Sec. 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 Sec. 73.54), and the square 
of the indicated unmodulated antenna current at that frequency, measured 
at the point where the resistance has been determined.

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    (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 Sec. 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 Sec. 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 
plus-minus10 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 x Ip x 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.

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    (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:

                                                                        
------------------------------------------------------------------------
                Method of        Maximum rated carrier      Class of    
Factor(F)       modulation               power              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......  ................
------------------------------------------------------------------------
\1\All 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, )

[37 307 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]