44670844 Khoa Luan Ve Anten

Kha lun tt nghip i hc

Ngnh: Thng tin v tuyn

LI CM N Em xin chn thnh cm n Tin s Trn Minh Tun, thy tn tnh hng dn, gip em trong sut thi gian thc hin kha lun. Khng c s gip v nhng li khuyn v gi ca thy, kha lun ca em khng th c hon thnh. Thm na, em cng mun by t li cm n ti GS. Phan Anh v Ths. Trn Th Thy Qunh kp thi tr li nhng cu hi v nhng thc mc ca em trong qu trnh thc hin kha lun, cng nh to iu kin thun li em s dng cc thit b, my mc trong phng th nghim thc hin ch to v o c thc nghim. Cui cng, em mun cm n su sc ti gia nh em. Gia nh yu thng, ng h v gip em khng ch trong thi gian lm kha lun m trong c kha hc.

H Ni, thng 06 nm 2008 Sinh vin Lu Vn Hoan

Sinh vin: Lu Vn Hoan

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Lp K49 Thng tin v tuyn



TM TT NI DUNG KHA LUN Kha lun tp trung nghin cu, thit k, ch to mt anten vi di bng rng, c kh nng hot ng ti nhiu di tn. Anten ny s dng cho cc thit b di ng hot ng trong cc di tn GSM, DCS, PCS, UTMS, WLAN. Anten c ch to trn cht nn c hng s in mi r = 4.4, dy l 0.8 mm v c thit k ti tn s 900 MHz v 2000 MHz. Ni dung kha lun bao gm 4 chng v phn nh gi kt qu kha lun v hng pht trin tip theo. Trong c th chia thnh hai phn vi ni dung nh sau: Phn u tin ca kha lun cp ti l thuyt c bn v anten, anten vi di v ng truyn vi di. Chng 1 gii thiu v nh ngha anten, nu ra mt s tham s c bn nh gi hiu sut ca anten nh: gin bc x, cng sut bc x, h s nh hng, h s tng ch, phn cc, tr khng vo, Tip theo l khi nim v anten vi di: cc u im, nhc im v mt s loi anten vi di c nghin cu v thit k. Mt s im tng qut v ng truyn vi di cng c trnh by. Chng 2 a ra cc phng php thit k anten vi di bng rng v anten vi di c th hot ng ti nhiu bng tn. Trong , mt s phng php thit k anten vi di c trnh by nh: phng php phi hp tr khng lin tc, phng php s dng cht nn dy hn, phng php kch thch a mode, phung php s dng nhiu patch xp chng nhau, ng thi a ra cc phng php thit lp anten vi di c kh nng hot ng ti nhiu di tn khc nhau. Vic thit k cc anten bng rng v a di tn p ng xu th tch hp, thu nh kch thc cho cc thit b di ng a nng. Phn th hai i vo thit k, m phng v o c cc tham s ca anten. Chng 3 trnh by phng php thit k cc thnh phn ca anten: thnh phn bc x, b phi hp tr khng di rng, v ng truyn vi di 50 . Chng 4 trnh by qu trnh m phng, qui trnh thit k v o c thc nghim. Phn m phng gii thiu v phn mm Ansoft HFSS, phn mm c s dng trong m phng cc bi ton in t 3D. Trong trnh by vic thit t cc tham s quan trng thc hin phn tch cu trc anten, kt qu m phng cu trc anten c nhnh iu chnh v khng c nhnh iu chnh. Sau khi qu trnh thit k, m phng hon thnh (t c cc tiu ch yu cu), tin hnh ch to v o c cc tham s ca anten s dng my Network Analyse v h o trng bc x ca anten. Kt qu o c thc nghim v kt qu m phng cho anten c thit k trong kha lun tng i ph hp nhau. Da vo vic phn tch kt qu v phn tch qui trnh ch to anten, kha lun a ra cc nguyn nhn gy ra sai lch. ng thi ra phng hng gii quyt v phng hng nghin cu tip theo nhm ci thin cc c tnh ca anten.


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MC LC LI CM N...................................................................................................................i TM TT NI DUNG KHA LUN ......................................................................... ii MC LC ..................................................................................................................... iii LI NI U.................................................................................................................1 Chng 1: L thuyt c bn v anten v anten vi di 1.1. L thuyt chung v anten .........................................................................................3 1.1.1. Gii thiu...........................................................................................................3 1.1.2. Cc tham s c bn ca anten ...........................................................................5 1.1.2.1. S bc x sng in t bi mt anten .......................................................5 1.1.2.2. Gin bc x ...........................................................................................6 1.1.2.3. Mt cng sut bc x ..........................................................................10 1.1.2.4. Cng bc x......................................................................................11 1.1.2.5. H s nh hng.....................................................................................12 1.1.2.6. H s tng ch ..........................................................................................13 1.1.2.7. Bng thng ...............................................................................................14 1.1.2.8. Phn cc...................................................................................................15 1.1.2.9. Tr khng vo ..........................................................................................17 1.2. ng truyn vi di v anten vi di .......................................................................18 1.2.1. ng truyn vi di ........................................................................................18 1.2.1.1. Cu trc ng truyn vi di ...................................................................18 1.2.1.2. Cu trc trng ca ng truyn vi di.................................................18 1.2.2. Anten vi di .....................................................................................................19 1.2.2.1. Gii thiu chung ......................................................................................19 1.2.2.2. Mt s loi anten vi di c bn................................................................20 1.2.2.3. Anten patch hnh ch nht .......................................................................22 Chng 2: Anten mch di bng rng v anten mch di nhiu bng tn 2.1. Gii thiu chung .....................................................................................................24 2.1.1. Di thng tn ...................................................................................................24 2.1.2. Di tn cng tc...............................................................................................25 2.2. M rng bng thng ca anten vi di .....................................................................25 2.2.1. Gii thiu.........................................................................................................25 2.2.2. nh hng ca cc tham s cht nn ti bng thng .....................................27 2.2.3. La chn hnh dng thnh phn bc x thch hp ..........................................28 2.2.4. La chn k thut tip in thch hp.............................................................29 2.2.5. K thut kch thch a mode ...........................................................................30 2.2.5.1. M rng bng thng s dng nhiu thnh phn bc x xp chng.........30 2.2.5.2. M rng bng thng s dng cc thnh phn k sinh ng phng ..........31 2.2.5.3. Cc k thut kch thch a mode khc .....................................................35 2.2.6. Cc k thut m rng bng thng khc ..........................................................35 2.2.6.1. Phi hp tr khng...................................................................................36 2.2.6.2. Mc ti in tr ........................................................................................37 Sinh vin: Lu Vn Hoan iii Lp K49 Thng tin v tuyn



