On first hunch, this looks much like a thermal issue, something is heating up and cools off to revert to normal. On to debugging my antenna system. A quick test with a dummy load hooked directly to the transceiver showed no problem with either PA nor power supply.

Rain, work (and skiing, I admit) kept me from looking closer at the outside antenna components; besides, with lighter load cycles e.g. in CW, no repro. But this weekend, there was yet another RTTY contest, CQ WPX RTTY.

Since it seems to be a thermal issue that only reproduces unter load, the antenna analyzer measurments didn’t yield anything other than solidifying my initial gut feel.

Several components were suspects: coax run to antenna base, remote tuner (AH-4), balun, lightning protector or antenna insulators. Since I only send barefoot with 100W at most, any antenna insulator breakdown seemed improbable; also, a defective coax (water, rodents?) would be a more clear and consistent failure.

With my XYL as the remote control in the shack to start/stop RTTY transmission (with a preconfigured text, including proper identification), I sat at the antenna base, observing tuner, balun and protectors. After ~90 secs, the SWR spike showed again. With TX off, the quick touch test showed the balun’s core to be very warm. So looks like the core is oversaturating, heating up and therefore changing its inductance. Appears the prospect’s “200 W” spec isn’t for 100% duty loads like RTTY. I’ll wind my own and sturdier balun and try again to fully proof my failure theory.