Indian Journal of Radio & Space Physics
Vol. 36, October 2007, pp. 418-422
Studies on radio frequency propagation characteristics for underground coalmine
communications
L K Bandyopadhyay, P K Mishra, S Kumar, D Selvendran & S K Chaulya
Central Institute of Mining & Fuel Research, Dhanbad 826 001, India
email: [email protected]
Received 19 June 2007; accepted 27 July 2007
A basic understanding of the behaviour of electromagnetic wave propagation through strata is the fundamental
requirement to design a suitable wireless communication system for underground mines. Background information on radio
propagation and its limitations, in a particular confined space can be known only after the measurement. Both
electromagnetic propagation studies and modelling of propagation coverage, ultimately help in selecting the best suitable
frequency and designing appropriate wireless communication system for underground mine. The paper discusses different
aspects for propagation modelling and the experiment conducted in the laboratory to understand the propagation
characteristics through coal. It is found that 6 MHz frequency is the best suitable frequency for propagation of
electromagnetic wave through coal.
Keywords: Transceivers, Miner, Radio frequency wave, Directional antenna, Underground coal mine
PACS No.: 84.40.Ba; 84.40.Ua; 41.20.Jb
1 Introduction
The radio wave propagation through coal and rock
strata suffers from dispersion, absorption and
scattering of electromagnetic waves due to its natural
properties and space limitations. The heterogeneous
and complex structure of coal and rock strata further
complicates the process of radio propagation. Radio
frequency (RF) waves get attenuated significantly
when traverse through coal strata due to absorption.
The attenuation of signal mainly depends upon the
dielectric constant and conductivity of coal strata. The
dielectric constant of different types of coal available
in Indian underground mines is given in Table 1. The
dielectric constant for coal with 15% moisture content
is 4. The conductivity of coal varies from 10–8 to
0.02 mho/m depending upon the physico-chemical
properties of the coal1.
To establish appropriate radio communication
system in underground coal mines2-4, studies of radio
propagation inside mine’s gallery and through coal
and rock strata are of paramount importance. Apart
from minewide communication, study of radio
propagation through coal strata is itself important to
establish communication through coal barrier.
Although propagation characteristics of radio waves
through tunnel have been studied by few researchers
in some developed countries5,6, but rigorous studies
have not been carried out so far in developing
countries such as India for radio wave propagation
through coal strata.
Accident due to roof fall and collapse of side
gallery is a regular occurrence in coal mines. The
radio propagation through coal strata is an important
technique to establish communication with the
miners, trapped under coal debris. Therefore, the
detecting system to locate the trapped miner is an
useful device for rescue and relief operation. To meet
the intrinsic safety criteria for hazardous zone7 (Indian
Standard, IS 5780: 2002), the power restriction of
transceiver to be used in underground mine is 2W,
which further limits the communication range.
Therefore, it is important to find out the suitable
frequency, which is attenuated the optimum when
Table 1 — Dielectric constant of different type of coal available
in Indian underground mines
Type of coal
Dielectric constant
Anthracite coal
3.2
Bituminous coal
2.8
Coal dust
2.5
Coal with 15 % moisture content
4.0
BANDYOPADHYAY et al.: CHARACTERISTICS OF RADIO FREQUENCY PROPAGATION IN COALMINE
passing through strata. This will ultimately help in
designing appropriate trapped miner locator and other
wireless communication devices for underground
mines.
An experimental study has been carried out to
analyze the radio wave propagation characteristics
and to find out the suitable frequency for getting
maximum signal strength, while passing through coal.
The theories of electromagnetic propagation and
laboratory experimental procedure along with the
results are discussed in subsequent sections.
2 Wave propagation through medium
In underground mine, the low frequency refractive
index is predominately real, and is also greater than
unity. Suppose, some fractions (f0) of the electrons are
free in the sense of having initial frequency ω0 = 0. In
this situation, the low frequency dielectric constant
takes the form8,9.
