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06. Transmission Systems

6.1 Introduction
Transmission is the process of dissemination of broadcast Radio and television. For this, Raw materials in the form of pre-recorded packages such as films, programmes and commercials are delivered to the operational centre and then sequenced together in a pre-defined method and delivered to the listener's and viewers. Normally, the requirements for a broadcaster in the transmission are:
  • A frequency license to broadcast
  • A right to broadcast the material
  • Commercial agreements covering the transmission of advertisements or other promotional material.
  • Published programme guides or programme schedule.

Broadcasters utilize part of the electromagnetic spectrum to transmit their signals. A natural resource, the electromagnetic spectrum, is composed of radio waves at the low-frequency end and cosmic rays at the high-frequency end. Electromagnetic waves carry broadcast transmissions (radio frequency) from station to receiver. It is the function of the transmitter to generate and shape the radio wave to conform to the frequency the station has been assigned by the concerned authority in the country. Electrical current carrying audio and video is sent by a line from the control room to the transmitter. The current then modulates the carrier wave. The antenna radiates the radio frequency. Receivers are designed to pick up transmissions, convert the carrier into sound or picture waves, and distribute them to the frequency tuned.

Depending upon the process of modulation and technology, the AM, FM mode of broadcasting is widely used for Analog Radio. Likewise, the digital versions of radio include different standards in the world. There is no named standard as such for basic AM and FM analog radio transmission. The AM and FM modulation and transmission systems use principles and technologies that are well defined and used throughout the world. The standards, however, specify various parameters, such as channels to be used, and the bandwidth of the transmitted signal.

6.2 AM  Transmission
Standard radio broadcasting refers to the transmission of voice and music received by the general public in the 55-1705 kHz frequency band. The band may differ in different regions identified by ITU such as region 1,2 and 3. Amplitude modulation is used to provide service using low power transmitters for small communities to high powered broadcast stations needed for larger regional areas. Generally, each station is allowed deviation no more than ±20 kHz from the allocated frequency to minimize heterodyning from two or more interfering stations.

In transmissions using amplitude modulation (AM), the program audio signal is used to modulate the amplitude of the carrier wave that will be transmitted by the station. Double-sideband full-carrier amplitude modulation (DSB AM), is used in standard broadcasting for sound transmission. Basically, the amplitude of an analog radio frequency (RF) carrier is controlled by an analog audio frequency (AF) modulating signal. The resulting RF waveform consists of a carrier wave plus two additional signals that are the result of the modulation process which is shown in figure 6.1. One is the upper-sideband signal which is equal in frequency to the carrier plus the audio modulating frequency. THe other sideband is a lower-sideband signal which equals the  carrier frequency minus the audio modulating frequency.

When the amplitude of the program signal is zero, the carrier remains unmodulated. As seen from figure 6.1, the instantaneous amplitude of the program signal increases up to its maximum and then the carrier amplitude varies accordingly. This may reach up to the maximum amount possible which is 100 percent modulation.
Figure 6.1 DSB amplitude modulation and amplitude modulated waveform

The power contained in each audio sideband, created in the process of modulating the carrier 100 percent, is equal to 25 percent of the carrier power. As a result, the overall power transmitted during periods of 100 percent modulation is 150 percent of the unmodulated carrier power. The overall bandwidth is equal to twice the highest audio modulating frequency. Although full fidelity is possible with amplitude modulation, the ITU recommends standard broadcast stations to limit the fidelity and, hence, restricts occupied bandwidth of the transmitted signal. Typical modulation frequencies for voice and music range from 50Hz to 10 kHz.

The filed strength produced by a standard broadcast station is a key factor in determining the primary and secondary service areas and interference limitations of possible future radio stations in the area. With thousands of AM stations licensed for operation by the international agreement, interference is a factor that significantly limits the service stations can provide. In the absence of interference, a daytime signal strength of 2 mV/m is required for reception in populated towns and cities, whereas a signal of 0.5mV/m is generally acceptable in rural areas without large amounts of man-made interference present. If the transmitted power, antenna radiation characteristics, and ground conductivity are known, the extent of coverage in a given direction for a particular station can be calculated with a high degree of accuracy. These calculations from the basis of the international station allocation system.

