Different Types of Loses in Underground Mine Communication

Transcription

1 CHAPTER 3 Different Types of Loses in Underground Mine Communication Introduction Difference between terrestrial and underground wireless communication is the communicating medium or more specifically the communication channel. Even though modulation formats, transceivers and antennas are same for both the type of communication, we have to consider additional factors like signal losses, noise effects and interference in signal strength estimation. Path loss, fading, scattering and bending are common in both the type of communication. In terrestrial communication scattering occurs due to tall buildings, hillocks etc, but in underground environment it occurs due to interior walls and other obstructions. In surface communication bending or diffraction occurs due to earth s horizon (curved nature of earth) and around buildings/towers etc. In underground communication bending occurs around mine or tunnel corners. 3.1 Path Loss Path loss (or path attenuation) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. This term is commonly used in wireless communications and signal propagation. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. Path loss is also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas. Generally for wireless communication amount of path loss that occurs for a transmitted signal can be M.N.Jayaram, Dept. of E&C, SJCE, Mysore

2 determined from Frii s transmission formula. This equation involves parameters like transmitted power, gain of transmitting and receiving antennas, distance between transmitter and receiver and wavelength of the signal.. In its simplest form, the Friis transmission equation is as follows. Given two antennas, the ratio of power available at the input of the receiving antenna, P r to output power to the transmitting antenna, P t is given by P r / P t = G t G r ( /4πR) 2 (3.1) Where G t and G r are the antenna gains (with respect to an isotropic radiator) of the transmitting and receiving antennas respectively, is the wavelength, and R is the distance between the antennas. The inverse of the factor in parentheses is the so-called free-space path loss. To use the equation as written, the antenna gain may not be in units of decibels, and the wavelength and distance units must be the same. If the gain has units of db, the equation is slightly modified to: P r = P t +G t +G r +20log10 ( /4πR)... (3.2) Gain has units of db, and power has units of dbm or dbw This simple form applies only under the following ideal conditions: R much greater than. If R is less than, then the equation would give the physically impossible result that the receive power is greater than the transmit power, a violation of the law of conservation of energy. The antennas are in unobstructed free space, with no multipath. P r is understood to be the available power at the receive antenna terminals. P t is understood to be the power delivered to the transmit antenna.. The antennas are correctly aligned and polarized. The bandwidth is narrow enough that a single value for the wavelength can be assumed. The ideal conditions are almost never achieved in ordinary terrestrial communications, due to obstructions, reflections from buildings, and most importantly reflections from the ground. One situation where the equation is reasonably accurate is in satellite communications when there is negligible M.N.Jayaram, Dept. of E&C, SJCE, Mysore

3 atmospheric absorption; another situation is in anechoic chambers specifically designed to minimize reflections. [81][82] 3.2 Penetration Loss Penetration loss is the one of the important loss in underground communication. This loss actually limits depth of communication of EM wave signals in the underground environment. When signal propagates in to the soil medium there will be decrease in the signal strength due to absorption. This attenuation due to penetrating in to the soil medium is called penetration loss. Penetration loss increases if there is more moisture content in the soil. [70] Concrete wall, wooden door, windows also contribute to this loss. These insulating materials behave as lossy dielectric materials, whose characterization is very important from the point of signal propagation. Penetration loss depends on parameters like frequency of the signal, power level and modulation format of the signal. From electromagnetic wave theory for propagation signal in lossy dielectric medium we have γ² = (α+jβ) ² = (σ+jωє) (jωμ) (3.3) Where γ is the complex propagation constant, α is the attenuation constant, β is the phase constant, σ is the conductivity of the medium, Є is the permittivity of the medium, μ is the permeability of the medium and ω is the angular frequency of the signal. Expanding the left hand side and equating real & imaginary parts respectively on either side we have, [77] α²-β²= - ω²μє (3.4) And 2αβ= ωμσ. (3.5) Substituting for β from eq (3.5) in eq (3.4) gives, α²-(ωμσ/2α) ² =-ω²μє... (3.6) After simplification, we have αexp4+ω²μєα²-(ωμσ/2)²=0... (3.7) On solving for α, we have M.N.Jayaram, Dept. of E&C, SJCE, Mysore

