Ly Thuyet Anten

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  • CHNG I. GII THIU 1.1 LCH S

    * Thng tin in : - Telegraphy (1884) - Telephony (1878) * Nn tng l thuyt : LT trng in t Maxwell (1854) * H thng Telegraphy khng dy dng bc x in t (Marconi 1897) * n in t v pht dao ng (1904 1915)

    Mng ni ht Anten pht anten thu

    Thnh ph ng dn c

    H thng yu cu ph TH hp (truyn thanh)

    Dng bc x in t

    Suy hao ph thuc khong cch theo quy lut ly tha

    C ly thng tin ln

    Dy i suy hao 23 dB(10 kHz/km) Truyn d liu tn s thp

    Cp ng trc Tn hiu Video Tn hao 4 5 dB, quy lut hm m Khong cch thng tin hn ch

    Thng tin di ng (tu b) Cc phng tin giao thng ng b, my bay.

    CC H THNG THNG TIN

    1

    - Thng tin v tinh - Kinh t - Bo mt * Nhc im : Hiu sut thp

  • 1.2 CC H THNG ANTEN

    + Anten thng dng : - Anten ru trn t - Anten tai th trn tivi - Anten vng cho UHF - Anten Log-chu k cho TV - Anten Parabol thu sng v tinh

    + Trm tip sng vi ba (Microwave Relay) - Anten mt - Anten Parabol bc nha

    + H thng thng tin v tinh : - H anten loa t trn v tinh - Anten cho thu sng v tinh - Mng cc loa hnh nn chiu x (20-30GHz)

    + Anten phc v nghin cu khoa hc QUY C V CC DI TN S Di tn s Tn, k hiu ng dng 3 30 kHz Very low freq. (VLF) o hng, nh v 30 300kHz Low freq. (LF) Pha v tuyn cho mc ch o

    hng 300 3000kHz

    Medium freq. (MF) Pht thanh AM, hng hi, trm thng tin duyn hi, ch dn tm kim

    3 30 MHz High Freq. (HF) in thoi, in bo, pht thanh sng ngn, hng hi, hng khng

    30 300MHz Very High Freq. (VHF) TV, pht thanh FM, iu khin giao thng, cnh st, taxi, o hng

    0,3 3 GHz Ultrahigh (UHF) TV, thng tin v tinh, do thm, Radar gim st, o hng

    3 30 GHz Superhigh freq. (SHF) Hng khng, Viba (microwave links), thng tin di ng, thng tin v tinh

    30 300GHz Extremly high freq. (EHF)

    Radar, nghin cu khoa hc

    2

  • 1.3 KHI QUT V TRUYN SNG IN T + Di pht thanh AM chun (0,55 1,6 MHz): Dng thp anten + Di sng di :

    - Anten n gin vi li thp, t trn mt t. - Mode truyn: sng mt, suy hao ~ R-4. - Mc nhiu cao do nhiu cng nghip - Cn my pht cng sut ln (50-500kW) - Mc nhiu v suy hao cao - C ly thng tin c vi trm dm - Suy hao tng nhanh theo tn s (khng s dng cho TS>20MHz) - Chiu cao ca anten cn c la chn thch hp. - C th c hin tng Fading trong thi gian hng giy, pht, chu nh

    hng ca nhit v m khng kh. khc phc Fading phn tp theo khng gian v tn s.

    + Di sng 30 40 MHz : - C th s dng s phn x t tng in ly - C ly thng tin hng ngn km cc dch v truyn thng quc t - S phn x ph thuc mt in t to bi bc x mt tri - Khng c s dng trn 40MHz (do xuyn qua v fading)

    +Trn 40MHz - Truyn thng (TV, Viba) - Kch thc anten phi ln gp mt s ln bc sng - di sng Viba ( 3 30cm) c th dng nhng anten gng c li cao

