Joe and Roger are correct about Bird Wattmeter accuracy. It's 5% of full
scale. I've not been following this thread closely, so I apologize if I'm
duplicating what others have said, but I find the only convenient way to
calibrate a wattmeter accurately is to use a good oscilloscope and measure
the p-p RF voltage across a 50 ohm dummy load. A calibrated Tektronix analog
scope has a rated accuracy of about 3%, which translates into a 6% error in
power. I have a scope calibrator and, if I use it as a calibration standard
and I'm careful, I can usually measure an RF voltage to about 2% on my Tek
2465B, which translates into a 4% power error. I doubt if lab calibration
methods using thermocouples, etc., can do better than that. RF power is
really hard to measure accurately.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
W4TV
> Sent: Sunday, May 03, 2015 6:44 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Time for New Power Meter
>
>
> > Rarely does Roger even need correcting, but this is one of those
> > times.
>
> No, Roger is correct. Bird's specification is 5% *of full scale*.
> That means the Bird's accuracy is +/- 125 Watts *anywhere* using a
> 2500 Watt element. While in practice the accuracy may be higher
> at other places on the scale, the Bird specifications allow that
> percentage accuracy may be worse than +/- 5% of the reading below
> full scale.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 2015-05-03 12:40 AM, Bill Turner wrote:
> > ------------ ORIGINAL MESSAGE ------------(may be snipped)
> >
> > On Sat, 02 May 2015 21:39:04 -0400, K8RI wrote:
> >
> >>
> >> The nearest slug I can find to the legal limit is 2500 Watts. 5% is 125
> >> watts, 125 watts is a bit over 8% (8.333%...)+/- 125 Watts, or
> >> 1500-125=1375 to 1500+125= 1625
> >
> > REPLY:
> >
> > Rarely does Roger even need correcting, but this is one of those
> > times.
> >
> > The 125 watt figure is applicable only at full scale, not part scale.
> >
> > Assuming the meter is linear across its full scale, the 5% spec
> > applies to any lower reading. For example, 5% of 1500 watts is 75
> > watts, not 125.
> >
> > Most RF meters are not perfectly linear (especially at low scale
> > readings), but for our purposes we can assume they are close.
> >
> > 73, Bill W6WRT
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> > Amps@contesting.com
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> >
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