|[AMRadio] Watt meters|
garyschafer at comcast.net
Sat Feb 16 12:07:45 EST 2008
>There is no such thing a an "rf power" meter, at least when we are >talking
about an instrument that most amateurs could afford. The so->called
"wattmeters" are really rf voltmeters calibrated to the read the >level of
power delivered when the measured voltage is imposed across a >known
resistance. Most rf wattmeters are calibrated to work into a 50 or >72-ohm
load. At any other load impedance, the reading is erroneous.
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. 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.
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