ADAPTATION OF EMPIRICAL ELECTRIC FIELD STRENGTH MODELS FOR TERRESTRIAL TELEVISION BROADCAST IN EKITI STATE, NIGERIA

ABSTRACT

This study investigates the coverage areas of VHF and UHF signals of two television stations in Ekiti State, Nigeria by quantitative measurement of the electric field strength. The signal levels of the Nigeria Television Authority (NTA), Ado Ekiti, Channel 5, (175.25 MHz) and Broadcasting Service of Ekiti State (BSES), Channel 41, (631.25 MHz) transmitters were measured   radially   along   several   routes   with   the   transmitting   stations   at   focus.   Their corresponding distances from the transmitting stations and locations were also measured and recorded. These measurements were taken using Digital Signal Level Metre, GE-5499, having a signal level range of 30 dBµV – 120 dBµV, and Global Positioning System (GPS) 72 – Personal Navigator. From the data obtained, Surfer 13 software application was used to draw the contour maps of the signal levels around the transmitting stations. The results obtained show that the present configurations of the transmitters of the two television stations do not give an optimal coverage of the state. Only 64.89 % of the entire land mass of the state has television signal coverage. Consequently, some areas in the state are completely out of television signal coverage. So, there is need to have repeater stations at certain intervals to provide reception of television signals throughout the state. Furthermore, this research alsoadapted some field strength models that are best suitable for Ekiti State, Nigeria. The models are free space, Hata, ITU-R P.529-3 and ERC Report 68 models. The results obtained show that the generalised free space model gives more accurate prediction for the field strength of the two television stations in Ekiti State, with the correction factor of -37.84 and Root mean square error of 6.28 dBµV/m for VHF signal(NTA Ado Ekiti) and the correction factor of  -25.48 and Root mean square error of 6.21dBµV/m for UHF signal(BSES, Ekiti).

CHAPTER ONE

1.0  INTRODUCTION

1.1  Background to the Study

Radio communications, like all other communications, rely on the atmosphere as the medium through which the signals travel from the transmitter to the receiver. As a result, the quality of the communications is dependent on the physical factors that influence the propagation of electromagnetic (EM) signals in this medium (Ajewole et al., 2013). The quality and high capacity networks together with accurate estimation of coverage is extremely important. Therefore, accurate design coverage of modern cellular networks and signal strength measurements must be taken into consideration in order to provide an efficient and reliable coverage area (Mardeni and Kwan, 2010).

At broadcast frequencies in Very high frequency (VHF) and Ultra high frequency (UHF) bands (30 MHz- 3 GHz), propagation is usually by ground waves which consist of direct wave, ground reflected and surface wave. Therefore, in these frequency bands, electrical parameters of the ground, curvature of the earth surface, height of the antenna and weather conditions influence wave propagation. The degree to which these factors affect propagation depends primarily on the frequency of the wave and the polarisation (Hall, 1991).

The electrical field strength at a given distance from the transmitter is attenuated by these parameters, with the result that radio services in VHF and UHF bands are limited to distances close to the transmitter. Electric field strength curves or propagation curves are essential parameters neccessary for the planning of VHF and UHF transmission especially for the determination of the coverage areas and the field strength signal levels desired.

The field strength of an antenna’s radiation at a given point in space, is equal to the amount of voltage induced in a wire antenna 1m long located  at that given point (Kennedy and Bernard, 1992). This field strength is affected by a number of conditions such as time of day, atmospheric conditions, transmitter-receiver distance, transmitter power and others like, terrain effect, transmitting and receiving antenna heights, and the gain of the transmitting antenna (Bothias, 1987). The present trend in broadcasting is to use widespread broadcast transmitter of VHF or UHF range of frequencies to serve areas not far away from the transmitter (Barclay, 1991).

The coverage areas of broadcast stations is the distance away from the transmitter in which the electric field transmitted signal; voice (audio) and picture (video) for television and voice alone for radio can be received by the veiwer or listener with the aid of a receiving antenna. All stations have their own expected coverage areas and their signals should not interfere with others (BON, 2010).

The coverage areas of broadcast stations are usually classified into primary, secondary and fringe areas (Ajewole et al., 2013).  Apart from weather conditions; the size of each of these areas also depends on the transmitter power, the directivity of the aerial, the ground conductivity and the frequency of propagation. The coverage area decreases with increase in frequency and reduction in the ground conductivity (Moses et al., 2013).

The primary coverage area is defined as a region about a transmitting station in which the signal strength is adequate to override ordinary interference in the locality at all times, and corresponds to the area in which the signal strength is at least  60 dBµV. The  quality of service enjoyed in this area can be regarded as grade A1. The appropriate value of the signal strength for this quality of service is also dependent on the atmospheric conditions and man-made noise in the locality. The signal strength also depends on whether the locality is rural, industrial or urban.

