AN INTRODUCTION TO BROADCAST TRANSMITTING ANTENNAS

Transcription

1 AN INTRODUCTION TO BROADCAST TRANSMITTING ANTENNAS Contents TELEVISION TRANSMITTING ANTENNAS... 2 OMNIDIRECTIONAL OR DIRECTIONAL SERVICE... 3 E.R.P. (EFFECTIVE RADIATED POWER)... 4 CHOICE OF VERTICAL RADIATION PATTERN... 5 PERFORMANCE SUMMARY TABLES... 8 ANTENNA IMPEDANCE (MATCHING) COVERAGE AND FIELD STRENGTH EXTRACTS FROM FCC PROPAGATION CURVES POWER AND VOLTAGE RATINGS... 23

2 TELEVISION TRANSMITTING ANTENNAS The transmitting antenna is the last link in the long complex chain between the programme subject and the viewer, over which the broadcast engineer has any control. It is as important a link in this chain as any other, perhaps more than some, yet it is frequently either ignored or at best rapidly dismissed as mere plumbing by those will otherwise discourse happily on anything else from a diode to a klystron. The fact that the antenna is frequently a thousand feet or more up in the air certainly helps to surround it with a cloud of mystery and it is hoped that the following notes may help to dispel that cloud just a little. Antenna systems vary enormously in type and complexity from a simple dipole serving a handful of viewers in a remote valley to the extremely large custom designed multiple arrays such as are on the Toronto CN Tower which transmits seven high powered television programmes and up to twelve FM radio programmes. Whatever their complexity however, all antennas have two fundamental requirements. Firstly, to radiate the required amounts of energy in the required directions and secondly to do it as efficiently as possible. The first requirement may be summed up under the term directivity. An antenna may be designed to concentrate energy in particular directions in just the same manner as a line source speaker system does for sound or a floodlight does for illumination. Since we are dealing with a three dimensional problem it is convenient to split this directivity into two parts. The first is determined largely by geographical considerations and is termed the Horizontal Directivity. The second is the Vertical Directivity and is determined by the limited call for a television service by high flying seagulls and by the lack of confidence amongst engineers in the flat-earth theory. The second fundamental requirement, efficiency, may also be divided into two parts. The first is simply connected with the losses within the system which produce heat. These are fairly easily minimised by careful cable design and by the correct choice of dielectric materials and by the use of high conductivity copper or sometimes by means of copper or silver plating. The second part is the consideration of electrical discontinuities in the system and is discussed in more detail in the section dealing with antenna matching. These topics are each briefly discussed in the following notes under the headings of "Omnidirectional or Directional Service", "Choice of Vertical Radiation Pattern" and "Antenna Impedance (Matching). A final section gives some thoughts on service areas. It should be emphasised that the approach adopted of necessity contains great simplification and is intended to give a broad understanding of the subject rather than a basis for antenna design.

3 OMNIDIRECTIONAL OR DIRECTIONAL SERVICE Ideally the transmitting site should provide an unobstructed line-of-sight over the area to be served, particularly for U.H.F. services. In the case of microwave links line-of-sight is essential..if such a site can be found central to the service area then an antenna radiating equal amounts of power in all directions (omnidirectional), when used with a transmitter of appropriate power, will provide the required service. See example (a) in Fig. 1 above. When a centrally located site is not available, as in example (b), if the same E.R.P. (effective radiated power) is used as was used in (a) then part of the service area remains uncovered whilst considerable power may be wasted in other directions. If the E.R.P. is increased to fully cover the original service area (dashed circle) then even more power is at best "thrown away" over unpopulated areas or possibly even reflected off surrounding hills to interfere with the direct signal. To the station owner "throwing away" energy is a serious economic consideration particularly on high power stations. Transmitter operating efficiencies are of the order of 50% to 60%, added to this he may well lose a further 30% of his power in the main feeder losses (150m of 3 1/8 ins. feeder at 600 MHz). If on top of this he radiates say 25% of his energy to unpopulated areas, he ends up with something like a 25% efficiency for his power utilisation. A third possibility is shown in (c). Here the same non-central site is used as in (b) but with a directional transmitting antenna. This is quite an effective solution to the problem but may also require an increase in transmitter power. The reason being that some of the boundaries of the service area are now further away from the antenna, and the received field falls off closely as the square of the distance.

