[Elecraft] Two KAT 500s Intermittent High SWR

[James Bennett]

Jack - while living in Folsom (no, not the Big House!) I put up a 102 foot long doublet fed with 450 ohm line. Horrible stuff - fog, rain, dew - they all caused drastic SWR changes and my KPA500 was quite unhappy. I built some home brew 600 ohm line using spreaders I found online. The problem was resolved. Of course, had that location been where we got snowfall enough to stay on the line, things may have been different. 

Now I live in an HOA neighborhood and am lucky to have put up a stealthy EFW fed with hidden coax. I’d love to revert to 600 ohm feeder but that just isn’t possible! :-(

Jim / K7TXA 

> On Jul 19, 2025, at 8:31 PM, Jack Brindle via Elecraft <elecraft at mailman.qth.net> wrote:
> 
> Sorry, but I have to ask this. If indeed this is the case, why does it not apply to open wire, where water has no effect on the line and thus there is no need to retune?
> As I recall from my fields and waves class (also 50 years ago), any change to the dielectric _will_ change the impedance of the line. Since it also plays the role of a matching section (as you note), then the impedance at the ATU will be changed by an about that we may be able to calculate given proper tools like a VNA. More explanation, please...
> 
> I bought 5 spools of open wire about 50 years ago just before I left Newington, and still have four full spools. Unfortunately the plastic spacers have deteriorated while in storage, so
> It isn’t as good as it used to be. I plan to rebuild the line, though. I find that the center plastic spool found in dog poop bags are perfect for this task. It doesn’t make the task of walking the dog a lot more fun, but it helps… <grin>
> 
> 73,
> Jack, W6FB
> 
>> On Jul 19, 2025, at 9:16 PM, David Gilbert via Elecraft <elecraft at mailman.qth.net> wrote:
>> 
>> 
>> 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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