4 α=ω (μє/2x { (1+ (σ/ωє) ²)-1}) (3.8) For σ/ωє << 1& using Binomial expansion for the radix term, we have (1+ (σ/ωє) ²) 1+1/2(σ/ωЄ) ² (3.9) So Attenuation constant α σ/2 (μ/ωє). (3.10) 3.2(a) Penetration loss calculation for the given mine condition There was a 35 cm thick concrete brick wall at the entrance which results in penetration loss. For concrete Є r =8.9 or 9 F/m, μ r =1H/m, σ =0.1 A/m² in GSM900 band with Є o =8.854x10-12 F/m, μ o =4πx10exp-7 H/m, substituting these values in eq (3.10), we have [69] α = 60πσ/ ( Є r ) or penetration loss in db = (20α) db = = 16 db. Hence penetration loss at the entrance of mine is 16 db. Effective signal strength just inside the mine after penetration is = Transmitted signal strength Penetration loss at the entrance due to concrete wall = dbm (corresponding to a signal level of 30 Watts) -16 db loss= dbm or μw. This is an appreciable loss and hence it must be minimized.possible solution is using a material with low penetration loss. In the next article we discuss some materials and their characteristics (Transmission and Reflection coefficient) based on computer simulation at different frequencies (as penetration loss is function of frequency). Transmission coefficient is directly related to signal attenuation through the material for an incident signal. 3.2(b) Lab simulation for attenuation loss considering some materials With continuing advances in the computing power of computer systems and the increasing availability of building and terrain data, deterministic propagation modeling approaches are becoming increasingly realistic for practical implementation. The accuracy of these methods depends on both the positional M.N.Jayaram, Dept. of E&C, SJCE, Mysore

5 accuracy of the building and terrain involved, and on the accuracy of the constitutive parameters of the media involved. This is true especially in environments where radio wave propagation is mainly due to propagation mechanisms without a strong line of sight contribution. The purpose of this research work is to provide a comprehensive review of the constitutive parameters of common building materials, reporting on the important composition characteristics and frequency dependent behavior of these materials. The importance of this review comes from the need to specify the constitutive parameters in a sensible way. A clear understanding of the involved processes can allow the propagation modeler to make reasonably accurate assumptions on the variation and validity of the constitutive parameters, for frequencies or frequency ranges that have not been reported in the literature. Some of the most common methods used to measure the dielectric properties of different samples are the following: microwave free space, two terminal measurements, time domain spectrometry, frequency domain, open resonator, closed cavity, dielectric probe, and waveguide techniques. From the above, the most frequently used technique for measuring the transmission and reflection coefficients under an angle of incidence, at microwave frequencies, is through the microwave free space method and the use of a turntable. This setup usually takes place in an anechoic chamber. This environment allows the absorption of unwanted reflections that can alter the received field. This method often uses a wideband technique to distinguish the path penetrating through the material from other contributions. Directional antennas with small beam widths are also used in order to avoid or further reduce any diffraction effects around the edges of the material under investigation. In order to evaluate the reflection and transmission coefficients, which quantitative the RF reflection and transmission loss, the constitutive parameters of the material of interest should be known. Since it is practically impossible to know the exact value of these parameters for all environments, one should be able to make sensible predictions. For this reason it is important to review a number of measurements carried out at different frequencies and for different materials. M.N.Jayaram, Dept. of E&C, SJCE, Mysore

