Studies on radio frequency propagation characteristics for underground coalmine communications

Transcription

1 Indian Jurnal f Radi & Space Physics Vl. 36, Octber 007, pp Studies n radi frequency prpagatin characteristics fr undergrund calmine cmmunicatins L K Bandypadhyay, P K Mishra, S Kumar, D Selvendran & S K Chaulya Central Institute f Mining & Fuel Research, Dhanbad , India laxmikb@yah.cm Received 19 June 007; accepted 7 July 007 A basic understanding f the behaviur f electrmagnetic wave prpagatin thrugh strata is the fundamental requirement t design a suitable wireless cmmunicatin system fr undergrund mines. Backgrund infrmatin n radi prpagatin and its limitatins, in a particular cnfined space can be knwn nly after the measurement. Bth electrmagnetic prpagatin studies and mdelling f prpagatin cverage, ultimately help in selecting the best suitable frequency and designing apprpriate wireless cmmunicatin system fr undergrund mine. The paper discusses different aspects fr prpagatin mdelling and the experiment cnducted in the labratry t understand the prpagatin characteristics thrugh cal. It is fund that 6 MHz frequency is the best suitable frequency fr prpagatin f electrmagnetic wave thrugh cal. Keywrds: Transceivers, Miner, Radi frequency wave, Directinal antenna, Undergrund cal mine PACS N.: Ba; Ua; 41.0.Jb 1 Intrductin The radi wave prpagatin thrugh cal and rck strata suffers frm dispersin, absrptin and scattering f electrmagnetic waves due t its natural prperties and space limitatins. The hetergeneus and cmplex structure f cal and rck strata further cmplicates the prcess f radi prpagatin. Radi frequency (RF) waves get attenuated significantly when traverse thrugh cal strata due t absrptin. The attenuatin f signal mainly depends upn the dielectric cnstant and cnductivity f cal strata. The dielectric cnstant f different types f cal available in Indian undergrund mines is given in Table 1. The dielectric cnstant fr cal with 15% misture cntent is 4. The cnductivity f cal varies frm 10 8 t 0.0 mh/m depending upn the physic-chemical prperties f the cal 1. T establish apprpriate radi cmmunicatin system in undergrund cal mines -4, studies f radi prpagatin inside mine s gallery and thrugh cal and rck strata are f paramunt imprtance. Apart frm minewide cmmunicatin, study f radi prpagatin thrugh cal strata is itself imprtant t establish cmmunicatin thrugh cal barrier. Althugh prpagatin characteristics f radi waves thrugh tunnel have been studied by few researchers in sme develped cuntries 5,6, but rigrus studies have nt been carried ut s far in develping cuntries such as India fr radi wave prpagatin thrugh cal strata. Accident due t rf fall and cllapse f side gallery is a regular ccurrence in cal mines. The radi prpagatin thrugh cal strata is an imprtant technique t establish cmmunicatin with the miners, trapped under cal debris. Therefre, the detecting system t lcate the trapped miner is an useful device fr rescue and relief peratin. T meet the intrinsic safety criteria fr hazardus zne 7 (Indian Standard, IS 5780: 00), the pwer restrictin f transceiver t be used in undergrund mine is W, which further limits the cmmunicatin range. Therefre, it is imprtant t find ut the suitable frequency, which is attenuated the ptimum when Table 1 Dielectric cnstant f different type f cal available in Indian undergrund mines Type f cal Dielectric cnstant Anthracite cal 3. Bituminus cal.8 Cal dust.5 Cal with 15 % misture cntent 4.0

