[Elecraft] OT: Bird Wattmeter "meter failures"

[Ron D'Eau Claire]

Bird instruments specs a Bird 43 at plus or minus 5% of full scale. If you have a calibrated 1000 watt slug and meter it will read within 100 watts of the real power. What many people miss is that the possible 100 watt error is constant over the range of the meter, so measuring 500 watts the meter will indicate something between 400 and 600 watts and at 100 watts the reading can be off by a full 100%! 

That's why they make a variety of slugs for various power readings. For anywhere near +/-5% accuracy you need a slug that produces a reading near full scale at the desired power output.  

Preparing radios for an FCC inspection I used a Bird 43 because the FCC field engineer used a Bird 43. All I cared about was whether when he hooked up his meter it read within specifications for the radio. 

A regular thread on the reflector is about how much accuracy is important. It's obviously a matter of opinion. If I voice any opinion it will trigger endless responses with other opinions. 

So I'll stop right here.

73, Ron AC7AC



-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of lmarion
Sent: Saturday, November 21, 2015 8:35 PM
To: Ron D'Eau Claire; elecraft at mailman.qth.net
Subject: Re: [Elecraft] OT: Bird Wattmeter "meter failures"

The Bird wattmeter is a coarse and loose measurement device.
It is the most overrated power meter, especially in the amateur community.
The calibration is only possibly  five percent at a single frequency, usually the center frequency of the slug. Away from that frequency it rapidly goes way off , I have seen as much as 30% in the range of the slug.
It’s a very rugged meter, used to confirm a rough level of RF. It is better at confirming a SWR, as the frequency error is the same in forward as in reverse.
I have seen many new slugs that could not be calibrated to >10%  over a 10Mhz range.
    Accurate power meters have frequency  calibration factors for each power head sensor., in modern ones it is sometimes in a PROM in the sensor head.
I have told this story many, many times in the ham community.
Bird marketing may be the reason for the ridiculous reverence for them in the ham community. You can drop one from the tower and it will probably work as poorly as it ever did,  one of their only strengths .

Leroy   AB7CE , retired NIST calibrations standards technician.