The secondary coverage area is a region where the signal  strength is often sufficient to be useful but is insufficient to overcome interference completely at all times. The service provided in this area may be adequate in rural areas where the noise level is low. The secondary coverage area corresponds to the area in which the signal strength is at least 30 dBµV but less than 60 dBµV. The quality of service enjoyed in this area can be regarded as Grade B1 (Moses et al., 2013).

The fringe service area can be regarded as that in which the electric field strength can be useful for some periods, but its service can neither be guaranteed nor be protected against interference. This is an area in which the electric field strength is greater than 0 dBµV but less than 30 dBµV. Such an area may be said to enjoy Grade B2 service (Ajewole et al., 2013).

1.2 Radio Propagation

Radio propagation is the behaviour of radio waves when they are transmitted, or propagated from one point on the earth or into various part of the atmosphere (Westman, 1968). As a form of electromagnetic radiation like light waves, radio waves are affected by the phenomena of reflection,  refraction,  diffraction,  absorption,  polarization  and  scattering  (Demetrius  and Kenneth, 1969). Radio signal propagation is  affected by the daily changes of water vapor in the troposphere and ionisation in the upper atmosphere, due to sun. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems.

Radio propagation is also affected by several other factors determined by its path from point to point. This path can be a direct line-of-sight path or an over-the-horizon path aided by refraction in the ionosphere, which is a region between approximately 60 and 600 km. Radio waves at different frequencies propagate in different ways. At extra low frequencies (ELF) and very low frequencies (VLF) the wavelength is very much larger than the separation between the earth’s surface and the D layer of the ionosphere, so electromagnetic waves may propagate in this region as  a  waveguide.  Indeed,  for  frequencies  below  20 kHz,  the  wave  propagates  as  a  single waveguide mode with a horizontal magnetic field and vertical electric field (Hall and Barclay, 1989).

1.2.1  Modes of radio wave propagation

The way that radio signal propagate, or travel from the radio transmitter to the receiver   is of great importance when planning a radio communication network. Signal at VHF and UHF signal bands can be propagated by a variety of modes, depending on the particular mode that is dominant at the time of   reception, the distances covered by the VHF and UHF signals can extend to hundreds or thousands of miles. Below are some of the modes for VHF and UHF propagation; such as ground mode (for ground or surface waves), direct mode(for direct or line- of-sight waves), ionospheric mode (for sky waves). Others are tropospheric and sporadic-E- propagations (Freeman, 2007). Figure 1.1 shows the mode of radio wave propagation.

1.2.2 Ground wave propagation

Ground wave propagation is a method of radio wave propagation that uses the area between the surface of the earth and the ionosphere for transmission. The ground wave can propagates a considerable distance over the earth’s surface particulaly in the low frequency and medium frequency portion of the radio spectrum. Ground wave   radio signal propagation is ideal for relatively short distance propagation on these frequencies during the daytime. Sky-wave ionospheric propagation is not possible during the day because of the attenuation of the signals on these frequencies caused by the D region in the ionosphere. Ground wave radio signal is made up of a number of constituent waves. If the antennas are in the line of sight then there will be direct wave as well as a reflected signal. Direct signal is one that travels directly between the two antennas and is not affected by the locality. There will also be a reflected signal as the transmission will be reflected by a number of objects including the earth’s surface and any hills,or large buildings that may be present.

In addition to this there is surface wave, this tends to followthe curvature of the earth andenables coverage beyond the horizon. It is the sum of all these components that is known as the ground wave.  Beyond the horizon the direct and reflected waves are blocked by the curvature of the Earth, and the signal is purely made up from the diffracted surface wave. It is for this reason that surface wave is commonly called ground wave propagation (Electronicsforu, 2019). Figure 1.2 shows the ground wave propagation.

1.2.3 Surface wave

The radio signal spreads out from the transmitter along the surface of the Earth. Instead of just travelling in a straight line the radio signals tend to follow the curvature of the Earth. This is because currents are induced in the surface of the earth and this action slows down the wave- front  in  this  region,  causing  the  wave-front  of  the  radio  communications  signals  to  tilt downwards towards the Earth. With the wave-front tilted in this direction it is able to curve around the Earth and be received well beyond the horizon (Freeman, 2007). Figure 1.3 shows the surface wave.

1.2.4 Line-of-sight propagation

Line-of-sight (LOS) propagation is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies. Radio signals at VHF are mainly used in LOS communication.