One of the major factors in the determination of field strength is the propogation characteristics. It describes the change in electric field intensity and orientation of the wave with an increase in distance from the broadcast station antenna. This variation depends on a number of factors including frequency, distance, surface dielectric constant, polarization, local topography, and time of day. Generally, surface-wave propagation occurs over shorter ranges both during day and night periods. Sky wave propagation in the AM broadcast band permits longer ranges and occurs during night periods. So, some stations must either reduce power or stop operation at night to avoid causing interference. Sky wave propagation is much less predictable than surface-wave propagation. The DSB emission is classified as A3E. The emission mask is the limit palced on the signal strength of the broadcast signal, and it is defined over a range of frequencies surrounding the carrier frequency. As the frequencies become farther away from the carrier, a broadcast station's signal strength at specific frequencies must decrease. For example, the -25dB figure is the limit that FCC in United States defines how much the reduction must be. The AM and FM mask level is depicted in figure 6.2.
Figure: 6.2 AM and FM mask
6.3 FM Transmission
In transmissions using frequency modulation (FM) the program audio signal is used to modulate the frequency of the carrier wave that will be transmitted by the station. When the amplitude of the program signal is zero, the carrier remains unmodulated. The greater the amplitude of the modulating frequency, the greater is the frequency deviation from the center carrier frequency. In FM modulation, multiple pairs of sidebands are produced. The actual number of sidebands that make up the modulated wave is determined by the modulation index (MI) of the system. The modulation index is a function of the frequency deviation of the system and frequency of the modulating signal. In amplitude modulation, the percentage of modulation is directly proportional to the carrier power. But, the percentage of modulation in FM is generally referenced to the maximum allowable occupied bandwidth set by regulation. Most commonly, FM broadcast stations are required to restrict frequency deviation to ±75 kHz from the main carrier. This is referred as 100 percent modulation for FM broadcast stations.

The FM broadcasting was developed to allow sound transmission of voice and music for reception by the general public. The audio frequencies from 50 to 15,000 Hz were contained within a ±75-kHz RF bandwidth. Pre-emphasis is employed in an FM broadcast transmitter to improve the received signal-to noise radio. There is significant signal-to-noise improvement at the receiver equipped with a matching de-emphasis circuit. The bandwidth allocated to FM channels is much wider than AM, and this allows the bandwidth of the audio signal that can be transmitted to extend to about 15 kHz. The wider frequency response combined with good signal-to-noise ratio and low interference makes FM capable of high-quality audio.

Stereophonic transmission is produced by adding the left-and right-channel stereo information together in the baseband signal. In addition, a left-minus-right channel is added and frequency multiplexed on a subcarrier of 38 kHz using double sideband suppressed carrier (DSSC) modulation. An unmodulated 19-kHz subcarrier is derived from the 38-kHz subcarrier to provide a synchronous demodulation reference for the stereophonic receiver. In the receiver, a synchronous detector at 38 kHz recovers the left-minus-right channel information. It is then combined with the left-plus-right channel information in sum and difference combiners to produce the original left-channel and right channel signals. In stereo broadcast systems, a composite FM signal is applied to the FM modulator as shown in figure 6.3.
Figure 6.3 Composite FM stereo signal
Modern FM systems utilize direct modulation. That is, the frequency modulation occurs in a modulated oscillator that operates on a center frequency equal to the desired transmitter output frequency. The direct-FM modulator is one element of an FM exciter which generates the composite FM waveform. The FM broadcasting coverage is, normally, limited by line-of-sight distances. As a result, FM coverage is limited to a maximum receivable range of about hundred kilometers depending on the antenna height above average terrain (HAAT) and effective radiated power (ERP). The actual coverage area for a given station can be reliably predicted after the power and the antenna height are known. Generally, either increasing the power or raising the antenna will increase the coverage area. Stations may not exceed the maximum power specified by the country authority, even if the antenna height is reduced. The classification of the station determines the allowable distance to other co-channel and adjacent channel stations.

6.4 Stereo Coding
Two-channel stereo sound, consisting of left and right program channels, is used almost universally at analog FM radio and TV broadcast stations. There are two program channels in FM radio techniques. A main program channel is transmitted that combines both the left and right audio signals together and it can be used by a monophonic receiver too. Similarly, a stereo program channel is transmitted that can be coupled with the main program channel to produce left and right program material at a stereo receiver. The technique is portrayed in figure 6.4

Figure 6.4 Stereo Coding and Decoding Process

At a stereo receiver, the sum and difference signals are added together, and subtracted from each other. This produces the original individual left and right channel signals which can be played over stereo loudspeakers or headphones. The stereo coding process is carried out using a stereo generator.

