The C HA-50HD is rated for 250 W SSB or 75 W for digital modes. It gets a flat SWR with resistive losses in the transformer. That is why the transformer has heat sink fins. A KPA500 is almost certainly overheating the transformer and causing failures. I would back off the power to match the ratings. For SSB with compression, I would run less than 250 W, maybe 150 W.
It is possible that overheating has caused permanent damage to the transformer. One detailed set of tests showed at least 75% of transmitter power being turned into heat in the transformer. Here is what Comet says about that antenna. CHA-250HD (New version of the CHA-250B) The Comet CHA-250HD is the revised CHA-250B. The top section of aluminum has been replaced with a solid whip for greater flexibility and less strain on the lower sections in high winds. A uniquely designed broadband vertical requiring NO GROUND RADIALS. This is EXTREMELY easy to assemble, requires no tuning or adjustments and VSWR is under 1.6:1 continuously from 3.5MHz – 57MHz! It is not a 1/4 wave vertical, but an end-fed long wire type antenna with a matching transformer in a vertical position. Broadband: TX 3.5 – 57MHz RX 2.0 – 90MHz VSWR: 1.6:1 or less Max Power: 250W SSB 75W Digital modes/FT8, etc… CHA-250HD Transformer Section This is a the “magic” behind the CHA-250HD, the transformer matching section. The transformer on the original CHA-250 had smooth sides. The current version has a heat sink to dissipate heat created inside the transformer. Yes, some of the RF that enters the power feeding section is turned into heat rather than transmitted as RF, but that is one of the compromises needed to create a broad-band, low SWR, multi-band HF antenna with minimal visual impact. https://cometantenna.com/amateur-radio/base-antennas/multi-band-hfvhfuhf-2/ wunder K6WRU Walter Underwood CM87wj http://observer.wunderwood.org/ (my blog) > On Jan 12, 2024, at 11:15 PM, Bruce Forsberg <bruce.forsb...@gmail.com> wrote: > > I have a CHA-250B antenna, but it is not currently up. This antenna is only > rated for 250W SSB which is probably 100W on CW or a digital mode. I think > this antenna has a matching network and at higher powers will saturate the > cores causing SWR to rise out of control. Same thing happens for me with an > EFHW antenna on 80M at 500W. This is a limitation with core matching > networks. > > Bruce, WB6IZG > > On Fri, Jan 12, 2024 at 9:29 PM Michael Reynolds, NO6O <mich...@no6o.com> > wrote: > >> Greetings, >> >> I joined this list today, after sending a message to Elecraft support, >> describing a similar problem. I have an Icom IC-7610, with the KPA500 >> and KAT500, and my SWR spontaneously climbs into the red sometimes >> during transmissions. It takes some effort to correct it, and I >> typically lose a QSO in the process. The relays go nuts trying to >> re-tune, and sometimes produces a fault condition. Usually, I can >> correct it by restarting both the amplifier and tuner. I have a Comet >> CHA-250HD, which has a almost a flat 1.1 SWR from 80m to 6m. I don't >> need a tuner while barefoot. However, when I first hooked up the >> KPA500, the SWR was not flat, and added the KAT500 this week, which led >> to my SWR issue. >> >> I hope the cause and resolution can be determined. >> >> Michael >> NO6O >> >> >> > ______________________________________________________________ > 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 wun...@wunderwood.org ______________________________________________________________ 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
