On Fri, 10 Dec 2010 19:52:55 +0000, Ian Jackson
wrote:

Do a Google on "Grease Spot Photometer" (back to school physics, over 50
years ago!). Very simple to make, and pretty accurate.

Hi Ian,

A very good suggestion which immediately led me to:
http://www.phy6.org/outreach/edu/greaspot.htm

A variation of this that I calibrated in the lab is an Optical
Pyrometer:
http://www.pyrometer.com/Pyro_Optical.html

Comparison measurements can be very accurate iff what you are
comparing to (aka standard) is known to sufficient accuracy. Both
references provide more than enough to inform the reader with the
essential details.

Slightly more elaborate methods of measuring RF power fall into the
Calorimetric method (actual heat). Hewlett Packard made the ultimate
heat transfer standard - the HP 434A power meter DC to 12GHz!

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Fri, 10 Dec 2010 19:52:55 +0000, Ian Jackson
wrote:

Do a Google on "Grease Spot Photometer" (back to school physics, over 50
years ago!). Very simple to make, and pretty accurate.

Hi Ian,

A very good suggestion which immediately led me to:
http://www.phy6.org/outreach/edu/greaspot.htm

A variation of this that I calibrated in the lab is an Optical
Pyrometer:
http://www.pyrometer.com/Pyro_Optical.html

Comparison measurements can be very accurate iff what you are
comparing to (aka standard) is known to sufficient accuracy. Both
references provide more than enough to inform the reader with the
essential details.

Slightly more elaborate methods of measuring RF power fall into the
Calorimetric method (actual heat). Hewlett Packard made the ultimate
heat transfer standard - the HP 434A power meter DC to 12GHz!


I think HP made a number of DC substitution/transfer sorts of power meters
The 434 was unique in using flowing oil and it could directly measure
watts, without needing attenuators (which have their own calibration
issues).

Didn't the 432 used the idea of DC power substituting for RF power to
bring the sum to a fixed temperature? The difference between the 432
and the 434 is that the thing measuring the temperature is also the RF
load in the 432. The 434 just uses the RF as a heater, and relies on
the DC powered heater and RF powered heater being matched.



A NIST Type IV power meter (like those from Arbiter) definitely does DC
power substitution, and uses the HP/Agilent thermistor heads.

Once you get away from "replacement heat" sorts of schemes, you'd be
into the classic calorimeter.. measure the temperature change over time,
and then turn that into energy. You'd calibrate it by putting DC on the
same sensor, essentially measuring the thermal capacity.


The thermocouple heads (8481A, 8482A for instance) work pretty much the
same way as the thermistor mounts.. measuring the heat dissipated by the
RF power coming in. They're not a substitution measurement though..
The actual sensor changes voltage in response to temperature (with a
clever compensation scheme so that overall temperature doesn't affect it)

But these are all basically thermal sensors (as opposed to, say, RF
voltmeters, like a diode detector as in a 8481D or 8484A head) and they
dissipate the RF power being measured.



73's
Richard Clark, KB7QHC

On Fri, 10 Dec 2010 17:09:25 -0800, Jim Lux
wrote:

Didn't the 432 used the idea of DC power substituting for RF power to
bring the sum to a fixed temperature? The difference between the 432
and the 434 is that the thing measuring the temperature is also the RF
load in the 432. The 434 just uses the RF as a heater, and relies on
the DC powered heater and RF powered heater being matched.

Hi Jim,

I worked with a world of different methods of measuring RF power.
Thermistors, Thermocouples (the 8481A, 8482 you identify - refer to HP
Application Note 64-1, "Fundamentals of RF and Microwave Power
Measurements"), and the older technologies of Thermopile, Wollaston
wire, Bolometers (which encompass the same things as those already
mentioned), and Barreters (a variation upon, or exactly the same as
the Wollaston wire - I've seen 10mA fuses used for the same purpose),
diodes certainly (generally for peak power). I have had the occasion
to burn out more than a couple of these.

Those in a bridge configurations (many in fact as the 434 is an
example) are temperature tracking. An excellent description can be
found on page 14 of:
http://www.hpmemory.org/an/pdf/an_64-1a.pdf
for a thermistor bridge that compensates for ambient heat.

"The fundamental premise in using a thermistor for power measurements
is that the RF power absorbed by the thermistor has the same heating
effect on the thermistor as the DC power." This from the HP432A which
has a dual thermistor, dual bridge design.

Pages 18 through 20 describe how heats are separated in a thermocouple
bridge to compensate for ambient. From page 26 is discussion of
diodes.

As for accuracies: "All thermocouple and diode power sensors require a
power reference to absolute power, traceable to the manufacturer or
national standards."

A good remainder of the application note goes into the issues of
accurate determination. Pages 51 and 60 each has a table of all the
various sources of error (rarely considered outside of the Metrology
Lab, but ever present nonetheless). This was standard consideration
and the examples the author offers yields roughly 5% accurate
measurements from the best of instrumentation - an accuracy figure
that I frequently read here as commonly available from the Bird RF
meter (and I have the experience to the matter having calibrated these
meters to know that is a fantasy of the first order).

73's
Richard Clark, KB7QHC