ε (ω)= n (ω) = n0 + iN e f0 /ε0 m ω (Γ – i ω)
2
2
2
0
2
…(1)
where, n0 is the contribution to the refractive index
from all the other resonances, N the number density of
electrons, Γ0 = limω0→0 ω0g0, where g0 is the dimensionless damping constant. But for a medium, the
contribution to the refractive index from the free
electrons is singular at ω = 0. Thus, using the
Maxwell’s field equation, the dielectric constant is
given by:
ε ( ω ) ≡ n 2 ( ω ) = n02 + i
σ
εoω
…(2)
f o Ne 2
m ( τ o − iω )
limit) is to write it in terms of a real normal dielectric
constant ( ε = n o 2 ) and a real conductivity (σ). Thus,
from Eq. (2), the following equation is derived:
n 2 (ω) = ε + i
σ
εoω
…(4)
It indicates that the field energy is almost entirely
magnetic in nature. It is clear that an electromagnetic
wave propagating through a good coal block has
markedly different properties to a wave propagating
through a conventional dielectric. For a wave
propagating in the x-direction, the amplitudes of the
electric and magnetic fields attenuate following the
 x
expression, exp  −  ;
 d
where, d =
2
µ o σω
…(5)
and is called the skin depth. These Eqs (4) and (5) and
parameters govern the RF wave propagation through
rock and coal strata. These parameters also vary from
place to place depending upon the geographical
region, geological formation, properties of strata and
local conditions. Therefore, these parameters must be
evaluated properly in a particular region for designing
effective wireless communication system to be
applied in underground mines.
3 Laboratory experiment
3.1 Experimental procedure
A comparison of this term with Eq. (1) yields the
following expression for the conductivity:
σ=
419
…(3)
Thus, at low frequencies conductors possess
predominately real part of conductivity. However, at
higher frequencies the conductivity becomes
complex. At these frequencies, there is little
meaningful distinction in coal barriers, since the
conductivity contribution to ε(ω) appears as resonant
amplitude just like the other contributions. The
conventional way to represent the complex refractive
index of a conducting medium (in the low frequency
The laboratory set-up to find the field strength of
radio waves passing through a coal block is shown in
Fig. 1. Different radio frequency signals of fixed
amplitude were generated by standard RF generator
and fed to the matched directional transmitting loop
antenna. The high ‘Q’ matched directional antennae
of different RF waves were designed in the
laboratory. The strength of input RF signals fed to the
antennae at different frequencies is depicted in Fig. 2.
The transmitting RF signals were passed through the
smoothen side of a coal block (having dimension
of 1 × 1 × 0.7 m3) placed about 15 cm away from the
transmitting antenna and the field attenuated waves
were received by a compatible receiving antenna
placed at the same distance as transmitting antenna,
i.e. at around 15 cm from another side of the coal
INDIAN J RADIO & SPACE PHYS, OCTOBER 2007
420
Fig. 1 — Experimental set-up of radio frequency signal propagation
Fig. 2 — Strength of Input RF signals at different frequencies
block. The gain of antennae and the attenuation of the
RF waves through coal block at different frequencies
are shown in Figs 3 and 4, respectively. It is evident
from Fig. 3 that the gain of the antenna is high at
around 6 MHz. The amplitude of attenuated RF
signals and field strength of the attenuated waves
were measured by Digital Oscilloscope (Make: Gould
Electronics, Model: 1425M) and Spectrum Analyzer
(Make: Anritsu, Model: MS2661C), respectively. The
values of attenuation of RF signal with respect to the
variation in transmitting frequencies were recorded.
The same experiment was repeated for another coal
block having dimension of 1 × 1 × 0.5 m3.
Fig. 3 — Gain of the loop antenna at different frequencies
3.2 Analysis of propagation signals
From the experimental data a graph was prepared
showing the value of output RF signal strength with
respect to the frequency variation for the two
Fig. 4 — Attenuation of RF signals through coal block at different
frequencies
BANDYOPADHYAY et al.: CHARACTERISTICS OF RADIO FREQUENCY PROPAGATION IN COALMINE
different coal blocks (Fig. 5). The graph represents a
polynomial
function.
Thus,
the
following
methodology was adopted to evaluate the best suitable
frequency, which generates maximum signal strength
while passing through the coal blocks.