At VHF, the bands are divided into channels and one channel is usually as  good as the next. This is in contrast to medium frequency (MF) and high frequency (HF) where the choice of a frequency channel may be crucial for good communications. For a wide coverage of Television (TV) signals, the transmitter should be as high as possible and free from obstructions. Moreso, the signals can travel through many non-mettalic objects and can be picked up through walls. To receive quality signals by a viewer, the receiving antenna should be positioned in such a way that it can see the transmitter very well and vice versa. That is the line of sight of the two antennas with respect to each other should be parallel (or the same).

Ground plane reflection effects are important factor in VHF line of sight propagation. Affecting this mode of propagation also, is the earth’s curvature. Shore stations are usually on the tops of hills to provide maximum range, but even the highest hills do not provide coverage beyond about 80 km, because of the Earth’s curvature. VHF signals also suffer from atmospheric noise during severe electrical storms in the atmosphere, in the absence of such storms, interference mainly results from many users wishing to the limited number of channels, and this can be significant problem in densely populated areas.

1.2.5 Ionospheric wave propagation

Ionospheric wave propagation, also referred to as sky wave propagation, is the mode of radio wave propagation that relies on the ionosphere, which is made up of one or more ionized layers in the upper atmosphere. F2-layer is the most important ionospheric layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers also play significant roles.

The D-layer, when present during sunlight periods, causes significant amount of signal loss, as does the E-layer whose maximum usable frequency can rise to 4 MHz and above and thus block higher frequency signals from reaching the F2-layer. The layers, or more appropriately “regions”, are directly affected by the sun on a daily diurnal cycle, a seasonal cycle and the 11-year sunspot cycle and determine the utility of these modes. During solar maxima, or sunspot highs and peaks, the whole HF range up to 30 MHz can be used usually around the clock and F2 propagation up to 50 MHz is observed frequently depending upon daily solar flux 10.7 cm radiation values. During solar minima, or minimum sunspot counts down to zero, propagation of frequencies above 15 MHz is generally unavailable (Hull, 1967).

1.2.6 Tropospheric ducting.

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic  weather.  In  this  propagation  method,  when  the  signal  encounters  a  rise  in

temperature in the atmosphere instead of the normal decrease(known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to  (1,300 km) (though some people have received “tropo” beyond 1,600 km), while with tropospheric-bending, stable signals with good signal strength from  (800 km) away are not common when the  index of the atmosphere is fairly high.

Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidity’s. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.

Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion, and the boundary between the two air masses may extend for (1,600 km) or more along a stationary weather front.

Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers. Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are generally more favorably propagated.

High mountainous areas and undulating terrain between the transmitter and receiver can form an effective  barrier  to  tropospheric  signals.  Ideally,  a  relatively  flat  land  path  between  the transmitter and receiver is ideal for Tropospheric ducting (Rijn, 2005).

1.2.7 Sporadic E

Sporadic E or Es  is an unusual form of radio propagation using characteristics of the Earth’s ionosphere. Whereas most forms of sky wave propagation use the normal and cyclic ionization properties of the ionosphere’s F region to refract (or “bend”) radio signals back toward the Earth’s surface, sporadic E propagation bounces signals off smaller “clouds” of unusually ionized atmospheric gas in the lower E region (located at altitudes of approximatelly 90 to 160 km).

This occasionally allows for long-distance communication at VHF frequencies not usually well- suited to such communication. Communication distances of 800–2200 km can occur using a single Es  cloud. This variability in distance depends on a number of factors, including cloud height and density .

1.3 Mechanism of Radio Wave Propagation

Ground  waves  exist  only for vertical  polarization,  produced  by vertical  antennas  when  the transmitting and  receiving antennas  are closed  to the surface of the  earth.  The transmitted radiation  induces  currents  in  the  earth’s  surface  being  attenuated  according  to  the  energy absorbed by the conducting earth (John and Smith, 1997). The ineffectiveness of horizontal electric field is due to the energy loss through the earth as the signal propagates.

Ground wave propagation is common for frequencies of a few MHz. Sky wave propagation is mainly dependent on reflection from the ionosphere, a region above earth’s surface of ratified air that is ionospheric by sunlight (primary ultraviolet radiation). The ionosphere is responsible for long distance communication in the high frequency band between 3 and 30 MHz, but it is very dependent on time of day, season, and longitude on the earth. It makes possible, long-range communication using very low power transmitters.

The most important propagation mechanism for short-range communication on the VHF and UHF bands is that which occurs in open fields, where the received signal is a vector sum of a direct  line-of-sight  signal  and  as  signal  from  same  source  that  is  reflected  off  the  earth (Rappaport, 1996). This shows that there exist a relationship between signal strength and range in line-of-sight signals and open field topographic. The range of line-of-sight when there is no reflection from the earth or ionosphere is a function of the dispersion of the waves from the transmitting antenna.