2 BANDYOPADHYAY et al.: CHARACTERISTICS OF RADIO FREQUENCY PROPAGATION IN COALMINE 419 passing thrugh strata. This will ultimately help in designing apprpriate trapped miner lcatr and ther wireless cmmunicatin devices fr undergrund mines. An experimental study has been carried ut t analyze the radi wave prpagatin characteristics and t find ut the suitable frequency fr getting maximum signal strength, while passing thrugh cal. The theries f electrmagnetic prpagatin and labratry experimental prcedure alng with the results are discussed in subsequent sectins. Wave prpagatin thrugh medium In undergrund mine, the lw frequency refractive index is predminately real, and is als greater than unity. Suppse, sme fractins (f 0 ) f the electrns are free in the sense f having initial frequency ω 0 = 0. In this situatin, the lw frequency dielectric cnstant takes the frm 8,9. ε (ω)= n (ω) = n 0 + in e f 0 /ε 0 m ω (Γ 0 i ω) (1) where, n 0 is the cntributin t the refractive index frm all the ther resnances, N the number density f electrns, Γ 0 = lim ω0 0 ω 0 g 0, where g 0 is the dimensinless damping cnstant. But fr a medium, the cntributin t the refractive index frm the free electrns is singular at ω = 0. Thus, using the Maxwell s field equatin, the dielectric cnstant is given by: σ ε( ω) n ( ω) = n0 + i () εω A cmparisn f this term with Eq. (1) yields the fllwing expressin fr the cnductivity: fne σ = m ( τ iω) (3) Thus, at lw frequencies cnductrs pssess predminately real part f cnductivity. Hwever, at higher frequencies the cnductivity becmes cmplex. At these frequencies, there is little meaningful distinctin in cal barriers, since the cnductivity cntributin t ε(ω) appears as resnant amplitude just like the ther cntributins. The cnventinal way t represent the cmplex refractive index f a cnducting medium (in the lw frequency limit) is t write it in terms f a real nrmal dielectric cnstant ( ε = n ) and a real cnductivity (σ). Thus, frm Eq. (), the fllwing equatin is derived: σ ( ω) =ε + i ε ω n (4) It indicates that the field energy is almst entirely magnetic in nature. It is clear that an electrmagnetic wave prpagating thrugh a gd cal blck has markedly different prperties t a wave prpagating thrugh a cnventinal dielectric. Fr a wave prpagating in the x-directin, the amplitudes f the electric and magnetic fields attenuate fllwing the x expressin, exp d ; where, d = (5) µ σω and is called the skin depth. These Eqs (4) and (5) and parameters gvern the RF wave prpagatin thrugh rck and cal strata. These parameters als vary frm place t place depending upn the gegraphical regin, gelgical frmatin, prperties f strata and lcal cnditins. Therefre, these parameters must be evaluated prperly in a particular regin fr designing effective wireless cmmunicatin system t be applied in undergrund mines. 3 Labratry experiment 3.1 Experimental prcedure The labratry set-up t find the field strength f radi waves passing thrugh a cal blck is shwn in Fig. 1. Different radi frequency signals f fixed amplitude were generated by standard RF generatr and fed t the matched directinal transmitting lp antenna. The high Q matched directinal antennae f different RF waves were designed in the labratry. The strength f input RF signals fed t the antennae at different frequencies is depicted in Fig.. The transmitting RF signals were passed thrugh the smthen side f a cal blck (having dimensin f m 3 ) placed abut 15 cm away frm the transmitting antenna and the field attenuated waves were received by a cmpatible receiving antenna placed at the same distance as transmitting antenna, i.e. at arund 15 cm frm anther side f the cal

3 40 INDIAN J RADIO & SPACE PHYS, OCTOBER 007 Fig. 1 Experimental set-up f radi frequency signal prpagatin Fig. Strength f Input RF signals at different frequencies blck. The gain f antennae and the attenuatin f the RF waves thrugh cal blck at different frequencies are shwn in Figs 3 and 4, respectively. It is evident frm Fig. 3 that the gain f the antenna is high at arund 6 MHz. The amplitude f attenuated RF signals and field strength f the attenuated waves were measured by Digital Oscillscpe (Make: Guld Electrnics, Mdel: 145M) and Spectrum Analyzer (Make: Anritsu, Mdel: MS661C), respectively. The values f attenuatin f RF signal with respect t the variatin in transmitting frequencies were recrded. The same experiment was repeated fr anther cal blck having dimensin f m Analysis f prpagatin signals Frm the experimental data a graph was prepared shwing the value f utput RF signal strength with respect t the frequency variatin fr the tw Fig. 3 Gain f the lp antenna at different frequencies Fig. 4 Attenuatin f RF signals thrugh cal blck at different frequencies

