Bird Wattmeters

[kd1na]

Working in the electronic calibration field for some years I have found that any mechanical meter movement has it's greatest accuracy at full scale only. the meter movement's accuracy diminishes as you go down in scale. So if a Bird Wattmeter is calibrated for 5% accuracy at full scale then it's accuracy will be reduced as the meter deflection is reduced. That is why for greatest accuracy with a Bird Wattmeter measuring 9 watts (for example) should be using a 10 watt slug for greatest meter deflection.

Digital readouts will always have the accuracy of the measuring circuitry + or - one count of the least segnifigent digit of the instrument or computer readout no matter how much of the selected range scale is used. Having said that you will have the most accuracy when approaching the upper scale range ( example 90 watts on the 100 watt range and not 9 watts on the 100 watt range).

See Below:

73 and Happy New Year
Dave KD1NA

Bird wattmeter's, the spec is 5% OF THE
FULL SCALE READING - and that is ONLY right after calibration.  That
specification says that if a BIRD wattmeter has been recently calibrated
and handled carefully afterwards, with a 100 watt slug, it can have an
error of 5 watts - and that applies to *anywhere* on the scale.

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[AD6XY]

Bird Trulines were designed a long time ago. Yes they are good but not that good. Certainly not a calibration standard. With a pure signal they give good results within their specification.

With this type of meter it is very important to make sure there are no harmonics in the signal as the element can give large errors if there are.  Even harmonics that are many dB below the fundemental can have a significant effect on the readings. This feature is probably behind some of the inflated VHF amplifier power output claims in the past. In extreme cases it is possible to exceed 100% amplifier efficiency according to the meter.

Mike

kd1na wrote

Working in the electronic calibration field for some years I have found that any mechanical meter movement has it's greatest accuracy at full scale only. the meter movement's accuracy diminishes as you go down in scale. So if a Bird Wattmeter is calibrated for 5% accuracy at full scale then it's accuracy will be reduced as the meter deflection is reduced. That is why for greatest accuracy with a Bird Wattmeter measuring 9 watts (for example) should be using a 10 watt slug for greatest meter deflection.

Digital readouts will always have the accuracy of the measuring circuitry + or - one count of the least segnifigent digit of the instrument or computer readout no matter how much of the selected range scale is used. Having said that you will have the most accuracy when approaching the upper scale range ( example 90 watts on the 100 watt range and not 9 watts on the 100 watt range).

See Below:

73 and Happy New Year
Dave KD1NA

Bird wattmeter's, the spec is 5% OF THE
FULL SCALE READING - and that is ONLY right after calibration.  That
specification says that if a BIRD wattmeter has been recently calibrated
and handled carefully afterwards, with a 100 watt slug, it can have an
error of 5 watts - and that applies to *anywhere* on the scale.

______________________________________________________________
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Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[e-mail]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html