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 > <[email protected]> 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 >> <[email protected]> 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. >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> Message delivered to [email protected] > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
