Interference Possibilities in TV Band-I due to Propagation of vhf Signals via Sporadic E

Transcription

1 Indian Jurnal f Radi. Space Physics Vl. 3, December 1974, pp Interference Pssibilities in TV Band-I due t Prpagatin f vhf Signals via Spradic E T. R. SUNDARAM, C. S. R. RAO & J. M. ROY Research Department, All India Radi. New Delhi Received 18 May 1974 Prpagatin f vhf signals by spradic E layer is well knwn and has been studied by a number f wrkers. Experimental and analytical investigatin in the Eurpean, Suth American (near magnetic equatr) and in the African and Far Eastern regins have shwn that the pssibility f interference due t Es prpagatin hhfsignals is cnsiderable. Hwever, n such data are available fr the Indian cnditins. In this paper an analysis f the spradic E data as btained frm vertical sundings has been made t estimate its interference ptentialities in TV and Band-I transmissins. A quantitative estimate f the percentage f time during which bjectinal interference can exist at different hurs f the day is given fr the trpical and temperate regins f India. These data are cnsidered essential fr the planning f TV net wrk in India. I. Intrductin PROPAGATION f vhf signals by reflectins frm spradic E layer is well knwn and has been studied by' a number f wrkers. Bwles and Chen- studied radi wave scattering frm spradic E near magnetic equatr in the American regin and have btained experimental evidence that irregularities in the E regin supprt strng blique scatter prpagatin f 50 MHz radiwaves bth during day and night time. Dieminger et au have analyzed vertical incidence Es data and shwn that due t the high prbability f c-channel interference which exists due t the prpagatin f vhf signals via the equatrial spradic E layer, the use f Band-I (41-68 MHz) fr TV shuld be severely discuraged in the equatrial regins. Miya and Sasaki' have carried ut bservatins n Es signals at vhf in the Far East ver several years and shwn that muf factrs much higher than thse estimated by secant law are pssible. Accrding t Beynn' calculatins based n vertical incidence measurements f{es cmbined with a simple secant law tend t underestimate the effectiveness f Es inizatin in blique prpagatin. frm the abve bservatins it wuld be seen that the interference ptentiality due t Es prpagatin f vhf (particularly in Band-I) is cnsiderable and n netwrk planning can be cmplete unless this factr is taken int cnsideratin. T cver a large cuntry like India, (;0- channel peratin cannt be avided in view f the limited number f channels available in the vhf band and it is very essential that the prblem f interference due t Es prpagatin be examined in detail. It may nt als be pssible t use data frm the African r American znes since it is knwn" that the "equatrial" belt extends t higher latitudes in India than in America. It is in this cntext that we analyze the Es data fr the Indian sub-cntinent and try t btain a quantitative estimate f the percentage f time during which bjectinable interference exists at different hurs f the day. 2. Data and Analysis S far as the ccurrence f spradic E is cncerned, Indian sub-cntinent can be divided int (i) the equatrial zne extending up t abut loon (gegr. lat.) and (ii) the temperate zne extending beynd the equatrial zne. A number f insnde statins have been perating in these tw regins fr a number f years and Es data cllected at these statins frm the basis f this analytical study. Of these statins, Kdaikanal, Tiruchi and Trivandrum (Thumba) are in the equatrial zne. Madras is n the brder between the tw znes and Hyderabad, Bmbay, Haringhata (Calcutta), Ahmedabad and Delhi are in temperate zne. 'Fr purpses f analyses in this paper, we have taken data frm Kdaikanal (77 29'E, lo I4'N; gemag. lat., 0044'N; dip, 3'S0N) as representative f equatrial regins and Hyderabad (78 21'E, Ir2I'N; gemag. lat., 7 38'N; dip, 21'5 N) and Ahmedabad (72 36' ; 'N; gemag. lat., 14 0!'N; dip, 34 0 N) as representative f temperate znes. Data regarding fes have been taken frm Inspheric Data-A-series published by the RRC, New Delhi. Observatins frm March 1964 t February 19 cvering a perid f six years were taken fr estimating the prbability f fes exceeding specific values, neglecting the slar cycle variatins, if any, 391

