Path mid-point latitude in degrees 50 00120016002000240028003200360040004400 Distance( Kilometers) FIGURE 69.10 Skywave propagation for standard AM broadcasting.( Source: FCC Rules and Regulations, 1982, vol. Ill, pt73.190,fg.2) where E is the electric field intensity, V/m, and r is the distance, m For a distance of 1 km(1000 m), the ERP to the square root of the power, field intensities may be determined at other powers intensity is proportiona quired to produce a field intensity of 100 mv/m is found to be 333. 3 w. Since the field intensity is proportional Skywave propagation necessarily involves some fading and less predictable field intensities and is most appropriately described in terms of statistics or the percentage of time a particular field strength level is found Figure 69.10 shows skywave propagation for a 100-mV/m field strength at a distance of 1 km for midpoint path latitudes of 35 to 50 degrees Transmitters Standards that cover AM broadcast transmitters are given in the Electronic Industry Association(ElA) Standard TR-101A. Electrical Performance Standard for Standard Broadcast Transmitters Parameters and methods for measurement include the following: carrier output rating, carrier power output capability, carrier frequency range, carrier frequency stability, carrier shift, carrier noise level, magnitude of radio frequency(RF)harmonics, ormal load, transmitter output circuit adjustment facilities, RF and audio interface definitions, modulation apability, audio input level for 100% modulation, audio frequency response, audio frequency harmonic distortion, rated power supply, power supply variation, operating temperature characteristics, and power input. Standard AM broadcast transmitters range in power output from 5 W up to 50 kW units. While solid-state devices are used for many models(especially the lower-powered units), several manufacturers still retain tubes in the final amplifiers of their high-powered models. This is changing, however, with the introduction in recent years of 50-kw fully transistorized models. a block diagram of a typical 1-kW solid-state transmitter is shown in Fig. 69.11 Antenna Systems The antenna system for a standard AM broadcast station typically consists of a quarter-wave vertical tower,a ground system of 120 or more quarter-wave radials buried a few inches underground, and an antenna tuning c 2000 by CRC Press LLC
© 2000 by CRC Press LLC where E is the electric field intensity, V/m, and r is the distance, m. For a distance of 1 km (1000 m), the ERP required to produce a field intensity of 100 mV/m is found to be 333.3 W. Since the field intensity is proportional to the square root of the power, field intensities may be determined at other powers. Skywave propagation necessarily involves some fading and less predictable field intensities and is most appropriately described in terms of statistics or the percentage of time a particular field strength level is found. Figure 69.10 shows skywave propagation for a 100-mV/m field strength at a distance of 1 km for midpoint path latitudes of 35 to 50 degrees. Transmitters Standards that cover AM broadcast transmitters are given in the Electronic Industry Association (EIA) Standard TR-101A, “Electrical Performance Standard for Standard Broadcast Transmitters.” Parameters and methods for measurement include the following: carrier output rating, carrier power output capability, carrier frequency range, carrier frequency stability, carrier shift, carrier noise level, magnitude of radio frequency (RF)harmonics, normal load, transmitter output circuit adjustment facilities, RF and audio interface definitions, modulation capability, audio input level for 100% modulation, audio frequency response, audio frequency harmonic distortion, rated power supply, power supply variation, operating temperature characteristics, and power input. Standard AM broadcast transmitters range in power output from 5 W up to 50 kW units. While solid-state devices are used for many models (especially the lower-powered units), several manufacturers still retain tubes in the final amplifiers of their high-powered models. This is changing, however, with the introduction in recent years of 50-kW fully transistorized models. A block diagram of a typical 1-kW solid-state transmitter is shown in Fig. 69.11. Antenna Systems The antenna system for a standard AM broadcast station typically consists of a quarter-wave vertical tower, a ground system of 120 or more quarter-wave radials buried a few inches underground, and an antenna tuning FIGURE 69.10 Skywave propagation for standard AM broadcasting. (Source: FCC Rules and Regulations, 1982, vol. III, pt. 73.190, fig. 2.)
