Sec.  73.14   AM broadcast definitions.

   AM broadcast band. The band of frequencies extending from 535 to 1705 kHz.

   AM broadcast channel. The band of frequencies occupied by the carrier and
   the upper and lower sidebands of an AM broadcast signal with the carrier
   frequency at the center. Channels are designated by their assigned carrier
   frequencies. The 117 carrier frequencies assigned to AM broadcast stations
   begin at 540 kHz and progress in 10 kHz steps to 1700 kHz. (See  Sec. 73.21 for
   the classification of AM broadcast channels).

   AM broadcast station. A broadcast station licensed for the dissemination of
   radio communications intended to be received by the public and operated on a
   channel in the AM broadcast band.

   Amplitude modulated stage. The radio-frequency stage to which the modulator
   is coupled and in which the carrier wave is modulated in accordance with the
   system of amplitude modulation and the characteristics of the modulating
   wave.

   Amplitude modulator stage. The last amplifier stage of the modulating wave
   amplitude modulates a radio-frequency stage.

   Antenna  current.  The  radio-frequency current in the antenna with no
   modulation.

   Antenna input power. The product of the square of the antenna current and
   the antenna resistance at the point where the current is measured.

   Antenna resistance. The total resistance of the transmitting antenna system
   at the operating frequency and at the point at which the antenna current is
   measured.

   Auxiliary facility. An auxiliary facility is an AM antenna tower(s) separate
   from the main facility's antenna tower(s), permanently installed at the same
   site or at a different location, from which an AM station may broadcast for
   short  periods without prior Commission authorization or notice to the
   Commission while the main facility is not in operation (e.g., where tower
   work necessitates turning off the main antenna or where lightning has caused
   damage to the main antenna or transmission system) ( See  Sec. 73.1675).

   Blanketing.  The interference which is caused by the presence of an AM
   broadcast signal of one volt per meter (V/m) or greater strengths in the
   area adjacent to the antenna of the transmitting station. The 1 V/m contour
   is referred to as the blanket contour and the area within this contour is
   referred to as the blanket area.

   Carrier-amplitude regulation (Carrier shift). The change in amplitude of the
   carrier  wave in an amplitude-modulated transmitter when modulation is
   applied under conditions of symmetrical modulation.

   Combined audio harmonics. The arithmetical sum of the amplitudes of all the
   separate harmonic components. Root sum square harmonic readings may be
   accepted under conditions prescribed by the FCC.

   Critical hours. The two hour period immediately following local sunrise and
   the two hour period immediately preceding local sunset.

   Daytime. The period of time between local sunrise and local sunset.

   Effective field; Effective field strength. The root-mean-square (RMS) value
   of the inverse distance fields at a distance of 1 kilometer from the antenna
   in all directions in the horizontal plane. The term “field strength” is
   synonymous with the term “field intensity” as contained elsewhere in this
   Part.

   Equipment performance measurements. The measurements performed to determine
   the overall performance characteristics of a broadcast transmission system
   from point of program origination at main studio to sampling of signal as
   radiated. (See  Sec. 73.1590)

   Experimental period. the time between 12 midnight local time and local
   sunrise, used by AM stations for tests, maintenance and experimentation.

   Frequency departure. The amount of variation of a carrier frequency or
   center frequency from its assigned value.

   Incidental phase modulation. The peak phase deviation (in radians) resulting
   from the process of amplitude modulation.

   Input power. Means the product of the direct voltage applied to the last
   radio stage and the total direct current flowing to the last radio stage,
   measured without modulation.

   Intermittent  service  area. Means the area receiving service from the
   groundwave of a broadcast station but beyond the primary service area and
   subject to some interference and fading.

   Last radio stage. The radio-frequency power amplifier stage which supplies
   power to the antenna.

   Left (or right) signal. The electrical output of a microphone or combination
   of microphones placed so as to convey the intensity, time, and location of
   sounds originated predominately to the listener's left (or right) of the
   center of the performing area.

   Left  (or  right)  stereophonic channel. The left (or right) signal as
   electrically reproduced in reception of AM stereophonic broadcasts.

   Main channel. The band of audio frequencies from 50 to 10,000 Hz which
   amplitude modulates the carrier.