2 INDIAN J. RADIO SPACE PHYS., VOL. 3, DECEMBER :; hrs 75 EMT Fig. 1- Prbability f fes exceeding different frequencies at Kdaikanal EQUI~OX SUMMER \NIHTER,;: >- 50 "75 \-...J ;{O en «CD :: 0... t rz 14 ts ie t hrs 75 0 EMT Fig. 2-Prbability f fes exceeding different frequencies at Ahmedabad 80 frm year t year. Hwever, t take int accunt seasnal variatins, each year was divided int three seasns: the equinx (March, April, September, Octber), the summer (May, June, July, August), and the winter (Nvember, December, January, February). This gives a ttal number f abut 730 pssible bservatins fr each seasn and fr each hur. The prbability ffes exceeding any specified frequency was estimated frm these fes values. In mst cases between 00 and 1900 hrs, the actual number f bservatins was a sufficiently high percentage f the ttal pssible number f bservatins and the prbability thus determined culd, therefre, be taken t be representative f the ppulatin. The diurnal variatins f fes exceeding specified frequencies are shwn ill Figs. I, 2 and 3 fr Kdaikanal, Ahmedabad and Hyderabad, all the three seasns fr Kdaikanal and Ahmedabad and summer nly fr Hyderabad. The data fr Hyderabad are based n fur years data nly frm Frm these figures it wuld s- 60 e-: > S r- -.J CD <t III 30 0 a:: a SUMMER 75 t t,---:-,.---'~:::::::;:===:::r:-----r"-"---.-j ~, fo te 20 hrs 75 EMT Fig. 3-Prbability f fes exceeding different frequencies at Hyderabad

3 SUNDARAM et al.: INTERFERENCE IN TV BAND-I DUE TO vhf SIGNALS.r====-O::::::------i FREQUENCY. t-lhl Fig. 4- Cumulative prbability distributin f fes at midday be seen that the prbability f fes exceeding 5 MHz ver Kdaikanal is cnsiderable all the year rund with very little seasnal variatins and that high values f fes during midday are assciated with fairly high prbability. Fig. 4 shws the cumulative prbability distributin f fes fr midday fr the three seasns at Kdaikanal. In cntrast, the summer curves f bth Ahmedabad and Hyderabad shw cnsiderably reduced spradic E phenmenn with the prbability at the lwest frequency f 5 MHz being less than 50% at midday. The cumulative distributin in summer at Ahmedabad and Hyderabad are als included in Fig. 4. As the bject f this paper is t estimate the effect f spradic E n vhf prpagatin in TV Band-I (41-68 MHz) prbabilities f fes exceeding 3 and 4 MHz were nt cmputed as these lw frequencies wuld nt be supprting vhf prpagatin. Figs. I, 2 and 3 frm the basis f estimating the prbability f mutual interference in the service areas f transmissins separated by different distances ranging frm 0 t 2000 km. The prbability f interference wuld be a functin f (a) the minimum useful signal strength in the absence f interference, (b) the minimum c-channel signal-t-interference rati which wuld be tlerated, (c) the effective radiated pwer in the path between tw c-channel transmissins, (d) the path lss which includes the spread lss due t distance and the lss suffered by the signal in the mechanism f spradic E prpagatin and (e) the rati f perating frequency t fes. While a cnsiderable amunt f investigatins f spradic E layer has been dne, the mechanism f prpagatin by spradic E layer is by n means clearly understd and we have t rely n statistical results fr estimating the attenuatin suffered by a vhf signal in its passage thrugh spradic E layer. In the absence f experimental data fr paths in India, the results btained by Miya and Sasaki fr vhf prpagatin by spradic E layer have been utilized in this paper. These results are based n a number f years f experimental s\ udy ver different paths in Japan and sme paths in the United States and are summed up in Fig. II f their paper. This figure gives the inspheric attenuatin by spradic E prpagatin as a functin f path distance fr different ratis f flfes wheref is the perating frequency andfes is the critical frequency at vertical incidence at the midpints. Ratis f flfes ranging frm 9 t 1 have been cnsidered fr bth single and duble hp prpagatin and emphasize the inadequacy f the secant law in vhf prpagatin by spradic E layer. Fig. II f the paper by Miya and Sasaki is reprduced as Fig. 5 in this paper. As regards signal requirements, a minimum signal f 48 db abve If V has been taken as the minimum useful signal in the absence f c-channel interference and' ther terrestrial nise interferences. The signalt-interference rati fr c-channel signals has been taken as 45 db. These figures are based n CCIR recmmendatin" and 418-\. An E. R. P. f 250 kw has been taken as the transmitter pwer as it is extremely unlikely that this pwer wuld be exceeded. Under these cnditins if an interfering cchannel signal exceeds 3 db there wuld be interference in the service area. As the lwest useful signal strength f 48 db wuld define the fringe f the Fig. 5- Inspheric attenuatin f Es prpagatin 393

