An Introduction to Broadcast Technology - Radio Frequency Systems
3.1.6 Transmitter Amplifiers for COFDM Modulation
3.2.2 VHF & UHF combiner systems
A variety of transmitters are used to broadcast services on a wide range of frequencies and at a wide range of power levels. The technology which is used is advancing continuously but since transmitters usually have a life in excess of 20 years, some of the transmitters in service use techniques which have now been superseded.
Until the 1980’s most high power LF and MF transmitter installations used a form of high level anode modulation. These transmitters are fed with an audio signal which is amplified to a high power. In essence, the signal at this point could be used to feed a large bank of loudspeakers (and the sound level would be comparable with a big rock concert!). In fact, the high power audio signal is used to vary the supply voltage to a high power radio frequency amplifier, with the result that its output is amplitude modulated. The radio frequency amplifier is fed from an extremely precise oscillator which produces the broadcast frequency.
The low power stages of both the modulating amplifier and the RF amplifier normally use solid state devices (transistors and integrated circuits) but many high power transmitters use valves in the other stages as individual transistors cannot handle the necessary power. (One way around this problem is to use a large number of transistors and this technique is mentioned below). A problem with the use of valves is that they have a life which is considerably shorter than transistors. The actual time varies considerably according to the precise circumstances, but between two and four years is typical. The valves used in transmitters are large and expensive, also, to prevent them overheating they usually have to be cooled with a flow of air or water. Therefore it is a fairly major task to change them and furthermore other parts of the transmitter sometimes need to be adjusted to compensate for differences between the old valve and its replacement.
Since the 1980’s most high power LF and MF transmitter installations have been entirely solid state. These transmitters have many RF amplifier modules that are switched on and off as required to build up the modulation cycle. This represented a significant innovation in transmitter design and a reduction in operating costs. If one of the amplifiers fails there is just a slight increase in distortion but no loss of service and so there is less need for a parallel transmitter, which would otherwise be required to maintain the service on reduced power in the event of an equipment failure.
3.1.3
All HF broadcast transmitters produce high powers and typically they range between 250 kW and 500 kW. The output stages invariably use valves and the older HF transmitters use conventional modulation amplifiers, but the newer transmitters introduced in the mid 1980s use modulation amplifiers that operate using a more efficient digital process.
The main distinguishing feature of HF transmitters is that they can easily be wave changed to operate on different frequencies at different times of the day. This is a significant complication and on the older transmitters it is achieved by physically changing large components – e.g. a ten centimetre diameter copper tube bent into a 1.5 meter loop! On these transmitters, the overall process takes one person about 10 minutes. More recent designs of transmitter have fully automatic tuning using large switches and motors to change the components, under computer control. This combination of high power, high frequency, frequency agility and computer control, makes these transmitters extremely complex and advanced pieces of technology.
Unlike most other transmitter systems, HF transmitters are 'single ended', which means that they can stop broadcasting if a single component fails. However, since HF transmitters are extremely flexible, cover can usually be arranged quickly by re-scheduling other transmitters. Although not ideal, this is by far the most economic approach and it has operated satisfactorily for many years.
Low power VHF transmitters are invariably solid state and often employ the transposer technique. This involves receiving an existing FM broadcast, changing its frequency, amplifying the signal and re-broadcasting. This simplifies the design as it avoids the need for a modulator.
All VHF transmitters, as opposed to transposers, take in two audio signals for the left and right channels of a stereo pair and these signals first pass into a stereo coder. The 'stereo multiplex' signal is then fed into the FM modulator or 'drive' which produces the frequency modulated carrier at low power e.g. 10W. The signal is then amplified and fed through a combining unit to the antenna. Various amplifier configurations are used to provide powers ranging from 200W to 20kW according to the service area requirements.
Since the signal to be broadcast is frequency modulated, the amplifiers are smaller and more efficient than their AM counterparts. At medium and high power stations the drive can feed two amplifiers which in turn feed separate antennas. This enables the service to continue, albeit with reduced coverage, in the event of any single component failing. It also enables the power to be removed fairly easily from half of the antenna for maintenance purposes.
