You are correct. Water has an Er up around 80 (!!!) and it changes the
velocity factor of a wire ... it doesn't have as large an effect on the
impedance. I recently did some experiments on dielectric loading of a
100 MHz dipole, and the detuning effect with a water filled cylinder
around just a two inch length at the end of the wire was amazingly large.
Dave AB7E
On 7/19/2025 6:48 PM, Bob McGraw via Elecraft wrote:
In further reading this series of postings, two things come to my
attention.
The G5RV incorporates a balun at the point where the coax connects to
the balanced line. That balun may be faulty as discussed about baluns
heating and the core material changing values. Hence the first half of
the FT-8 QSO was good but the ATU faulted in the latter part.
My measurements on balanced window line, such as used with the G5RV,
indicates the velocity factor changes with moisture on the line.
(Someone stated changed impedance. I don't find this to be correct.)
With a velocity factor change, the electrical length of the line
changes. This section of line is a matching section and thus its
electrical length is very important. Yes if the VF changes, the
electrical length changes, and it will require to ATU to implement a
change in matching component values to resolve the desired SWR.
Candidly, I doubt there is anything wrong with the KAT500 tuners.
73
Bob, K4TAX
P.S. For an excellent dissertation on the G5RV antenna
see:https://www.bvarc.org/pdf/g5rv_w5rh.pdf
Here we learn the length is 102 ft. or 3/2 wavelength at 14 MHz. The
center feed Z is ~90 ohms +/- j0. The matching section is 34 ft of
OPEN WIRE line which is 1/2 *electrical*wavelength to a 1:1 balun
which is fed via *75 ohm*coax to the transmitter.
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