"Richard Clark" wrote in message
...
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,

Yea... Then I give only a partial description since trying to give
it completely would take pages, I decided to "overview" it.

However...


... and wholly absent from
amateur activities.

OOPS! "wholly"?? I've got one. A really nice (but un temp
compensated) thermistor mount) (but don't tell anyone that I have nothing
to drive it with -- the rest of the bridge.)

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.

I gotta go back & look, (this was from the 60's) but there are two and
they are in series for the low freq and parallel for the RF. On the low
freq side it is a 200ohm mount. Now I hafta' remember how the RF was
handled .... musta' been to only one...let me think about this...


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).

OOPS again! You DO get absolute because you know how much low freq
power you remove. Isn't that the principle of the HP437 & 438's? or are
they doing something else...diode ... er...uh crystal if you're across the p
ond? I always thought they were, caloric, as you say.

--
Steve N, K,9;d, c. i My email has no u's.


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