ORIENTATION IN AM-FM BROADCASTING [AM-FM Broadcasting Equipment, Operations, and Maintenance (1974)]

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This guide is concerned with basic training in AM and fm broadcast engineering, where the following definitions apply: The abbreviation AM stands for "amplitude modulation," in which the transmitted rf carrier is fixed in frequency but is increased or decreased in amplitude (modulated) by the applied sound. The abbreviation fm stands for "frequency modulation," in which the rf carrier remains constant in amplitude, but the frequency is varied (modulated) by the applied sound. In this Section we will first consider specifics of AM transmission, then fm transmission. After the characteristics of each have been explored, we will examine studio and transmitter requirements common to both systems.


The following three definitions describe the foundation for the standard AM station assignments. Standard broadcast station means a broadcasting station licensed to operate on a channel in the frequency band of 535 to 1605 kilohertz (kHz) and to transmit radiotelephone emissions primarily intended to be received by the general public. Standard broadcast band means the band of frequencies extending from 535 to 1605 kHz. Standard broadcast channel means the band of frequencies occupied by the carrier and two sidebands (upper and lower) of a broadcast signal with the carrier frequency at the center. Channels are designated by their assigned carrier frequencies. There are 107 carrier frequencies assigned to standard broadcast stations, beginning at 540 kHz and continuing in successive steps of 10 kHz to 1600 kHz.

When a radio carrier is modulated in amplitude, the original carrier plus upper and lower sidebands exist. The extent of the sidebands depends on the highest frequency that modulates the carrier wave at the specific time.

Thus, if the highest modulating frequency is 5000 Hz (5 kHz) , the limits of the transmission bandwidth are the carrier frequency plus and minus 5 kHz. For example, a carrier of 540 kHz amplitude modulated by a 5-kHz tone would have an upper sideband of 545 kHz and a lower sideband of 535 kHz.

When AM broadcasting was first established, 5 kHz was about the highest audio frequency that could be expected from either microphones or phono pickups. Telephone lines carrying program transmissions between studio and transmitter, and overland routes carrying network radio pro grams, were equalized to 5 kHz, which was entirely adequate to meet the needs of the times. Each AM station was assigned a channel of 10 kHz (plus and minus 5 kHz), as described above.

Fig. 1-1. Channels in standard broadcast band.

For example, Fig. 1-1 shows a station with an assigned carrier frequency of 1000 kHz. By FCC definition, a station on 540 kHz has a lower sideband limit of 535 kHz, and a station on 1600 kHz has an upper sideband limit of 1605 kHz, thus fixing the assigned channel width at ±5 kHz. In Fig. 1-1, the 1000 kHz carrier, when modulated with a 5000 Hz tone, would have sidebands from 995 kHz to 1005 kHz, which would just meet the sideband limits from the adjacent (upper and lower) assigned carriers.

Modern technology allows the gamut of audible frequencies to be picked up, recorded, or reproduced. This covers a frequency range of 30 to 15,000 Hz. Practically all modern AM broadcast transmitters are capable of an audio frequency response within 1 dB from 30 Hz to 12 kHz. This is entirely practicable in spite of the theoretical bandwidth limitation on AM channels. In practice, certain minimum mileage separations must exist be tween stations on the same or adjacent channels. This is considered in greater detail in Section 9.


Table 1-1 presents a general outline of the assignment of stations to channels in the standard broadcast band in the United States. Caution:

Always keep abreast of current FCC Rules and Regulations, which are subject to change. (NOTE: Some stations authorized under earlier allocation standards may not be operating in accord with Table 1-1.) Clear Channels

A clear channel is one on which the dominant station or stations render service over wide areas. These stations are protected from objectionable interference within their primary service areas and over all or a substantial portion of their secondary service areas. Stations operating on these channels are classified as follows.

Class I Stations--A class I station is a dominant station operating on a clear channel and designed to render primary and secondary service over an extended area and at relatively long distances. Its primary service area is free from objectionable interference from other stations on the same and adjacent channels, and its secondary service area is free from interference except from stations on adjacent channels, and from stations on the same channel in accordance with the channel designation in current FCC rulings.

The operating power must not be less than 10 nor more than 50 kW.

