Television Transmitters for The' Ultra-High Frequency Band
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256 PHILIPS TECHNICAL REVIEW VOLUME 26 Television transmitters for the' ultra-high frequency band J. A. van der Vorm Lucardie 621.397.61.029.6 A number of frequency bands in the very high and In addition to this need, however, a far greater one ultra-high frequency regions of the radio spectrum has been created by the desire to transmit a second have been set aside by international agreement for or even a third television programme. Fortunately, the the provision of television and sound broadcasting much greater width of bands IV and V amply allows programmes. In these regions, which are generally the demand to be met in these bands. referred to by their initials VHF and UHF, a total of In designing television transmitters for the UHF five bands are available. Bands I, IJ and III are in the range a number of problems are encountered which VHF spectrum and bands IVand V in the UHF. Band are more or less peculiar to these high frequencies. We 11 is used for sound broadcasting, while the others shall devote particular attention to such points in the have been assigned to television broadcasting. present article. For frequency-band allocation the world is di- . The power required for transmitters in bands IV and V vided into three regions, and the location' of the frequency bands varies slightly for each of these A certain minimum field strength is necessary to regions. That for Europe, the Near East and North ensure a good-quality television picture. An increase Africa was last defined in 1961 during the Stockholm in the field strength need not involve increasing the conference [11. As the use of frequencies in UHF transmitting power but can also be achieved by bands IV and V was still in a very early stage, it proved arranging that the RF power is not radiated uni- possible to definethe bandwidth available per television formly in all directions but strongly concentrated in channel - 8 Mcfs - and the positions of the vision the horizontal plane. A number of aerial designs are carrier frequencies within bands IV and V in a uni- available for this. It is consequently customary, in form manner. describing television transmitters, to speak of their For the sake of completeness the positions of the "effective radiated power", which is defined as the five bands are here quoted: product of the power applied to the aerial and a factor Band I : 41 - 68 Mcfs (7.3 - 4 m), dependent on the type or design of aerial employed. Band 11: 87.5 - 100 Mcfs (3.4 - 3 m), This factor is expressed in decibels and is called the Band Ill: 162 - 230 Mcfs (1.85 - 1.3 m), aerial power gain. Band IV: 470 - 582 Mc/s €64 - 51 cm), When allowance has been made for the effective Band V : 582 - 960 Mcfs (51 - 31 cm). aerial height, the gain of the receiving aerial and the It will be seen from this table that bands I and III noise contribution of the receiver, it is found that the are relatively narrow and they can therefore accom- effective radiated power of a UHF station has to be modate only a limited number of TV channels. approximately 10dB higher than that of a VHF sta- The service area of a television transmitter - i.e. tion for the same quality of reception. The effective the area in which the field strength is sufficient to radiated power of large stations in bands I and III ensure good picture quality - depends on the height being 30-100 kW, an ERP of 300-1000kW is neces- of the aerial and is confined within a radius of ap- sary in bands IV and V. proximately 35 miles (60 km) for powerful stations. At the frequencies with which we are concerned The interference area, however, extends much further here the limit of the service area more or less coin- and therefore the distance between two transmitters cides with the optical horizon as seen from the aerial, which it is intended to operate on the same frequency which is therefore erected in as elevated a position has to be several hundred kilometres. It is consequently as possible. The short wavelengths in bands IV and impossible, even in countries where there is only one V make it possible to obtain much greater aerial gain television programme, to obtain a completely closed than in bands I, 11and Ill, while keeping the size of pattern of service areas, as is desirable in Europe, using the transmitting aerial within economic bounds. The only channels which are available in bands I and Ill. limit is determined by mechanical considerations. If this is to be achieved, a number of additional chan- The maximum gain factor that can be attained is nels in the UHF spectrum are necessary. approximately 50. If, finally, allowance is made for [1) Final acts of the European VHF/UHF Broadcasting Confer- Ir. J. A. vall der Vorm Lucardie is 011 the staff of N. V. Philips'" ence, Stockholm, 1961, published by the International Tele- Telecommunicatie Industrie, Hilversum. communication Union, Geneva. .. 