2.3. Anten vi di nhiu bng tn....................................................................................37 2.3.1. Anten vi di 2 tn s cng hng ...................................................................37 2.3.2. Anten vi di nhiu hn 2 tn s cng hng ..................................................38 2.4. Phi hp tr khng di rng ...................................................................................39 2.4.1. ngha ca vic phi hp tr khng ..............................................................39 2.4.2. Phi hp tr khng di rng............................................................................39 2.4.3. Mt s b phi hp tr khng di rng...........................................................42 2.4.3.1. B phi hp tr khng lin tc dng hm m .........................................42 2.4.3.2. B phi hp tr khng lin tc dng tam gic.........................................43 2.4.3.3. B phi hp tr khng lin tc Klopfenstein...........................................44 2.4.4. Tiu chun Bode Fano .................................................................................46 Chng 3: Thit k anten dt cu trc xon, tip in dng ng truyn vi di 3.1. Gii thiu ................................................................................................................48 3.2. Thit k thnh phn bc x ....................................................................................49 3.3. Thit k thnh phn phi hp tr khng di rng ..................................................50 3.3.1. So snh mt s b phi hp tr khng di rng..............................................50 3.3.2. La chn b phi hp tr khng di rng.......................................................52 3.4. Thit k ng truyn vi di 50 .........................................................................53 3.4.1. Thit k vi Ansoft Designer 2.0 ....................................................................53 3.4.2. Thit k da vo l thuyt ng truyn vi di ..............................................54 3.4.2.1. Tr khng c trng Z0 ............................................................................54 3.4.2.2. Bc sng trn ng vi di .................................................................55 3.4.2.3. Cng sut cho php trung bnh Pav ..........................................................57 3.4.2.4. Cng sut cho php ti a Pp ...................................................................58 Chng 4: M phng, ch to v o c cc tham s ca anten 4.1. M phng cu trc anten vi phn mm Ansoft HFSS..........................................59 4.1.1. Phn mm HFSS phin bn 9.1.......................................................................59 4.1.2. Kt qu m phng vi HFSS 9.1 ....................................................................61 4.2. Ch to anten ..........................................................................................................67 4.3. o c cc tham s ca anten.................................................................................69 PH LC ......................................................................................................................73 A. Ph lc 1: Thut ton chia li thch nghi ca Ansoft HFSS 9.1 ........................73 B. Ph lc 2: Mt s lu v thit t cc tham s trong HFSS..............................74 B.1. Solution Setup ...............................................................................................74 B.2. Mesh Operations ...........................................................................................77 B.3. Radiation Boundary ......................................................................................78 TI LIU THAM KHO .............................................................................................80


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LI NI U Truyn thng khng dy pht trin rt nhanh trong nhng nm gn y, theo cc thit b di ng ang tr nn cng ngy cng nh hn. tha mn nhu cu thu nh cc thit b di ng, anten gn trn cc thit b u cui cng phi c thu nh kch thc. Cc anten phng, chng hn nh anten vi di (microstrip antenna) v anten mch in (printed antenna), c cc u im hp dn nh kch thc nh v d gn ln cc thit b u cui, ; chng s l la chn tha mn yu cu thit k trn. Cng bi l do ny, k thut thit k anten phng bng rng thu ht rt nhiu s quan tm ca cc nh nghin cu anten. Gn y, c bit l sau nm 2000, nhiu anten phng mi c thit k tha mn cc yu cu v bng thng ca h thng truyn thng di ng t bo hin nay, bao gm GSM (Global System for Mobile communication, 890 960 MHz), DCS (Digital Communication System, 1710 1880 MHz), PCS (Personal Communication System, 1850 1990 MHz) v UMTS (Universal Mobile Telecommunication System, 1920 2170 MHz), c pht trin v xut bn trong nhiu cc ti liu lin quan. Anten phng cng rt thch hp i vi ng dng trong cc thit b truyn thng cho h thng mng cc b khng dy (Wireless Local Area Network, WLAN) trong cc di tn 2.4 GHz (2400 2484 MHz) v 5.2 GHz (5150 5350 MHz). Anten vi di vn c bng thng hp, v m rng bng thng thng l nhu cu i vi cc ng dng thc t hin nay. Do , vic gim kch thc v m rng bng thng ang l xu hng thit k chnh cho cc ng dng thc t ca anten vi di. Nhiu s ci tin ng k thit k anten vi di nn vi c tnh bng rng, nhiu bng tn, hot ng vi c hai loi phn cc, phn cc trn v tng ch cao c bo co trong mt vi nm gn y. Kha lun tp trung thit k mt anten vi di bng rng v a di tn. ng thi s dng phn mm Ansoft HFSS thit k v m phng. HFSS s dng phng php phn t hu hn (Finite Element Method, FEM), k thut chia li thch nghi (adaptive meshing) v giao din ha p mang n s hiu bit su sc i vi tt c cc bi ton trng in t 3D. Kha lun gm 4 chng: Chng 1: L thuyt c bn v anten v anten vi di Chng 2: Anten mch di bng rng v anten mch di nhiu bng tn Chng 3: Thit k anten dt cu trc xon, tip in dng ng truyn vi di Chng 4: M phng, ch to v o c cc tham s ca anten Bng nhng nghin cu l thuyt v thc nghim, kt hp vi m phng kha lun thc hin c nhng ni dung chnh sau y:


