Low Power Output for Hendricks BitX20A - Watt meters and Scope readings do not agree.... #bitx20


I solved my stability problem for the Hendricks Bitx20A  kit by applying the Martiens mod.     Now I need to resolve a low power output problem.   I had been just calculating the power output from my peak to peak O-Scope readings with a 10x probe.   However I notice that while I had 70 VPP on the scope with the 10x probe ( ~ 12.2W = (70^2)/400 ); my wattmeters both read a much lower 4 to 6 watts.   My wattmeters are a NorCal QRP Club watt meter and a Heathkit HM-9 QRP Wattmeter.   I am driving the microphone input with a 600 Hz phase shift sine wave oscillator.

Interestingly, when I switch to 1X on the probe my reading drops to 42 VPP which calculates out to 4.4 Watts and is more inline with the external wattmeters.   My scope is a new Siglent 200 MHz dual trace digital unit, with stock 10x/1x probes.

I have been reading up on scope probes and it seems that my 200 MHz probes are good to 200 MHz only in the 10x position.  In 1X the spec says 6 MHz, well below the 14 MHz carrier.    Based on this alone I would tend to accept the 70 VPP reading as accurate and accept that I am getting 12W out.    However given the lower readings from the external wattmeters I am not sure what to believe.

I have a homemade -20 db coupler that when terminated to 50 ohms at the tap and fed to the scope with RG-58 also agrees with the lower power output.

My older 20 MHz analog scope agrees with the lower 42 VPP / 4.4W readings independent of the 10x/1x setting.  (Calibration of this scope is reasonable but not traceable. )

All readings into a 50 ohm dummy load.

What am I missing?

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