[Elecraft] Two KAT 500s Intermittent High SWR

[David Gilbert]

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.