National Association of Broadcasters Engineering Handbook: NAB Engineering Handbook
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A wide range of related topics that engineers and managers need to understand are also covered, including broadcast administration, FCC practices, technical standards, security, safety, disaster planning, facility planning, project management, and engineering management. Basic principles and the latest technologies and issues are all addressed by respected professionals with first-hand experience in the broadcast industry and manufacturing.
This edition has been fully revised and updated, with chapters and over pages. The Engineering Handbook provides the single most comprehensive and accessible resource available for engineers and others working in production, postproduction, networks, local stations, equipment manufacturing or any of the associated areas of radio and television. S Dollarhite National Association of Broadcasters.
Cavell Cavell Mertz and Davis. Ralph S Blackman Rees Associates. DaneE Ericksen Hammett Edison. Stanley Salek Hammett Edison. David Maxson Broadcast Signal. PaulHeartyRyerson University. Richard G Hickey Flash Technology. The vertical scale is greatly exaggerated for convenience in displaying significant angles and path differences. Under these conditions, vertical dimensions are measured along vertical parallel lines rather than along radii normal to the curved surface, and the propagation paths appear as straight lines. The field to be expected at a low receiving antenna at A from a high transmitting antenna at B can be predicted by plane-earth methods, by drawing a tangent to the profile at the point at which reflection appears to occur with equal incident and reflection angles.
The heights of the transmitting and receiving antennas above the tangent are used in conjunction with Figure 1. A similar procedure can be used for more. Propagation over a sharp ridge, or over a hill when both the transmitting and receiving antenna locations are distant from the hill, may be treated as diffraction over a knife edge, shown schematically in Figure 1. The height of the obstruction H is measured from the line joining the centers of the two antennas to the top of the ridge.
As shown in Figure 1. When the direct ray clears the obstruction, H is negative, and the shadow loss approaches 0 dB in an oscillatory manner as the clearance is increased. Thus, a substantial clearance is required over line-of-sight paths in order to obtain free-space transmission. There is an optimum clearance, called the first Fresnel-zone clearance, for which the transmission is theoretically 1.
Physically, this clearance is of such magni-.
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Note: When accuracy greater than 1. The locations of the first three Fresnel zones are indicated on the right-hand scale on Figure 1. At MHz, for example, the direct ray should clear all obstructions in the center of a 40 mi 64 km path by about ft 36 m to obtain full first-zone clearance, as shown at C in Figure 1. The corresponding clearance for a ridge ft 30 m in front of either antenna is 4 ft 1.
The locus. When there are two or more knife-edge obstructions or hills between the transmitting and receiving antennas, an equivalent knife edge can be represented by drawing a line from each antenna through the top of the peak that blocks the line of sight, as in Figure 1. Alternatively, the transmission loss can be computed by adding the losses incurred when passing over each of the successive hills, as in Figure 1.
The height H1 is measured from the top of hill 1 to the line connecting antenna 1 and the top of hill 2. Similarly, H2 is measured from the top of hill 2 to the line connecting antenna 2 and the top of hill 1.
The nomogram given in Figure 1. This procedure applies to conditions for which the earth-reflected wave can be neglected, such as the presence of rough earth, trees, or structures at locations along the profile at points where earth reflection would otherwise take place at the frequency under consideration; or where first Fresnel-zone clearance is obtained in the foreground of each antenna and the geometry is such that reflected components do not contribute to the field within the first Fresnel zone above the obstruction.
If conditions are favorable to earth reflection, the base line of the diffraction triangle should not be drawn through the antennas, but through the points of earth reflection, as in Figure 1. H is measured vertically from this base line to the top of the hill, while d1 and d2 are measured to the antennas as before. In this case, Figure 1. Under conditions where the earth-reflected components reinforce the direct components at the transmitting and receiving antenna locations, paths may be found for which the transmission loss over an obstacle is less than the loss over spherical earth.
This effect may be useful in establishing VHF relay circuits where line-of-sight operation is not practical. Little utility, however, can be expected for mobile or broadcast services . An alternative method for predicting the median value for all measurements in a completely shadowed area is as follows : 1.
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The roughness of the terrain is assumed to be represented by height H, shown on the profile at the top of Figure 1. This height is the difference in elevation between the bottom of the valley and the elevation necessary to obtain line of sight with the transmitting antenna. The difference between the measured value of field intensity and the value to be expected over plane earth is computed for each point of measurement within the shadowed area.
These empirical relationships are summarized in the nomogram shown in Figure 1. The scales on the right-hand line indicate the median value of shadow loss, compared with planeearth values, and the difference in shadow loss to be expected between the median and the 90 percent values.
This analysis is based on large-scale variations in field intensity, and does not include the standing-wave effects that sometimes cause the field intensity to vary considerably within a matter of a few feet.
National Association of Broadcasters Engineering Handbook, 10th Edition [Book]
Effects of Buildings Built-up areas have little effect on radio transmission at frequencies below a few megahertz, since the size of any obstruction is usually small compared with the wavelength, and the shadows caused by steel buildings and bridges are not noticeable except immediately behind these obstructions. However, at 30 MHz and above, the absorption of a radio wave in going through an obstruction and the shadow loss in going over it are not negligible, and both types of losses tend to increase as the frequency increases.
The attenuation through a brick wall, for example, can. Consequently, most buildings are rather opaque at frequencies of the order of thousands of megahertz. For radio-relay purposes, it is the usual practice to select clear sites; but where this is not feasible the expected fields behind large buildings can be predicted by the preceding diffraction methods.
