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  ±5% accuracy) or temperature and
   coolant flow indicator (within  ±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 × Ip × 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.

   (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, 303))

   [ 44 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]