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Spatial variability of VHF/UHF electric field strength in Niger State, Nigeria.

1 INTRODUCTION

At broadcast frequencies in the VHF and 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, the 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 polarization [1]. The electric field strength at a given distance from the transmitter is attenuated by these parameters, with the result that radio services in the VHF and UHF bands are limited to distances close to the transmitter.

The coverage areas of broadcast stations are usually classified into primary, secondary and fringe areas. 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 [2].

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 dB[micro]V. 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 dB[micro]V, but less than 60 dB[micro]V. The quality of service enjoyed in this area can be regarded as Grade B1.

The fringe service area is that in which the signal strength can be useful for some periods, but its service can neither be guaranteed nor protected against interference. This is an area in which the signal strength is greater than 0 dB[micro]V, but less than 30 dB[micro]V. Such an area may be said to enjoy Grade B2 service [3].

This study investigates the coverage areas of the Nigeria Television Authority, (NTA), Minna, Channel 10 (210.25 MHz); Nigeria Television Authority (NTA), Kotangora, Channel 8 (196.25 MHz); and Niger State Television, Minna, UHF 25 (503.25 MHz), by means of quantitative measurement of the signal levels of the corresponding signals at increasing distances from each transmitter until the signals completely fade out.

2 THEORETICAL BACKGROUND

All electromagnetic waves obey the inverse-square law in free space. 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. Also, the electromagnetic waves coming from a transmitter may experience three other phenomena: reflection, diffraction, and scattering. All of these factors affect the transmitted signal as it is "carried" through the air medium to the distant receiving antenna [4].

The range of a 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 waves 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.

An approximation to calculate the line-of-sight horizon distance (on Earth) is given as [5]:

d = [square root of 12.746 x [h.sub.b]] (km) (1)

where [h.sub.b] is the height of the antenna in meters.

This approximation is only valid for antennas at heights that are small compared to the radius of the Earth. In communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station [5].

2.1 Influence of Clear-Air Aspect of the Troposphere on VHF and UHF Propagation

In a vacuum, electromagnetic waves propagate along straight lines with velocity c (velocity of light in vacuum). The electromagnetic properties of the air are slightly different from that of a vacuum, and are characterized at each point by the refractive index, n=c/v, with v as the local electromagnetic wave propagation velocity [6]. Clear-air effects refer to atmospheric influences that do not involve condensed water vapour (such as cloud, rain, wet, haze, hail, and so on); but the effects of dry haze and dust particles are included [7].

Electromagnetic waves propagating through the troposphere are refracted and scattered by variations in the radio refractive index, n which in turn are caused by variation in pressure P(mb), temperature T(K), and water vapour pressure 'e' (mb) according to the formula:

N = (n--1) [10.sup.6] = 77.6 P/T + 3.73 x [10.sup.5] e/[T.sup.2] (2)

Here N is the refractivity which is a practical quantity often used in preference to 'n'. The refractive index at the earth's surface, denoted by '[n.sub.s]' is about 1.0003. It falls to the value of unity at great heights. The little difference in the value 'n' from unity in the troposphere is the reason for the use of practical unit N. Its value, denoted by '[N.sub.s]' at earth's surface is about 300. Values of N are often expressed in "N-units" though it is a dimensionless quantity.

The water vapour pressure 'e' is usually calculated from the relative humidity, and saturated water vapour, using the expression:

e = H x 6.1121exp (17.502t/t + 240.97)/100 (3)

where H = relative humidity (%) and t = temperature ([degrees]C) [8].

The macroscopic structure of the troposphere varies much more rapidly vertically than horizontally. For this reason, the troposphere is said to be vertically stratified. The same vertical stratification may persist over a horizontal region tens or hundreds of kilometers in extent. As temperature decreases with altitude, the air above a certain level is saturated and excess moisture precipitates out. This water vapour pressure, e, decreases more rapidly with height than pressure, P, and to all intents and purposes is negligible above 2 or 3 km [9].

The net effect of the variation in pressure P, temperature T and water vapour pressure e is that N decreases with height. The average behaviour of N in the troposphere is the exponential decrease given as:

N = [N.sub.s]exp(-z/Z) (4)

where [N.sub.s] is the surface value of N, z is height above the earth's surface and Z is a scale height.

The two terms in the refractivity N in equation (2) are often separated into dry term D and wet term W:

D = 77.6 P/T (5)

W = 3.73 x [10.sup.5] e/[T.sup.2] (6)

Both terms decay with height but at different rates leading to the bi-exponential model with corresponding surface values Ds, Ws and scale heights [Z.sub.d], [Z.sub.w] respectively:

N = [D.sub.s]exp(-z/[Z.sub.d]) + [W.sub.s]exp(-z/[Z.sub.w]) (7)

Values [Z.sub.d] = 9 km and [Z.sub.w] = 2.5 km are typical. The dry term [D.sub.s] makes a relatively constant contribution to [N.sub.s] of about 265 whilst the wet term Ws provides most of the variability of N.