    (40-50dB), cng sut my pht gim, nhiu kh quyn gim, c th dng tn hiu bin nh

    + Di sng mm : - Suy gim sng do kh quyn hoc do ma tng - C ly thng tin b gii hn

    3

  • CHNG 2 C S L THUYT AN TEN, CC THNG S C BN CA ANTEN

    2.1 M U Mt s qui c v k hiu: ch nt mvector, ch nghingthng s + nh ngha anten: l mt cu trc c lm t nhng vt liu dn in tt, c thit k c hnh dng kch thc sao cho c th bc x sng in t theo mt kiu nht nh mt cch hiu qu. + Nguyn l hot ng: dng in thay i theo thi gian trn b mt anten bc x sng in t Anten l mt cu trc m dng thay i theo thi gian, c cp t mt ngun thch hp qua ng truyn hoc ng dn sng, c th b kch thch vi bin ln trn b mt anten. + Yu cu v cu trc anten: n gin, kinh t (v d : anten na sng) + Bi ton chnh ca l thuyt v k thut anten: xc nh phn b mt dng in J trn b mt anten sao cho trng bc x tha mn cc iu kin bin trn anten. Bi ton ny thng ch c th gii gn ng. + Phn b dng trn anten c th c xc nh chnh xc hn khi xc nh c c trng tr khng ca anten. + T c tnh tuyn tnh ca h phng trnh Maxwell, v nguyn tc c th xc nh c phn b trng tng khi bit phn b trng ca phn t dng. + Cc phng trnh Maxwell, th vector v th v hng l nhng cng c ton hc ch yu gii bi ton v anten. + Cc c trng c bn ca mt anten:

    - Kiu bc x (hm phng hng). - rng tia, h s nh hng, in tr bc x.

    + Cc phn t bc x c bn: Phn t dng in nguyn t, vng in nguyn t, dng t nguyn t, vng t nguyn t.

    4

  • 2.2 PHNG TRNH MAXWELL V CC IU KIN BIN 2.2.1 H PHNG TRNH MAXWELL + i tng ch yu ca thuyt v k thut anten l kho st s bc x v thu trng iu ha ~ejwt. + Dng in v trng s c biu din di dng cc vector m cc thnh phn ca chng l cc s phc. Khi , trng thc c dng:

    tjt )e(Re),( rEr = (2.1) + Cc phng trnh Maxwell: (2.2.a e)

    + Trong chn khng :

    j

    Dj

    j

    ===

    +==

    J

    B

    D

    JH

    BE

    0

    (2.2a)

    (2.2b)

    (2.2e)

    (2.2d)

    (2.2c)

    (2.3a);B (2.3a); , 00 ,HE ==D + );/(36

    10 90 metFara

    = )/(10.4 70 metHenry= + Trong mi trng c hng s in mi v dn in : dng dn EJc = (2.2b) => ( ) JJH +

    +=++= E

    jjEj

    2.2.2 CC IU KIN BIN BIN CA MT VT DN L TNG ( = ): (2.5) Bn trong vt dn: E , H = 0 Trn b mt: n x E = 0, n . H = 0 Mt dng in mt: sJ = n x H Mt in tch mt: Dns .=BIN CA MT VT DN KHNG L TNG: Trng in t xuyn qua b mt vi bin gim theo hm m: e-z/ ( = (2/o)1/2 vi ng , = 6.6x10

    mS /108.5 7=-3cm tn s 1MHz, v 2.1x10-4cm 1GHz (2.7)

    V d: vi ng, = 5.8x107 S/m, = 6.6x10-3 cm tn s 1MHz, v 2.1x10-4 cm tn s 1GHz.

    Trong a s cc trng hp thc t c th coi trng in t khng xuyn qua cc vt dn tt nh kim loi. Tuy nhin, khi tnh n in tr ca cc vt dn kim loi

    5

  • th cn tnh ti tn hao Joule theo nh lut Ohm (tn hao ca ng truyn, ng dn sng) TNH TN HAO: T trng H to ra dng mt HnJ s = ( nh lut Ampere) Thnh phn tip tuyn ca in trng lin quan vi mt dng in mt:

    ss JnZEn = (L Ohm) (2.8) Trong Zs l tr khng b mt ca vt dn: ( )

    ss

    jZ += 1 (Ohm/dt) (2.9) Bao gm thnh phn thun tr 1/s (in tr ca lp da c chiu su s) v thnh phn cm ng do s xuyn qua ca t trng. Tn hao trn n v din tch c cho bi phn thc ca vector Poynting hng vo vt dn ti b mt vt dn:

    s

    sJP

    2

    21

    = (2.10) - Nu = v cng, th chiu su lp da, v do tr khng b mt v tn hao = 0 - Thng ngi ta so snh tr khng b mt vi tr khng ca khng gian t do:

    OhmZ 3772

    1

    0

    00 =

    = (2.11)

