Study on the Problem of VLF Underwater Emission

Transcription

1 Intrnationa Cor Journa of Engrg Vo.4 No.4 8 ISSN: Study on th Probm of VLF Undrwatr Emission Xichuang n a, Bg Liu b and Ji Cui c Nava Univrsity of Engrg, Bijg 84, Cha. arxcxxx@6.com Abstract Considr adoptg th way of VLF mission sourc undr watr VLF communication, with vry ow frquncy symmtric dipos antnna as an xamp to cacuat th undrwatr radiation of th vry ow frquncy signa sourc probm, givn symmtric dipos antnna undrwatr aunch ratd paramtrs, cacuatg th currnt distribution on th transmittg antnna, and accordg to th currnt torqu cacuation of diffrnt distanc of ctromagntic fid tnsity on th surfac. Kywords VLF; communication; tnsity.. Th Introduction Whn a submar is undrwatr, VLF communication is th ma communication mthod. Whn th VLF ctromagntic wavs ntr th sa from th air, thy wi hav a strong attnuation du to th action of th sa watr, so th ctromagntic wavs mittd by th transmittg sourc abov th watr surfac ar difficut to ntr th watr. In ordr to sov this probm, th communication distanc and communication dpth of VLF to submar ar improvd by crasg transmittg powr and antnna transmittg fficincy. A VLF communication station buit on and has a arg siz of transmittg antnna, which is vunrab to damag wartim, and usuay has a arg attnuation distanc from th station to th ocan. Anothr way is considrd this papr, th way of ocatg VLF communication mission sourc undr watr. Th undrwatr symmtric dipos antnna aunch ratd paramtrs is givn, th currnt distribution on th transmittg antnna is cacuatd, thn accordg to th currnt momnt, th ctric fid tnsity abov th ocan surfac of diffrnt distanc is cacuatd. Thn accordg to atmosphric nois v, communication distanc and dpth is stimatd.. Cacuation of Paramtrs of Vry Low Frquncy Undrwatr Transmittg Antnna Takg th VLF symmtric dipos antnna pacg undr th sa surfac as an xamp, th put impdanc, th currnt distribution on th antnna, and th currnt momnt of th antnna ar cacuatd.. Th Propagation Constant of Ectromagntic Wavs Aong th Antnna Assum that transmittg antnna is symmtric dipos antnna, ocatd bow th ocan surfac dpth h, arm ngth, wir radius for a, sa watr transmittg antnna to b coatd a ayr of suation, suation outr radius to b, for rativ dictric cofficint, can xport th ctromagntic wav propagation constant aong th antnna[] 94

2 Intrnationa Cor Journa of Engrg Vo.4 No.4 8 ISSN: Among that b a n b. A 4 B / j nb a nb a n () f () rprsnts dpth of th watr, rprsnts th conductivity of sawatr, rprsnts th wavngth of fr spac, A () 6 h.8 h B.h h. h 4. Whn h, th paramtrs A and B quation () can b ignord. Th ra part of th propagation constant is th attnuation cofficint, xprssd as, and th imagary part is th phas cofficint, xprssd as. j (5). Antnna Impdanc Th cacuation formua of charactristic impdanc of symmtric dipos antnna undr sa surfac is b j b Z c j n n (6) a a Th put impdanc of th symmtric dipos antnna can b cacuatd as foows[]: Z Zc In th abov quation, is th put rsistanc of th antnna, and is th put ractanc of th antnna. For symmtric dipos antnna with short ngth, th put impdanc is gnray arg capacitiv ractanc, ndd to tun with cois.. Antnna Currnt Distribution If th put powr of th antnna is P, and th antnna is tund to rsonanc with a tung coi, th antnna puts currnt can b xprssd I t jx X P (8) X Q is rsistanc of th tung coi, Q is quaity factor of th tung coi. t / Th currnt distribution on th antnna sh x sh x I x I I sh I sh sh j cos x jch xs x sh cos jch s xcos xsh cos ch xs x sh cos ch s xcos xch s ch xs xsh sh cos ch s ch s cos In th abov quation, x is th distanc from th pot th symmtric osciator. Th momnt of currnt th antnna is qua to (4) (7) (9) 95

