The Cantenna in question (70 ohms) is no good. As others said, replace
the element, someone cooked it too hard.
Dr. Dave K. is spot on, Bird 43 is not a laboratory instrument. Just a
rough indicator. Certainly useful when trimming antennas to watch
reflected power, or to tune the input of an amplifier, or the output for
maximum. When measuring efficiency, gain, anything where you do need
better than 5%, use directional couplers and attenuators, and bench
power meters like the old HP 436/437/438 series. Other manufacturers
such as Boonton also make decent meters. These can be gotten on epay for
reasonable cost. If you have access to a network analyzer, even the
TenTec kit, or a good gain/loss set, you can check and calibrate your
own couplers and pads at the exact frequency being tested.
Good power metering is obviously needed if you are making commercial
rigs and amplifiers. Calorimetric dummy loads can be fashioned from
standard plumbing, if you can find an accurate flow meter and a pair of
thermometers in wells. But working with only a few deg C rise in water
temperature requires that all systematic errors be removed first. Like
offsets between thermometers, recording this value with no RF applied.
And if the load resistor that is being cooled has flat response down to
DC or 60 Hz, you can apply power at these ranges and measure the applied
power using ammeter and voltmeters, to do a transfer calibration check
with your RF/calorimeter measurement.
I'd guess that the cooling air measurement would be fraught with errors.
We used small temperature sensors in the exhaust of the Broadcast
Electronics cavity amplifiers for FM, that would readout on the screen
and give you a warm feeling about how you were tuning the PA, for best
efficiency or far off of normal. But nothing was accurate enough to
calculate power.
Using calorimetric water loads, meters on the plate DC power, and
directional coupler measurements at work, I can usually get within 5%
on power balance, which is considered good. I trust the couplers and
power meters the most, as I can measure the coupling of a line section
to better than 0.02 dB using a network analyzer. And the power meters
can be checked, send out for calibration. This is acceptable to the gov't.
73
John
K5PRO
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