The MB-1 quotes its performance compared with a Mini Circuits USB
power sensor, which itself quotes an uncertainty of up to 7% in
power reading. Typically it will be better than that, but 7% is
the only guaranteed figure.
Similarly, LP100A refers to comparison with HP436. From memory,
that has an uncertainty in the order of 3%. N8LP clearly
understands the significance of directivity in the couplers
(something which other high profile manufacturers clearly don't)
but even the excellent 40dB typical which he quotes can give an
error of up to 2% under some circumstances, and the 30dB minimum
an error of up to 6% on top of the basic calibration accuracy.
Basically, there's lots of reasons not to expect better than 5%
accuracy in power measurement, and don't be surprised if you see
closer to 10% variation between meters.
Steve
I have an LP100A wattmeter and like it very much. But how do any of
you know how accurate it really is? The only way to know is with a
calibration standard, and RF power is notoriously difficult to measure
accurately. I've compared the LP100A to the Alpha 4510, and a
MeterBuilder MB-1, and by measuring the RMS RF voltage (with a
calibrated Tek oscilloscope) across a 50 ohm dummy load. There's no
obvious consistency among these wattmeters. The readings vary from one
another with frequency and power level, by as much as five percent. My
guess is that the MB-1 is likely the most accurate, based on the
calibration curves and methodology supplied by the manufacturer. But
that is still just a guess.
73,
Jim w8zr
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