The decrease of N with height bends rays towards the earth. In average conditions, N decreases by about 40 N-units/km in the lowest kilometer of the troposphere. However, the troposphere is a varying medium and quite significant deviations from average condition occur. Over a given height range, refractivity profiles can be designated as: sub-refractive, normal, superrefractive or ducting, according to [9]:

[partial derivative]N/[partial derivative]Z > -40 Sub-refractive (8)

[partial derivative]N/[partial derivative]Z [approximately equal to] -40 Normal (9)

[partial derivative]N/[partial derivative]Z < -40 Super-refractive (10)

[partial derivative]N/[partial derivative]Z < -157 Ducting/Trapping (11)

Typical propagation ray profiles under these refractivity conditions are as shown in Figure 1.

3 STUDY AREA

Niger State is a State in North central Nigeria and the largest state in the country. It is named from the River Niger and Minna is the State capital. Niger State is located between latitudes 8[degrees]20'N and 11[degrees]30'N and longitude 3[degrees]30'E and 7[degrees]20'E. Niger State is bordered to the north by Zamfara State, to the northwest by Kebbi State, to the south by Kogi State, to the southwest by Kwara State; while Kaduna State and the Federal Capital Territory border the State to the northeast and southeast, respectively (Figure 2).

Furthermore, Niger State shares a common international boundary with the Republic of Benin at Babanna in Borgu Local Government Area of the state. Currently Niger State covers a total land area of 76,363 sq. km, or about 9 percent of Nigeria's total land area. This makes the State the largest in the country. Niger State has a population of 3,950,249 (2006 Population Census) and population density of 52/[km.sup.2] [11, 12].

Climate and Vegetation: Niger State experiences dry and wet seasons within a year. The annual rainfall varies from 1,100mm in the northern parts to 1,600mm in the southern parts. The maximum temperature is recorded between March and June which is usually not more than 37 [degrees]C, while the minimum temperature is between December and January (usually not less than 19 [degrees]C), when most parts of the state come under the influence of the tropical continental air mass which blows from the north. The rainy seasons last for about 150 days in the northern parts to about 120 days in the southern parts of the state. The fertile soil and hydrology of the state support the cultivation of most of Nigeria's staple crops and still allows sufficient opportunities for grazing, fresh water fishing and forestry development [13].

The Southern Guinea Savannah vegetation covers the entire landscape of the state. Like in other states of similar vegetation, it is characterized by woodlands and tall grasses interspersed with tall dense species. However, within the Niger trough and flood plains are taller trees and a few oil palm trees. In some areas, traces of rain forest species can be seen [13].

4 DATA COLLECTION AND ANALYSIS

This research was carried out for three different television stations, namely: Nigeria Television Authority (NTA), Minna, Channel 10, Nigeria Television Authority (NTA), Kotangora, Channel 8 and Niger State Television, Minna, Channel 25.

NTA Minna transmits at 210.25 MHz for video signal and 215.25 MHz for audio signal. The output power of the transmitter during the period of this work was substantially constant at 7.5 kW. NTA Kotangora transmits at 196.25 MHz for video signal and 201.75 MHz for audio signal with an output power of 2 kW. Niger State Television, Minna, transmits at 503.25 MHz for video signal and 508.75 MHz for audio signal. The output power of the transmitter in this case fluctuates between 100 and 500 W.

The signal levels of the transmitters of the three television stations were taken along some radial routes from the transmitting stations as shown in Figure 3, using Digital Signal Level Meter GE5499. The corresponding transmitter-receiver distances, elevations above the sea level and locations were also measured using GPS 72 Personal Navigator. Measurements were taken (at distances further from each transmitter) around the towns and villages in all the local government areas in Niger State until all the signals faded away completely. Data obtained along one of the radial routes in Minna is shown in Table 1.

From the data obtained, Surfer 8 software application was used to draw contour maps of the signal levels around the transmitting stations to determine the coverage areas of the stations. The coverage area is divided into three different areas based on the following classification of signal strength E:

i. Primary Coverage Area, E > 60 dB[micro]V

ii. Secondary Coverage Area, 60 dB[micro]V > E > 30 dB[micro]V

iii. Fringe Coverage Area, 30 dB[micro]V > E > 0 dB[micro]V

5 RESULTS

Figures 4 to 7 show the contour maps of the signal levels around the three transmitting stations and their coverage areas in the state, while Figure 8 shows elevation of the ground around the coverage areas of the stations under study. Tables 2 to 5 show the television signal coverage areas as percentage of the total land mass and the local government areas. The results obtained show that:

i. The present configurations of the transmitters of the three television stations do not give an optimal coverage of the state. Only 25.82% of the entire land mass of the state has television signal coverage. So, greater percentage of Niger State is completely out of television signal coverage.

ii. The contour maps show the need for repeater stations at appropriate intervals to provide reception of television signals for the entire state.

