It sounds to me like Bill has the answer. As the torodial core heats up it eventually reaches its Curie temperature at which its magnetic properties (and so inductance of the coil wound on it) changes dramatically. If the torodial core doesn't get so hot it cracks, it will return to "normal" when it cools.
Personally, I like air wound coils wherever there may be a wide range of impedances. Air wound coils are bigger, but air does not saturate and dissipate RF as heat. All one needs to worry about with air wound coils is big enough wire to avoid excessive resistive losses at high RF currents and sufficient spacing to avoid arcing or corona discharges when the RF voltage is high, but torodial inductors have the same issues was well. 73, Ron AC7AC -----Original Message----- From: Elecraft [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Nr4c Sent: Saturday, May 13, 2017 10:19 AM To: Terry Brown Cc: elecraft@mailman.qth.net Subject: Re: [Elecraft] Strange SWR change on 20 at What's the antenna impedance before the ATU is activated? You may be heating up the BALUN due to big mis-match. Sent from my iPhone ...nr4c. bill > On May 13, 2017, at 12:07 PM, Terry Brown <n...@comcast.net> wrote: > > I recently started using my horizontal loop that is about a full > wavelength on 80, for 20 meters. I took down my yagi as we are > moving; drat! My Horizontal Loop is at 40 ft. height and fed with > 450 window line into a 4:1 voltage balun with about 15 ft. of coax to rig. > > > > I am running 100 watts through my KXPA100. I tune a frequency in the > CW portion of 20 to about 1.2 :1 and start sending CQ. After about 3 to 4 3x3 > CQ's,, the SWR goes off the charts and power drops to zero. I can retune > the antenna and the process repeats itself. 20 meters is the only > band affected. > > > > I am wondering if I am having RF problems in that part of the 20 meter > bands This only occurs on 20 meters. The Amp is grounded, but the KX3 > is not bonded to the Amp. > > > > I would appreciate any advice. > > > > Thanks and 73's, > > > > Terry de N7TB > > ______________________________________________________________ > 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 > n...@widomaker.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 r...@cobi.biz ______________________________________________________________ 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
