M.N.Jayaram, Dept. of E&C, SJCE, Mysore - 06 66

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 - 06 67

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, Pr to

output power to the transmitting antenna, Pt is given by

Pr / Pt = Gt Gr ( /4πR) 2 ……… (3.1)

Where Gt and Gr 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:

Pr = Pt +Gt +Gr+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.

Pr is understood to be the available power at the receive antenna terminals.

Pt 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

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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 - 06 69

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 Єr=8.9 or 9 F/m, μr=1H/m, σ =0.1 A/m² in GSM900 band with

Єo=8.854x10-12 F/m, μo=4πx10exp-7 H/m, substituting these values in eq (3.10), we

have [69]

α = 60πσ/ (√ Єr) or penetration loss in dB = (20α) dB = 15.96 = 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 = 44.77 dBm

(corresponding to a signal level of 30 Watts) -16 dB loss= -28.77 dBm or 1.327 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

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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.

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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 Permittivity Farad/meter

Permeability Henry/meter

Conductivity Ampere/meter2

Thickness Centimeter

Concrete thin wall

9 1 0.1 35

Wooden 5 1 1x10 -15 3

Glass 2.4 1 1x10 -12 0.3

Copper 1 1 5.7x10 7 1.3

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)

f1= 100 MHz α1=13.87 dB

f2= 500 MHz α2=15.79dB

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f3= 940 MHz α3=15.9dB

For wood

f1= 100 MHz α1= -1.587 dB

f2= 500 MHz α2= -1.495 dB

f3= 940MHz α3= -1.491 dB

For Silica glass

f1= 100 MHz α1= 1.16dB

f2= 500 MHz α2= 1.58 dB

f3=940 MHz α3= 1.655 dB

For copper

f1=100MHz α1= 103.56 dB

f2=500MHz α2= 110.466 dB

f3=940MHz α3= 113.3 dB

For concrete thin wall results are plotted using MAT LAB code for various

frequencies (in MHz) as in Fig 3.1.

Fig 3.1: Attenuation (in dB) vs. frequency (in MHz) for Concrete Wall.

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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

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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

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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) & Variat ion w.r.t distance

results in path loss .[68], [71],[73]

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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: large-

scale 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.).

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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

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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 σ2 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 σ2 = 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

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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 d1 from the transmitter and d2 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<<d1, d2 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)

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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 from

which diffraction loss is calculated from Fresnel’s diffraction equations

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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) = 15.94 m

Fresnel’s diffraction factor is v = - h√ [2(D1+D2)/ (λ D1 D2)] = - 12.12

From v, diffraction loss or gain can be calculated.

Diffraction loss or gain is given by,

Gd (dB) =20log (-0.225/v) = -34.63dB (-ve sign indicates loss)

Diffraction or Bending loss at a depth of 30 m = -34.63dB

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(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 (−ik3z) ------------------ -(3.15)

to E’x = F(x, y) exp [−ik3 (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 klx cos k2y

for F, we find it more convenient to use an equivalent Gaussian function:

------------------------ (3.19)

Integrating over infinite limits, we have

------------------------- (3.20)

Next, we assume that F2 falls to l/e at the point x = d1/2, y = 0, which is at the surface

of the waveguide. Then a2 = 1/2d12 and

…………………….(3.21)

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Likewise, tilting of the floor or roof gives a coupling factor:

…………….. (3.22)

The loss factor for a distance z is

----------- (3.23)

Where k3 is replaced by k0. 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.

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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 from

the wireless system is given as:

LFI= [4.8x10-5f2–(0.094f+2)] dB ………… (3.25)

Where, Electric field E = -0.094f+95 ……….. (3.25b)

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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 Rt = 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 = Grf (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.

Hence the total machinery obstruction loss is = 8dB at the entrance of the

mine.

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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 Єr = 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

(Єr-jx) Sin θ – (√ (Єr-jx)-(Cosθ) ²)

Kv = ------------------------------------------……………..(3.26)

(Єr-jx) Sin θ+ (√ (Єr-jx)-(Cosθ) ²)

Where x=σ/ωЄ=18x10³σ /f (in MHz) = 3.913m, θ is angle of incidence & σ =

conductivity of earth

For θ =90o (Vertical incidence), Kv = 0.532 & for θ=0 (Horizontal

incidence), Kv = 0.9106. 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 - 06 87

2. For horizontally polarized wave the reflection coefficient is

Sin θ–(√ (Єr-(Cosθ) ²)

Kh = -------------------------------- ………. (3.27)

Sin θ + (√ (Єr-(Cosθ) ²)

For θ = 90, |Kh| = 0.52 & for θ = 0, |Kh| = 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]