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Nghin cu l thuyt v anten v anten vi di. Nu ra nguyn l v cc phng php xy dng anten vi di bng rng v anten c kh nng hot ng ti nhiu di tn. Thit k, m phng v ch to anten vi di dt c cu trc xon, tip in dng ng truyn mch di. o c v nh gi cc c tnh ca anten c thit k nh: tn s cng hng, bng thng, tr khng vo, gin bc x.


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CHNG 1

L THUYT C BN V ANTEN V ANTEN VI DI

Tm tt Chng ny trnh by cc vn sau: nh ngha anten Cc tham s c bn ca anten ng truyn vi di Anten vi di, m t c th anten vi di c patch hnh ch nht 1.1. L thuyt chung v anten 1.1.1. Gii thiu Thit b dng bc x sng in t (anten pht) hoc thu nhn sng (anten thu) t khng gian bn ngoi c gi l anten. Ni cch khc, anten l cu trc chuyn tip gia khng gian t do v thit b dn sng (guiding device), nh th hin trong hnh 1.1. Thng thng gia my pht v anten pht, cng nh gia my thu v anten thu khng ni trc tip vi nhau m c ghp vi nhau qua ng truyn nng lng in t, gi l fide. Trong h thng ny, my pht c nhim v to ra dao ng in cao tn. Dao ng in s c truyn i theo fide ti anten pht di dng sng in t rng buc. Ngc li, anten thu s tip nhn sng in t t do t khng gian bn ngoi v bin i chng thnh sng in t rng buc. Sng ny c truyn theo fide ti my thu. Yu cu ca thit b anten v fide l phi thc hin vic truyn v bin i nng lng vi hiu sut cao nht v khng gy ra mo dng tn hiu.

Hnh 1.1. Anten nh mt thit b truyn sng [3] Sinh vin: Lu Vn Hoan 3 Lp K49 Thng tin v tuyn



Phng trnh tng ng Thevenin h thng anten trong hnh 1.1 lm vic ch pht c th hin trong hnh 1.2, trong ngun c th hin bi b to dao ng l tng, ng truyn dn c th hin bi ng dy vi tr khng c trng Zc, v anten c th hin bi ti ZA, trong ZA=(RL + Rr)+jXA. Tr khng ti RL th hin s mt mt do in mi v vt dn (conduction and dielectric loss), 2 thnh phn mt mt ny lun gn vi cu trc anten. Tr khng Rr c gi l tr khng bc x, n th hin s bc x sng in t bi anten. in khng XA th hin phn o ca tr khng kt hp vi s bc x bi anten. Ngoi sng in t bc x ra khu xa, cn c trng in t dao ng gn anten, ging buc vi anten. Phn cng sut ny khng bc x ra ngoi, m khi th chuyn thnh nng lng in trng, khi th chuyn thnh nng lng t trng thng qua vic trao i nng lng vi ngun. Cng sut ny gi l cng sut v cng, v c biu th thng qua in khng XA. Trong iu kin l tng, nng lng to ra bi ngun s c truyn hon ton ti tr khng bc x Rr. Tuy nhin, trong mt h thng thc t, lun tn ti cc mt mt do in mi v mt mt do vt dn (ty theo bn cht ca ng truyn dn v anten), cng nh ty theo s mt mt do phn x (do phi hp tr khng khng hon ho) im tip in gia ng truyn v anten.

Hnh 1.2. Phng trnh tng ng Thevenin cho h thng anten trong hnh 1.1 [3] Sng ti b phn x ti im tip in gia ng truyn dn v u vo anten. Sng phn x cng vi sng truyn i t ngun thng ti anten giao thoa nhau to thnh sng ng (standing wave) trn ng truyn dn. Khi trn ng truyn xut hin cc nt v bng sng ng. Mt m hnh sng ng in hnh c th hin l ng gch t trong hnh 1.2. Nu h thng anten c thit k khng chnh xc, ng truyn c th chim vai tr nh mt thnh phn lu gi nng lng hn l mt thit b truyn nng lng v dn sng. Nu cng trng cc i ca sng ng ln, chng c th ph hy ng truyn dn. Tng mt mt ph thuc vo ng truyn, cu trc anten, sng ng. Mt mt do ng truyn c th c ti thiu ha bng cch chn cc ng truyn mt mt thp, trong khi mt mt do anten c th c gim i bng cch gim tr khng bc x RL trong hnh 1.2. Sng ng c th c gim i v kh nng lu gi nng lng ca ng truyn c ti thiu ha bng cch phi hp tr khng ca anten vi tr khng c trng ca ng truyn. Tc l phi hp tr khng gia ti vi ng truyn, y ti chnh l anten. Mt phng trnh tng t nh hnh 1.2 c s dng th hin h thng anten trong ch thu, ngun c thay bng mt b thu. Tt c cc phn khc ca