4 BANDYOPADHYAY et al.: CHARACTERISTICS OF RADIO FREQUENCY PROPAGATION IN COALMINE 41 different cal blcks (Fig. 5). The graph represents a plynmial functin. Thus, the fllwing methdlgy was adpted t evaluate the best suitable frequency, which generates maximum signal strength while passing thrugh the cal blcks. Let f(x) = ax 3 + bx + cx + d, be a plynmial functin, which rightly fits int the experimental data f change in signal strength (db µv) with respect t variatin f radi frequencies (MHz). The negative sign f f (x) indicates the lss in signal strength and the psitive sign indicates the amplificatin in the signal transmitted thrugh the cal blcks. The suitable frequency fr getting ptimized signal may be mathematically estimated using the maxima and minima methd as explained belw. The slutins f the equatin f (x) = 3ax + bx + c = 0 are b + b 3ac x1 = (6) 3a and x b b 3ac = (7) 3a Nw, f (x 1 ) = b 3ac > 0, i.e. f(x) is minimum at x = x 1 and f (x ) = b 3ac < 0, i.e. f(x) is maximum at x = x Therefre, the recmmended frequency fr bth transmitting and receiving, the maximum signal passing thrugh the cal barrier using transceiver is x = ( b b 3ac)/3a MHz (8) Based n the statistical analysis f data using Statistica Sftware (SPSS15.0), it was fund that the graph as shwn in Fig. 5 was best fitted with the fllwing plynmial equatins: Fr cal blck-1: y = x x x Fr cal blck-: (9) y = 0.04 x x x (10) If we put the cefficients f Eqs (9) and (10) in Eq. (8), we get the value f x as 6.1 and 5.93 MHz, respectively. The average f these tw values is 6.05 MHz. Thus, it may be cncluded that the best suitable frequency fr getting maximum signal strength thrugh cal blcks is arund 6 MHz. It is als evident frm Fig. 5 that the signal strength is maximum at arund 6 MHz. Nw if the cefficients f Eqs (9) and (10) are put t Eq. (6) t get the frequency fr minimum signal strength, ne can get it as MHz and 17.5 MHz, respectively, fr bth the cal blcks. The average f these tw values is MHz. The result f labratry experiments als indicates that the field strength is lwest at arund 16 MHz (Fig. 5). 4 Results and discussin The experiment was carried ut in the frequency range 100 khz-0 MHz. Fascinatingly it was fund that there was appreciable high signal strength in the frequency range 1 MHz-11 MHz. The frequency level, at which the absrptin is critically lw, the signal strength decreases with increase in frequency due t respective lw gain f antennae and high attenuatin f RF waves (Figs 3 and 4). It was als bserved that there was a significant lss f signal strength after the frequency level f 11 MHz. It seems that the graph f signal strength with increasing frequency value frms a peridic r cyclic functin as shwn in Fig. 6. Fcusing the bjective f the present study, the exact frequency level where the signal Fig. 5 Plt f utput RF signal strength versus transmitting frequency Fig. 6 Radi wave travelling in a given directin

5 4 INDIAN J RADIO & SPACE PHYS, OCTOBER 007 strength gets high in the cycle falls within the frequency range f 1-11 MHz. Based n the mathematical analysis it was fund t be 6 MHz. 5 Cnclusins Prpagatin f electrmagnetic wave thrugh strata is a cmplex phenmenn. The same can be analyzed using mdelling technique and experimental studies. Based n the experimental results it was fund that 6 MHz frequency is the best suitable frequency fr generating maximun signal strength while transmitting thrugh cal strata. Therefre, the wireless cmmunicatin system, which will be develped fr cmmunicatin thrugh cal strata, shuld be designed at 6 MHz frequency. The plarizatin in cal mlecules at this particular frequency may be ne f the reasns fr amplified signal strength. Therefre, a deliberative study n plarizatin mechanism f cal mlecules with varying frequency is warranted t establish the relatinship between the signal strength and plarizatin in cal mlecules. Acknwledgements The authrs are thankful t the Directr, Central Institute f Mining and Fuel Research, Dhanbad, fr his encuragement and supprt. They are als grateful t the Department f Infrmatin Technlgy, Ministry f Cmmunicatin and Infrmatin Technlgy, Gvernment f India, New Delhi fr spnsring the study. References 1 Singh K K K, Using resistivity t delineate surface areas at risk f subsidence ver Kamptee clliery, Maharastra, India, The Leading Edge (USA), 5 (006) Bandypadhyay L K, Mishra P K, Kumar Sudhir & Narayan A, Radi Frequency Cmmunicatin System in Undergrund Mines, Paper presented in the Internatinal Seminar n XXVIIIth General Assembly f Internatinal Unin f Radi Science (URSI), Vigyan Bhawan, New Delhi, India, 3-9 Octber, Bandypadhyay L K, Kumar S, Mishra, P K, Narayan, A & Sinha M K, Studies n wireless cmmunicatin systems fr undergrund cal mines in Prceedings f Internatinal seminar n Cal Science and Technlgy Emerging Glbal Dimensins, edited by A K Singh and K Sen (Allied Publishers, New Delhi, India), 005, pp Undergrund Mine Cmmunicatins, Cntrl and Mnitring, IC 8955, Bureau f Mines Infrmatin Circular, Mariage Ph, Lienard M & Degauque P, Theretical and experimental apprach f the prpagatin f light frequency waves in rad tunnels, IEEE Trans Antennas & Prpag. (USA), 4 (1994) Zhang Y P & Hwang Y, Thery f radi wave prpagatin in railway tunnels, IEEE Trans Veh Technl (USA), 47 (1998) Bureau f Indian Standards, Electrical apparatus fr explsive gas Atmspheres Intrinsic safety specificatin, IS 5780 : 00, BIS, New Delhi. 8 Ellitt R S, Electrmagnetics (McGraw-Hill Internatinal Bk Cmpany), Bagguley D M S, Electrmagnetism and linear circuits (Van Nstrand Reinhld Cmpany, Lndn), 1973.