UHF transmitters are required at various power outputs in order to cover different population sizes. In general the higher the power, the further the transmissions extend, and thus the greater the population covered. The lowest power normally used is around two watts and these stations typically cover communities of about 200 people. As there are many small communities which are screened from higher power stations by mountains, hills, or buildings, large numbers of low power stations are required in a typical network. These stations use transposers, which receive a parent station, change the frequency, amplify the signal and transmit it again (as opposed to transmitters which take baseband audio and video signals as inputs).
Transposers are all fully solid state (transistor) and are normally housed in small buildings or GRP cubicles. Although most transposers have an output power of about 2 watts, some have powers up to about 1kW.
UHF Transmitters in an analogue TV network extend typically from 1kW up to 80kW. Most of these transmitters use valve or klystron output stages, although since about 1990 many new transmitters have used solid state techniques.
Klystrons have been used in broadcasting since the inception of UHF TV. They are large evacuated tubes with an electron beam inside. They are able to amplify very low signals to high output powers and are very reliable. In the 1990’s new devices called IOT’s came on the scene to challenge the dominance of the klystron. IOT’s use technology borrowed from the klystron, and also from high power valves, and use less electrical power than either. This makes them attractive as it makes the transmitter cheaper to run.
3.1.6 Transmitter Amplifiers for COFDM Modulation
The output from a COFDM modulator (used for digital terrestrial TV) consists of many signals with closely spaced frequencies and in the limit distortion caused by the amplifier could make it impossible to demodulate the signals. Therefore a very linear (i.e. low distortion) amplifier is needed.
No amplifier is perfectly linear but one way to reduce unwanted frequencies from a given amplifier is to operate with a reduced input RF power. This is because the products of distortion decrease by a disproportionately large amount as the power is reduced. As a result of this it is possible to use relatively non-linear UHF analogue television transmitter amplifiers for digital terrestrial television (COFDM), by backing off the Radio Frequency power.
These are basically radio frequency multiplexers used for combining several services at a particular site so they can share a common antenna. Combining Units perform a very simple function - to combine the outputs of different transmitters - but the principles on which they work are anything but simple!
One of the main problems when combining the outputs of two or more transmitters is to ensure that the power from each transmitter does not feed through to the output components of the other transmitters (both for safety and to avoid the generation of intermodulation products). Fundamentally this can be achieved by using tuned circuits, as they can have the very useful property of acting rather like a switch which is off for one frequency and on for others..
Fig 34 Basic characteristic of parallel tuned circuit
A tuned circuit is constructed from an inductor (often made of metal formed into a coil) and a capacitor (often made of metal surfaces placed close to one another). By choosing the correct dimensions and materials a tuned circuit can be made to "resonate" at the desired frequency and, for signals at this frequency, act like a switch that is turned off. Then, as signals move away from the resonant frequency, the tuned circuit acts more and more like a switch that is turned on.
Considering two transmitters of frequencies f1 and f2, they can be isolated from one another using tuned circuits with resonant frequencies of f2 and f1 as shown below.
Fig 35 Basic method of combining transmitter outputs
MF combiners usually consist of combinations of inductors and capacitors to isolate the different transmitters, as shown above. In addition there are inductors and capacitors to "match the impedance of the antenna", which is basically similar to the process performed by a mains transformer used to provide the voltage needed for a domestic appliance such as a car battery charger.
3.2.2 VHF & UHF combiner systems
At VHF and above the dimensions of the inductors and capacitors would need to be physically small in order to make them resonate at the required frequency and an alternative approach is needed, particularly for high powers where small components would overheat. The solution is to use "transmission line" techniques where the inductance and capacitance is physically distributed rather than lumped all in one place. For instance, a coaxial cable of the type normally plugged into a television set is a "transmission line" and it has precise dimensions to give it certain properties.