4 Since field strength is proportional to P, we have the useful relationship that the radius, r, to a given field strength contour is related to the power, P, radiated in that direction by:- r 4 P Some of the extra power requirement will be offset by the increase in the horizontal gain of the antenna, the exact amount depending on just how directional the system has been made. There are of course other reasons for using directional systems. The service area may well have an irregular shape as in a valley, on a coastal plane, or on an island for example. National boundaries, adjoining service areas, long distance co-channel interference etc., may also require the use of reduced power in specific directions. E.R.P. (EFFECTIVE RADIATED POWER) PEAK E.R.P. = (TRANSMITTER POWER) x (PEAK ANTENNA GAIN) The antenna gain figure here includes allowances for the main feeder loss, distribution feeder loss, null fill and beam tilt loss, cross polarisation loss, etc. The main feeder loss is frequently applied to the transmitter separately, when we then speak of the transmitter power available at the antenna input, i.e. at the top of the mast. Strictly speaking gain should be quoted for a specific direction, when it is not, and when no qualification is made, it is usually the peak gain that is being referred to, that is the gain in the direction of maximum power. Antenna gain has two components, one due to the directivity (if any) of the horizontal radiation pattern (H.R.P.), the other due to the vertical radiation pattern (V.R.P.). The peak gain is the product of the two components and is therefore the antenna gain in the direction of maximum radiation in the HRP and on the peak of the VRP. This must be remembered when estimating service areas. When checking service in any direction other than that of maximum (peak) ERP, correction must be made for the appropriate values of the HRP and VRP in that direction. In making these corrections care must be taken not to confuse relative field strengths (a voltage measurement) and relative ERP (a power measurement). If corrections are made in db the problem should take care of itself. For example, in a direction where the HRP has fallen to half its maximum value (relative voltage) i.e. - 6dB, the ERP will also be reduced by 6dB (relative power) i.e. to one quarter of its maximum value. This is hardly surprising since remembering that P V2, if V falls to 0.5V then P falls to (0.5)2P, or 0.25P. In broadcasting applications gain is normally quoted with respect to a half-wave dipole (one of the most basic radiating elements in use) and refers to the increase in power which will be radiated in any specific direction by the antenna in question compared to the power that would be radiated in the same direction if the same transmitter were fed into a single half-wave dipole. Sometimes, (generally in communications work, including microwave links) though rarely in broadcasting, gain is specified or quoted with respect to an isotropic source. That is a hypothetical point source which radiates uniformly in all directions over a sphere. The gain of a half wave dipole is 1.64 (2.16dB) relative to an isotropic source. Thus the gain of any antenna will be approximately 2.2dB higher when quoted with respect to an isotrope (sometimes written as dbi) than it will be when quoted with respect to a half-wave dipole.

5 CHOICE OF VERTICAL RADIATION PATTERN 1. GAIN A multiple element antenna covers a greater area with less transmitter power. In Fig. 2 above, the effects of three combinations of antenna gain and transmitter power are illustrated. In (a) a single low gain antenna element is shown with a representation of its vertical plane radiation pattern. It is clear that some energy is wasted by being directed upwards. The illustration in (b) shows a multiple element antenna which concentrates the energy into a narrower beam in the vertical plane. Less energy is wasted in unwanted directions thereby extending the coverage area for the same, or even lower, transmitter power. In (c) the single element antenna is shown with the transmitter power increased to extend the coverage out to the same distance as in (b), clearly however there is even more energy wasted above the horizon. On the face of it, it would thus appear that the object should be to use as high a gain antenna system as possible. There are several precautions which must however be taken, and indeed an eventual practical limit.