In the engineering of mobile- and broadcast-radio systems it has not been found practical in general to relate measurements made in built-up areas to the particular geometry of buildings, so that it is conventional to treat them statistically. However, measurements have been divided according to general categories into which buildings can readily be classified, namely, the tall buildings typical of the centers of cities on the one hand, and typical two-story residential areas on the other. Buildings are more transparent to radio waves than the solid earth, and there is ordinarily much more backscatter in the city than in the open country.
Both of these factors tend to reduce the shadow losses caused by the buildings. On the other hand, the angles of diffraction over or around the buildings are usually greater than for natural terrain, and this factor tends to increase the loss resulting from the presence of buildings. Quantitative data on the effects of buildings indicate that in the range of 40 to MHz there is no significant change with frequency, or at least the variation with frequency is somewhat less than the square-root relationship noted in the case of hills.
The median field strength at street level for random locations in New York City is about 25 dB below the corresponding plane-earth value. The corresponding values for the 10 percent and 90 percent points are about 15 and 35 dB, respectively [1, 15]. Measurements in congested residential areas indicate somewhat less attenuation than among large buildings. Effects of Trees and Other Vegetation When an antenna is surrounded by moderately thick trees and below treetop level, the average loss at 30 MHz resulting from the trees is usually 2 or 3 dB for vertical polarization and negligible with horizontal polarization.
However, large and rapid variations in the received field strength can exist within a small area, resulting from the standing-wave pattern set up by reflections from trees located at a distance of as much as ft 30 m or more from the antenna. Consequently, several nearby locations should be investigated for best results. At MHz, the average loss from surrounding trees may be 5 to 10 dB for vertical polarization and 2 or 3 dB for horizontal polarization. The tree losses continue to increase as the frequency increases, and above to MHz they tend to be independent of the type of polarization.
Above MHz, trees that are thick enough to block vision present an almost solid obstruction, and the diffraction loss over or around these obstructions can be obtained from Figures 1. There is a pronounced seasonal effect in the case of deciduous trees, with less shadowing and absorption in the winter months when the leaves have fallen. However, when the path of travel through the trees is sufficiently long that it is obscured, losses of the above magnitudes can be incurred, and the principal mode of propagation may be by diffraction over the trees.
When the antenna is raised above trees and other forms of vegetation, the prediction of field strengths again depends upon the proper estimation of the height of the antenna above the areas of reflection and of the applicable reflection coefficients. For growth of fairly uniform height and for angles near grazing incidence, reflection coefficients will approach 1 at frequencies near 30 MHz.
As indicated by Rayleighs criterion of roughness, the apparent roughness for given conditions of geometry increases with frequency so that near MHz even such low and relatively uniform growth as farm crops or tall grass may have reflection coefficients of about 0. The distribution of losses in the immediate vicinity of trees does not follow normal probability law but is more accurately represented by Rayleighs law, which is the distribution of the sum of a large number of equal vectors having random phases. Effects of the Lower Atmosphere Troposphere Radio waves propagating through the lower atmosphere, or troposphere, are subject to absorption, scattering, and bending.
The index of refraction of the atmosphere, n, is slightly greater than 1 and varies with temperature, pressure, and water vapor pressure, and. An exponential model showing a decrease with height to 37 to 43 mi 60 to 70 kin is generally accepted [18, 19]. For this model, variation of n is approximately linear for the first kilometer above the surface in which most of the effect on radio waves traveling horizontally occurs. For average conditions, the effect of the atmosphere can be included in the expression of earth diffraction around the smooth earth without discarding the useful concept of straight-line propagation by multiplying the actual earths radius by k to obtain an effective earths radius, where.
Stratification and Ducts As a result of climatological and weather processes such as subsidence, advection, and surface heating and radiative cooling, the lower atmosphere tends to be stratified in layers with contrasting refractivity gradients . The radio energy thus is trapped in a duct or waveguide. The wave also may be trapped between two elevated layers, in which case energy is not lost at the ground reflection points and even greater enhancement occurs. Radio waves thus trapped or ducted can produce fields exceeding those for free-space propagation because the spread of energy in the vertical direction is eliminated as opposed to the free-space case, where the energy spreads out in two directions.
Ducting is responsible for abnormally high fields beyond the radio horizon. These enhanced fields occur for significant periods of time on overwater paths in areas where meteorological conditions are favorable. Such conditions exist for significant periods of time and over significant horizontal extent in the coastal areas of southern California and around the Gulf of Mexico. Over land, the effect is less pronounced because surface features of the earth tend to limit the horizontal dimension of ducting layers .
Tropospheric Scatter The most consistent long-term mode of propagation beyond the radio horizon is that of scattering by small-scale fluctuations in the refractive index resulting from turbulence. Energy is scattered from multitudinous irregularities in the common volume which consists of that portion of troposphere visible to both the transmitting and receiving sites. There are some empirical data that show a correlation between the variations in the field beyond the horizon and N, the difference between the reflectivity on the ground and at a height of 1 km .
Procedures have been developed for calculating scatter fields for beyond-the-horizon radio relay systems as a function of frequency and distance [22, 23]. These procedures, however, require detailed knowledge of path configuration and climate. The effect of scatter propagation is incorporated in the statistical evaluation of propagation considered previously in this chapter , where the attenuation of fields beyond the diffraction zone is based on empirical data and shows a linear decrease with distance of approximately 0.
Atmospheric Fading Variations in the received field strengths around the median values are caused by changes in atmospheric conditions. Field strengths tend to be higher in summer than in winter, and higher at night than during the day, for paths over land beyond the line of sight. As a first approximation, the distribution of long-term variations in field strength in decibels follows a normal probability law. Measurements indicate that the fading range reaches a maximum somewhat beyond the horizon and then decreases slowly with distance out to several hundred miles.
Also, the fading range at the distance of maximum fading increases with frequency, while at the greater distances where the fading range decreases, the range is also less dependent on frequency.