6 Although the literature presents a number of measurements for different materials, we concentrate on those frequencies, which will be of most use. For the purpose of this review the building materials are classified in 3 categories. First category includes bricks concrete etc, materials which are usually used for hardened walls. Second category refers to glass since glass usually provides a relatively low loss propagation path and third category refers to wood and wood based materials. Parameters presented and discussed here will cover mainly the VHF, UHF and lower microwave range In order to find the penetration loss in all the models we have to consider some of the parameters like conductivity, permittivity and permeability of the different materials. The Table 3.1 [68], [72], [73] shows the conductivity, permittivity and permeability parameters for different materials for calculation of the penetration loss in underground wireless communication in mines. Table 3.1 Dielectric Constants for different materials Type of wall Concrete thin wall Permittivity Farad/meter Permeability Henry/meter Conductivity Ampere/meter 2 Thickness Centimeter Wooden 5 1 1x Glass x Copper x For lossy dielectric medium, using eq (3.10), the attenuation loss in db for different materials at different frequencies is given below: For concrete thin wall (thickness up to 35 cm) f 1 = 100 MHz f 2 = 500 MHz α 1 =13.87 db α 2 =15.79dB M.N.Jayaram, Dept. of E&C, SJCE, Mysore

7 f 3 = 940 MHz α 3 =15.9dB For wood f 1 = 100 MHz f 2 = 500 MHz f 3 = 940MHz α 1 = db α 2 = db α 3 = db For Silica glass f 1 = 100 MHz f 2 = 500 MHz f 3 =940 MHz α 1 = 1.16dB α 2 = 1.58 db α 3 = db For copper f 1 =100MHz f 2 =500MHz f 3 =940MHz α 1 = db α 2 = db α 3 = db For concrete thin wall results are plotted using MAT LAB code frequencies (in MHz) as in Fig 3.1. for various Fig 3.1: Attenuation (in db) vs. frequency (in MHz) for Concrete Wall. M.N.Jayaram, Dept. of E&C, SJCE, Mysore

8 The graph shows that the attenuation increases initially with frequency up to 500 MHz. At higher frequencies (i.e. after 500 MHz) the loss becomes almost constant. Hence at higher frequencies even though concrete has higher attenuation loss it is frequency independent or constant. As concrete is having high tensile strength it is used in underground structures. For wood results are plotted using MAT LAB codes for various frequencies (in MHz) as in Fig 3.2. Fig 3.2 Attenuation vs. Frequency for Wood From Fig 3.2 we can conclude that the conductivity for wood is almost zero and hence the loss is too high and it is constant. Wood is not so durable as compared to concrete. The above procedure can be repeated for silica glass and copper. 3.3 Multipath Loss It is well known that multipath delay spread in the wireless channel limits data rates due to transmission errors caused by inter symbol interference (ISI). One method M.N.Jayaram, Dept. of E&C, SJCE, Mysore

9 to mitigate the effects of multipath propagation is to use directional antennas. The radiation pattern or beam width of a directional antenna accepts only multipath signals that arrive within the beam pattern of the antenna, which therefore limits the amount of multipath received in the channel, resulting in less delay spread and the ability to achieve higher data rates. Hence we have used dish and loop antennas for practical measurements. Other method to overcome the multi path loss effect is to use diversity reception. Path loss is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space as generally given by Frii s transmission formula. Path loss is a major component in the analysis and design of the link budget of a telecommunication system. This term is commonly used in wireless communications and signal propagation. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. Path loss is also influenced by terrain contours, environment, propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas. Path loss normally includes propagation losses caused by the natural expansion of the radio wave front in free space, absorption losses (sometimes called penetration losses), when the signal passes through media not transparent to electromagnetic waves, diffraction losses when part of the radio wave front is obstructed by an opaque obstacle, and losses caused by other phenomena. The signal radiated by a transmitter may also travel along many and different paths to a receiver simultaneously; this effect is called multipath. Multipath can either increase or decrease received signal strength, depending on whether the individual multipath wave fronts interfere constructively or destructively. The total power of interfering waves in a Rayleigh fading scenario vary quickly as a function of space (which is known as small scale fading), resulting in fast fades which are very sensitive to receiver position. Dropped mobile phone connections, missing wireless data packets, and lost radio reception at traffic lights are all examples of the problems that can result from M.N.Jayaram, Dept. of E&C, SJCE, Mysore