6 SUMMARY AND CONCLUSION

The coverage areas of the transmitting stations show that less than 1% of the entire land mass of Niger State has television signals strong enough to override ordinary interference in the locality at all times, and comprise the primary coverage area. About 9.21% of the state also enjoys good television signals but not strong enough to overcome interference completely at all times, thus within the secondary coverage area. But the service provided in this area may be adequate in rural areas where the noise level is low. Also, 18% of the state is in fringe service areas. In such areas, the service can neither be guaranteed nor protected against interference, and antennas with high antenna gain and heights higher than the surrounding buildings and obstacles are needed to receive good signals. In summary, only 28.82% of the entire land mass of Niger State has television signals coverage. More than 71% of Niger State does not receive television signals from any of the three television stations in the state. Thus, the present configurations of the transmitters of the three television stations do not give optimal coverage of the total land mass of Niger State.

The rate of attenuation of the VHF and the UHF signals is very high in Niger State. This may be as result of high loss of television signals due to diffraction by some physical features like hills and vegetations covering about 60% of the entire land mass and irregular elevation of the surface of the ground. The ground elevation is extremely high in some places and extremely low in some other parts of the state as shown in Figure 8.

It is also noted that none of the two NTA coverage areas overlap at any point. Therefore, there is no town or village in Niger State that can receive television signals from the two NTA television stations at the same time. So, installation of repeater stations at certain intervals of distance to provide reception of television signals for the entire state is necessary.

The coverage areas also show that none of the television stations in Niger State constitutes potential interference to any of the television stations in the neighboring states. Hence, the stations are in compliance with the Nigeria Broadcasting Corporation (NBC) regulation.

7 REFERENCES

[1.] Hall, M.P.M.: Overview of Radio wave propagation. In: Hall, M.P.M. and Barclay, L.W. (Ed); Radio Wave Propagation--(IEE Electromagnetic Wave Series). Peter Peregrinus Ltd, London, United Kingdom, pp. 121 (1991).

[2.] Ajayi, G.O. and Owolabi, I.E.: Medium Wave Propagation Curves (for use in medium wave transmission planning and design). Technical Report of the Radio wave propagation Research Group, Department of Electronic and Electrical Engineering, University of Ife, Nigeria, pp. 3-4 (1975).

[3.] Ajayi, G.O. and Owolabi, I.E.: Coverage Area of the 10kW, 702 kHz Medium Wave Transmitter at Minna and Feasibility Studies for full Radio Coverage of Niger State". Technical Report of the Electrical Communication Consultancy Unit (ECCU), Department of Electrical and Electronics Engineering, University of Ife, Nigeria, pp. 1-2 (1979).

[4.] Propagation Characterization, taken from http://transition.fcc.gov/pshs/techtopics/techtopics17.ht ml

[5.] Very High Frequency, taken from http://en.wikipedia.org/wiki/Very high frequency

[6.] ITU-R, Document 3/11-E, 10 March, 1995, Working Party 3J: Draft Handbook on Radiometerology, pp. 2568.

[7.] Oyedum, O.D.: Effects of Tropospheric Conditions on Microwave Propagation in Nigeria. Nigerian Journal of Space Research (NJSR). Vol. 3, 81-100 (2007).

[8.] Adediji, A.T. and Ajewole, M.O.: Vertical Profile of Radio Refractivity Gradient in Akure South-West Nigeria. Progress in Electromagnetic Research C, Vol. 4, 157-168 (2008).

[9.] Rotheran, S.: Clear air aspects of the troposphere and their effects on propagation mechanisms from VHF to millimetre waves. In: Hall, M.P.M. and Barclay, L.W. (Ed); Radio Wave Propagation--(IEE Electromagnetic Wave Series). Peter Peregrinus Ltd, London, United Kingdom, pp. 150-172 (1991).

[10.] Bruce, W.: Atmospheric Refraction: How Electromagnetic Waves Bend in the Atmosphere and Why It Matters.

www.weather.nps.navy.mil/.../Atmospheric_Refractio n_of_EM_Waves.doc

[11.] Niger State, taken from http://en.wikipedia.org/wiki/Niger State

[12.] About Niger, taken from http://www.nigerstate.gov.ng/about-niger.html

[13.] Nigeria: Physical setting--Niger State, taken from http://www.onlinenigeria.com/links/nigeradv.asp7blurb =330

Moses Oludare Ajewole (1), Onyedi David Oyedum (2), Adekunle Titus Adediji (1), Abiodun Stephen Moses (2) and Julia Ofure Eiche (2)

(1) Department of Physics, Federal University of Technology, Akure, Ondo State, Nigeria

