Power level fluctuation in PGXL output

Adrian · September 24

I am seeing the power level fluctuate on the output of my PGXL. I am not seeing it from the Flex 8600 up to 100W. I see it on the PGXL with any transmission over 200 watts output.

I can observe it on AM, FM, CW (slow dashes @ 5wpm for the test), and FT8. Presumably it is happening on other modes but of course it's harder to see on modes like SSB.

Fluctuation occurs on anything over 200 watts, is around 20-30 watts @ 1kW output.
It happens regardless of antenna, SWR, etc.

I posted a short video clip showing it https://www.youtube.com/shorts/39EectU030A

Curious to know what is going on and how it might be remedied.

73, Adrian VE7NZ

David Decoons, wo2x · September 24

in the video, what mode?

Adrian · September 24

FM. But it is the same on any constant carrier mode, unmodulated, such as AM or steadily modulated such as a tune tone on FT8.

Bill NE1B · September 24

Possibly dirty filter=overheating=software power slump?

Mike-VA3MW · September 24

That is actually normal and would not worry about it.

VE7FM · September 25

Mike - do you think the power level is actually changing or is it a limitation of the sampler resolution?

Mike-VA3MW · September 25

It really depends on the digital meter sampling rate.

To really know 100% you either need to watch it on a O-Scope or even a Bird (analogue) watt meter. If you are concerned with it, feel free to open a support ticket.

Then, send a steady tone (not FT8) through the radio, do not use WSJT.

What you can't tell is it the audio levels that are not a constant voltage or is the rf level?

Adrian · September 30

Mike - re your last question, an unmodulated AM carrier, FM carrier or a slow CW "dah" would presumably suggest it's the RF level that is not constant, not the audio driving it. Thx for the replies.

Mike-VA3MW · September 30

The most reliable way to check this is by sending a steady TUNE carrier directly from the radio and adjusting the carrier level as needed. Internally, this produces a clean, single-frequency carrier that removes outside variables.

This approach is important because if you’re modulating with another source (like WSJT or audio input), there are many things that can introduce variation — for example, sound card levels, ALC action, or software gain adjustments. These can make it look like the RF power is fluctuating, when in reality it’s the audio levels driving it that are changing.

It’s also essential to use a dummy load for the test. Antenna or connector issues — even something as simple as weather-related SWR shifts — can cause small but noticeable changes in RF power.

By starting with a stable TUNE carrier into a dummy load, you can rule out these variables and confirm whether any fluctuation is truly in the RF stage, rather than being caused elsewhere in the chain.

Make sense?

Power level fluctuation in PGXL output

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