6 2. NULL FILL AND BEAM TILT The above diagram, Fig. 3, shows that when radiating elements are stacked up to produce a narrower main beam then side lobes or minor lobes are produced at the same time. Between each lobe the radiated power theoretically falls to zero and hence the field strength received on the ground at the corresponding distance from the tower is also zero. The higher the gain of the antenna and the taller the tower on which it is mounted the further out into the service area will these areas of zero and very low field strength occur. Ideally one would like to provide an equal field strength at all points on the ground, from the mast right out to the edges of the coverage area. It is the shaping of the vertical radiation pattern to fill the null regions in order to approach this aim that occupies a considerable amount of the antenna design engineer s time. Shaping is achieved by judicious choice in the way the transmitter power is divided, in both amplitude and phase, between the individual radiating elements of the system. If all elements are fed with equal power and in phase (the condition for maximum practical gain) then nulls are produced as already outlined. Also, as shown in the sketch, the main beam of the antenna is directed straight out parallel to the earth s surface at the mast base. Because the earth is curved much of the transmitted energy would never reach the ground under these conditions despite refraction but would be lost straight out over the horizon. For this reason the main beam of the antenna is normally tilted downwards one or two degrees, the exact amount depending on the gain of the system and the local topography. Beam tilting is normally achieved electrically by the correct choice of relative phase between the individual radiating elements. When the nulls are filled the required energy has to come from somewhere, it does in fact come from the main beam of the pattern thereby reducing slightly the peak gain of the system. This reduction is referred to as the "null fill loss". Correct design of an antenna should normally achieve a vertical gain of between 1 and 1.2 per wavelength of aperture.

7 Although it would thus appear that we can go on stacking elements one above the other to produce as high a gain as we please, various practical limits impose themselves. As the gain increases the number of nulls increases (in all practical systems) and the areas they affect move further out into the main service area. Filling them not only becomes more difficult but the null fill loss increases to use up more and more of the additional gain. If the beam is made too narrow then insufficient energy is directed towards areas closer in towards the mast to provide the required field strength. This could be corrected by tilting the beam downwards even more; this in turn would reduce the outer limits of the coverage area. In addition, as the beam gets narrower it becomes more and more sensitive to movement of the supporting mast and field strength fluctuations on the ground may become unacceptably large. This latter point is tied up with the whole economic consideration not only of the increased cost of higher gain antennas themselves, but also of the structure to carry the extra weight and wind load. Figure 4 above shows the relative radiated field strength plotted against degrees below the horizontal for the same transmitter power being fed into antennas having the same beam tilt and null fill, but one having approximately twice the gain of the other. It clearly shows that whilst the high gain system provides greater field strength and hence greater coverage over the peak area of the main beam, over quite considerable areas closer in there is a decrease in field strength. Considering the complete service area it is therefore normally desirable to increase transmitter power to some extent at the same time as increasing antenna gain. Figure 5 indicates quite simply how changing the height of an antenna, whilst keeping the gain constant, can greatly affect the field strength received, particularly at points relatively close to the transmitting site.

8 PERFORMANCE SUMMARY TABLES Figures 8 and 9 are examples of ADBL's standard Performance Summary tables for TV and FM antennas. Whilst some items in the tables are self-evident, a few might be made clearer by the following comments. Other than in exceptional circumstances where possibly a non-standard screening arrangement is employed, or where radiating elements such as Yagis are used, the Electrical Aperture is simply a translation of the Physical Aperture into wavelengths and the physical aperture is the distance from the top of the highest antenna panel to the bottom of the lowest panel. The Intrinsic Gain is the maximum theoretical gain available from the electrical aperture (before consideration of any losses) using the type of antenna, number of elements and spacing between them, being described. The intrinsic gain assumes that the HRP is a perfect circle (unity gain); it is therefore purely a function of the vertical radiation pattern and as such is sometimes referred to as the VRP gain. The intrinsic gain (strictly the intrinsic power gain) is calculated from the properties of the individual radiating elements making up the antenna array. In practise it generally varies between about 1.0 and 1.2 per wavelength of aperture. Null fill has been described in some detail in the section dealing with VRP's. The Null Fill Loss is a direct measure of the energy that has been extracted" from the main beam to provide null fill. Distribution Feeder Loss or distribution harness loss is the total of losses within the power dividers and small cables feeding the individual antenna panels from the output of the main feeder(s). Antenna Mean Gain is the intrinsic gain less the antenna internal losses, i.e. the distribution feeder and null fill losses. It is still referring to the gain of an omnidirectional antenna, hence the use of the word "mean. HRP Max/Mean Gain is a measure of how the antenna HRP concentrates energy in a particular direction or directions, compared with a perfectly omnidirectional service. In practise it is measured simply by comparing the area of the particular HRP with the area of a circle having the same radius as the HRP maximum. These comments on Max/Mean gain are straight forward when applied to an antenna which is intentionally designed to provide a directional service. In almost all practical cases even antennas which are designed to be omnidirectional do in fact depart from perfect circles, in some cases quite considerably.