(2) Department of Physics, Federal University of Technology, Minna, Niger State, Nigeria

*E-mail: mosesdotcom@gmail.com

Table 1. Data obtained along one of the radial routes in Minna, Niger
State

Distance (km)     Signal         Location                    Elevation
(from the         Levels                                        (m)
transmitting    (dB[micro]V)    Latitude       Longitude
Station)                       ([degrees]N)   ([degrees]E)

0                   90.0          9.610          6.559          298
3.35                80.1          9.573          6.572          256
6.97                73.0          9.553          6.582          236
8.54                65.4          9.542          6.582          231
14.29               51.4          9.526          6.581          277
18.08               56.5          9.467          6.638          309
21.97               42.5          9.422          6.621          307
23.11               45.3          9.429          6.662          367
25.62               38.1          9.422          6.694          374
29.06               45.2          9.406          6.724          392
31.28               42.0          9.402          6.750          381
34.02               39.6          9.393          6.777          378
37.48               32.5          9.392          6.819          363
40.48               47.3          9.395          6.856          373
45.49               27.0          9.398          6.913          393
50.53               27.0          9.390          6.969          454
60.26                0            9.278          6.995          294

Table 2. Percentage Of The Coverage Areas Of The Television Stations
relative To The Total land Mass Of Niger State

Stations                 % of Primary    % of Secondary
                         coverage area   coverage area

NTA Minna                    0.65%           4.15%
NTA Kotangora                0.22%           4.74%
Niger State Television      0.005%           0.32%

Stations                  % of Fringe     Total % of
                         coverage area   coverage area

NTA Minna                    9.38%          14.18%
NTA Kotangora                6.68%          11.64%
Niger State Television       1.97%          2.295%

Table 3. Percentage of the local government areas covered by the NTA
Minna station in Niger State

L.G.A       Average Distance   % of L.G.A.   % of L.G.A.
             (km) (from the       with          with
              transmitting       primary      secondary
                station)        coverage      coverage
                                  area          area

Chanchaga         3.82            100%           --
Bosso            16.73            7.14%        64.29%
Paikoro          30.37            3.80%        46.20%
Shiroro          36.06            2.80%        22.20%
Kaicha           44.93             --           2.27%
Muya              39.7             --           5.00%
Wushishi         54.55             --            --
Gurara           60.38             --            --
Gbako             59.3             --            --
Rafi             78.41             --            --
Lavun            99.13             --            --
Agaie            79.05             --            --
Lapai            72.79             --            --

L.G.A       % of L.G.A.   Total % of
            with fringe     L.G.A.
             coverage      coverage
               area          area

Chanchaga       --           100%
Bosso         28.57%         100%
Paikoro        7.70%        57.70%
Shiroro       19.40%        44.40%
Kaicha        45.45%        47.72%
Muya           2.50%         7.50%
Wushishi      69.20%        69.20%
Gurara        25.00%        25.00%
Gbako         73.10%        73.10%
Rafi          13.60%        13.60%
Lavun          5.26%         5.26%
Agaie          3.85%         3.85%
Lapai          6.70%         6.70%

Table 4. Percentage of the local government areas covered by the NTA
Kotangora station in Niger State

L.G.A       Average Distance   % of L.G.A.   % of L.G.A.
             (km) (from the       with          with
              transmitting       primary      secondary
                station)        coverage      coverage
                                  area          area

Kotangora        16.22            7.70%        61.50%
Magama           28.27             --          37.90%
Mashegu          26.46             --           3.60%
Mariga           44.63             --           2.60%

L.G.A       % of L.G.A.   Total % of
            with fringe     L.G.A.
             coverage      coverage
               area          area

Kotangora     23.10%        92.30%
Magama        27.60%        65.50%
Mashegu       29.10%        32.70%
Mariga         7.90%        10.50%

Table 5. Percentage of the local government areas covered by the
Niger State Television station in Niger State

L.G.A       Average Distance   % of L.G.A.   % of L.G.A.
             (km) (from the       with          with
              transmitting       primary      secondary
                station)        coverage      coverage
                                  area          area

Chanchaga         7.88            3.00%        47.00%
Bosso            11.93             --           3.13%
Paikoro          20.54             --           6.67%
Shiroro          25.34             --            --

L.G.A       % of L.G.A.   Total % of
            with fringe     L.G.A.
             coverage      coverage
               area          area

Chanchaga     50.00%         100%
Bosso         50.00%        50.13%
Paikoro       20.00%        26.67%
Shiroro        7.50%         7.50%
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Author:Ajewole, Moses Oludare; Oyedum, Onyedi David; Adediji, Adekunle Titus; Moses, Abiodun Stephen; Eiche
Publication:International Journal of Digital Information and Wireless Communications
Date:Jul 1, 2013
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