[Amps] History of Grounded Grid Amplfiers

[Paul Christensen]

>"Was it not an FCC requirement to meter the plate current of the final amplifier in series with the high voltage and plate of the final amplifier on broadcast transmitters? So it was a TRUE reading for plate input power?"

Although possible, I don't recall that as a requirement because AM broadcast station power in the U.S. is determined by the so-called "direct method."  The direct method of power is determined: (1) directly from measurements of RF voltage, RF current, and phase; or (2) by calculating the product of the licensed antenna or common point resistance at the operating frequency and the square of the unmodulated antenna current as measured at the point where the resistance has been determined.  The latter method is the type most commonly used.  

In a prior life, I recall calibrating our remote antenna current ammeter by walking out the to the ATU, killing modulation, then breaking the knife switch across antenna RF ammeter long enough to take a reading and match it to the remote meter.

The Indirect method of using the more common power formula of P(out) = Ep x Ip x F is only temporarily allowed when the direct method cannot be used.   The FCC gives some latitude as to the computation used for indirect method measurements.  By contrast, nearly all analog FM broadcast stations measure power by the indirect method where F (percent efficiency) is determined by the transmitter manufacturer during testing. 

Paul, W9AC