9 The question arises under these circumstances whether even an omnidirectional antenna has a max/mean gain. From the method of measuring HRP gain by comparing areas, the practical omnidirectional antenna clearly has a max/mean gain. In the illustration on the left, Fig. 7, Ac/Ap = approx. 1.3 (power gain), or 1.2dB. In drawing up a Performance Summary table each case is evaluated in accordance with the specification. For example in the USA any station licensed to operate an omnidirectional pattern must file its performance data based on a perfectly circular pattern, i.e. a max/mean power gain of 1.0 (0 db). In Canada on the other hand, the actual HRP max/mean gain of the practical antenna must be included. It is common practise in many countries to base the specification of an omnidirectional antenna on the mean gain (as in the USA), but each case must be treated individually. Antenna Max Gain is simply the mean gain (i.e. the omnidirectional gain after allowing for internal losses) plus the HRP max/mean gain (if any). CP loss or Polarisation Loss is a factor used with circularly polarised antennas. If the system is designed to provide equal ERP's for both the horizontal and vertical components, which is the usual case, the CP loss is 3 db. The figure simply indicates that the available transmitter power is equally divided between the two components. Some manufacturers (notably North American suppliers of CP FM antennas) make this allowance in step one by quoting only half the normal intrinsic gain. Either way the end result is to produce an ERP, in the Performance Summary table, which is provided in both the horizontal and vertical planes of polarisation. Individual specifications must be noted for the rare instances in which a total combined CP ERP is specified. Main Feeder Loss is the loss in the main transmission line connecting the output of the transmitter to the input of the antenna array. For a multi-channel antenna a Combining Unit Loss will also appear at this point in the performance summary table. Sometimes either a Max or Mean ERP is specified for the system. In this case the performance summary table will quote the Transmitter Power required to produce this ERP. If on the other hand the specification simply quotes a transmitter power then the table will quote the max and mean ERP's produced by the system. i.e. the product of the max or mean system gain and the available transmitter power. CIRCULAR POLARlSATION Circular polarisation has been mentioned in the above comments on Performance Summary tables. No mention has been made of CP in the main sections, either to specifically include or exclude it. In fact virtually everything that has been said applies equally well to both linearly and circularly polarised antennas. It might be worth mentioning that linear polarisation simply refers to the fact that such antennas produce an electric field which lies almost entirely in one plane.

10 This is normally either horizontal or vertical although much more rarely slant (45 degree) polarisation is occasionally used. In circular polarisation the transmitting antenna is of a special design which produces an electric field which rotates continuously through all planes, including of course horizontal and vertical. Strictly speaking true circular polarisation refers to a system where the electric field is always exactly the same magnitude whatever the plane of polarisation at any instant. For most practical antennas the field tends to vary with angle and should really be referred to as elliptical. The degree by which an antenna departs from perfect circular polarisation is usually described by two parameters, the polarisation ratio and the axial ratio. The polarisation ratio refers to the ratio of the horizontally polarised component to the vertically polarised component whereas the axial ratio refers to the ratio of the maximum to minimum values of the electric field as it rotates; these need not necessarily occur in the horizontal and vertical planes.

