Hi Hans, well, perhaps I did not explain my observations well. When doing the SWR scans we transmit a preset portion of RF power into a load/antenna. In my case it is 45%. And the scan stops before the LPF begins to attenuate. Here, on the other hand, when doing the LPF scan we transmit over 1W of RF power thru the LPF into a load but up to the frequencies which are well beyond the cut-off of the LPF. So, most of the scan time we get great VSWR. I get quite a current increase (up to the current limit) when that happens. I feel the 1/4 of full RF power is too much for such a measurement.
I'm not sure it's so bad. Problem is, if you use lower power, the noise floor hits you sooner. So the range of the measurement is less. If you use more power, then as you say, VSWR hits. But there is a difference too - the SWR sweep is done into an antenna. The LPF sweep should be done into a more innocent resistive dummy load. I did check the BS170 suffering when sweeping the LPF, done at 25% power output... it seemed harmless to my judgement... of course, I could make it configurable; so the constructor owns his responsibility. Which is also a danger... but I did try at 100% also and still had no fails?or anything, and some extra dB of vertical range of course. But chose the 25% power (50% voltage) setting as it seemed conservative. But maybe I am still too much of a cowboy, I don't know.?
