Hi, I thought I understood this but recent discussions left me wondering.
A Bird Wattmeter provides a voltage that is proportional to the forward power
minus the reflected power. This assumes the output impedance is known and
constant, usually 50+-j0. Assume a Bird has a 0-10vdc meter for indication. I
don't know what it is, but for discussion.
Input 100 watts into a 50 ohm load, and you get 70.7 volts, the Bird scales
this to 1 volt, and you get 10% deflection on your wattmeter, or 100 watts.
Input 1000 watts and you get 223.6 volts which the Bird scales to 3.16 volts,
or 316 watts. 316 does not equal 1000, so the scale on the meter has to have
V**2 relationship to indicate 1000 watts. So 1000 watts is 31.6% of full
scale. I thought all Ham wattmeters did this. Of course it is highly
dependent on the Z the wattmeter sees. Wattmeters that actually multiply V and
I are another subject, it is hard to keep them in line without causing some
insertion loss, is it not?
73 Gary N4AST