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13 ANTENNA IMPEDANCE (MATCHING) The last of the three properties of transmitting antennas to be briefly considered is the antenna impedance, frequently referred to as the matching of the system. Whilst the concept of matching is simple enough, it is a difficult topic to approach without the risk of becoming over technical. Fortunately for the present purpose of giving a broad understanding of the place of antennas in the broadcast chain most of the detail is unnecessary. The transmission line, or main feeder, carrying the energy from the transmitter to the antenna input has definite 'characteristic' impedance, most frequently this is nominally 50 ohms. In North America where rigid line is used more often than in Europe 75 ohms is also quite common. 75 ohm line employs marginally less copper so is less expensive than 50 ohm line and also allows 6 1 /8 line to be used at higher UHF frequencies. The recommended upper frequency for 6 1 /8 50 ohm line is 816MHz whilst that for 75 ohm line is 919MHz. These frequencies apply strictly only to straight line sections; elbows and U links may cause problems at lower frequencies. The relationship between impedance mismatch and reflection coefficient is quite simple: (Zr-Z0)/(Zr+Z0) = kt or since VSWR = (1+kt)/(1-kt) we have VSWR = Zr/Z0 Where Z0 is the characteristic impedance of the line, Zr is the terminating impedance and kt is the reflection coefficient. For example, terminating a 50 ohm line with 52 ohms would give a VSWR of 1.04:1 that is a reflection coefficient of 1.96%. It is the matching of the junction at the top of the mast between the main feeder and the antenna distribution feeder that is most important. If the feeder is 50 ohms then the antenna input must be made to look like 50 ohms, if the feeder is 49 1/2 ohms then the antenna must be 49 1/2 ohms etc. There are two main reasons for this. Firstly, energy can only be completely transferred from a source (the transmitter) to a load (the antenna), if the two have identical impedances. Any mismatch whatever will cause some of the energy to be reflected back towards the transmitter. When this energy reaches the input to the main feeder at the bottom of the mast again most of it will be re-reflected and will travel back up to the antenna and be radiated as a delayed, or ghost, image, the delay depending on the time taken for the signal to travel back down the feeder and up again. The undesirable effect of this is to produce a ghost picture on the viewer s screen, displaced to the right of the main picture. If you consider a reflection at the end of a 300m long feeder, the total path length for any reflected signal travelling down to the transmitter and back up to the antenna is 2 x 300m and the time taken to travel this path is 2 μsec. In system I with 625 lines and 50 frames per second the time for a single line scan is 64 μsec. Reflections from hills or tall buildings within the service area may also cause delayed images but these, other than as mentioned under directional antenna systems, are outside the engineer s control. Just how objectionable such a delayed image appears depends on both its magnitude and delay, or displacement. The second main reason for matching is really an extension of the first. A television signal is made up of information in the form of pulses and a pulse occupies a band of frequencies rather than a single spot frequency. Admittedly the bulk of the picture information is concentrated very closely around the vision carrier frequency, but the details and fine structure may extend several megahertz into the side

14 band. Some analogy may be drawn with the fact that the greater part of a musical score was intelligently conveyed to the listener by a 78 r.p.m. record, but most of us appreciate the additional information carried in the extended frequency response of the modern compact disc. This illustrates why the antenna must be matched over a band of frequencies rather than just at the carrier frequency, in order to ensure the transmission of all the picture detail in an undistorted manner. The exact nature required of this matching has been built up largely from an extensive subjective testing of picture quality by the major broadcasters. In colour systems there is the additional consideration of the completely separate colour sub carrier frequency, but this is really an extension of the principle rather than anything completely new. In the overall design of the antenna system a few other points have to be considered in connection with the impedance matching. The total combination of antenna, feeders, combining unit, filters and switching frame must present a reasonable match to the transmitter output, not merely to ensure maximum energy transfer to avoid waste, but in order to prevent energy being reflected back into the output stage of the transmitter and producing distortion. Within the distribution feeder components of the antenna itself care must be taken over the matching to prevent either excessive heating or the possibility of voltage breakdown. COVERAGE AND FIELD STRENGTH At the end of the day this is what it's all about. We have to provide sufficient field strength for domestic receivers to give good noise free pictures and distortion free sound. The quality of received information depends very largely on the signal/noise ratio provided, not simply on the sensitivity of the receiver. For television, a viewer is considered to have an adequate signal if, using an average receiving antenna system, it is possible to receive a picture with no impairment worse than that which might be described as falling between 'perceptible but not annoying' and 'slightly annoying'. The principal impairments which limit a service are signal to noise ratio, as already mentioned, multipath interference (ghosting) and co-channel interference. The recommended field strengths for planning purposes vary slightly from authority to authority but the table in Fig. 10 gives frequently quoted values. The actual field received at any point depends on the propagation conditions between the transmitter and the specific point in question. Unfortunately these conditions depend not only on fixed features such as ground conductivity, e.g. sea paths or land paths, hills and other obstructions and average ground 'roughness', but also on time dependent factors such a temperature and humidity. Whilst it is possible, though very time consuming, to carry out a mathematical analysis for some of these effects over a specific path, this is the province of the specialised service area planner and is beyond our normal needs. For most practical purposes the best guide for general use is the intelligent application of the various Coverage Curves published by such organisations as the FCC and the CCIR. There are various sets of curves but the most useful for general assessment are the 50/50 curves, referring to 50% of locations for 50% of the time. The 50/50 curves published by the FCC for FM Radio, VHF Television and UHF Television are shown in Figs These give the received field at 10m above ground at varying distances from the transmitter site for a 1 kw ERP. The user then applies a correction for the ERP of the actual antenna under consideration. Thus if the antenna has an ERP of 50kW, the received field predicted by the propagation curves must be increased by 17dB. There are some relatively simple graphical corrections which may be applied for other percentages of time or locations and for other than 'average' terrain roughness. The corrections recommended by the