Let f(x) = ax3 + bx2 + cx + d, be a polynomial
function, which rightly fits into the experimental data
of change in signal strength (dB µV) with respect to
variation of radio frequencies (MHz). The negative
sign of f (x) indicates the loss in signal strength and
the positive sign indicates the amplification in the
signal transmitted through the coal blocks. The
suitable frequency for getting optimized signal may
be mathematically estimated using the maxima and
minima method as explained below.
The solutions of the equation f´(x) = 3ax2 + 2bx + c
= 0 are
x1 =
−b + b 2 − 3ac
3a
−b − b 2 − 3ac
3a
…(6)
421
For coal block-1:
y = 0.016 x3 − 0.445 x2 + 3.1165 x − 4.4234
…(9)
For coal block-2:
y = 0.0242 x3 − 0.6391 x2 + 4.448 x − 7.1401 …(10)
If we put the coefficients of Eqs (9) and (10) in
Eq. (8), we get the value of x as 6.12 and 5.93 MHz,
respectively. The average of these two values is 6.025
MHz. Thus, it may be concluded that the best suitable
frequency for getting maximum signal strength
through coal blocks is around 6 MHz. It is also
evident from Fig. 5 that the signal strength is
maximum at around 6 MHz. Now if the coefficients
of Eqs (9) and (10) are put to Eq. (6) to get the
frequency for minimum signal strength, one can get it
as 14.85 MHz and 17.25 MHz, respectively, for both
the coal blocks. The average of these two values is
16.055 MHz. The result of laboratory experiments
also indicates that the field strength is lowest at
around 16 MHz (Fig. 5).
Based on the statistical analysis of data using
Statistica Software (SPSS15.0), it was found that the
graph as shown in Fig. 5 was best fitted with the
following polynomial equations:
4 Results and discussion
The experiment was carried out in the frequency
range 100 kHz-20 MHz. Fascinatingly it was found
that there was appreciable high signal strength in the
frequency range 1 MHz-11 MHz. The frequency
level, at which the absorption is critically low, the
signal strength decreases with increase in frequency
due to respective low gain of antennae and high
attenuation of RF waves (Figs 3 and 4). It was also
observed that there was a significant loss of signal
strength after the frequency level of 11 MHz. It seems
that the graph of signal strength with increasing
frequency value forms a periodic or cyclic function as
shown in Fig. 6. Focusing the objective of the present
study, the exact frequency level where the signal
Fig. 5 — Plot of output RF signal strength versus transmitting
frequency
Fig. 6 — Radio wave travelling in a given direction
and x2 =
…(7)
Now, f′′ (x1) = 2√b2 − 3ac > 0, i.e. f(x) is minimum
at x = x1 and f′′ (x2) = − 2√b2 – 3ac < 0, i.e. f(x) is
maximum at x = x2
Therefore, the recommended frequency for both
transmitting and receiving, the maximum signal
passing through the coal barrier using transceiver is
x = (− b − √b2 − 3ac)/3a MHz
…(8)
422
INDIAN J RADIO & SPACE PHYS, OCTOBER 2007
strength gets high in the cycle falls within the
frequency range of 1-11 MHz. Based on the
mathematical analysis it was found to be 6 MHz.
5 Conclusions
Propagation of electromagnetic wave through strata
is a complex phenomenon. The same can be analyzed
using modelling technique and experimental studies.
Based on the experimental results it was found that 6
MHz frequency is the best suitable frequency for
generating maximun
signal strength while
transmitting through coal strata. Therefore, the
wireless communication system, which will be
developed for communication through coal strata,
should be designed at 6 MHz frequency. The
polarization in coal molecules at this particular
frequency may be one of the reasons for amplified
signal strength. Therefore, a deliberative study on
polarization mechanism of coal molecules with
varying frequency is warranted to establish the
relationship between the signal strength and
polarization in coal molecules.
Acknowledgements
The authors are thankful to the Director, Central
Institute of Mining and Fuel Research, Dhanbad, for
his encouragement and support. They are also grateful
to the Department of Information Technology,
Ministry of Communication and Information
Technology, Government of India, New Delhi for
sponsoring the study.
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