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phng trnh tng ng l tng t. Tr khng pht x Rr c s dng th hin trong ch thu nhn nng lng in t t khng gian t do truyn ti anten. Cng vi vic thu nhn hay truyn pht nng lng, anten trong cc h thng khng dy thng c yu cu l nh hng nng lng bc x mnh theo mt vi hng v trit tiu nng lng cc hng khc. Do , anten cng cn phi c vai tr nh mt thit b bc x hng tnh. Hn na, anten cng phi c cc hnh dng khc nhau ph hp cho cc mc ch c th. Anten l mt lnh vc si ng. Cng ngh anten l mt phn khng th thiu trong cc gii php truyn thng. Nhiu s ci tin c a ra trong thi gian cch y hn 50 nm vn cn s dng ngy nay; tuy nhin cc kt qu mi v nhng thay i c a ra ngy nay, c bit l nhu cu hiu sut h thng ngy cng ln hn. 1.1.2. Cc tham s c bn ca anten Phn ny trnh by mt s khi nim v cc quan h c bn v anten nh: s bc x sng, trng bc x v gin trng bc x, phn cc sng bc x, nh hng, tn s cng hng, tr khng, bng thng, tng ch, 1.1.2.1. S bc x sng in t bi mt anten Khi nng lng t ngun c truyn ti anten, 2 trng c to ra. Mt trng l trng cm ng (trng khu gn), trng ny ging buc vi anten; cn trng kia l trng bc x (trng khu xa). Ngay ti anten (trong trng gn), cng ca cc trng ny ln v t l tuyn tnh vi lng nng lng c cp ti anten. Ti khu xa, ch c trng bc x l c duy tr. Trng khu xa gm 2 thnh phn l in trng v t trng (xem hnh 1.3).

Hnh 1.3. Cc trng bc x ti khu xa [8] C hai thnh phn in trng v t trng bc x t mt anten hnh thnh trng in t. Trng in t truyn v nhn nng lng in t thng qua khng gian t do. Sng v tuyn l mt trng in t di chuyn. Trng khu xa l cc sng phng. Khi sng truyn i, nng lng m sng mang theo tri ra trn mt din tch ngy cng ln hn. iu ny lm cho nng lng trn mt din tch cho trc gim i khi khong cch t im kho st ti ngun tng. Sinh vin: Lu Vn Hoan 5 Lp K49 Thng tin v tuyn



1.1.2.2. Gin bc x Cc tn hiu v tuyn bc x bi anten hnh thnh mt trng in t vi mt gin xc nh, v ph thuc vo loi anten c s dng. Gin bc x ny th hin cc c tnh nh hng ca anten. Gin bc x ca anten c nh ngha nh sau: l mt hm ton hc hay s th hin ha ca cc c tnh bc x ca anten, v l hm ca cc ta khng gian. Trong hu ht cc trng hp, gin bc x c xt trng xa. c tnh bc x l s phn b nng lng bc x trong khng gian 2 chiu hay 3 chiu, s phn b l hm ca v tr quan st dc theo mt ng hay mt b mt c bn knh khng i. H ta thng c s dng th hin trng bc x trong hnh 1.4.

Hnh 1.4. H thng ta phn tch anten [3] Trong thc t, ta c th biu din gin 3D bi hai gin 2D. Thng thng ch quan tm ti gin l hm ca bin vi vi gi tr c bit ca , v gin l hm ca vi mt vi gi tr c bit ca l a ra hu ht cc thng tin cn thit. Gin ng hng v hng tnh Anten ng hng ch l mt anten gi nh, bc x u theo tt c cc hng. Mc d n l l tng v khng th thc hin c v mt vt l, nhng ngi ta thng s dng n nh mt tham chiu th hin c tnh hng tnh ca anten thc. Anten hng tnh l anten c c tnh bc x hay thu nhn sng in t mnh theo mt vi hng hn cc hng cn li.


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Mt v d ca anten vi gin bc x hng tnh c th hin trong hnh 1.5. Ta nhn thy rng gin ny l khng hng tnh trong mt phng cha vector H (azimuth plane) vi [ f ( ), = / 2 ] v hng tnh trong mt phng cha vector E (elevation plane) vi [ g ( ), = const ] .

Hnh 1.5. Gin bc x v hng ca mt anten [3] Mt phng E c nh ngha l mt phng cha vector in trng v hng bc x cc i, v mt phng H c nh ngha l mt phng cha vector t trng v hng bc x cc i. Trong thc t ta thng chn hng ca anten th no t nht mt trong cc mt phng E hay mt phng H trng vi mt trong cc mt phng ta (mt phng x hay y hay z). Mt v d c th hin trong hnh 1.6. Trong v d ny, mt phng x-z (vi = 0 ) l mt phng E v mt phng x-y (vi = / 2 ) l mt phng H.

Hnh 1.6. Gin bc x trong mt phng E v mt phng H cho anten loa [3] Sinh vin: Lu Vn Hoan 7 Lp K49 Thng tin v tuyn



Cc bp sng ca gin bc x hng tnh Cc bp sng khc nhau ca gin bc x nh hng hay cn gi l cc thy (lobe) c th phn loi thnh thy chnh, thy ph, thy bn v thy sau. Hnh 1.7(a) minh ha gin cc 3D i xng vi mt s thy bc x. Mt vi thy c cng bc x ln hn cc thy khc. Nhng tt c chng u c gi l cc thy. Hnh 1.7(b) th hin gin 2D (mt mt phng ca hnh 1.7(a)).