   Maximum percentage of modulation. The greatest percentage of modulation that
   may be obtained by a transmitter without producing in its output, harmonics
   of  the  modulating  frequency  in  excess of those permitted by these
   regulations. (See  Sec. 73.1570)

   Maximum rated carrier power. The maximum power at which the transmitter can
   be  operated  satisfactorily  and  is  determined by the design of the
   transmitter and the type and number of vacuum tubes or other amplifier
   devices used in the last radio stage.

   Model I facility. A station operating in the 1605–1705 kHz band featuring
   fulltime operation with stereo, competitive technical quality, 10 kW daytime
   power,  1  kW  nighttime  power,  non-directional antenna (or a simple
   directional  antenna  system),  and separated by 400–800 km from other
   co-channel stations.

   Model II facility. A station operating in the 535–1605 kHz band featuring
   fulltime  operation,  competitive technical quality, wide area daytime
   coverage with nighttime coverage at least 15% of the daytime coverage.

   Nighttime. The period of time between local sunset and local sunrise.

   Nominal  power.  The antenna input power less any power loss through a
   dissipative network and, for directional antennas, without consideration of
   adjustments specified in paragraphs (b)(1) and (b)(2) of  Sec. 73.51 of the
   rules. However, for AM broadcast applications granted or filed before June
   3, 1985, nominal power is specified in a system of classifications which
   include the following values: 50 kW, 25 kW, 10 kW, 5 kW, 2.5 kW, 1 kW, 0.5
   kW, and 0.25 kW. The specified nominal power for any station in this group
   of stations will be retained until action is taken on or after June 3, 1985,
   which involves a change in the technical facilities of the station.

   Percentage modulation (amplitude) 

   In a positive direction:

   M = MAX−C × 100

         ————––—––—

             c

   In a negative direction:

   M = C−MIN × 100

         ————–—––—

             c

   Where:

   M = Modulation level in percent.

   MAX = Instantaneous maximum level of the modulated radio frequency envelope.

   MIN = Instantaneous minimum level of the modulated radio frequency envelope.

   C = (Carrier) level of radio frequency envelope without modulation.

   Plate modulation. The modulation produced by introduction of the modulating
   wave into the plate circuit of any tube in which the carrier frequency wave
   is present.

   Primary service area. Means the service area of a broadcast station in which
   the groundwave is not subject to objectionable interference or objectionable
   fading.

   Proof  of  performance  measurements  or  antenna proof of performance
   measurements. The measurements of field strengths made to determine the
   radiation pattern or characteristics of an AM directional antenna system.

   Secondary service area. Means the service area of a broadcast station served
   by the skywave and not subject to objectionable interference and in which
   the signal is subject to intermittent variations in strength.

   Stereophonic channel. The band of audio frequencies from 50 to 10,000 Hz
   containing the stereophonic information which modulates the radio frequency
   carrier.

   Stereophonic crosstalk. An undesired signal occurring in the main channel
   from  modulation  of the stereophonic channel or that occurring in the
   stereophonic channel from modulation of the main channel.

   Stereophonic  pilot tone. An audio tone of fixed or variable frequency
   modulating the carrier during the transmission of stereophonic programs.

   Stereophonic separation. The ratio of the electrical signal caused in the
   right (or left) stereophonic channel to the electrical signal caused in the
   left (or right) stereophonic channel by the transmission of only a right (or
   left) signal.

   Sunrise and sunset. For each particular location and during any particular
   month, the time of sunrise and sunset as specified in the instrument of
   authorization (See  Sec. 73.1209).

   White area. The area or population which does not receive interference-free
   primary service from an authorized AM station or does not receive a signal
   strength of at least 1 mV/m from an authorized FM station.

   [ 47 FR 8587 , Mar. 1, 1982, as amended at  47 FR 13164 , Mar. 29, 1982;  47 FR 13812 , Apr. 1, 1982;  50 FR 18821 , May 2, 1985;  50 FR 47054 , Nov. 14, 1985;
    56 FR 64856 , Dec. 12, 1991;  62 FR 51058 , Sept. 30, 1997;  66 FR 20755 , Apr.
   25, 2001]