§ 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.

   (Secs. 4, 5, 303, 48 Stat., as amended, 1066, as amended, 1068, 1082, as
   amended; 47 U.S.C. 154, 155, 303. Interpret or apply secs. 301, 303, 307, 48
   Stat. 1081, 1082, as amended, 47 U.S.C. 301, 303, 307)

   [ 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]

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