[Amps] Bird accuracy, etc.

[Will Matney]

Peter,

What I've found is there's a lot of meters out there just as good as, if not better than a Bird 43. Bird uses the simple principle of using a strip line (their term thru-line) to measure power. That's also the reason they have all those different slugs for different bands as the stripline method isn't very widebanded. A transformer type is more widebanded and accurate over a wider bandspread than a stripline type. Even they have their band limits and the toroid core material has to be changed when that happens. Some of the old Drake Heathkit, and other meters that used the same method I've found just as accurate. The "peak reading" ones without the true reading circuit merely added a small electrolytic capacitor to charge up so the meter had time to read the peak power so to speak. True peak circuits though use op-amps to measure it. Philco had a peak reading meter that used tubes along with a thru-line arrangement like Bird. I have one of those here. Still, if the meter face is off, or the meter itself not calibrated right on along with the percentage allowed, they're not that accurate. A calorimeter as you mention is the closest possible way to do this.

Best,

Will


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On 3/8/06 at 5:04 PM Peter Chadwick wrote:

>Roy said:
>> Accuracy is essentially that of the voltmeter, typically 3 percent.<
>Have you calibrated that?
>For Type Approval testing, many standards call for the measurement of
>power to +/-0.75dB for a measurement certainty of 95%. That's in a
>certifed laboratory, with traceable calibrations, and works out to
>+/-18.9%. Now if it is that easy to measure to 3%, why don't the standards
>call for better?
>For radiated power, a measurement to 95% certainty requires +/-6dB - a
>value detrmined by taking aknown tx and antenna round a string of
>certified labs and measuring. Frightening.
>The most fundamental way of measuring power must be calorimetric. I
>believe that if you have a thermally insulated load and use it to heat
>water, and repeat with DC in the load to get the same temperature rise,
>(with constant ambient) you can probably get within a few percent....
>73
>Peter G3RZP
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