10 multipath loss. This phenomenon can also produce losses in underground mobile communication. Above methods mentioned may not completely eliminate path loss. Multipath loss can occur when an antenna receives a transmitted signal that is the sum of the desired line-of-sight (LOS) signal plus one or more non-line-of-sight (NLOS) signals. (NLOS signals are caused by reflections off of structures and diffraction off of obstacles.) In the example of Fig 3.3(a) the receiving antenna is assumed to be directional while the transmitting one has no restrictions. If the LOS and NLOS signals are received with nearly equal amplitude and 180 out of phase as in Fig 3.3(b) then, destructive interference occurs, which results in a loss of carrier power to the receiver feed. Fig 3.3 a&b Multi path reception 3.3 (a) Representation of mobile or wireless signals Wireless/mobile signal can be represented as a random process P (t, s). Signals not only vary w.r.t time but also vary w.r.t distance.if we make measurements at a fixed instance of time random process converge in to a random variable. Variation of signal w.r.t time results in fading (short term/long term) & Variation w.r.t distance results in path loss.[68], [71],[73] M.N.Jayaram, Dept. of E&C, SJCE, Mysore

11 3.3(a.1) Fading In wireless communications, fading is deviation of the attenuation that a carrier-modulated signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modeled as a random process. A fading channel is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading. Fig3.3c gives different channel fading types. Fig3.3c Channel fading types Reflection, diffraction, and scattering have a great impact on the signal power, and they constitute the main reasons for signal attenuation (fading). The interaction between the waves derived by reflection, diffraction and scattering cause multipath fading at a specific location. Fading can be categorized into two main types: largescale fading and small-scale fading. Large-scale fading is due to motion in a large area, and can be characterized by the distance between transmitter and receiver. Small-scale fading is due to small changes in position (as small as half wavelength) or to changes in the environment (surrounding objects, people crossing the line of sight between transmitter and receiver, opening or closing of doors, etc.). M.N.Jayaram, Dept. of E&C, SJCE, Mysore

12 In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all frequency components of the signal will experience the same magnitude of fading. In frequency-selective fading, the coherence bandwidth of the channel is smaller than the bandwidth of the signal. Different frequency components of the signal therefore experience décor related fading. We are using two distributions for multipath loss namely Rayleigh and Gaussian. Rayleigh is well suited for noisy environment like underground wireless communication channel and most of the natural occurring phenomenon fit to Gaussian distribution. 3.3(a.2) Rayleigh Distribution In probability theory and statistics, the Rayleigh distribution is a continuous probability distribution. As an example of how it arises, the wind speed will have a Rayleigh distribution if the components of the two-dimensional wind velocity vector are uncorrelated and normally distributed with equal variance. The distribution is named after Lord Rayleigh. The Rayleigh probability density function is given by. (3.11) For multipath propagation Rician model is more appropriate than Rayleigh distribution.it takes into consideration line of sight component.since line of sight component signal componenent reception is very remote in underground mine communication, we have considered Rayleigh distribution only.also Rician distribution with direct path component zero leads to Rayleigh distribution.curve fitting for the multipath scattering loss is done in Chapter6. 3.3(a.3) Gaussian Distribution In probability theory and statistics, the normal distribution or Gaussian distribution is a continuous probability distribution that often gives a good description of data that cluster around the mean. The graph of the associated probability density M.N.Jayaram, Dept. of E&C, SJCE, Mysore

