[AMRadio] Watt meters


Thomas Adams quixote2 at ix.netcom.com
Sat Feb 16 12:54:37 EST 2008


> >>> 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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