{"data":{"id":"us/47-cfr-73.14","jurisdiction":"us","citation":"47 CFR 73.14","heading":"AM broadcast definitions.","body":"AM broadcast band. The band of frequencies extending from 535 to 1705 kHz.\nAM 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 § 73.21 for the classification of AM broadcast channels).\nAM 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.\nAmplitude 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.\nAmplitude modulator stage. The last amplifier stage of the modulating wave amplitude modulates a radio-frequency stage.\nAntenna current. The radio-frequency current in the antenna with no modulation.\nAntenna input power. The product of the square of the antenna current and the antenna resistance at the point where the current is measured.\nAntenna resistance. The total resistance of the transmitting antenna system at the operating frequency and at the point at which the antenna current is measured.\nAuxiliary 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 § 73.1675).\nBlanketing. 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.\nCarrier-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.\nCombined 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.\nCritical hours. The two hour period immediately following local sunrise and the two hour period immediately preceding local sunset.\nDaytime. The period of time between local sunrise and local sunset.\nEffective 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.\nEquipment performance measurements. The measurements performed to determine the overall performance characteristics of a broadcast transmission system from point of program origination to sampling of signal as radiated. (See § 73.1590)\nExperimental period. the time between 12 midnight local time and local sunrise, used by AM stations for tests, maintenance and experimentation.\nFrequency departure. The amount of variation of a carrier frequency or center frequency from its assigned value.\nIncidental phase modulation. The peak phase deviation (in radians) resulting from the process of amplitude modulation.\nInput 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.\nIntermittent 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.\nLast radio stage. The radio-frequency power amplifier stage which supplies power to the antenna.\nLeft (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.\nLeft (or right) stereophonic channel. The left (or right) signal as electrically reproduced in reception of AM stereophonic broadcasts.\nMain channel. The band of audio frequencies from 50 to 10,000 Hz which amplitude modulates the carrier.\nMaximum 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 § 73.1570)\nMaximum 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.\nModel 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.\nModel 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.\nModulation dependent carrier level (MDCL) control technologies. Transmitter control techniques that vary either the carrier power level or both the carrier and sideband power levels as a function of the modulation level.\nNighttime. The period of time between local sunset and local sunrise.\nNominal 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 § 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.\nPercentage modulation (amplitude)\nIn a positive direction:\nM = MAX−C × 100\n————--—--—\nc\nIn a negative direction:\nM = C−MIN × 100\n————-—--—\nc\nWhere:\nM = Modulation level in percent.\nMAX = Instantaneous maximum level of the modulated radio frequency envelope.\nMIN = Instantaneous minimum level of the modulated radio frequency envelope.\nC = (Carrier) level of radio frequency envelope without modulation.\nPlate 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.\nPrimary service area. Means the service area of a broadcast station in which the groundwave is not subject to objectionable interference or objectionable fading.\nProof 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.\nSecondary 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.\nStereophonic channel. The band of audio frequencies from 50 to 10,000 Hz containing the stereophonic information which modulates the radio frequency carrier.\nStereophonic 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.\nStereophonic pilot tone. An audio tone of fixed or variable frequency modulating the carrier during the transmission of stereophonic programs.\nStereophonic 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.\nSunrise 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 § 73.1209).\nWhite 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.","path":["Title 47—Telecommunication","CHAPTER I—FEDERAL COMMUNICATIONS COMMISSION","SUBCHAPTER C—BROADCAST RADIO SERVICES","PART 73—RADIO BROADCAST SERVICES","Subpart A—AM Broadcast Stations"],"source_url":"https://www.ecfr.gov/api/versioner/v1/full/2026-08-25/title-47.xml","current_through":"2026-08-25","vintage":"","retrieved_at":"2026-08-27T02:26:28Z","sha256":"91bcd4f1cea50b6e9b834722849481c72eedf347c2feb187e7da0917fefd22ff","source_id":"us-cfr","stale":true,"prev":"us/47-cfr-73.1","next":"us/47-cfr-73.21"},"notice":"GroundRules: Original legal text. Not legal advice."}