6.5 Transmitter Systems
The location of the transmitter site must be established at an early stage as site costs and planning considerations can vary considerably from site to site. Some concept of the structure necessary to support the proposed antenna loading must be established prior to visiting the proposed site. A detailed topographical survey of the site should be undertaken either to check values or update the existing site plan or to produce a new one. The best location for the transmitting antenna for a radio or television station is often different from the preferred location for the studio facilities for different reason such as:

  • antenna coverage,
  • zoning,
  • convenience of access and  
  • space requirements,

The transmitter and antenna will usually be located at a separate site with the program feed provided by a studio-transmitter link. On occasions, it may be possible for the transmitter equipment and antenna to share the same site as the studios. In this case the studios and transmitter facilities are said to be co-sited.

Generally, most transmitter sites will have at least the following categories of equipment:

  • Studio-transmitter link (STL)
  • Processing equipment to prepare the signal for transmission
  • Transmitter exciter to create the low-power version of the RF signal to be transmitted
  • Transmitter power amplifier(s) to produce the high-powered signal for broadcast
  • Transmission line to carry the signal from the transmitter to the Antenna
  • Transmitting antenna
  • Tower to support the antenna

All signals, whether they are audio or video, analog or digital, arrive at the transmitter site via the studio-transmitter link (STL). Both analog and digital STLs are still used for radio and television stations. But for DTV, the STL has to be digital. Audio signals out of the STL are typically stereo left and right channels for FM and TV stations, or there may be a mono feed for an analog AM station.
Figure 6.5 Block Diagram of transmitter site facility

6.5.1 Audio Processing
The audio processor needed to sweeten the sound of the station may be located at the studio or at the transmitter site. Both for analog FM radio and analog TV stations, the stereo generator is used to create the composite stereo signal. The stereo generator has two main inputs i.e. left and right channel audio. If t the video signal arrives at the site on an analog STL, it will be in a composite video from ready to feed direct to the video exciter. If it arrives as component digital video, it needs first to be converted to analog composite video. The DTV compressed bitsream sent to the transmitter contains the complete and final video and audio program signals.

6.5.2 Exciter
The exciter is the device that converts the baseband audio, video or digital bitstream baseband signal to a radio frequency signal with the appropriate method of modulation. The output of the exciter is at a low level and has to be amplified further to produce the high power needed to feed the antenna. In analog television, there are two exciters: one for video and one for audio. The video exciter takes the incoming composite video signal and amplitude modulates it onto the station's video carrier. The audio exciter takes the incoming audio signal and frequency modulation it onto the station's aural carrier. In digital television, a single exciter takes the digital ATSC data stream and modulates it onto the station's carrier. For AM radio, the exciter is only used to amplify the signal to a level enough to drive the final amplifier.

6.5.3 Power Amplifier
The second main part of a broadcast transmitter is the power amplifier. This takes the modulated input from the exciter and amplifies it to the high-power radio frequency signal needed to drive the antenna. It is the main section, in AM radio, to modulate the carrier. Transmitter power output may vary from as small as a few hundred watts to many tens or even hundreds of kilowatts. The most common amplifying devices are solid state devices, tetrodes, klystrons, and inductive output tubes (IOTs). Solid-state power amplifiers use the same principles as transistors and integrated circuits and are very reliable. Some of the high-power amplification components can still fail and require replacement at times.

The earliest radio frequency tubes were triodes and multi-grid tubes. At low freqencies, the principle of operation of these tubes is quite simple. The RF signal is applied to the grid facing the cathode, which is a source of electrons sensitive to the applied electric field, connected to the negative pole of a power supply. As the field changes, the current emitted by the cathode changes in proportion. The electron flow, crossing the grid, reaches an electrode connected via the impedance of the useful load to the positive pole of the power supply. This impedance thus conducts the electronic current and develops an RF voltage at its terminals. This voltage is usually much larger than the one applied on the grid, so that the tube presents a large gain (15-20dB). However, as the frequency of operation is increased, at frequencies of the order of 100MHz, the behaviour of the amplifier begins to deteriorate.

6.5.4 Cooling System
The tube or solid-state components in a power amplifier get very hot during operation and must be cooled. This is done either with cold air being blown through the transmitter or with a liquid cooling system. Several types of cooling may be used depending on the power to be evacuated and the environmental conditions of tube use.