13 function is bell-shaped, with a peak at the mean, and is known as the Gaussian function or bell curve. The Gaussian distribution is one of many things named after Carl Friedrich Gauss, who used it to analyze astronomical data and determined the formula for its probability density function. However, Gauss was not the first to study this distribution or the formula for its density function. The probability density function (pdf) of a random variable describes the relative frequencies of different values for those random variables. The pdf of the Gaussian distribution is given by the formula: -- (3.12) This is a proper function only when the variance σ 2 is not equal to zero. In that case this is a continuous smooth function, defined on the entire real line, and which is called the function. When σ 2 = 0, the density function doesn t exist. However we can consider a generalized function that would behave in a manner similar to the regular density function. Curve fitting for the multipath scattering loss is done in Chapter6 3.4 Bending loss or Diffraction Loss There are two types of losses (i) Conventional bending or diffraction loss and (ii) Tunnel tilt loss. (i) Conventional bending or diffraction loss Diffraction allows radio signals to propagate around the curved surface of earth, beyond the horizon and to propagate behind obstructions. Although the received field strength decreases rapidly as receiver moves deeper into the obstructed (shadowed) region, the diffraction field still exists and often has sufficient strength to produce a useful signal The phenomenon of diffraction can be explained by Huygens s principle, which states that all points on a wave front can be considered as point sources for the production of secondary wavelets, and that these wavelets combine to produce a new wave front in the direction of propagation. Diffraction is caused by the propagation of secondary wavelets into a shadowed region. The field strength of diffracted wave in M.N.Jayaram, Dept. of E&C, SJCE, Mysore

14 the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle. 3.4a Fresnel zone geometry Consider a transmitter and receiver separated in free space as shown in Fig.3.4 let an obstructing screen of effective height h with finite width (going into and out of paper) be placed between them at a distance d 1 from the transmitter and d 2 from the receiver. It is apparent that the wave propagation from transmitter to the receiver via the top of the screen travels a longer distance than if a direct line of sight path (through the screen) existed. Assuming h<<d 1, d 2 and h>>λ, Fig 3.4: Knife-Edge Diffraction Geometry The point T denotes the transmitter and R denotes the receiver with an infinite knife-edge obstruction blocking the line of sight path. Dimensionless Frensnel- Kirchoff diffraction parameter v which is given by.. (3.13) The diffraction gain due to the presence of a knife edge, as compared to the free space E-field, is given by.. (3.14) M.N.Jayaram, Dept. of E&C, SJCE, Mysore

15 Where F (v) is a complex Fresnel integral. The Fresnel integral F (v) is functions of the Fresnel Kirchhoff diffraction parameter v. In practice, graphical or numerical solutions are relied upon to compute diffraction gain, a graphical representation of as a function of v is given in Fig 3.5. Fresnel diffraction parameter v Fig 3.5: Knife Edge Diffraction Gain as function of Fresnel Diffraction Parameter v An approximate solution for above equation provided by Lee as Approximate elevation geometry inside the mine is given in Fig 3.6 which diffraction loss is calculated from Fresnel s diffraction equations from M.N.Jayaram, Dept. of E&C, SJCE, Mysore

16 Fig 3.6 Terrain profile inside the mine. In Fig 3.6, X-axis is distance inside the mine & Y-axis is gradient. Undulations on the terrain due to digging also causes scattering, but here we assume this loss is negligible. It was observed that terrain is almost flat with negligible curvature between points A & D, i.e. point A to D can be considered as a straight tunnel. From Fig 3.6, Height of knife edge is h ((15 2)²-14²) = m Fresnel s diffraction factor is v = - h [2(D 1 +D 2 )/ (λ D 1 D 2 )] = From v, diffraction loss or gain can be calculated. Diffraction loss or gain is given by, G d (db) =20log (-0.225/v) = dB (-ve sign indicates loss) Diffraction or Bending loss at a depth of 30 m = dB M.N.Jayaram, Dept. of E&C, SJCE, Mysore

