In a message dated 22/03/2005 19:58:05 GMT Standard Time, jtml@lanl.gov
writes:
Use a Delta Electronics TCT current transformer to get the current
component. Or use a Pearson current transformer. Use a voltage
divider (like the Jennings glass vacuum capacitor unit) and a meter
with 2 MHz accuracy (such as a good old Hp 400C) to get the voltage
component at the base. These would give you the VA. Use a network
analyzer, impedance meter, antenna analyzer, or the knowledge of 10
ohms in series with 250 pF if that is accurately determined) to get
the phase angle, and use that to correct the calculation for real
watts.
But the question is: what accuracy?
Now this wasn't such a hypothetical case. For Type Approving marine
transmitters, that was the load into which you measured the transmitter.
Remember
it's also several kV of RF, and you have to be very careful with capacitive
currents. The usual method was to use a thermo-ammeter, but when you had a
customer (as we did) who wanted 400 watts indicated and not 390 (!), it got a
bit
fraught.
For a more practical view, the UK law (Merchant Vessels Radio Regulations)
said that if you had an antenna where the horizontal top was more than twice
the vertical, then if the product of the length of the vertical in meters
times
the antenna current in amps exceeded 7.5, you could communicate over 75
miles by day on 2182kHz! If the top was shorter, then the product had to be
12.5.
But as we said earlier, for us, does it matter? And the answer in most
cases is 'probably not'. Just as I don't worry about SWR provided it's a)
below
2:1 (there's the LP filter to think of), and more importantly b) hasn't
changed appreciably since last time I tuned up.
A well calibrated power meter in the milliwatt range would be nice, though,
for confirming the signal generator calibrations. Admittedly, all 6 of them
are within a 2dB window, and they tie up in that respect with the spectrum
analyser calibrator, but still, it would be nice.....
73
Peter G3RZP
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