[Radio Wrangler]

Most will give rated power into VSWRs of about 1.5 and below. Above that you can expect them to start reducing output power. It's not purely a matter of the scalar VSWR, it also depends on the phase angle of the reflected signal at the transmitter.

One transmitter I've designed for the day job has to meet requirements to put >=50% of rated power into a 2:1 VSWR load of any angle for the US market, and >=40% of rated power into a 3:1 VSWR load of any angle for European market . Unfortunately, it's not possible to say that if you pass one requirement, you'll pass the other, so both have to be designed for and tested. For a transmitter with ALC control of power combined with active feedback linearisation, it gets rather involved and the testing involves slide lines extended by lots of different lengths of feeder. Oh, and there are specs for modulation depth, distortion and RF spectrum under all tested load conditions. I've become more familiar than I ever wanted to be with the effects of mis-loads on transmitters!

In amateur radio terms, though, your transmitter will start folding back power into worsening loads. You could fix this with an ATU, but then you get the ATU losses as well as magnified losses of the mismatched feeder run. You could fix it by changing the antenna, but this is also not lossless and may move your direction of best radiation (in 3-dimensions)

These are all interlinked compromises and any general advice might not be valid under your circumstances.

Don't overlook ATU losses. Georg Burt, GM3OXX had worked DXCC (and then some!) with a 1-watt transmitter before he discovered the ATU he was using had 3dB loss on most of the bands he used. So he'd really done it on 500mW. He got interested in low-loss ATUs.

David