§  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
   § 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]