§  73.51   Determining operating power.

   (a) Except in those circumstances described in paragraph (d) of this
   section, the operating power shall be determined by the direct method.
   The direct method consists of either:

   (1) using a suitable instrument for determining the antenna's input
   power directly from the RF voltage, RF current, and phase angle; or

   (2) calculating the product of the licensed antenna or common point
   resistance at the operating frequency (see § 73.54), and the square of
   the indicated unmodulated antenna current at that frequency, measured
   at the point where the resistance has been determined.

   (b) The authorized antenna input power for each station shall be equal
   to the nominal power for such station, with the following exceptions:

   (1) For stations with nominal powers of 5 kW, or less, the authorized
   antenna input power to directional antennas shall exceed the nominal
   power by 8 percent.

   (2) For stations with nominal powers in excess of 5 kW, the authorized
   antenna input power to directional antennas shall exceed the nominal
   power by 5.3 percent.

   (3) In specific cases, it may be necessary to limit the radiated field
   to a level below that which would result if normal power were delivered
   to the antenna. In such cases, excess power may be dissipated in the
   antenna feed circuit, the transmitter may be operated with power output
   at a level which is less than the rated carrier power, or a combination
   of the two methods may be used, subject to the conditions given in
   paragraph (c) of this section.

   (i) Where a dissipative network is employed, the authorized antenna
   current and resistance, and the authorized antenna input power shall be
   determined at the input terminals of the dissipative network.

   (ii) Where the authorized antenna input power is less than the nominal
   power, subject to the conditions set forth in paragraph (c) of this
   section, the transmitter may be operated at the reduced power level
   necessary to supply the authorized antenna input power.

   (c) Applications for authority to operate with antenna input power
   which is less than nominal power and/or to employ a dissipative network
   in the antenna system shall be made on FCC Form 302. The technical
   information supplied on section II-A of this form shall be that
   applying to the proposed conditions of operation. In addition, the
   following information shall be furnished, as pertinent:

   (1) Full details of any network employed for the purpose of dissipating
   radio frequency energy otherwise delivered to the antenna (see § 73.54).

   (2) A showing that the transmitter has been type accepted or notified
   for operation at the proposed power output level, or, in lieu thereof:

   (i) A full description of the means by which transmitter output power
   will be reduced.

   (ii) Where the proposed transmitter power output level(s) is less than
   90% of the rated power of the transmitter, equipment performance
   measurements must be made to confirm that the station transmissions
   conform to the emission limitation specified in § 73.44, under all
   conditions of program operation.

   (iii) A showing that, at the proposed power output level, means are
   provided for varying the transmitter output within a tolerance of ±10
   percent, to compensate for variations in line voltage or other factors
   which may affect the power output level.

   (d) When it is not possible or appropriate to use the direct method of
   power determination due to technical reasons, the indirect method of
   determining operating power (see paragraphs (e) and (f) of this
   section) may be used on a temporary basis. A notation must be made in
   the station log indicating the dates of commencement and termination of
   measurement using the indirect method of power determination.

   (e) The antenna input power is determined indirectly by applying an
   appropriate factor to the input power to the last radio-frequency power
   amplifier stage of the transmitter, using the following formula:

   Where:

   Antenna input power = Ep × Ip × F

   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 F applicable to each mode of operation must be
   determined and a record kept thereof with a notation as to its
   derivation. This factor is to be established by one of the methods
   described in paragraph (f) of this section and retained in the station
   records.

   (f) The value of F is to be determined by one of the following
   procedures listed in order of preference:

   (1) If the station had previously been authorized and operating by
   determining the antenna input power by the direct method, the factor F
   is the ratio of the antenna input power (determined by the direct
   method) to the corresponding final radio frequency power amplifier
   input power.

   (2) If a station has not been previously in regular operation with the
   power authorized for the period of indirect power determination, if a
   new transmitter has been installed, or if, for any other reason, the
   determination of the factor F by the method described in paragraph
   (f)(1) of this section is impracticable:

   (i) The factor F as shown in the transmitter manufacturer's test
   report, if such a test report specifies a unique value of F for the
   power level and frequently used; or

   (ii) The value determined by reference to the following table:
   Factor(F) Method of modulation Maximum rated carrier power Class of
   amplifier
   0.70 Plate 1 kW or less
   .80 Plate 2.5 kW and over
   .35 Low level 0.25 kW and over B
   .65 Low level 0.25 kW and over BC^1
   .35 Grid 0.25 kW and over

   ^1All linear amplifier operation where efficiency approaches that of
   class C operation.

   [ 37 FR 7516 , Apr. 15, 1972, as amended at  42 FR 36827 , July 18, 1977;
    42 FR 61863 , Dec. 7, 1977;  44 FR 36036 , June 20, 1979;  47 FR 28387 ,
   June 30, 1982;  48 FR 38477 , Aug. 24, 1983;  48 FR 44805 , Sept. 30, 1983;
    49 FR 3999 , Feb. 1, 1984;  49 FR 4210 , Feb. 3, 1984;  49 FR 49850 , Dec.
   24, 1984;  50 FR 24521 , June 11, 1985;  52 FR 10570 , Apr. 2, 1987;  83 FR 48963 , Sept. 28, 2018]