4 .. IN \0 t 90 BO 600 -m JO ~ >- I- 10 A -l 0 CD 100 <! 90 CD 0 BO ~ ca. iood u-iz 1900-= J -e r 60 S 3Q tr1 tn s: td tr1 fq 29 t E F 'l f % OF SUMMER DAY TIME GO Fig. 6-Prbability f interference-free service: A-summer, Kdaikanal, 45 MHz; B-summer. Hyderabad, 45 MHz; C-summer, Ahmedabad, 45 MHz; D-s ummer, Kdaikanal, 65 MHz; E-summer, Hyderabad, 65 MHz; F-summer, Ahmedabad, 65 MHz

5 SUNDARAM et al.: INTERFERENCE IN TV BAND-I DUE TO vhf SIGNALS service area, interfering signals f mre than 3 db wuld prgressively reduce the service area. Fr example, interfering signal strength f 9 db wuld reduce the service area in the absence f such interference by 50% fr as lng as the interfering signal remains at 9 db, while an interfering signal f 15 db wuld reduce the service area t 25%. The data in Figs. 1 t 3 can be utilized alngwith the inspheric attenuatin f Fig. 5 t estimate the. prbability that a 250 kw transmitter wuld set up a field strength nt greater than 3 dbhn at any given distance and at any given time f the day. Figs. 6 (A-F) have been drawn frm such cmputatins and they illustrate the prbability f interference-free service fr different percentages f summer day time, the wrst perid f the year. Ideally, the curves in these figures shuld be straight lines indicating 100% prbability fr different percentages f time, the departure frm this ideal shwing the effect f spradic E prpagatin in reducing the service prbability. In these figures n accunt has, hwever, been taken f time variability f the inspheric attenuatin values f Fig. 5. Frm these figures it can be seen that spradic E prpagatin causes greater interference at lnger distances and at lwer end f Band-I. Fr instance, frm Fig. 6(C) which shws the effect f spradic E prpagatin f 45 MHz when midpint f Es crrespnds t that ver Ahmedabad (23 01'N) c-channel transmissins lcated within abut 900 km wuld be free f interference all the time with a high prbability f 90% which prgressively reduces t abut 55% at lnger distances. The effects f spradic E are, hwever, negligible at 65 MHz as can be seen frm Fig. 6(F). Figs. 6(B) and 6(E) shw that at lwer latitu des crrespnding t Hyderabad the psitin is similar t that at Ahmedabad. Nearer the gemagnetic equatr, hwever, incidence f spradic E can severely affect the service areas due t mutual interference even at shrt distances f the rder f 0 t 600 km as is evident frm Figs. 6(A) and 6(D). 3. Cnclusins This study indicates that there is nt much f a prblem in c-channel allcatins in Band-I in India abve a latitude f 17 N but allcatins f channels in the suthern regin wuld be seriusly restricted during daytime due t the high incidence f spradic E. Cnsidering that neighburing cuntries such as Burma, Malaysia and Ceyln are all in the distance ranges f severe interference, use f Band-III ( MHz) seems t be the nly pssibility fr prviding interference-free service all the year rund in these regins. Acknwledgement This frms part f the wrk f prpagatin sectin f the Research Department f All India Radi and is published with the permissin f the Research Engineer, All India Radi, New Delhi. References 1. BOWLES, K. L. & COHEN, R., Inspheric spradic E, edited by E. K. Smith and Sadami Matsushita (Pergamn Press Inc., New Yrk), (1962), DIEMINGER, W., ROSE, G. & WIDDET, R. U., Telecmmun.J., 30 (1963), MIYA, K. & SASAKI, T., Radi sct., 1 (1966), BEYNON, W. 1. G., B. B. C. Mngraph, 61 (1966), RAO, C. S. R., J. atms, terr. Phys., 26 (1964), C. C. 1. R. (19), Vl. V, Part