Sec. 73.267  Determining operating power.

    (a) The operating power of each FM station is to be determined by 
either the direct or indirect method.
    (b) Direct method. The direct method of power determination for an 
FM station uses the indications of a calibrated transmission line meter 
(responsive to relative voltage, current, or power) located at the RF 
output terminals of the transmitter. This meter must be calibrated 
whenever there is any indication that the calibration is inaccurate or 
whenever any component of the metering circuit is repaired or replaced. 
The calibration must cover, as a minimum, the range from 90% to 105% of 
authorized power. The meter calibration may be checked by measuring the 
power at the transmitter terminals while either:
    (1) Operating the transmitter into the transmitting antenna, and 
determining actual operating power by the indirect method described in 
Sec. 73.267(c); or
    (2) Operating the transmitter into a load (of substantially zero 
reactance and a resistance equal to the transmission line characteristic 
impedance) and using an electrical device (within &177;5% 
accuracy) or temperature and coolant flow indicator (within 
&177;4% accuracy) to determine the power.
    (3) The calibration must cover, as a minimum, the range from 90% to 
105% of authorized power and the meter must provide clear indications 
which will permit maintaining the operating power within the prescribed 
tolerance or the meter shall be calibrated to read directly in power 
units.
    (c) Indirect method. The operating power is determined by the 
indirect method by applying an appropriate factor to the input power to 
the last radio-frequency power amplifier stage of the transmitter, using 
the following formula:

Transmitter output power=Ep x Ip x F

Where:

Ep=DC input voltage of final radio stage.
Ip=Total DC input current of final radio stage.
F=Efficiency factor.

    (1) If the above formula is not appropriate for the design of the 
transmitter final amplifier, use a formula specified by the transmitter 
manufacturer with other appropriate operating parameters.

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    (2) The value of the efficiency factor, F, established for the 
authorized transmitter output power is to be used for maintaining the 
operating power, even though there may be some variation in F over the 
power operating range of the transmitter.
    (3) The value of F is to be determined and a record kept thereof by 
one of the following procedures listed in order of preference:
    (i) Using the most recent measurement data for calibration of the 
transmission line meter according to the procedures described in 
paragraph (b) of this section or the most recent measurements made by 
the licensee establishing the value of F. In the case of composite 
transmitters or those in which the final amplifier stages have been 
modified pursuant to FCC approval, the licensee must furnish the FCC and 
also retain with the station records the measurement data used as a 
basis for determining the value of F.
    (ii) Using measurement data shown on the transmitter manufacturer's 
test data supplied to the licensee; Provided, That measurements were 
made at the authorized frequency and transmitter output power.
    (iii) Using the transmitter manufacturer's measurement data 
submitted to the FCC for type acceptance and as shown in the instruction 
book supplied to the licensee.

(Secs. 4, 5, 303, 48 Stat., as amended, 1066, 1068, 1082 (47 U.S.C. 154, 
155, ))

[44 303 FR 58731 , Oct. 11, 1979, as amended at  45 FR 28141 , Apr. 28, 1980; 
 48 FR 38479 , Aug. 24, 1983;  49 FR 4210 , Feb. 3, 1984;  49 FR 49851 , Dec. 
24, 1984]