Radiation cooling is the simplest. The anode radiates waste heat. As the temperature may reach several hundred degrees, this types of cooling may not be used when the anode is under vaccum. The anode will be made of a refractory material such as graphite, nickel, tantalium or molybdenum, whose surface will be blackened to increase the radiation efficiency.

Conduction cooling can be used for low power dissipation by strong mechanical contact between the anode and and external heat sink. It electrical insulation is required, the thermal conductor to the heat sink may be made of berylium oxide. Forced air cooling can be used for power levels up to above 30kW. In order to improve the thermal exchange with the moving air, cooling fans are welded to the anode to increase the surface area. This method becomes more troublesome at higher power levels as the air flow rates become large. Fans for such air flow rates are noisy, bulky, power hungry and vibrating.

Water cooling allows the dissipation of higher power levels by immersion of the anode in water. As the water heats up, it is replaced by an incoming cool water flow, and the heated water is pumped to a thermal exchanger to be cooled and reintroduced into the circuit. To avoid the formation of thermally insulating deposits on the anode, the water must be distilled. In order to provide electrical insulation of the anode, all water connections in the vicinity of the tube are of insulating materials.

6.5.5 Transmission Line
The transmission line that connects the  transmitter or diplexer to the antenna is usually a length of coaxial feeder or waveguide. A type called open wire feeder is occasionally used mostly for shortwave stations. All VHF and UHF antennas are constructed so that they can be fed by coaxial feeders. Feeders vary in size from about 170mm diameter main feeders to 15mm diameter feeders. Small size feeders may be used to feed the elements of a low-power antenna. The 170mm diameter feeders are needed to carry large amounts of energy over large distances with a minimum of attenuation. Most antennas and transmitters are standardized to an impedance of 50 ohms.

The mechanical and electrical properties of some of the semi flexible coaxial feeders are available in the manufacturer's catalogues. The power rating and attenuation of the feeders are inversely proportional to square root of frequency and directly proportional to square root of frequency respectively. The power rating also depends on the ambient temperature and standing wave ratio. Any antenna which forms the topmost part of a mast should be equipped with 1m lighting protection spikes electrically bonded to the mast structure. In addition, the outer conductors of all coaxial feeders inside a mast should be electrically bonded to the mast at antenna and where they leave the mast at ground level.

6.5.6 Dummy Loads and Diplexers
The large switch or connection panel allows the transmitter signal to be fed to a dummy load at the transmitter building. The dummy load accepts the RF power and turns it into heat. It may be used for testing the transmitter without transmitting over the air. A diplexer allows two signals from the transmitter to be combined together that come from two different transmission lines. Generally, diplexer are used for combining the output of transmitters operating on the same carrier frequency.

Likewise, channel combiners are used for combining the signals of several channels into a single antenna. They are required at ground level when more than one channel is fed into a single antenna. The input ports for the several frequencies must be isolated from each other by at least 30dB to keep intermodulation to an acceptable level. The input voltage reflection coefficient must be maintained at a low level by means of suitably connected absorber load. This load will help to ensure the stability of transmitters by providing a sink for any spurious frequencies and intermodulation products.

6.5.7 Antenna
Antenna differs significantly from one type of broadcast service to the next i.e. AM, FM or TV. The antenna system comprises all the equipment that is necessary to carry radio frequency energy from the transmitter and to propagate it into space. Transmitters may consist of pairs of amplifiers with common drives. In the event of failure of an amplifier, the service can continue to run with a reduction in effective radiated power. There is a need to diplex the outputs of two amplifiers rather than feed them directly to separate half-antennas. Regardless of its basic structure, any antenna design must focus on three key electrical considerations:

  • Adequate power handling capability
  • Adequate signal stregngth over the coverage area
  • Distortion-free emissions characteristics.

In order to operate efficiently, the length of any antenna must be related to the wavelength of the transmitted signal. The wavelength for AM transmissions is several hundred feet, depending on the frequency. Therefore, an AM radio station's transmitting antenna is usually simply the station's  tower. The actual metal structure of the tower is hot. Vertical polarization of the transmitted signal is used for AM broadcast stations because of its superior ground wave propagation, and because of the relatively simple antenna designs. AM signals propagate further at nighttime than during the day. The different day/night operating powers are designed to provide each AM station with a specified coverage area that is free from interference. A single tower will have a non-directional horizontal radiation pattern transmitting the same amount of energy in all directions. Sometimes, AM radio towers are used together as part of directional antenna system called an array.