Hnh 1.7. Cc bp sng ca anten bc x hng tnh [3] (a). Thy bc x v rng chm ca anten (b). th ca gin cng sut v cc thy v cc rng chm kt hp vi n Thy chnh (cng c gi l chm chnh) c nh ngha l thy cha hng bc x cc i. Trong hnh 1.7, thy chnh ang ch theo hng = 0 . C th tn ti nhiu hn mt thy chnh. Thy ph l bt k thy no, ngoi tr thy chnh. Thng thng, thy bn l thy lin st vi thy chnh v nh x bn cu theo hng ca chm chnh. Thy sau l thy bc x m trc ca n to mt gc xp x 180 so vi thy chnh. Thng th thy ph nh x bn cu theo hng ngc vi thy chnh. Sinh vin: Lu Vn Hoan 8 Lp K49 Thng tin v tuyn



Thy ph th hin s bc x theo cc hng khng mong mun, v chng phi c ti thiu ha. Thy bn thng l thy ln nht trong cc thy ph. Cp ca thy ph c th hin bi t s ca mt cng sut theo hng ca thy vi mt cng sut ca thy chnh. T s ny c gi l t l thy bn hay cp thy bn. Trng khu gn v trng khu xa Khng gian bao quanh mt anten c chia thnh 3 vng; (a) trng gn tc ng tr li (reactive near-field), (b) trng gn bc x (radiating near-field, Fresnel) v (c) trng xa (Fraunhofer) nh ch ra trong hnh 1.8. Cc vng trng c phn nh nh vy xc nh cu trc trng trong mi vng. Khng c s thay i trng t ngt no khi i qua bin gii gia cc vng ni trn. Cc bin phn gii cc vng trng khng phi l duy nht, do c nhiu tiu chun khc nhau s dng xc nh cc vng trng. Vng trng gn tc ng tr li (reactive near-field region) c nh ngha l phn khng gian trng gn trc tip bao quanh anten, xt kha cnh trng tc ng tr li chim u th. Trng ny ging buc vi ngun bc x v trao i nng lng vi ngun. Vi hu ht cc anten, bin ca vng ny c tnh ti khong cch R < 0.62 D 3 / tnh t mt phng anten, l bc sng v D l ng knh ln nht ca anten.

Hnh 1.8. Cc vng trng ca mt anten [3] Vng trng gn bc x (radiating near-field (Fresnel) region) c nh ngha l phn khng gian nm gia trng gn tc ng tr li v trng xa, xt kha cnh trng bc x chim u th. Nu ng knh cc i ca anten khng ln hn so vi bc sng, vng ny c th khng tn ti. Bin trong c tnh khong cch R 0.62 D 3 / v bin ngoi khong cch R < 2 D 2 / , trong D l kch thc ln nht ca anten.


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Vng trng xa (Far-field (Fraunhofer) region). Nu anten c kch thc ln nht l D (D phi ln hn bc sng, D > ), vng trng xa thng c xem l tn ti khong cch ln hn 2 D 2 / tnh t anten. Trong vng ny, trng l trng in t ngang. Bin bn trong c xem nh khong cch R = 2 D 2 / v bin ngoi v cc. Trong vng trng xa, dng ca gin bc x hu nh khng thay i khi dch chuyn im quan st ra xa dn. iu ny c minh ha trong hnh 1.9.

Hnh 1.9. Gin bc x trng xa ca anten parabol ti cc khong cch R khc nhau [3] 1.1.2.3. Mt cng sut bc x Sng in t c s dng truyn ti thng tin qua mi trng v tuyn hay cu trc dn sng, t im ny ti im khc. i lng c s dng m t nng lng kt hp vi sng in t l vector Poynting tc thi c nh ngha nh sau: , vector Poynting tc thi (W/m ) cng in trng tc thi (V/m) cng t trng tc thi (A/m)2

(2.1)

Tng cng sut i qua mt mt kn c th thu c bng cch tch phn thnh phn php tuyn vi mt kn ca vector Poynting trn ton mt kn . Phng trnh l: ( 2.2) Trong , tng cng sut tc thi (W) vector n v php tuyn vi b mt vi phn din tch ca b mt (m2) 10 Lp K49 Thng tin v tuyn




Khi trng bin i theo thi gian, ta thng tm mt nng lng trung bnh bng cch tch phn vector Poynting tc thi trong 1 chu k v chia cho chu k. Khi trng bin i tun hon theo thi gian c dng e j t , ta nh ngha cc trng phc E v bi cng thc: H, chng c quan h vi cc thnh phn tc thi v (2.3) (2.4)1 Ta c Re[ Ee jt ] = [ Ee jt + E * e jt ] . Khi (2.1) c th c vit li l: 2

(2.5) Thnh phn u tin ca (2.5) khng bin i theo thi gian, v thnh phn th hai bin i theo thi gian c tn s bng 2 ln tn s cho trc. Vector Poynting trung bnh theo thi gian (mt cng sut trung bnh) c th c vit li l: (2.6) Thnh phn xut hin trong (2.5) v (2.6) bi v cc trng E v H tnh theo bin . Da trn nh ngha (2.6), cng sut trung bnh bc x bi anten (cng sut bc x) c th c nh ngha l:

(2.7) 1.1.2.4. Cng bc x Cng bc x theo mt hng cho trc c nh ngha nh sau: nng lng c bc x t anten trn mt n v gc c. Cng bc x l tham s trng xa, v c tnh bng cch n gin l nhn mt bc x vi bnh phng ca khong cch. (2.8) U = r 2Wrad , U l cng bc x (W/n v gc c). Wrad l mt bc x (W/m2). Cng bc x cng c quan h vi in trng trong trng xa ca anten bi:2 r2 r2 2 2 [ E (r , , ] [ E (r , , ) + E (r , , ) ] U ( , ) = 2 2 2 2 1 [ E o ( , ) + E o ( , ) ] 2 : l tr khng sng ca mi trng.

(2.8a)


11


Kha lun tt nghip i hcE (r , , ) = E o ( , )


e jkr l cng in trng trong trng xa ca anten. r

E , E l cc thnh phn in trng trong trng xa ca anten.

Tng cng sut bc x nhn c bng cch tch phn cng bc x, nh c cho bi (2.8) trn ton gc c 4 . Do : (2.9)

, d = sin d d l mt vi phn gc c.

