>>> While a true wattmeter is an absorption type of instrument, the
wattmeters
like the Bird and many other thru line meters measure voltage and current to
determine power on the line. <<<
Quite true... thus the market for the Bird TERMALINE units, which combine a
line voltage measurement device with a terminating resistor of known
characteristics.
Combine a Bird 43 with a well designed 50 ohm dummy load and you've essentially
got a TERMALINE instrument... even tho the 43 and similar "slug" wattmeters use
a different principle for measuring line voltage than a TERMALINE.
They will accurately measure at impedances
other than 50 ohms too. You just subtract the amount of reflected power
indicated from the forward power indicated for true forward power. It also
doesn't matter if the load is reactive either as these type wattmeters will
read true within their capability.
Yep, so sayeth the Book of Bird. I use that one all the time because
I ran into an
offer I couldn't refuse on 1200 feet of RG11/U coax! ;o)
Mr. T. W9LBB
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