The transmission line equivalent of a parallel tuned circuit is a specific length of (usually) coaxial cable, shorted at the far end. (For ease of illustration the diagram below shows a transmission line as two wires running parallel to one another.)
Fig 36 Basic tuned circuit made from a transmission line "stub"
The relationship between wavelength and frequency is explained earlier in the notes.
Stubs such as this (or "resonators", which are similar devices) can be used to provide isolation between transmitters and similar techniques can be used to combine signals together and provide matching.
Combiners often use "3dB couplers" formed from two transmission lines placed close together so that half the power is coupled from one transmission line to the other. One basic type of combiner used on VHF transmitting stations is illustrated below, but note that there are many types of combiner used to meet various requirements.
Fig 37 Basic combiner used at VHF transmitting stations
In the above combiner the "combined output of other transmitters" is not affected by the resonators and, due to the properties of the 3 dB couplers, virtually all of the power appears at the output of the combiner. Power from the transmitter f1 can't get to the other transmitters as it is reflected back by the resonators to the output of the combiner.
Several different types of antennas are used for broadcasting and differences in their physical construction are a major distinguishing factor between different types of site - an HF transmitting station and a television relay station look totally different. All antennas have the same function, which is to transmit (or receive) electromagnetic waves. The main distinguishing factors are determined by the frequency of transmission, the amount of power to be radiated, and the direction in which the signal is to be radiated. Briefly, frequency determines the antennas size, power determines its type of construction and directivity determines its complexity, although all these factors are inter-related.
There are many variations, but basically an antenna consists of two conductors which are arranged in line, with the transmitter connected to the point where the conductors are closest together. In order to work efficiently the length of the rods must be related to the wave length of the transmission, which is inversely proportional to the frequency. As a result LF antennas are large and UHF antennas are small. The following paragraphs outline some of the characteristics and practical problems associated with each type of antenna.
A "T" antenna supported from 150 metre masts is typical and an alternative is a mast radiators with an umberella. Smaller versions of these types of antennas are used for MF transmissions as described below.
Perhaps the most obvious form of antenna is the "mast radiator". In this case the entire mast is mounted on an insulator and to work efficiently the height of the mast needs to be about a quarter the wavelength of the carrier frequency to be transmitted.
Fig 38 Mast radiator antenna used at MF sites
The other conductor is the earth and a connection is made using an "earth mat" which is a set of copper wires buried in the ground and arranged radially from the foot of the mast. At this point there is an "aerial tuning hut" (ATH) where the feeder from the transmitter is connected to both the earth mat and the mast. A feeder isimply a cable of carefully controlled dimensions which joins a transmitter to an antenna, which is usually some distance away. Large components in the ATH also "match" the transmitter to the antenna, which is a process very roughly analogous to a battery charger which converts mains voltage to the correct voltage for charging a battery.
Since the transmitter applies high voltages to the steelwork of the mast itself, insulators are also placed in the stays which support the mast and it is obviously necessary to switch the transmitter off before ascending the mast.
A single mast radiator radiates evenly in all directions in the horizontal plane. Sometimes this is the required radiation pattern but there are circumstances where a higher signal strength is required in a particular direction (e.g. MF antennas at Orfordness which serve Europe). In order to achieve this one or more additional masts of the correct height can be placed in a precise position nearby to act as reflectors and directors (like the rods on a domestic TV antenna).
Sometimes (especially at LF) it is not economic to provide a mast about a quarter wavelength high, so an alternative is to use a shorter vertical conductor with some means of extending the conductor laterally at the top. One common method is to use a "T" antenna as shown below. The masts themselves do not form part of the antenna, but are simply there to support it. However a major practical difficulty is that the electromagnetic field strength in the vicinity of the masts is very high and it is unsafe to climb the masts while the transmitter is on.
Fig 39 "T" antenna used at LF and MF sites
(There are some situations where a mast that supports a 'T' antenna is also a mast radiator itself for another service on a different frequency.)