1965, No. 8/9 TELEVISION TRANSMITTERS FOR BANDS IV AND V 257 the fact that very considerable losses occur in the coaxial cable connecting the transmitter and the aerial in its high position at band IV and band V frequencies, it is finally found that transmitter powers of 10-40kW are necessary. On the basis of this result Philips' Telecommunica- tie Industrie first developed a 10 kW transmitter. This was followed by a 20 kW transmitter, while for smaller service areas a 2 kW transmitter is now also- Fig. 1. Diagram showing the main parts of a vision-transmitter. available. All of these are suitable for colour television. A crystal oscillator; B frequency multipliers; C modulated am- plifier; D final amplifier; E video modulator. When necessary, double power can be obtained by connecting two transmitters in parallel. An advantage is called a filterplexer. In the transmitter to be described • of this arrangement is that if one transmitter develops below, a combining unit is adequate since a klystron is a fault, the transmission can continue without inter- used as the final power amplifier. Suppression of the ruption, though at reduced power. We shall return to lower sideband is effected with a simple coaxial filter this point later. inserted before the final amplifier input. In keeping with normal practice, the sound-trans- General arrangement of a television transmitter mitter employs frequency modulation. Once again, What is generally called a television transmitter is the process starts with the generation of a signal with in fact a combination of two transmitters, one of a frequency much lower than the transmitting fre- which transmits the vision signal and the other the quency. This is done in the oscillator Al (see fig.2), accompanying sound signal. As the aerial and its whose frequency is directly modulated with the sound feeder cable form a very costly element, both trans- signal, the latter being amplified in F. Oscillator Al mitters are always connected to the same aerial by cannot therefore be crystal-controlled and as a result means of a coupling network, for which the term its stability is limited. If the frequency of Al were to combining unit has been generally adopted in television be raised to its final value solely by multiplication, practice. The combining unit prevents the sound- as is done in the vision-transmitter, the large multipli- transmitter and vision-transmitter from interacting cation factor required would result in equally large on each other. magnification of the fluctuations of the mean fre- The essential parts of a vision-transmitter are shown quency of oscillator Al in relation to its nominal value. infig. J. A crystal oscillator A generates a signal whose The output' frequency from Al is first doubled in VI frequency is a sub-multiple of the transmitting fre- and then compared with the frequency of a crystal quency. The transmitting frequency is attained by oscillator A2 in circuit C. This comparison circuit multiplication in stage B. In C the radio-frequency delivers a control voltage which corrects the mean signal is modulated by the video signal, which has frequency of Al when deviations are observed. Stage previously been amplified in the video modulator E. V2 triples the output frequency of VI and the output As a rule, modulation takes place as close as possible signal from V2 is then mixed in stage M with that to the output stage, and sometimes in the output stage from a crystal oscillator Aa whose frequency is equal itself. The effect of this is to simplify transmitter tuning to half the mean transmitting frequency, minus the and improve the stability. In output stage D the mo- output frequency of Va. The frequency of the output dulated signal is amplified to output level. signal from M is therefore ---:-after the suppression of Amplitude modulation is used for picture trans- unwanted products of mixing - half the mean trans- mission. The bandwidth occupied is limited by sup- pressing. a large part of the lower sideband of the modulated signal. In tele- vision transmitters in which modulation is effected in the output stage, the general practice is to have the out- put stage followed by a filter Fig. 2. Diagram showing the main parts of an FM sound-transmitter. Al frequency-mod- ulated oscillator; F sound-amplifier/modulator; Vl doubler; C comparison circuit; A2 combined with the combi- crystal oscillator; V2 tripler; M mixer circuit; A3 crystal oscillator; V3 doubler; D power ning unit. This combination amplifier; E output amplifier.