Measuring milliwatts

One K3 setup adjustment that I have never done is to check the power level of my K3 transverter output, because I don’t have an RF millivoltmeter. I do, however, have an Elecraft DL1 dummy load which has a test point fed by a 1N5711 diode and capacitor. You can take voltage readings there, and then calculate power using the formula:

Power = (( Voltage * 1.414) + 0.15) ^2 / 50

and I normally use this whenever I want an accurate power measurement rather than rely on the readings of analogue meters.

What I don’t know is whether this is accurate enough to measure power at levels as low as 1mW. So I asked the question on the Elecraft email reflector, rashly forgetting that the function of of the reflector is for users to flame each other and speculate on or redesign Elecraft products. Don, W3FPR, who was usually good for an answer to a technical question, has left the reflector after he was flamed for some imagined breach of netiquette. Frankly, the reflector is now worthless. I wish Elecraft would create an announcements-only mailing list so that one could stay informed of new developments and firmware updates without having to see all the ego clashes and endless questions about problems with USB serial adapters.

When I measured, using the DL1 and my DVM, the output from the K3 transverter port with the level set to 1mW, I got a reading of 36mV. Plugging that into the formula, I get 0.8mW. But the 0.15 is, I presume, a “fudge factor” to compensate for the voltage drop in the detector diode, which gives me a result of 0.45mW even if zero voltage was detected. So I’m wondering if my 0.8mW is within the limits of error of my measurement method and that I should leave my transverter drive level as it is. I assembled my K3 myself and the transverter board was added later so I don’t believe the level has ever been set at the factory.

Julian Moss, G4ILO, is a regular contributor to and writes from Cumbria, England. Contact him at [email protected].

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