An alternative to a "T" antenna is to use a mast radiator with an "umbrella" consisting of several metal rods fixed radially to the top of the mast:-
Fig 40 Mast radiator with an "umbrella" to allow a shorter mast to be used
Transmission's HF antennas are invariably supported between masts, because the antenna elements are horizontal rather than vertical. This "horizontal polarisation" is used because it provides a better signal in the service area which can be thousands of miles away. In this case, the signal needs to be concentrated in a narrow beam which points at a few degrees above the horizon up to the sky so that it can be bounced off the ionosphere and reflected down to the service area. In order to achieve this narrow beam an "array" of antenna dipoles are used. They are stacked on top of one another a precise distance apart and usually there are two or more stacks side by side.
Fig 41 HF antenna array (two stacks of dipoles are shown, but no supporting cables)
Physically, the result is that there is a complex arrangement of wires with some of them being used as antennas and others being used for supporting the antennas. Insulators are used to separate the antennas from the supporting structure and the entire assembly is supported from a "triatic" which is a wire which stretches between the top of two adjacent masts. In some cases the same triatic will support several complete antenna systems and the overall tension on the triatic can be several tons. The word "wire" is not very precise as a range of conducting and non-conducting materials are used, as well as a variety of diameters to provide the required tensile strength. Maintenance is carried out by lowering the entire assembly to the ground - not a trivial task!
VHF antennas, as used for FM broadcasting, consist of assemblies which are bolted to a mast. Each assembly is in the region of a metre in each direction and there is usually a stack of them mounted one above the other and all around the mast. They are mounted one above the other to focus the beam and avoid unnecessary radiation towards the sky. They are mounted around the mast to ensure that a good service is provided in all directions i.e. to provide a good "horizontal radiation pattern". Normally there are two stacks of VHF antennas mounted one above the other and fed by separate feeders which run up the mast. Under normal circumstances both stacks are powered but for maintenance purposes the upper and/or the lower stack can be de-powered.
Subject to stringent checks and procedures, it is safe to climb up the mast through the middle of a VHF antenna stack which is de-powered, even though the other stack is powered. In order to ascend to the top of the mast it is necessary to wait above the de-powered lower stack until it is re-powered and the upper stack is de-powered. The physical process of climbing through the antenna is difficult because the space is very restricted due to diplexers which join the feeder onto each antenna element. These diplexer systems include many coaxial cables which have to be a certain length for electrical rather than physical reasons and they have to be bulky in order to carry sufficient power. It is sometimes necessary to carry highly specialised test equipment up the mast so that measurements can be carried out on diplexers. The hazards of height and high power combined with technical complexity and cramped space make this a very difficult task.
Fig 42 UHF TV and VHF/FM antennas on the same mast (which is often the case)
UHF television antennas are similar in some ways to VHF antennas, but each element is smaller due to the higher frequency involved. On high and medium power sites, UHF antennas are mounted at the top of the mast and they often shrouded by a fibreglass cylinder which provides a distinctive appearance and makes it possible to identify that the station is broadcasting UHF television. For reasons of economy, service coverage and to allow domestic TV and radio antennas to point in the same direction (usually), high and medium power UHF TV antennas are often mounted on the same mast above VHF radio antennas.
The majority of sites which broadcast UHF television are small relay stations where the power level is low and the antennas are fairly simple. There is one antenna pointing at the parent transmitting station and another antenna pointing towards a local community. They are not much larger than domestic antennas.
SHF antennas on broadcast towers are used for point-to-point communication as opposed to direct broadcasting to the public. They are similar to domestic satellite TV antenna dishes although they are often much larger and surrounded by "skirts" forming a short cylinder around the rim to reduce unwanted radiation in the wrong directions. The actual radiating element is comparatively small (due to the "Super High Frequency"), and is mounted at the focal point of the dish which forms a "parabolic" reflector similar to that found in a torch.