1.1.2.5. H s nh hng H s nh hng ca anten c nh ngha nh sau: t l ca cng bc x theo mt hng cho trc so vi cng bc x trung bnh trn tt c cc hng. Cng bc x trung bnh bng tng cng sut bc x bi anten chia cho 4 . Nu hng khng c xc nh, hng ca cng bc x cc i c chn. n gin hn, h s nh hng ca mt ngun bc x hng tnh bng vi t l ca cng bc x theo mt hng cho trc (U) v cng bc x ca mt ngun ng hng (U0): 4U U (2.10) = D=U0 Prad

Hng bc x cc i (hng tnh cc i) c biu din nh sau: U max U max 4U maxDmax = D0 = U0 = U0 = Prad

(2.10a)

, D l hng tnh (khng c th nguyn). D0 l hng tnh cc i (khng c th nguyn). U l cng bc x (W/n v gc c). Umax l cng bc x cc i (W/n v gc c). U0 l cng bc x ca ngun ng hng (W/n v gc c). Prad l tng cng sut bc x (W). Vi ngun ng hng, hin nhin t (2.10) hay (2.10a) ta nhn thy rng hng tnh bng 1 khi U, Umax v U0 bng nhau. Vi anten c cc thnh phn phn cc trc giao, chng ta nh ngha h s nh hng ring (partial directivity), theo mt phn cc cho trc v mt hng cho trc, l t l ca cng bc x tng ng vi mt phn cc cho trc chia cho tng cng bc x trung bnh trn tt c cc hng. Vi nh ngha ny, th theo mt hng cho trc h s nh hng tng l tng ca cc h s nh hng ring. Trong h ta cu, hng tnh cc i D0 vi cc thnh phn ta v ca anten c th c vit l: (2.11) D0 = D + D


12


Kha lun tt nghip i hcD =


Trong h s nh hng ring D v D c biu din bi: 4U ( Prad ) + ( Prad ) 4U ( Prad ) + ( Prad ) D =

(2.11a) (2.11b)

, U l cng bc x theo mt hng cho trc ch ph thuc . U l cng bc x theo mt hng cho trc ch ph thuc . ( Prad ) l cng sut bc x theo tt c cc hng ch ph thuc vo . ( Prad ) l cng sut bc x theo tt c cc hng ch ph thuc vo . 1.1.2.6. H s tng ch Mt n v khc m t hiu sut ca anten l h s tng ch (G). H s tng ch ca anten c quan h vi h s nh hng, v l n v dng tnh ton hiu sut ca anten cng nh kh nng hng tnh ca n. Trong khi h s nh hng ch th hin c c tnh hng tnh ca anten. H s tng ch c xc nh bng cch so snh mt cng sut bc x ca anten thc hng kho st v mt cng sut bc x ca anten chun (thng l anten v hng) cng hng v khong cch nh nhau, vi gi thit cng sut t vo hai anten bng nhau, cn anten chun l anten c hiu sut bng 1 (khng tn hao). Cng bc x ca anten ng hng bng vi cng sut t vo anten chia cho 4 (do ta gi thit anten chun c hiu sut bng 1, nn cng sut bc x bng cng sut t vo anten). Do , ta c: Cng bc x ca anten thc ti hng kho st G = 4 Cng bc x ca anten anten v hng U ( , ) => G = 4 (Khng th nguyn) (2.12) Pin

Tng cng sut bc x (Prad) c quan h vi tng cng sut t vo anten (Pin) bi: Prad = ecd Pin (2.13) , ecd l hiu sut bc x ca anten (khng th nguyn). S dng (2.13) bin i (2.12) thnh: U ( , ) G ( , ) = ecd 4 (2.14) Prad

S dng cng thc (2.10), ta c: G ( , ) = ecd D( , ) Gi tr cc i ca h s tng ch c quan h vi h s nh hng cc i bi: Sinh vin: Lu Vn Hoan 13

(2.15)


Kha lun tt nghip i hcG 0 = G ( , ) max = ecd D( , ) max = ecd D0

Ngnh: Thng tin v tuyn (2.16)

Cng nh i vi h s nh hng, ta nh ngha h s tng ch ring (partial gain) ca anten theo mt phn cc cho trc v mt hng cho trc nh sau: phn cng bc x tng ng vi mt phn cc cho trc chia cho tng cng bc x khi anten bc x ng hng. Vi nh ngha ny, th theo mt hng cho trc tng h s tng ch l tng ca cc h s tng ch ring. Trong h ta cu, h s tng ch cc i G0 theo cc thnh phn trc giao v ca anten c th c vit nh sau, theo dng tng t nh h s nh hng cc i trong (2.11a) v (211b): (2.17) G 0 = G + G Trong khi cc h s tng ch ring G v G c biu din bi: 4U G = (2.17a) PinG = 4U Pin

(2.17b)

Trong : Pin l tng cng sut a vo anten. U l cng bc x theo mt hng cho trc cha trong thnh phn trung E . U l cng bc x theo mt hng cho trc cha trong thnh phn trng E . Thng th h s tng ch c biu din theo khi nim dB thay v khng c th nguyn nh trong cng thc (2.16). Cng thc tng ng c cho bi: G0 (dB) = 10 log10 [ecd D0 ] (2.18) 1.1.2.7. Bng thng Bng thng (BW) ca anten c nh ngha nh sau: khong tn s m trong hiu sut ca anten tha mn mt tiu chun nht nh. Bng thng c th c xem xt l khong tn s, v hai bn ca tn s trung tm (thng l tn s cng hng), cc c tnh anten (chng hn nh tr khng vo, gin , rng chm, phn cc, cp thy bn, h s tng ch, hng chm, hiu sut bc x) t gi tr c th chp nhn c. Vi cc anten di rng, bng thng thng c biu din l t s ca tn s trn v tn s di khi anten hot ng vi cc c tnh c th chp nhn c. V d, bng thng 10:1 ch ra rng, tn s trn ln hn 10 ln tn s di.BW = f max f min