3 Intrnationa Cor Journa of Engrg Vo.4 No.4 8 ISSN: r h m I sh Idx j () xcos xsh cos ch xs x sh cos ch s xcos xch s ch xs x sh cos ch s r i ch s dx sh sh cos i I dx 5m 4S / m For xamp, th numrica cacuation assums,, a. 5mm, b 7. 5mm,,th ctrica conductivity of sawatr, frquncy f 5kz. Du to th arg attnuation of VLF sawatr, th dpth of antnna cannot b vry dp. Accordg to quation (), th sk dpth of sa watr to 5kz ctromagntic wavs was givn. Accordg to quation (), th func of trfac is ignord[]. Accordg to th quation (6), th Z c charactristic impdanc can b cacuatd 5.6 j. 7 Ω. Accordg to th quation (7), cacuat th put impdanc Ω. Assumg that th powr of th transmittr is qua to 5kW and th tung coi is tund, th put currnt of th antnna is cacuatd accordg to quation (8). Accordg to quation (9), w draw th currnt distribution curv of th ra part on th vibrator arm (figur ) and th virtua part currnt distribution curv (figur ). Both figur and figur ar normaizd rativ to th maximum currnt at th ra part. It can b sn that th virtua currnt is much smar than th ra currnt and can b ignord. In prcip, th currnt momnt of th antnna shoud b cacuatd accordg to quation () ~ (), but sc w hav cacuatd th currnt distribution; It is asir to cacuat th currnt momnt from th currnt distribution [4,5]. Th imagary part of th currnt can b ignord, and th ra part of th currnt is basicay a straight, so th momnt of currnt is qua to th put currnt tims th arm ngth, which is qua to 686.5A.m. () () Fig. Th actua currnt distribution on th vibrator arm (rativ to th maximum normaization) Fig. Th currnt distribution on th imagary part of th vibrator arm (rativ to th maximum of th ra part currnt) 96

4 Intrnationa Cor Journa of Engrg Vo.4 No.4 8 ISSN: Cacuation of Ectromagntic Fid Intnsity of VLF Undrwatr Emission Accordg to th itratur[,], th formua of th ctromagntic fid componnt ovr th sa surfac gnratd by th horizonta momnt of currnt th ocan is givn. In gnra, th distanc is much gratr than th dpth h of th transmittg sourc, so ths formuas ar simpifid to E p jkh jk jk k Ez cos () k k k k p jk h jk jk j k k k k k E k p jk h jk j jk k k k k k p jk h jk j jk k k k k k p jk h jk jk j k k k k k whr is Frsn tgra,dfd as[6] 97 cos s s cos (4) (5) (6) (7) k p jk h jk j z s 4 k (8) k k Ths formuas a conta mutipication factors jt jt j dt dt t t (9) k () k jkh, wav numbr of sa watr k j () Th conductivity of sawatr is 4S / m, for 5kz ctromagntic wavs, thr is k.4867 j Th ctromagntic fid dcays by about tims through m ayr of sawatr, and by 688 tims through m sawatr. This dicats that with th cras of dpth h, th attnuation of factors jk h crass rapidy, so th antnna dpth cannot b vry dp. As can b sn from quation (), for 5kz frquncy, k /(k ) j.75. With th communication rang on th ground, is vry sma, and th Frsn tgra is approximaty as jt j dt t In addition, for vrtica ctric fid, radia ctric fid and azimuth magntic fid, whn th distanc For azimuth ctric fid and radia magntic fid, whn th distanc () k () k

5 Intrnationa Cor Journa of Engrg Vo.4 No.4 8 ISSN: k (4) k Th vaus of trms cudg Frsn tgras ar far ss than thos of th first trm, so th formr can b ignord. For 5kz frquncis. It can b sn that th Frsn trm th formua abov th VLF frquncy band can b ignord. Accordg to quation (), th absout vau of vrtica ctric fid with diffrnt distancs th maximum dirction is cacuatd, as shown tab. Tab. Diffrnt distanc maximum absout vau dirction vrtica ctric fid Distanc (km) E( z V / m) (db) E z 4. Concusion Puttg VLF communication mission sourc undrwatr can improv th concamnt of th aunchg sit, th aunch of th vry ow frquncy undrwatr communication mod, can b usd as backup of and VLF communication station, and usd as mrgncy communication mod. But rction transmittg antnna undrwatr, such as antnna shoud not b too big, th mar nvironmnta factors, this thsis discusss th symmtric dipos antnna undrwatr transmittg antnna propagation constant, th cacuation mthod of antnna impdanc and currnt distribution, and accordg to th currnt momnt cacuation of diffrnt distanc of ctromagntic fid tnsity on th surfac of th procss, and 5 mtrs symmtric dipos antnna as an xamp for th numrica cacuation. frncs [] Modrn prformanc VLF atmosphric nois. K. Omura, P.D. Shaft. IEEE Transactions on Communications. 97. [] Word Distribution and Charactristics of Atmosphric adio Nois. CCI [] "Prcips of Communication Systms Simuation with Wirss Appications". Wiiam. Trantr,K. Sam Shanmugan,Thodor S. appaport, and Kurt L. Kosbar.. 4. [4] An Optimum Dtctor for Cohrnt M-ary Signag th Prsnc of Impusiv Nois. anjan K Maik,and Anup K Gogoi. IEEE Transactions on Communications.. [5] onod W. P. Kg.Micha Frank Brown. Latra ctromagntic wavs aong pan boundaris: a summarizg approach[j]. Procdgs of th IEEE (5). [6] OWE E. Extrmy ow frquncy (ELF) communication to submars [J]. IEEE Trans. on Commun. 974, (4):