15 CCIR are shown in Fig. 15 and Fig. 16. The comments made earlier in the section dealing with Gain and ERP must be noted when checking coverage in order to ensure that the correct ERP for the appropriate direction is used. The accompanying examples are taken from the FCC curves. As a salutary warning to the pitfalls which can arise in taking such curves too literally, one piece of raw data used in compiling the curves is also shown. The very large variations in the data should of themselves serve as sufficient warning. Similar sets of curves are published for MF propagation. For anyone interested in pursuing the subject a little further the following approximate relationships might be of interest: r 4 (P) A (P) P h2 Where r is the distance to a given field contour, A is the area contained within a given contour, P is the ERP (Effective Radiated Power) and h is the height of the transmitting antenna above the service area. The free space field produced at a distance d metres from a halfwave dipole radiating a power of P watts is E1=7( P)/d volts/metre. A frequently used field strength reference is that occurring at 1 km for a power of 1 kw fed into a halfwave dipole, this is E1=7( 103)/(103) volts/m = 220 mv/m or 107 dbμ Remember that this is for propagation in "free space". The main reduction which occurs over any unobstructed path is due to the interference of ground reflected signals with the direct free space signal. A correction for this may be applied to the free space field by using the formula E = E1 x 4πhthr/(λd2) In this relationship ht and hr are the heights of the transmitting and receiving antennas in metres and d is the distance in km, λ is the wavelength in metres. The ground reflection coefficient has been taken as -1 that is total reflection with a phase change of 180 deg.

16 These relationships are given largely for interest, comparison with the propagation curves will show that the theoretical increase of field strength with frequency does not really appear to happen in practise because it is largely compensated by increased diffraction and clutter losses at higher frequencies. This is in fact the main reason for the production of two sets of curves by both the FCC and the CCIR, one for VHF frequencies from 30 to 250 MHz and one for UHF frequencies from 450 to 1000 MHz Computational Methods It is not within the scope of this Introduction to go into any detail regarding the various analytical methods now available for predicting field strengths and coverage contours. Suffice it to say that the general availability of powerful computing facilities has made such computations much more feasible. There is a variety of proprietary and public domain software available and the calculations have two distinct and importantly different parts to them. Given the terrain profile between transmitting and receiving locations the calculation of received field starts from the simple free space prediction which is then modified for losses introduced by the various forms of "obstruction" along the path. These may include diffraction over single and multiple "smooth" or "knife edge" obstructions, general "terrain roughness", sections of the path across water, clutter losses in urban environments, etc. One of the best known of these procedures is the Longley- Rice method, but there are many others. Some methods give more accurate results for urban areas and some for relatively open country. Some incorporate corrections that have been learned from experience rather from pure calculation, for example absorption in large forested areas and the previously mentioned clutter losses of urban environments. There is little in these methods that have not been employed for many years in tedious hand calculations. Their main value lies in the speed with which multiple calculations may be made for known path profiles. The great difficulty in fully implementing these methods is the availability of path profile data that may readily be interfaced with the calculations. Accurately digitised contour maps are required in sufficient detail. While these are available for Western Europe, North America and many other areas their cost often makes their use non-viable. For many parts of the world, frequently where it would be of most use, such data is simply not available.

