How is the (tr)uSDX adjusted for minimal power output?

[Arnold]

From these two items https://forum.dl2man.de/viewtopic.php?t=674 where a 5 volt supply resulted in a 1 watt output and https://forum.dl2man.de/viewtopic.php?t=720 where a 50 ohm resistor attenuates by 50% the power sent to a 50 ohm antenna, I learned two ways to reduce output, for my interest is in weak signal digital modes at QRPP levels. The meter, seen to be close to the accuracy of a Bird wattmeter in a YouTube video, has a minimum indication of 10mW. Clearly, adding attenuators to get well below 1 watt is one way to go. But what is the lowest voltage that can be used before breakdown and failure overcome the desired reduction in power? And are these two the only practicable ways of getting into the milliwatt range?

[dl6sez]

Hello OM,

it can't be adjusted really! It's a class-E PA and digitally only pushed on and off.
You can only reduce voltage down to the 5V by USB at abt 0,3W to 0,7W there normally.
You could modify the voltage at the output FETs only by changing the circuit for example at L15, but think about you change the matching of the FETs also with reducing Vdd.
This will also result in worse lpf behaviour.

73 de Chris

[Arnold]

Dear Chris

Very many thanks for that. I feared it was so. Looks like I'll be going the 5 volt route with some attenuators at the far end, monitoring output with an Oak Hills Research wattmeter, which goes down to 5mW, although clumsy for portable use.

Does anyone know of a compact digital wattmeter going this low? The QRPGuys one doesn't! Any home brew designs about, if not commercial?

Kind regards
Arnold