Class II Stations--A class II station is a secondary station that operates on a clear channel and is designed to render service over a primary service area, which is limited by and subject to such interference as may be received from class I stations. Whenever necessary, a class II station must use a directional antenna or other means to avoid interference with class I stations and with other class II stations.

Class II stations are divided into three groups as follows: A class II--A station is an unlimited-time class II station operating on one of the clear channels listed in Table 1-1 and assigned to a community within a state specified in current FCC rulings. A class II-A station must operate with power of not less than 10 kW at night nor more than 50 kW at any time.

A class II-B station is an unlimited-time class II station other than those included in class II-A. A class II-B station must operate with power not less than 250 watts nor more than 50 kW. A class II-D station is a class II station operating daytime or limited time. A class II-D station must operate with power not less than 250 watts nor more than 50 kW.

Regional Channels


Table 1-1. Channels for AM Stations; Table 1-1. Channels for AM Stations-cont

'C = Clear, R = Regional, L = Local


1. For Class I stations which will not deliver over 5 microvolts per meter ground wave or 25 microvolts per meter 10 percent time sky wave at any point on the Canadian border, and provided that such stations operating nighttime (i.e., during hours between sunset and sunrise at the location of the Class II station) are located not less than 650 miles from the nearest point on the Canadian border.

2. For Class II stations which operate daytime only, which will not deliver at any point on the Mexican border over 5 microvolts per meter ground wave, and which operate with no more than the following powers:

(A) If not located within the areas specified below, 5 kilowatts.

(B) If operating on any of the following frequencies within the following specified areas, no more than 1 kilowatt:

(1) 800 kHz: less than 1319 kilometers (820 miles) from Ciudad Juarez, Chihuahua.

(2) 1050 kHz: less than 998 kilometers (620 miles) from Monterrey, Nuevo Leon.

(3) 1570 kHz: less than 998 kilometers (620 miles) from Ciudad Acuna, Coahuila.

3. For Class II stations which will not deliver a signal of more than 5 microvolts meter ground wave or 25 microvolts per meter 10 percent sky wave at any point on the Canadian border, nor more than 10 microvolts per meter daytime or 50 microvolts per meter night time at any point on the Mexican border: Provided, that stations operating at night shall be located:

(A) Within the continental United States including Alaska; and (B) Not less than 650 miles from the nearest point on the Canadian border; and (C) North of the parallel 35° N if west of the meridian 93° W, or north of the parallel 30° N if east of said meridian.


A regional channel is one on which several stations may operate with powers not in excess of 5 kW. The primary service area of a station operating on any such channel may be limited to a given field-intensity contour as a consequence of interference.

A class III station is a station that operates on a regional channel and is designed to render service primarily to a principal center of population and the surrounding rural area. Class III stations are subdivided into two classes. A class 3-A station is a class III station that operates with power not less than 1 kW nor more than 5 kW and whose service area is subject to interference in accordance with current FCC rulings. A class III-B station is a class III station that operates with power not less than 500 watts nor more than 1 kW nighttime and 5 kW daytime, and whose service area is subject to interference in accordance with current FCC rules.

Local Channels

A local channel is one on which several low-power stations operate. The primary service area of a station operating on any such channel may be limited to a given field-intensity contour as a consequence of interference.

Such stations operate with power no greater than 250 watts nighttime and 1 kW daytime. The daytime power may be no greater than 250 watts if the station is located 100 kilometers (62 miles) or closer to the Mexican border, or in the area of the state of Florida south of 28° north latitude and between 80° and 82° west longitude.

A class IV station is a station operating on a local channel and designed to render service primarily to a city or town and the immediately surrounding suburban and rural areas. The power of a station of this class cannot be less than 250 watts, and it cannot be more than 250 watts nighttime or 1 kilowatt daytime. Its service area is subject to interference in accordance with current FCC rules. (Stations which were licensed to operate with 100 watts day or night prior to this ruling may continue to do so.)

Special Class II-A Station Rulings

Some of the FCC rulings applicable to class II--A stations are as follows:

No class II-A station shall be authorized unless at least 25 percent of its interference-free service area or at least 25 percent of the population residing therein receives no other interference-free nighttime primary service.