17 EXTRACTS FROM FCC PROPAGATION CURVES

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23 POWER AND VOLTAGE RATINGS Power and voltage ratings were not covered in the original text. They were always important considerations but the introduction of digital broadcasting services has emphasised some interesting differences between analogue and digitally encoded signals. Components need to have sufficient rating to ensure that the power they are required to carry does not cause excessive heating and that the peak (or maximum) voltage applied to them does not cause breakdown. It is Alan Dick Broadcast policy to build in a 1.5:1 safety factor above manufacturer s published ratings for transmission lines and other components. VOLTAGE CALCULATIONS Analogue television One of the first things of which to be aware when calculating voltages is that even single channel analogue television signals are composed of more than one carrier signal, in this case the vision and sound carriers (stereo carriers are a further minor variation that need not concern us here). The voltages produced by each separate carrier must be individually calculated and then summed. This is true for both single and multiple channel operation. Analogue TV transmitters are rated by their peak synch power and it is this peak power that must be used in calculating the peak, or maximum, voltage that will occur in the system. By the simple application of the relationship between power, voltage and resistance (P=V2/R) a transmitter with a rating of P watts will produce a voltage of V= (Px50) x 2, across a resistance of 50 Ohms. Although the term peak synch power is in universal use it is sometimes misleading in this calculation. It is referring to the fact that the highest power output of an analogue TV transmitter occurs during the relatively short sync pulse at the beginning of each line of a TV picture. While this sync power is considerably higher than the power during the transmission of the general picture content the transmitter rating is still in terms of the mean power produced during the pulse. More accurately it should possibly be referred to as Mean peak synch power. Hence the introduction of the additional 2 term, this being the ratio of the peak to mean voltage of a simple sine wave. Thus a 10kW transmitter would produce a voltage of V= (10,000x50) x 2 = 1,000V or 1kV during each synch pulse. Now remember that a TV transmitter will have an associated sound transmitter generally operating at a carrier level of either 10% or 20% of the vision power. For the UHF service in the UK the level is 10%. In the case of our 10kW visual transmitter there will therefore be an additional 1kW of aural power which will produce a voltage of V= (1,000x50) x 2 = 316V or 0.32kV. The total peak voltage produced by this 10kW TV service is therefore 1.32kV. Similarly a 20kW TV service would produce a total voltage of 1.86kV. The total voltage of a multi-channel system having two 20kW and two 10kW services would therefore be 6.36kV. If we had fallen into the trap of first adding up all the transmitter powers (2 x x 20 = 60kW plus 10% sound = 66kW) and then calculating the voltage from 66kW we would have come up with only 2.57kV. Remember also that we will not always be dealing with 50 Ohms and the appropriate value must be used in the calculations.

24 FM Radio Calculation of the voltage produced by an FM radio transmitter is marginally simpler than that for a TV transmitter in that there is only the one sound carrier as opposed to the vision and sound carriers of the TV transmitter. Thus a 10kW FM transmitter produces a peak voltage of 1kV across 50 Ohms. Digital Television A digital transmitter is different again in that although the vision and sound signals are now effectively combined in the single modulation system there is a very large difference between the mean and peak powers of a digital transmitter. The rating of a digital transmitter is always quoted as its mean power but in the UK s COFDM system the peak power is up to 10x (10dB) the mean power. In the USA the ratio generally used for their ATV service is 5x (7dB). Care must clearly be taken to use the correct ratio for the service being considered. Thus a 10kW COFDM transmitter is capable of a peak power output of 100kW and may therefore produce a voltage of V= (100,000x50) x 2 = 3.16kV. The voltages for a number of digital services or a combination of analogue and digital services must be calculated individually before being added up, in exactly the same way as for a multi-channel analogue or FM system. POWER CALCULATIONS Unlike voltages, for which the peak or maximum values must be taken into account, power ratings are generally related to heating affects and average values must be considered. Analogue Television For an analogue TV signal, power level actually varies with picture content and maximum power occurs when the picture is at black level. For both 625 line PAL systems and the 525 line NTSC system the average power for a black level transmission is approximately 62% of the transmitter s peak synch rating. When a 10% sound carrier signal is also included this level becomes 72% and this is the ratio most generally used in calculations for these systems. This ratio generally errs on the safe side as black level requires more power than an average picture for which the ratio is closer to 65%. On this basis a 10kW analogue TV transmitter produces an average power of 10 x 0.65 = 6.5kW. Unlike voltages, average powers for multi-channel operation may be calculated by first adding all the peak powers together and then converting to average power. The example of the 2 x 10kW plus 2 x 20kW transmitters used in the example of voltage calculations therefore produces a total average power of 60 x 0.65 = 39kW. FM Radio The quoted ratings of FM transmitters are their average powers so no conversion is necessary. 5 x 10kW FM transmitters therefore produce a total average power of 50kW. Digital Television As stated earlier, the quoted powers of digital transmitters is, like FM transmitters, the average power, Again therefore no conversion is necessary and the average power of multi-channel digital services may simply be added. Remember however that for combined analogue and digital services the analogue powers must be converted to average values before being added to the digital powers.