Mark has an excellent point, especially if you have a ferrite core transformer (balun, etc.) somewhere in the antenna system. One thing that can cause that behavior is heating of the toroid core. At a certain temperature, the core's magnetic properties change dramatically, changing the inductance and so causing the SWR to change.
Ron AC7AC -----Original Message----- What happens when you run the KPA-500 into a dummy load? If all is well, I would guess that something is changing under high power and affecting the load. -----Original Message----- From: ai6ii <[email protected]> To: [email protected] Date: Mon, 4 Jun 2012 10:00:54 -0700 (PDT) Subject: [Elecraft] [K3-KPA500] SWR jumps up with 5 second key down I have installed a new antenna and have strange changing swr. When testing with RigExperts meter I find good resonance at say 15m (21.200). With the K3 at 100 watts reading is 1 bar on K3 meter and 1.0 on KPA500 (STBY). When I bring the amp online and drive it at 200 watts, all is still good even with a 15 second key down period. Driving the KPA500 to 275 watts, still okay with key down, although the KPA500 meter now shows 1.1. BUT when I increase the drive a little more (300 watts) the swr jumps up to 2.5+ and the KPA500 shows REFL HI and drops into STBY. And here is the kicker...NOT ALL THE TIME! So I think it has to be in the coax. But the 75feet of RG213 is brand new premade from Cable Xperts with the connectors. I will try a different coax. Any other suggestions? ..mike ______________________________________________________________ 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
