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I've read the PA bias study from Hans and tried to tweak the PA of my U3S for optimum harmonics suppression etc. The spectrum is measured using an SIGLENT SSA3021X analyzer. The power is measured using a 40dB tap and a cheap 20w Dummyload from Diamond: The tap is based on the work of OZ2CPU.
Inspired from some work found on the Internet and the full functionality of my U3S I've build an automatic spot collector. The spot collector pulls every 5 minutes my spots from WSPRnet.org and puts it into a MYSQL database for later analysis. See the schematic for this setup
Measurement of the signal quality of QRP-Labs U3s with low-pass filters.Transmitter output is fed into a homemade 40dB power pad. The spectrum is measured using an SIGLENT SSA3021X analyzer.
The bias voltage was about 2.6VDC and the PA voltage was abt. 7VDC.