It is possible that the ferrites are heating. If that is the case, you could test it by transmitting a steady carrier for a minute while watching the SWR. If it’s heating, there should be a gradual rise. Twin lead will change characteristics significantly when it gets wet. So will ladder line, although not as much. So if this happens when it rains, that’s the issue. There is also the possibility that there is a badly corroded connection inside the balun which opens when something expands due to heat. I once had this problem with what seemed to me to be a very well-constructed bead balun. The connection to the coax connector was the culprit. It was hard to find because it was intermittent. If you replace the twinlead with ladder line, it will need to be slightly longer because the ladder line has a greater velocity factor than the twinlead. The Vf of twinlead is about 78% and of ladder line 91% so it will be longer by a factor of 1.17.
Victor 4X6GP > On 7 Aug 2019, at 5:00, Mike Lichtman via Elecraft <elecraft@mailman.qth.net> > wrote: > > Every so often at random after I have been on SSB for a while ( 80m with > about 450w and a 1.1 to 1 SWR) my tuner jumps to retune or the amp faults > and switches to standby. I am using a Van Gorden full size (102’) G5RV as > an inverted vee. The 31’ matching section is 300 ohm twin lead and, even > though it has been up for 10yrs, is still flexible. Van Gordon is now out of > business. The twin lead goes into a 3/4” pvc pipe and coax comes out the > other end. > I am thinking that the 12” long pipe section has ferrite beads to cut common > mode currents. I don’t know what wattage the ferrites are rated for. > Someone suggested that the ferrites might be heating up and causing my > problem since the problem occurs after being on the air a while and could be > thermal. > My questions are: > I have some new 450 ohm ladder line. Should I replace the 300 > ohm with it? I know that most G5RV’s on the market today use 450 ohm not > twinlead. If so what kind of line isolator is needed or what balun? Should I > just replace the ferrite beads — and if so with what no. and mix. Can I just > use an ugly balun (coiled coax). All thoughts are appreciated. The antenna > itself has worked fine for me on all bands (80 thru 10) and I have worked all > continents except Antarctica. Thank you in advance and 73 Mike KF6KXG > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to k2vco....@gmail.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