(2.19)

Vi anten di hp, bng thng c th hin bi t l phn trm ca s sai khc tn s (tn s trn tn s di) so vi tn s trung tm ca bng thng. V d, bng thng 5% th hin rng, s sai khc tn s l 5% tn s trung tm ca bng thng.BW = f max f min f0

(2.20)


14




Bi v cc c tnh nh tr khng vo, gin bc x, h s tng ch, phn cc, ca anten khng bin i ging nhau theo tn s, nn c nhiu nh ngha bng thng khc nhau. Ty tng ng dng c th, yu cu v cc c tnh ca anten c chn th no cho ph hp. 1.1.2.8. Phn cc Phn cc ca anten theo mt hng cho trc c nh ngha nh sau: l phn cc ca sng c truyn i bi anten. Ch : khi khng cp ti hng no, phn cc c xem l phn cc theo hng c h s tng ch cc i. S phn cc ca sng bc x c th hin bi u mt ca vector in trng tc thi, v hng m n vch theo khi quan st dc theo hng truyn sng. Mt ng vch theo bi u mt ca vector in trng l hm ca thi gian c th hin trong hnh 1.10(a) v (b).

(a)

(b) Hnh 1.10. S quay ca sng in t phng phn cc elip l hm theo thi gian [3] (a). S quay ca vector in trng (b). Phn cc elip z = 0 Sinh vin: Lu Vn Hoan 15 Lp K49 Thng tin v tuyn



Phn cc phn thnh 3 loi: thng, trn v ellip. Nu u mt vector in trng mt im trong khng gian lun hng theo mt ng thng, trng ny c gi l phn cc tuyn tnh. Tng qut, u mt vector in trng vch ra l mt elip, v trng c gi l phn cc ellip. Phn cc tuyn tnh v trn l trung hp c bit ca phn cc elip. u mt vector in trng quay theo chiu kim ng h (clockwise, CW) gi l phn cc phi v ngc kim ng h (counterclockwise, CCW) gi l phn cc tri. Trng tc thi ca sng phng khi sng ny truyn theo chiu m trc z, c th c biu din nh sau: (2.21) Cc thnh phn tc thi c quan h vi cc thnh phn phc ca n bi: (2.22) (2.23) , Exo v Eyo tng ng l bin cc i ca cc thnh phn trng theo trc x v y. Phn cc thng sng bc x phn cc thng, lch pha theo thi gian gia 2 thnh phn phi l: (2.24) Phn cc trn Phn cc trn c th t c ch khi bin ca 2 thnh phn l ging nhau, v s . Tc l: khc pha theo thi gian gia chng phi bng s l ln (2.25) (2.26) (2.27) Nu hng truyn sng b o ngc (v d, theo hng +z), cc lch pha trong (2.26) v (2.27) cho hng CW v CCW phi trao i cho nhau. Phn cc ellip Phn cc ellip c th t c ch khi lch pha theo thi gian gia 2 thnh phn l v bin ca chng khng bng nhau hay lch pha gia 2 thnh mt s l ln phn khng l bi ca (khng quan tm n bin ca chng). l: (2.28)

(2.29) Hay: Sinh vin: Lu Vn Hoan 16 Lp K49 Thng tin v tuyn


Ngnh: Thng tin v tuyn (2.30)

Vi phn cc ellip, ng cong c qut mt v tr cho trc nh mt hm theo thi gian, v ni chung n l mt ellip, nh c ch ra trong hnh 1.10(b). T l bn trc ln v bn trc nh ca ellip c gi l h s trc (axial ratio, AR), v bng vi: (2.31) : (2.32) (2.33) nghing ca ellip, c quan h vi trc y, c th hin bi gc cho bi: (2.34) Khi ellip thng vi cc trc chnh vi Ex0 (Eyo) hay Eyo (Exo) v t l trc (AR) bng: 1.1.2.9. Tr khng vo Tr khng vo c nh ngha nh sau: tr khng ca anten ti im u vo ca n hay t s in p so vi dng in ti u vo hay t s ca cc thnh phn tng ng ca in trng so vi t trng mt im. Trong phn ny, chng ta quan tm ch yu ti tr khng vo ti u vo ca anten. T s in p trn dng in u vo ny, khng c ti, xc nh tr khng ca anten nh sau: (2.35) ZA = RA + jXA Trong , ZA l tr khng ca anten cc u vo (Ohm). RA l in tr ca anten cc u vo (Ohm). XA l in khng ca anten cc u vo (Ohm). Ni chung, thnh phn in tr trong (2.35) bao gm 2 thnh phn l: R A = Rr + RL Trong , Rr l tr khng bc x (radiation resistance) ca anten. RL tr khng mt mt (loss resistance) ca anten. (2.36) trc chnh (ph) bng

Tr khng vo ca mt anten ni chung l hm ca tn s. Do , anten ch c phi hp tt vi ng tip in ch trong cng mt di tn no . Thm na, tr khng vo ca anten ph thuc vo cc yu t nh: hnh dng ca anten, phng php tip in cho anten, v nh hng ca cc i tng bao quanh n. Do s phc tp ca chng, ch mt lng gii hn cc anten thc t c nghin cu v phn tch t m. Vi cc loi anten khc, tr khng vo c xc nh bng thc nghim. Sinh vin: Lu Vn Hoan 17 Lp K49 Thng tin v tuyn



1.2. ng truyn vi di v anten vi di 1.2.1. ng truyn vi di 1.2.1.1. Cu trc ng truyn vi di ng truyn vi di c s dng nhiu nht trong mi trng truyn dn l cc mch tch hp siu cao tn. ng truyn vi di l cu trc mch in cp cao, bao gm mt di dn in bng ng hoc kim loi khc trn mt cht nn cch in, mt kia ca tm in mi cng c ph ng gi l mt phng t. Ta thy mt phng t l mt phn x. Do , ng truyn vi di c th c xem nh l ng truyn gm 2 dy dn.