    - Vi Cu, ti 1MHz, Zs = 2.6x10-4(1+j) Ohm - Kt qu trn c th p dng cho cc vt dn tt khc v cho cc b mt c bn knh cong ln hn nhiu so vi su lp da. BIN GIA HAI IN MI: 21 EnEn = , 21 HnHn = , 21 DnDn = 2.2.3 TH VECTOR V TH V HNG T (2.2a), (2.2b) v (2.3) => ,020 JjEkE = (2.12) Vi l s sng ca khng gian t do ( ) 2/1000 =k- Theo phng trnh ny in trng c th c tm trc tip khi bit phn b dng. Trong thc t c th n gin ha bi ton nh th vect A v th v hng : Mt khc bt c vect no vi zero curl u c th biu din di dng gradient ca mt hm v hng. Do c th t : AB = (2.13) - V 0= A nn A c gi l th vector. - S dng cng thc ca gii tch vector => ( )++=+ 000202 . jAJAkA (2.14) - n gin ta chn : = 00jA (iu kin Lorentz) (2.15) - Khi (2.14) tr thnh : JAkA 0202 =+ (2.16) - Thay cc phng trnh (2.14) v (2.15) vo (2.2c) => (2.17) 0202 / =+ k 6

  • - S dng iu kin Lorentz v (2.14) => 00/. jAAjE += (2.18)

    - Trng hp ngun dng : zz aJJ .= th zz aJJ .= v ( ) zz JAk 0202 =+ (2.19)

    2.3 BC X CA PHN T DNG IN - nh ngha phn t dng in: dlI thng, rt mng, rt ngn. Gi thit d liu //

    (z). - Th vector ch c mt thnh phn theo phng (z) tun theo PT (2.19). trong

    Jz=I/dS, vi dS l tit din ca phn t dng. Th tch dV

  • - Cc mt sng ng pha c dng hnh cu bn knh r, tm = gc to .

    - Vn tc pha = (cng thc) - Bc sng

    fC

    wC

    koo===

    2

    2 (2.25)

    Tm biu thc ca ca trng: - S dng (2.13) v (2.18) v h to cu. - Biu din A

    r theo cc thnh phn trong h to cu v lu rng:

    Ta c: ( )Aaer

    IdlA rjkt sina-Acos

    400

    = (2.26) Dng (2.13):

    aerrjkIdAH rjk02

    0

    0

    14sin.1

    +== l (2.27)

    T (2.18) => aEaEjAAjE rr +=+=

    00

    . (2.28)

    - Nu r rt ln so vi bc sng th : (vng xa) b qua cc s 21r

    , 31r

    arekIdjZE

    rjk

    4sin

    0

    00

    = l (2.29a)

    arekjIdH

    rjk

    4sin

    0

    0

    = l (2.29b)

    * Nhn xt: - Vy khu xa, trng bc x ch c thnh phn ngang, in trng v t trng

    vung gc vi nhauv vung gc vi phng truyn sng. t s bin ca chng

    chnh bng tr khng sng ca khng gian t do Z0; 21

    0

    00

    = Z

    - Dng vector:

    HaZE r = 0 (2.30a) EaYH r = 0 (2.30b) Trong : 100 Z=Y- Trng khng c tnh i xng cu. ( E Z v H ph thuc sin ) * Vector Poynting phc:

    ( ) 2222020** 32sin.21

    rakdZIIHE rl= (2.31b)

    8

  • C dng thun thc, (trng bc x) c hng trng vi hng lan truuyn, v cng sut bc x gim t l nghch vi r2

    * Cc s hng cn li ca (2.27) v (2.28): chim u th khi r < o v to ra trng phn ng khu gn v tnh thun o ca vector Poynting.

    - Nu kor rt nh sao cho c th thay th: (khu gn) 10 rjke

    a

    rkIdH4

    sin0l= (2.32a)

    ++

    += arjkrarjkr

    IdZE r0

    20

    20 11sin11cos24

    l (2.32b)

    Cho k0rphng trnh (2.32b) tr thnh

    += ararQdE r 3

    sincos24 3

    l (2.32c)

    Lu : - Tng t nh phn b trng tnh ca mt dipole in. - Mc d trng khu gn khng ng gp vo cng sut bc x, ch

    lin quan n s tch t nng lng khu vc bao quanh ngay gn anten, nhng cn c tnh n khi tnh tr khng anten.

    - Biu thc ca vector Poynting phc, c tnh bi vic s dng cc biu thc tng qut ca trng s c phn thc (phn lin quan trc tip n bc

    x) ch bao gm trng bc x cho bi biu thc (2.31) __________________________________________________

    2.4 MT S CC THNG S C BN CA ANTEN Bc x ca mt phn t dng in cn c gi l bc x lng cc. c dng

    nh ngha cc thng s c bn