Class II-A stations shall operate with a power of not less than 10 kilowatts nighttime.

The co-channel class I station shall be protected by the Class II-A station to its 0.1 millivolt per meter (mV/m) contour daytime and its 0.5 mV/m 50 percent sky-wave contour nighttime. All other stations of any class authorized on or before October 30, 1961 shall normally receive protection from objectionable interference from class II-A stations as provided in current FCC rules.

A class II-A station shall normally receive daytime protection to its 0.5 mV/m ground-wave contour and nighttime protection to the contour to which it is limited by the co-channel class-I station.


The following terms, as used in this text, are defined as indicated.

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

Antenna 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 sys tem at the operating frequency and at the point at which the antenna cur rent is measured.

Blanketing: That form of interference which is caused by the presence of a broadcast signal of one volt per meter (V/m) or greater intensity 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 con tour is referred to as the blanket area.

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.

Effective field: The root-mean-square (rms) value of the inverse distance fields at a distance of 1 mile from the antenna in all directions in the horizontal plane.

Grid modulation: Modulation produced by introduction of the modulating wave into any of the grid circuits of any tube in which the carrier-frequency wave is present.

High level modulation: Modulation produced in the plate circuit of the last radio stage of the system.

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

Low level modulation: Modulation produced in an earlier stage than the final.

Maximum percentage of modulation: The greatest percentage of modulation that may be obtained in a transmitter without producing in its out put harmonics of the modulating frequency in excess of those permitted by the regulations.

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

Modulated stage: The radio-frequency stage to which the modulator is coupled and in which the continuous wave (carrier wave) is modulated in accordance with the system of modulation and the characteristics of the modulating wave.

Modulator stage: The last amplifier stage of the modulating wave which modulates a radio-frequency stage.

Operating power: The power that is actually supplied to the antenna.

Percentage modulation (amplitude): The ratio, expressed in percentage, of half the difference between the maximum and minimum amplitudes of the amplitude-modulated wave to the average amplitude.

Plate input power: The product of the direct plate voltage applied to the tubes in the last radio stage and the total direct current flowing to the plates of these tubes, measured without modulation.

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

Service areas: The primary service area of a broadcast station is the area in which the ground wave is not subject to objectionable interference or objectionable fading. The secondary service area is the area served by the sky wave and not subject to objectionable interference. The signal is subject to intermittent variations in intensity. The intermittent service area is the area receiving service from the ground wave but beyond the primary service area and subject to some interference and fading.

Table 1-2. FM Broadcast Channels--- *Class A channel.


The frequency 108.0 MHz may be assigned to VOR test stations subject to the condition that interference is not caused to the reception of fm broadcasting stations, present or future.


The fm broadcast band consists of that portion of the radio-frequency spectrum between 88 and 108 megahertz (MHz) . It is divided into 100 channels of 200 kilohertz each. For convenience, the frequencies available for fm broadcasting (including those assigned to noncommercial educational broadcasting) are given numerical designations, which are shown in Table 1-2.

The frequency band 88.1-91.9 MHz (channels 201-220) is for noncommercial educational fm broadcasting, except in Alaska, where the frequency band 88-100 MHz is allocated exclusively to government radio services and the nongovernment fixed service. However, the frequencies 100.1 107.9 MHz ( channels 261 through 300, inclusive) are available for use by noncommercial educational stations in Alaska.

Except as provided in current and special FCC rulings, the frequencies shown by asterisks in Table 1-2 are designated as class A channels and are assigned for the use, in all zones, of class A stations only. A class A station is designed to render service to a relatively small community, city or town, and the surrounding rural area. (Most communities designated for fm broadcast service by the FCC were assigned class A channels.) A class A station may be authorized to operate with as much as 3 kW effective radiated power (erp) in the horizontal plane, with an antenna height above average terrain up to 300 feet. For improved reception and coverage, class A stations may also radiate a full 3 kW erp in the vertical plane.

Class B fm broadcast stations provide service to principal cities and larger communities. In general, these stations may radiate up to 50 kW erp in the horizontal plane (and also in the vertical plane) , with antenna height up to 500 feet above average terrain.

