[AMPS] power meter errors

[John T. M. Lyles]

A few years ago I designed in a diode-based relected power sensor on a
directional coupler, where it was in the intermediate coax driving the
final PA input tuning circuit. It drove a digital meter readout, the
rectified peak voltage. I could not get a null in reflected power beyond
some minimum, in this case, about 20 kW. Forward power was 125 kW. When we
put a Mini circuits LP filter on the signal before the detector diode, the
null became much sharper, could tune the input to get < few kW of
reflected. The harmonics were getting reflected as well, and were showing
up as out of band response in the detector. Also, directional coupler has a
6 dB/octive rising frequency response.

Jon's experience using a power meter, and watching the needle change as he
tunes, could also be partly attributed to the change in harmonic content of
the power amplifier as he tunes and loads. While the harmonics should be
20-40 dbc, this might make some 'wiggle' on the meter pointer. Directional
couplers and diode detectors are notoriouslly broadband and respond to the
energy. If it was after a LPF, then it would not be a problem. Phil's
comment about watching the reflected and forward power, and subtracting
them to get the actual delivered power to the load is correct. That is
probably the biggest source of changing readings.


73
John
K5PRO

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