In case anyone might be interested, I looked out some data on a couple of the
power meter systems I use here.
The 'better', more modern, one uses HP8481A sensor which appears to be a
bolometer - the sensor is a thermocouple attached to a resistor.
The older one uses HP478A sensors, which uses a technique similar to one
described by G8WRB. Four thermistors are mounted close together. The rf to be
measured heats up one pair and a feedback system ends up applying a similar
amount of dc power to the second to maintain balance. The dc power is
measured and used to indicate the rf power.
Steve
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