
much less affected by water, rain, ice, snow and dirt. It is
specifically 1/2 electrical wavelength on 20M. If one uses window
line, as in my tests, the electrical length will change from dry to
wet, due to VF change, and thus the matching section of the line will
no longer be electrically correct. Thus the ATU must be adjusted to
account for the change in electrical length. So if you are using
window line, one should expect the ATU to be required to change values
as the weather changes.
Also with the G5RV, there is a 1:1 balun at the point where the
balanced line transitions to coax. Again researching the original
documentation for the G5RV, the coax line should be 75 ohm coax. The
feed point of the G5RV is ~ 90 ohms +/-j0.
73
Bob, K4TAX
REF: G5RV_w5rh.pdf
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