17 (ii) Loss due to tilt in the tunnel /mine walls Suppose that a ray of the EH mode encounters a portion of a side wall that is tilted through a small angle θ about a vertical axis. Then the reflected beam is rotated through an angle 2θ. This means that the electric field is changed from Ex = F (x, y) exp ( ik 3 z) (3.15) to E x = F(x, y) exp [ ik 3 (z cos 2θ + x sin 2θ) (3.16) The power coupling factor g1 of the disturbed field (1.2) back into the mode (1.1) is given by.. (3.17) Where the integrations are over the cross section of the tunnel. The bar over E x indicates complex c conjugate. Since θ is small, we can replace cos 2θ by 1 and sin2θ by 2θ. Then Eq. (1.3) becomes (3.18) Where F is a Gaussian function. Instead of using the actual function cos k l x cos k 2 y for F, we find it more convenient to use an equivalent Gaussian function: Integrating over infinite limits, we have (3.19) (3.20) Next, we assume that F 2 falls to l/e at the point x = d 1 /2, y = 0, which is at the surface of the waveguide. Then a 2 = 1/2d 1 2 and.(3.21) M.N.Jayaram, Dept. of E&C, SJCE, Mysore

18 Likewise, tilting of the floor or roof gives a coupling factor: The loss factor for a distance z is.. (3.22) (3.23) Where k 3 is replaced by k 0. The loss in db is, therefore, (3.24) 3.5 Low Frequency Interference Loss The need for improved communication systems in mines is a long standing problem, during normal operation of a mine, the machinery used creates a wide range of many types of intense EM interference (EMI), and ambient EM1 is, therefore, a major limiting factor in the design of a communication system. However, under emergency conditions when all the power in a mine is cut off, the residual EM noise is not a problem. EM noise generated in mines is generally a non stationary, random process. Therefore, the most meaningful parameters for EM noise generated in mines are statistical ones. In the work by the National Bureau of Standards, five time and amplitude statistics have been used in order to unravel the complexities included in the EM manmade noise in mines. Ambient magnetic-field noise spectra covering frequencies from 100 Hz to 100 KHz are given for several underground coal mine locations. Data have been developed for magnetic field noise on the surface above the mine, noise in the mine face area, noise radiated by specific equipment, the voltage spectrum found on a 600-V dc trolley wire, and noise picked up simultaneously on loops and on roof support bolts. Extensive work has been conducted in the development of data collection techniques suitable for underground mines and in the qualification of noise conditions from representative mines. M.N.Jayaram, Dept. of E&C, SJCE, Mysore

19 The EM noise amplitude decreases with increasing frequency, however, three propagation mechanisms must be considered, Through the strata. Through the entries supported by metallic structures and conductors. Through the entries where they serve as a "waveguide." Each of these mechanisms is discussed below. For the latter two cases, it would appear from the data presented above that selection of frequencies>>l00 KHz would be desirable, however, for situations in which the propagation is through the strata, attenuation varies inversely with frequency. Because of the lower attenuation, the lower the frequency, and the better the signal-to-noise ratio will generally be, despite the higher amplitude noise levels. In-mine noise levels at higher frequencies are typically the same as in other industrial operations. Power lines are divided into 3 classes with respect to radio noise: lines with voltages below 70 KV and lines with voltages above 110 KV. Extra-high-voltage (EHV) transmission lines have operating voltages of 345 KV or greater.[73],[74] 3.5 (a) EM coupling loss Similar charges repel each other. Therefore, if a charge density exists at a point along a wire, it will tend to repel like charges away from that point in adjacent wires. Those elements of charge that are repelled away are moving. By definition this is a current. Changing electric field in one wire (as charge density changes) causes a changing current in another adjacent wire. This effect is often referred to as electric coupling, charge coupling, or capacitive coupling.the electric and magnetic fields from the power cables near the antenna units induce certain residual fields which affect the power patterns of the antennas, degrades the signal due to fluctuations and introduces loss in the system. [74], [85] The low frequency interference (LFI) loss for a power line of 345KV the wireless system is given as: from LFI= [4.8x10-5 f 2 (0.094f+2)] db (3.25) Where, Electric field E = f+95.. (3.25b) M.N.Jayaram, Dept. of E&C, SJCE, Mysore