The purpose of the directional antenna system is to direct the transmitted energy towards the are of license and to reduce the energy travelling elsewhere. AM radio waves travelling across the surface of the earth depend on ground conductivity. They need an excellent connection to the ground at the transmitter site to give the signal a conductive path as it leaves the transmitter. This is achieved with a series of ground radials, which are copper wires buried in the ground, extending outward from the base of the antenna for several hundred feet in a certain pattern.

Wavelengths in the FM and TV wavebands range from just over a foot for the highest UHF TV frequencies to about 10 feet for FM radio. The antenna tower is located on the highest terrain in the area often a hilltop. There is essentially no difference between day and night FM propagation. FM stations have relatively uniform day and night service areas with the same operating power. A wide variety of antenna is available for use in the FM broadcast band. Nearly all employ circular polarization. This is because the short wavelength of FM and TV band signals tend to travel in straight lines and do not flow around obstacles. The antennas are sometimes made up of multiple elements which affect the directional pattern.

The antenna elements are mounted on their supporting structure with clamps. The antenna is fed with the signal from the transmitter by the transmission line that extends from the transmitter up the tower to the antenna. Multiple bay antennas are often used in FM radio and TV transmission because they make the transmission system more efficient by changing the vertical radiation pattern to focus the beam and reduce the amount of energy sent up into the sky. If the impedance of an antenna and its distribution network at the top of the mast is not sufficiently well matched to the characteristics impedance of the main feeder, a fraction of the signal applied to the antenna will be reflected back to the transmitter. In general, the output stage of the transmitter will not absorb this signal, and a large percentage of it will be returned to the antenna. Then it will be transmitted, but delayed in time and attenuated by two traversals through the main feeder and equipment at ground level. The reflected signal may then be seen by viewers as a delayed image or ghost in case of TV.

Directional couplers may be used at various points in an antenna system to monitor the forward and reverse flows of power. They are particularly useful if situated at the upper or lower ends of the main feeders, where a change in the ratio of reverse to forward power may indicate a fault in the antenna. For this purpose, the directivity of the reverse coupler needs to be about 40dB. A directional coupler splits power equally between its direct output and its coupled line. There are other forms of coaxial hybrid where the power is split equally, but is either co-phased or 180° out of phase, depending on which input port is used. Hybrids are used to diplex the power of two transmitters, to split power equally between two loads, to provide quadrature phase feeds in phase rotating systems, or to form parts of channel combiners.

6.5.8 Towers
They are of two main types such as self-supporting towers and guyed towers which are held up with guy wires. The tower is used to support the antenna, either fastened to the top of the tower or to the side of the tower. An antenna nd high-quality receiver is installed at a suitable high location that can receive the main station. The output of the receiver is converted directly to a new RF frequency and retransmitted with a directional antenna toward the area where it is needed. In some cases, the fill-in transmitter or a repeater operates on the same frequency as the main transmitter. The structure may be a simple wooden pole or a tall guyed mast, but the principles in selection will remain the same. They are that the structure shall:
  • be strong enough to withstand the maximum design wind speed with the specified antenna loading;
  • be stiff enough to limit the deflection of each antenna;
  • be safe to be climbed by those staff trained to do so;
  • be constructed within the budget and time scales allocated;
  • be maintainable for its intended life-span;
  • not impose unacceptable environmental or physical conditions on the locality.

Self-supporting towers can vary in height from few tens of meters to hundreds of meters. The base width of the structure should be as large as possible to minimize the foundation forces. The tower legs are usually supported on individual foundations. Cylindrical poles of wood, steel or aluminum can support light antennas up to few tens of meters.

Guyed mast column will be supported at various levels by sets of tensioned stays. The ratio of the height between stay levels and the face width of the column should not exceed 40:1. The face width of the column should conform to those parameters given for towers. The bracing shapes and number designs will also conform to the parameters. The normal stay arrangements are for three stay lanes 120° apart for triangular mast columns and four stay lanes 90° apart for square mast columns. These stays will be anchored to foundations so that the vertical angle between the stay and the ground plane is between 30° and 60°. The stay anchors are usually blocks of concrete of sufficient weight to resist the uplift forces, and sufficient width and depth to resist the sliding and overturning forces.

Roof-mounted structures are potentially the easiest and cheapest to utilize. It is important that the antenna support structure loads are transmitted directly to the building frame. The assumption that the building can transmit the loads even short distances onto its structural frame can lead to local failures. The main loading on the structure will be the wind force exerted against the structural frame and all the ancillary ladders, platforms, antennas and feeders attached to it.