I would certainly second Peter's findings.
In a previous occupation. I had some involvement with acceptance testing of
UHF solid-state power amplifiers for military ground/air uses.
The amplifier specification called for 1000 W -0,0 + 0,5 dB output
over the full ambient temperature range of -20 to +55 C.
A previous amplifier contractor had tried to "cheat" the customer by presenting
power output readings suitable for his purpose from (you guessed it)
uncalibrated Bird wattmeters,
so the use of anything else than a third-party calibrated calorimetric setup
was a "no-no".
It was possible to calibrate the system to an accuracy of about 1 % of reading
in the whole frequency and temperature range.
Somewhat later, I supervised the site acceptance test procedures of 2,5 kW
Rockwell/Collins
HF amps. There were no reasons to suspect any "cheating", but to be consistent
with the factory test
procedures Bird power attenuators that had been calibrated against attenuation
standards and a network analyzer and a HP435/8481A thermal wattmeter with
traceable calibration were used.
The accuracy of this system was found to be in the range of 2 % of reading in
the 2 - 30 MHz frequency range,
so it was quite easy to verify the promised amplifier ratings of 2500 W - 0,5 +
1,0 dB over the frequency range.
I used the opportunity to check a few 2500H and 5000H Bird "slugs" of unknown
ancestry against this setup, and a maximum
spread of about 10% of full scale was observed.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: <G3rzp@aol.com>
To: <wa6fgi@sbcglobal.net>; <david.kirkby@onetel.net>
Cc: <amps@contesting.com>
Sent: Monday, March 21, 2005 4:57 PM
Subject: Re: [Amps] Bird Element Calibration?
>
> In a message dated 21/03/2005 15:27:42 GMT Standard Time,
> wa6fgi@sbcglobal.net writes:
>
> The readings of power out on bird wattmeter have been as accurate as can be
> obtained.
>
>
> That is a matter of some contention!
>
> Firstly, as someone said, it doesn't really matter that much as far as we're
> concerned. Secondly, 5% is 0.22 of a dB: especially at V/UHF, mismatch
> errors and cable losses can eat all that up, so exactly what is meant by 5%
> needs
> considering. Plus the Bird is rated to 5% of full scale, so the real
> accuracy
> depends where on the meter scale the power happens to be sitting.
>
> I suspect that ratios are fairly accurate, so if you measure 50 watts one
> way and 40 watts the other, the actual SWR is fairly accurate (something I
> did
> today to check the cables going up the tower, with the ends open) and you
> can
> get a fair intimation of the cable loss. But primary or even secondary
> standard, it ain't!
>
> One of these days, I might get me an HP power meter: I believe they are more
> accurate, as they work on a different principle. By the time you've put in
> all the attenuation that needs, I suspect you won't end up with a much better
>
> answer than the Bird.
>
> 73
>
> Peter G3RZP
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>
>
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