G3rzp@aol.com wrote:
> For real fun, how do you measure the power (with what accuracy?) into
> a load such as marine antenna at 2MHz? The dummy load is typically 10
> ohms in series with 250pF: a DC calorimetric method might do to
> calibrate the resistive part, but the RF and DC resistances are
> probably slightly different.
Peter,
I might have an answer to that one.
In the book "Principles of Microwave Measurements" by GH Bryant, IEE
(1998). It mentions several variants of the Caliormeter for high power
measurements, but interestingly one uses an idea I never thought of and
would address this issue of DC and RF not causing the same heating effect.
The idea of this Calorimeter is that there is a DC heater in addition to
the RF load. DC is applied at the same time as RF power. The total
dissipation in the Calorimeter is kept fixed at all times - the more RF,
the less DC. So if you know 20kW produces a rise of 30 deg C, you
adjust the DC power until there is a 30 deg C rise. If that needs 5 kW
of DC, then the RF power dissipated in the load is 20-5=15kW. Quite a
neat idea - just even more complex to build.
It's interesting to look at what the National Physical Laboratory (NPL)
can do in the UK for a calibration service. For RF power in the range
1-10W in a coaxial system (at 100MHz, 300MHz, 1GHZ ...) they do a
calibration service of 3% at a 95% confidence level.
http://www.npl.co.uk/electromagnetic/rfmgw/rfmwgwpower/mwpower.html
At higher frequencies, at lower powers, they offer services of 0.2% or
so. Now if NPL offer a service of 3%, at fixed frequencies, it seems
hard to believe a field type instrument like the Bird can do 5% of FSD
overing a 15:1 frequency range on the 2-30MHz device. I'm sorry if that
shatters the dreams of hams all over the World as suggested by Paul G4DCV!
Perhaps the person who wants his Bird elements calibrated will phone NPL
and ask why they only manage a little bit more accuracy than a Bird.
I believe NPL's *primary standard* would be a lot better than 3%, but
don't quote it on their web page as a service. They would I am sure use
a Calorimeter for the most accurate measurements, which is not what they
would use for a calibration service they sell.
There's an interesting paper on the NIST web site, which addresses lots
of RF measurements (noise, power, attenuation etc)
http://ts.nist.gov/ts/htdocs/230/233/calibrations/Electromagnetic/pubs/met29.pdf
says:
"Calorimeters form the basis of primary standards of microwave power
measurements and provide the highest quality calibration of other
power-measuring devices. They have the advantage that measurements may
be referred to fundamental physical constants, and that the measurement
technique can be subjected to a thorough analysis and difinative error
evaluation. On the other hand, they are bulky, expensive to construct,
require highly trained personnel, slow, difficult to use, have limited
dynamic range, and after therefore unsuitable for field use outside the
laboratory"
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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