Section

[Code of Federal Regulations]

[Title 47, Volume 4]

[Revised as of October 1, 2005]

From the U.S. Government Printing Office via GPO Access

[CITE: 47CFR73.51]

[Page 21-22]

TITLE 47--TELECOMMUNICATION

CHAPTER I--FEDERAL COMMUNICATIONS

COMMISSION (CONTINUED)

PART 73_RADIO BROADCAST SERVICES--Table of Contents

Subpart A_AM Broadcast Stations

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.

(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.

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 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

[[Page 22]]

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.

(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, 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]