The most powerful fm broadcast facility permitted by the FCC is the class C station. These stations are authorized to radiate up to 100 kW erp at an antenna height above average terrain up to 2000 feet. This applies to both the horizontal and vertical planes of polarization.

NOTE: For all classes, where the height of the antenna exceeds that specified, the maximum radiated power must be reduced. These conditions are explored more fully in Section 10.

The minimum required erp for a class A fm station is 100 watts. For a class B station, the minimum is 5 kW, and for a class C station it is 25 kW.

No minimum antenna height above average terrain is specified.


As used in this text, the following terms related to fm broadcasting are defined as indicated.

Antenna height above average terrain: The average of the antenna heights above the terrain from 2 to 10 miles from the antenna for the eight directions spaced evenly for each 45° of azimuth, starting with true north.

(In general, a different antenna height will be determined in each direction from the antenna.) The average of these various heights is considered the antenna height above the average terrain. In some cases, fewer than eight directions may be used. Where circular or elliptical polarization is employed, the antenna height above average terrain shall be based upon the height of the antenna which transmits the horizontal component.

Antenna power gain: The square of the ratio of the root-mean-square free-space field strength produced at 1 mile in the horizontal plane, in millivolts per meter for 1 kilowatt antenna input power, to 137.6 mV/m. This ratio should be expressed in decibels (dB) . (If specified for a particular direction, antenna power gain is based on the field strength in that direction only.) Center frequency: (1) The average frequency of the emitted wave when modulated by a sinusoidal signal. (2) The frequency of the emitted wave without modulation.

Cross talk: An undesired signal occurring in one channel and caused by an electrical signal in another channel.

Effective radiated power: The product of the antenna power (transmitter output power less transmission line loss) times (1) the antenna power gain or (2) the antenna field gain squared. Where circular or elliptical polarization is employed, the term effective radiated power is applied separately to the horizontal and vertical components of radiation. For allocation purposes, the authorized effective radiated power is the horizontally polarized component of radiation only.

Field strength: The electric field strength in the horizontal plane.

Fm broadcast band: The band of frequencies extending from 88 to 108 MHz, which includes those assigned to noncommercial educational broad casting.

Fm broadcast channel: A band of frequencies 200 kHz wide and designated by its center frequency. Channels for fm broadcast stations begin at 88.1 MHz and continue in successive steps of 200 kHz to and including 107.9 MHz.

Fm broadcast station: A station employing frequency modulation in the fm broadcast band and licensed primarily for the transmission of radiotelephone emissions intended to be received by the general public.

Fm stereophonic broadcast: The transmission of a stereophonic program by a single fm broadcast station, utilizing the main channel and a stereo phonic subchannel.

Free-space field strength: The field strength that would exist at a point in the absence of waves reflected from the earth or from other reflecting objects.

Frequency modulation: A system of modulation in which the instantaneous radio frequency varies in proportion to the instantaneous amplitude of the modulating signal (amplitude of modulating signal to be measured after pre-emphasis, if used) and the instantaneous radio frequency is independent of the frequency of the modulating signal.

Frequency swing: The instantaneous departure of the frequency of the emitted wave from the center frequency.

Left (or right) signal: The electrical output of a microphone or combi nation of microphones that are placed so as to convey the intensity, time, and location of sounds originating 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 fm stereophonic broadcasts.

Main channel: The band of frequencies from 50 to 15,000 Hz that frequency modulate the main carrier.

Multiplex transmission: The simultaneous transmission of two or more signals within a single channel. Multiplex transmission as applied to fm broadcast stations means the transmission of facsimile or other signals in addition to the regular broadcast signals.

Percentage modulation: The ratio of the actual frequency swing to the frequency swing defined as 100 percent modulation, expressed in percent age. For fm broadcast stations, a frequency swing of ±75 kHz is defined as 100 percent modulation.

Pilot subcarrier: A subcarrier serving as a control signal for use in the reception of fm stereophonic broadcasts. The pilot subcarrier has a frequency of 19 kHz.

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.

Stereophonic subcarrier: A subcarrier having a frequency which is the second harmonic of the pilot subcarrier frequency and which is employed in fm stereophonic broadcasting.

