Yes, Dan, that 5% means anywhere on the scale. So for a 2500 watt slug,
it's +/- 125 watts, even if you're reading 100 watts. Your own
measurement bear this out. On the 10 meter frequency, you had slugs
reading both 1450 watts and 1600 watts. These two readings do not bear
out +/-5% at 1500 watts, it's more like +/- 5.2% at 1500 watts, which
would be out of Bird's spec. The "% of full scale" sorts of
specifications arise from the inclusion of a diode-based detection
circuit in the measurement. You'll find this sort of spec in any
instrument that uses a diode in the "over-square-law" region for
measurement. The old VTVMs, for the same reason, had accuracies quoted
as "% of full scale".
I suspect that Bird calibrates these slugs at 2.5kW at some frequency
with a known standard (caloric would be nice, but is definitely not
required at these frequencies), and the actual accuracy at full scale is
far better than 5%, but they choose to not publish the full-power spec
because it might be misinterpreted. I'm actually impressed at how good
the slugs are, and how repeatable, given the relatively low cost. To
obtain a measurement system that can do 1.5kW at ham frequencies using
thermocouple technology, you'd be shelling out about $5000, and add to
it another couple of grand for a cal lab to validate your attenuator's
actual attenuations. At the price of a Bird 43 and slugs, we're getting
a good bargain in ham radio.
Dave W8NF
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