POWER SUPPLY EXCITER CRCUIT AM-1A BLOCK DIAGRAM FIGURE 69.11 agram of typical 1-kW solid-state AM transmitter. ( Source: Broadcast Electronics Inc, Quincy, Ill. unit to"match" the complex impedance of the antenna system to the characteristic impedance of the transmitter and transmission line so that maximum transfer of power may occur. Typical heights for AM broadcast towers range from 150 to 500 ft. When the radiated signal must be modified to prevent interference to other stations or to provide better service in a particular direction, additional towers may be combined in a phased array to produce the desired field intensity contours. For example, if a station power increase would cause interference with existing stations, a directional array could be designed that would tailor the coverage to protect the existing tations while allowing increases in other directions. The protection requirements can generally be met with arrays consisting of 4 towers or less, but complex arrays have been constructed consisting of 12 or more towers to meet stringent requirements at a particular location. An example of a directional antenna pattern is shown in Fig. 69.12. This pattern provides major coverage to the southwest and restricts radiation(and thus interfer ence) towards the northeast. Frequency Modulation Frequency-modulation( FM) broadcasting refers to the transmission of voice and music received by the general public in the 88-to 108-MHz frequency band FM is used to provide higher-fidelity reception than is available with standard broadcast AM. In 1961 stereophonic broadcasting was introduced with the addition of a double- sideband suppressed carrier for transmission of a left-minus-right difference signal. The left-plus-right sum hannel is sent with use of normal FM. Some FM broadcast systems also include a subsidiary communications authorization(SCA) subcarrier for private commercial uses. FM broadcast is typically limited to line-of-sight ranges. As a result, FM coverage is localized to a range of 75 mi(120 km)depending on the antenna height and ERP Frequency allocations The 100 carrier frequencies for FM broadcast range from 88. 1 to 107.9 MHz and are equally spaced every 200 kHz. The channels from 88. 1 to 91.9 MHz are reserved for educational and noncommercial broadcasting and ose from 92.1 to 107.9 MHz for commercial broadcasting. Each channel has a 200-kHz bandwidth. The maximum frequency swing under normal conditions is +75 kHz. Stations operating with an SCA may under certain conditions exceed this level, but in no event may exceed a frequency swing of +82.5 kHz. The carrier frequency is required to be maintained within +2000 Hz. The frequencies used for FM broadcasting generally limit the coverage to the line-of-sight or a slightly greater distance. The actual coverage area is determined by ne ERP of the station and the height of the transmitting antenna above the average terrain in the area. Either increasing the power or raising the antenna will increase the coverage area Station Classifications In FM broadcast, stations are classified according to their maximum allowable ERP and the transmitting antenna height above average terrain in their service area. Class a stations provide primary service to a radius of about c 2000 by CRC Press LLC
© 2000 by CRC Press LLC unit to “match” the complex impedance of the antenna system to the characteristic impedance of the transmitter and transmission line so that maximum transfer of power may occur. Typical heights for AM broadcast towers range from 150 to 500 ft. When the radiated signal must be modified to prevent interference to other stations or to provide better service in a particular direction, additional towers may be combined in a phased array to produce the desired field intensity contours. For example, if a station power increase would cause interference with existing stations, a directional array could be designed that would tailor the coverage to protect the existing stations while allowing increases in other directions. The protection requirements can generally be met with arrays consisting of 4 towers or less, but complex arrays have been constructed consisting of 12 or more towers to meet stringent requirements at a particular location. An example of a directional antenna pattern is shown in Fig. 69.12. This pattern provides major coverage to the southwest and restricts radiation (and thus interference) towards the northeast. Frequency Modulation Frequency-modulation (FM) broadcasting refers to the transmission of voice and music received by the general public in the 88- to 108-MHz frequency band. FM is used to provide higher-fidelity reception than is available with standard broadcast AM. In 1961 stereophonic broadcasting was introduced with the addition of a doublesideband suppressed carrier for transmission of a left-minus-right difference signal. The left-plus-right sum channel is sent with use of normal FM. Some FM broadcast systems also include a subsidiary communications authorization (SCA) subcarrier for private commercial uses. FM broadcast is typically limited to line-of-sight ranges.As a result, FM coverage is localized to a range of 75 mi (120 km) depending on the antenna height and ERP. Frequency Allocations The 100 carrier frequencies for FM broadcast range from 88.1 to 107.9 MHz and are equally spaced every 200 kHz. The channels from 88.1 to 91.9 MHz are reserved for educational and noncommercial broadcasting and those from 92.1 to 107.9 MHz for commercial broadcasting. Each channel has a 200-kHz bandwidth. The maximum frequency swing under normal conditions is ±75 kHz. Stations operating with an SCA may under certain conditions exceed this level, but in no event may exceed a frequency swing of ±82.5 kHz. The carrier frequency is required to be maintained within ±2000 Hz. The frequencies used for FM broadcasting generally limit the coverage to the line-of-sight or a slightly greater distance. The actual coverage area is determined by the ERP of the station and the height of the transmitting antenna above the average terrain in the area. Either increasing the power or raising the antenna will increase the coverage area. Station Classifications In FM broadcast, stations are classified according to their maximum allowable ERP and the transmitting antenna height above average terrain in their service area. Class A stations provide primary service to a radius of about FIGURE 69.11 Block diagram of typical 1-kW solid-state AM transmitter. (Source: Broadcast Electronics Inc., Quincy, Ill. Reprinted with permission.)