On Wed, 25 Feb 2004 12:22:32 -0600, "Steve Nosko"
wrote:

[...] you simply measure the caloric result and ignore
shape altogether.

I always thought that the common method of measuring RF power was pretty
cool! The Thermistor or bolometer. Here you balance a bridge with DC or
low freq AC. It heats the thermistor to the correct resistance. Then, when
you add RF power, the thing heats up more and changes resistance. So, you
remove some DC power to get back to the correct resistance and that amount
is easy to figure. That is how much RF you put in. Cool. I think it is
correct to say that you absolutely cannot measure power *directly*. You
must measure something else which is affected/caused by the power...comment?


Hi Steve,

There are many classes of caloric devices, two of which you identify
that are common within the Metrologist's art, and wholly absent from
amateur activities. So here I must make a slight correction of your
description. Power meters contain two (2) such devices which form the
balanced halves of a bridge. One side is exposed to the RF, the other
side is exposed to the simpler DC or AC power that is known to a high
degree of accuracy. What you describe is the detector implementation
of the same devices (which exhibit non-linearity to perform
detection). They would, in the fashion you describe, offer good
"relative" power indication, but not absolute power (except through
substitution methods). As such, they are fairly common in precision
VSWR instrumentation especially when they are driven by 1KHz modulated
power sources, and in turn drive special AC VTVM's scaled to present
dB and VSWR to very high resolution.

A list of the methods:
The Crystal: 1N21/23/25/26...
The Bolometer (low power caloric)
The Barretter (a Bolometer): Sperry 821, PRD 630A
The Wollaston wire (a Barretter): actually a 0.01A glass fuse
The Carbon filament (a Barretter)
The Thermistor (a Bolometer): Western Electric 28A
The Thermocouple
The Thermopile (lotsa Thermocouples)

73's
Richard Clark, KB7QHC