For this free-space case, the signal strength decreases in inverse proportion to the distance away from the transmitter antenna (Seybold, 2005).

1.4 Free Space Propagation

In free space, all electromagnetic waves obey the inverse-square law. The inverse-square law states that the power density of an electromagnetic wave is proportional to the inverse of the square of the distance from the source. That is, if the distance from a transmitter is doubled, the power density of the radiated wave at the new location is reduced to one-quarter of its previous value. The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of their received field strengths over a free-space path by one-half (Westman, 1968). Also, the electromagnetic waves coming from a transmitter may experience three other phenomena: reflection, diffraction, and scattering. All of these factors affect the tranmitted signal as it is “carried” through the air medium to the distant receiving antenna.

The range of VHF transmission depends on the transmitting antenna height, transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a near line-of-sight phenomenon.Radio wave are weakly bent back toward the Earth  by the atmosphere,so  the distance to  the  radio  horizon  is  slightly extended  over the geometric line-of-sight to the horizon (Beasley and Miller, 2010).

1.5 Radio Frequency Spectrum

The radio frequencies spectrum ( as shown in Table 1.1) is the part of the electromagnetic spectrum with frequencies from 30 Hz to 300 GHz. Electromangetic waves in this frequency range called radio waves and widely used in modern technology particulary in telecommunication. The generation and transmission of radio waves to prevent interference between different users is strictly regulated by national laws and coordinated by an international body; the International Telecommunication Union (ITU). The radio spectrums of different parts are allocated by the ITU for different radio transmission technologies and applications. Parts of the radio spectrum are sold or licenced in some cases to operators of private radio transmission services (for example, broadcast television stations or cellular telephone operators). Allocated frequency ranges are often referred to by their provisioned use (for example, cellular spectrum or television spectrum) (Robinson, 2003).

1.6 Statement of the Research Problem

The major challenge facing the coverage area of television signal is the unwanted reduction in the  signal  strength  due  to  environmental  factors (such  as  terrain  and  building pattern) and atmospheric factors (such as temperature, pressure, humidity and water vapours) which may reduce the strength of the signal. Therefore, it is important to understand how these factors impact on the propagating radio signal. There are numerous propagation prediction models but none of these models can be generalised for all environments and localities. Propagation models are usually suitable for particular areas such as (urban, suburban and rural), terrain and climate. To overcome these drawbacks, a propagation models parameter (such as height of the mast, frequency, power of transmitter, distance, and mobile antenna height) can be adjusted according to the targeted environment to achieve minimal error between predicted and measured signal strength.

1.7 Justification of the Study

For a proper coverage area prediction, propagation models are necessary. Use of inaccurate propagation models result in high co-channel interference and poor network coverage. hence, this research work will provide a model for ascertaining the electric field strength of the study area, which will   enhance optimal performance of Television (TV) broadcasting system within the coverage area.

1.8 Aim and Objectives of the Study

The aim of this research is to determine the coverage areas of VHF/UHF television signal in

Ekiti State, and adapt the existing empirical electric field models to suit the state. The objectives of this research are to:

(i)    modify some field strength models and determine the one that is most suitable for the state; and

(ii)  determine the coverage areas of the VHF and UHF television stations in Ekiti State.

1.9 Study Area

The study was carried out in Ekiti State, Nigeria. The State is located in the South Western part of Nigeria between latitude 7°40’N and latitude 7.667°N and longitude 5°15’E and 5.250°E with the capital at Ado-Ekiti. The State is bounded in the North by Kwara State and Kogi State while Osun State occupies the West and Ondo State lies in the South and extends to the eastern part. Ekiti State has sixteen local government areas with an overall population of about 2,384,212 people that spread over an approximately 88.7 𝑘�2. The region lies at about 250 m above the sea level and rhythmically undulating surface. The landscape consists of ancient plains broken  by steep-sided out cropping dome rocks. These rocks may occur singularity or in groups or in ridges and the most notable of these are to be found in Efon-Alaye, Ikere Ekiti and Okemesi Ekiti. The State is dotted with rugged hills, notable ones being Ikere-Ekiti hills in the south, Efon-Alaye hills on the western boundary and Ado-Ekiti hills in the center. The state  enjoys  a tropical climate with two distinct seasons. These are the rainy season (April-October) and the dry season (November-March) and the temperature ranges between 21°C and 28°C with high humidity (Salau, 2016). Figure 1.4 shows the map of Ekiti State in Nigeria.

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