20 Electrical cables or power lines are used to source different electrical equipments inside the mine.power line interference loss/ low frequency interference loss is predominant where as the other electrical installation losses due to fan / lighting / blowers / exhaust are neglected. Power line noise depends on KVA of the cable, distance at which it is located from measuring equipment, height of the measuring antenna, frequency at which measurements are done, gain of measuring antenna. Half wave dipole is used for the measurement of low frequency noise. Measured electric field strength will be in μv/m/mhz or dbμv /m/mhz. This is converted to dbm noise power and then can be converted to db loss. Field strength measurements are done at same distance from cable at point s mouth of the mine (A), an intermediate point (D) and at a depth of 30m (C). This is necessary because effect of noise on signal must be investigated at different depths For a standard measuring instrument having R t = 50 ohm (terminating resistance), 0dB μv corresponds to -107 dbm [73]. Hence db loss in signal power at any point due to low frequency interference noise is = G rf (db) = E-107 is the gain factor due to power cable= Measured electrical field strength due to noise at that point-107 dbm. 3.6 Machinery obstruction loss Large machines like generators /motors are used for sourcing the electrical installation like trolleys, blowers, lighting, exhaust fans etc. If the area of these machines is comparable to the wavelength of the signal obstruction loss occurs. [73].For a machinery of medium area < 15 sqm obstruction loss will be around 4 db in the UHF band. For two machines (one is used as back up or redundancy) the total machinery obstruction loss is = 8dB. mine. Hence the total machinery obstruction loss is = 8dB at the entrance of the M.N.Jayaram, Dept. of E&C, SJCE, Mysore

21 3.7 Other losses Apart from the six losses that occur in a mine environment there are also other losses, like ground reflection attenuation loss for a given polarization and angle of incidence & effective antenna height loss. 3.7 (a) Ground reflection attenuation loss Whenever electromagnetic waves propagate from one antenna to other, if distance of separation between them is large there will be line of sight signal, ground reflected signal and multi path signal will be present. Ground reflection also causes attenuation of signal. Rocky soil is common inside mine. They have Є r = 10, σ = 20 mho/cm in the operating frequency range of UHF. Reflection co-efficient at air earth interface 1. For vertically polarized wave is given by (Є r -jx) Sin θ ( (Є r -jx)-(cosθ) ²) K v = (3.26) (Є r -jx) Sin θ+ ( (Є r -jx)-(cosθ) ²) Where x=σ/ωє=18x10³σ /f (in MHz) = 3.913m, θ is angle of incidence & σ = conductivity of earth For θ =90 o (Vertical incidence), K v = & for θ=0 (Horizontal incidence), K v = Vertical incidence is possible in open terrain. In mines with closed geometry vertical incidence is remote & hence possible incidence must be horizontal. Also as height of transmitting antenna is > the receiving antenna θ will be small catering horizontal incidence. M.N.Jayaram, Dept. of E&C, SJCE, Mysore

22 2. For horizontally polarized wave the reflection coefficient is Sin θ ( (Є r -(Cosθ) ²) K h = (3.27) Sin θ + ( (Є r -(Cosθ) ²) For θ = 90, K h = 0.52 & for θ = 0, K h = 1. From the above calculations for both vertical and horizontal polarization, it is clear that for small θ (incidence angle), reflection coefficient is 1(i.e. K 1). This indicates that the entire incident signal will be reflected from ground without attenuation [73]. 3.7 (b) Antenna effective height loss In normal flat terrain terrestrial communication transmitting antenna and receiving antenna are located on the same plane. Here physical antenna height and effective antenna height are same. If mobile unit is climbing a hill or moving down a hill or in the present situation (inside the mine) where trolley is constantly moving up and down effective height of receiving antenna varies. This is because at each point mobile unit will be in a different reference plane as compared to the ground reference plane of transmitting or base station antenna outside the mine. Hence effective height of receiving antenna is important in all calculations. In a mine receiving antenna is always below the ground plane, effective height of receiving antenna is always greater than the physical size of the antenna. [75], [76] M.N.Jayaram, Dept. of E&C, SJCE, Mysore