[Amps] Measuring RF Power

[Will Matney]

I've been setting back and watching this and hope some aren't confusing rms and average voltage as the same thing. RMS or (root mean square) is calculated as 0.707 X peak voltage. The average voltage is calculated by 0.637 X peak voltage. This is the way I was taught and is in several of my books here which I already have double checked myself. There is only one power formula for wattage (really two, one for DC and one for AC) but boils down to the same thing. One can be fooled but rms and average are not the same. This can be confused when talking about meter movements and them being designed to read avarage or rms values (some average reading calibrated to read rms).

Quoted from the "Modern Dictionary of Electronics";

"Average power output of an AM transmitter: The RF power delivered to the transmitter output terminals, averaged over a modulation cycle".

"Average value: The value obtained by dividing the sum of a number of quantities by the number of quantities. The average value of a sine wave is 0.637 times the peak value".

"Average voltage: The sum of the instantaneous voltages in a half cycle waveshape, divided by the number of instantaneous voltages. In a sine wave, the average voltage is equal to 0.637 times the peak voltage"

The ac rms value is known as the "effective value" or the same value that would produce the same work as a DC value.;

"Rms voltage: The effective value of a varying or alternating voltage. That value which would produce the same power loss as if a continuous voltage were applied to a pure resistance. In sine wave voltages, the rms voltage is equal to 0.707 times the peak voltage".

"Watt: A unit of the electric power required to do work at the rate of 1 joule per second. It is the power expended when 1 ampere of direct current flows through a resistance of 1 ohm. In an alternating current circuit, the true power in watts is effective volt amperes multiplied by the circuit power factor".

The above explains the use of two wattage formulas. Notice it says the watts in an ac circuit are the "effective" volt amperes which means the rms value, not average.

Hope this helps reveal things through all the smoke.

Will