Stereophonic subchannel: The band of frequencies extending from 23 to 53 kHz, and containing the stereophonic subcarrier and its associated side-bands.


Fig. 1-2 illustrates an example of studio and control-room layout that meets the requirements of the "medium-size" station, whether AM, fm, or both. In this example, the transmitter is installed at a location remote from the studio.

Continuous operation is carried out in the main control room. The control console allows operation of all microphones, turntables, and cartridge and reel-to-reel tape machines, and it provides for switching of remote or network signals. It may handle operations of both the AM and fm transmitters. Normally, all rack-mounted tape machines and possibly automation equipment may be controlled from the operating control console. If the transmitter (s) is remotely controlled, control and monitoring equipment is included, as shown at the right of the control console in the main control room.

The production control room normally handles all special recording facilities (cartridge and reel-to-reel tape machines) , and it may also be used in hours of split am-fm programming. Many times, a complete "disc jockey" show may be put together in this location, then played back at the regular program time from the main control room. Or delayed broadcasts from the studio, remote sources, or the network may be recorded here for future playback. A view of the ABC Radio Network production control operating center in Hollywood is shown in Fig. 1-3.

The record and tape library contains all the records (discs) and tapes used by the station. Turntables, tape equipment, and suitable monitoring facilities are installed in this room for the purpose of auditions and, in some cases, editing of tapes and programs.

Fig. 1-2. Example of studio and control-room layout.---- Legend: TT-Turntable M-Microphone, desk or floor-stand AM-Announce microphone

Fig. 1-3. ABC production control, Hollywood, California.

Fig. 1-4. Example of transmitter equipment layout.


An example of typical transmitter equipment layout is shown in Fig. 1-4. When the transmitter is located remotely from the studio, the program is normally carried over telephone company (Telco) land lines, with a separate line to provide direct communication with the studio. In the case of an fm transmitter that is located atop a mountain, a radio-frequency studio-to-transmitter link (stl) is often employed. Program lines to AM transmitters are usually equalized to 8 kHz or 10 kHz. Program lines to fm sites must be equalized to 15 kHz.

The rack equipment includes program amplifiers, power supplies, monitoring equipment, and, if authorized, remote-control equipment.

The transmitter control desk contains switches to control final-stage power adjustment and switches for "going directional" when required. It normally also houses metering equipment for antenna currents, remote indication of modulation percentage, and incoming audio level indicators.

The phasing equipment provides the common-point feed to all towers; it is required only when a directional antenna system must be employed.

When a transmitter is operated by remote control, the following FCC rules must be satisfied:

(1) The equipment at the operating and transmitting positions shall be so installed and protected that it is not accessible to or capable of operation by persons other than those duly authorized by the licensee.

(2) The control circuits from the operating positions to the transmitter shall provide positive on and off control and shall be such that open circuits, short circuits, grounds, or other line faults will not actuate the transmitter, and any fault causing loss of such control will automatically place the transmitter in an inoperative position.

(3) A malfunction of any part of the remote-control equipment and associated line circuits resulting in improper control or inaccurate meter readings shall be cause for the immediate cessation of operation by remote control.

(4) Control and monitoring equipment shall be installed so as to allow the licensed operator at the remote-control point to perform all the functions in a manner required by the Commission's Rules.


Q1-1. Define (A) amplitude modulation, (B) frequency modulation.

Q1-2. Give the frequencies covered by the standard broadcast band.

Q1-3. Give the lowest and highest assigned carrier frequencies in the standard broadcast band.

Q1-4. If an AM transmitter on 890 kHz is modulated with a tone of 10 kHz, what is the (A) lower sideband and (B) upper sideband?

Q1-5. What are the minimum and maximum operating powers allowed by the FCC for (A) class I stations and (B) class IV stations?

Q1-6. Define the fm broadcast band in terms of frequency.

Q1-7. How wide is each fm channel?

Q1-8. When an fm station is authorized for a given maximum power, what else is specified?

Q1-9. What is a "pilot subcarrier" and when is it used?

Q1-10. Give the frequency band of a stereophonic subchannel.

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Updated: Thursday, 2021-09-09 13:48 PST