Hnh 1.11. Cu trc ca ng truyn vi di [9] C hai tham s chnh l rng di dn in W v chiu cao tm in mi h. Mt tham s quan trng khc l hng s in mi tng i ca cht nn r. dy ca di dn in l t v in dn sut l l cc tham s km quan trng hn v i khi c th b qua. 1.2.1.2. Cu trc trng ca ng truyn vi di Sng truyn trn ng truyn vi di l sng c dng gn vi TEM (quasi-TEM). iu ny c ngha rng c mt vi vng trong ch c mt thnh phn in trng hoc t trng theo hng truyn sng. Hnh 1.12 th hin gin trng in t ca mt ng truyn vi di c bn.

Hnh 1.12. Gin trng ca mt ng vi di [9]


18




Trn cu trc ng vi di, gin quasi-TEM xut hin, bi v mt tip gip gia cht nn in mi v khng gian xung quanh l khng kh. Cc ng sc in trng khng lin tc ti mt tip gip ny. iu kin bin cho in trng l thnh phn tip tuyn ca in trng phi lin tc khi truyn xuyn qua bin; do mt cht nn c hng s in mi l 10, th in trng s gim t ngt 10 ln so vi trong khng kh. Mt khc, thnh phn tip tuyn (song song vi b mt di dn in) ca in trng cng phi lin tc khi xuyn qua bin. Do , mt phn nng lng in trng c lu tr trong khng kh v mt phn c lu tr trong in mi. Hng s in mi hiu dng i vi cc sng trn ng truyn nm gia gi tr hng s in mi khng kh v hng s in mi ca cht nn. 1.2.2. Anten vi di 1.2.2.1. Gii thiu chung Khi nim anten vi di ln u tin c a ra bi Deschamps vo nm 1953, Gutton v Bassinot vo nm 1955. Tuy nhin mi ti tn nm 1972 ngi ta mi i vo ch to cc anten vi di, bi v thi im ny mi xut hin cht nn c cc c tnh tt. Nh c ch ra trong hnh 1.13, anten vi di vi cu hnh n gin nht bao gm mt patch pht x nm trn mt mt ca cht nn in mi (r f min 1

f < 50% f0

T s ca tn s cc i v tn s cc tiu ca di tn cng tc fmax/fmin gi l h s bao trm di sng. 2.2. M rng bng thng ca anten vi di 2.2.1. Gii thiu Anten vi di c nhiu c tnh rt hu ch, nhng mt trong cc hn ch ng k ca cc anten loi ny l c tnh bng hp ca chng. Bng thng tnh theo tr khng (impedance bandwidth) ca anten vi di in hnh thng trong khong nh hn 1% ti vi % i vi cc cht nn mng tha mn tiu chun h/0 < 0.023 v c r 10 , Sinh vin: Lu Vn Hoan 25 Lp K49 Thng tin v tuyn



cho ti h/0 < 0.07 v c r 2.3 . iu ny hon ton tri ngc vi bng thng t 15% ti 50% ca cc anten nh l dipole, khe v loa. Cc nh nghin cu i vo nghin cu cc anten mch di ny ci thin gii hn bng hp trong gn 20 nm, v thnh cng trong vic ci thin bng thng tnh theo tr khng ti 90% v bng thng tnh theo h s tng ch (gain bandwidth) ti 70%. Hu ht nhng s i mi ny u s dng nhiu hn 1 mode, iu lm tng kch thc, chiu cao hoc th tch, v ng thi lm gim st mt s c tnh khc ca anten. Mong mun lm tng bng thng cng c th t c bng vic chn k thut tip in v mng phi hp tr khng thch hp. Chng ta bt u bng vic nh ngha bng thng (bandwidth). Mt s c tnh ca anten chng hn nh VSWR (hoc S11), rng chm (beamwidth), bc thy bn (sidelobe level), h s tng ch (gain), phn cc (polarization), . Cc c tnh ny u bin i theo tn s. Do , c th c nhiu nh ngha khc nhau v bng thng tng ng vi cc c tnh trn. Tr khng vo ca anten vi di bin i nhanh hn theo tn s, do vic gii hn di tn gip cho thnh phn bc x c th c phi hp tr khng vi ng tip in. Chng ta s s dng bng thng tnh theo tr khng trong phn cn li ca chng nh ngha bng thng ca anten. Bng thng tnh theo tr khng ca mt anten vi di cng hng c th c xc nh t p ng tn s ca mch tng ng ca n. i vi mch tng ng cng hng song song, bng thng na cng sut (BW) c cho bi:

BW =

2G dB 0 d

(2.1)0

Trong , Y = G + jB l dn np u vo ti tn s cng hng 0 . i vi mch tng ng cng hng ni tip G c thay th bi R v B c thay th bi X trong (2.1), trong Z = R + jX l tr khng vo ti tn s cng hng. Mt vi nh nghin cu nh ngha bng thng tr khng (impedance bandwidth) theo khi nim bng thng vi VSWR = 2. Bng thng na cng sut ca (2.1) tng ng vi bng thng vi VSWR 2.4 khi anten c tip in bi ng truyn mch di. S chuyn i bng thng t mt gi tr VSWR ny sang mt gi tr VSWR khc c th c thc hin thng qua quan h sau: VSWR 1 (2.2)

BW =

Q VSWR

Trong Q l h s phm cht ca anten. Cng thc (2.2) ch ra rng, bng thng vi VSWR 2 bng khong 78% bng thng na cng sut. Cc yu cu i vi bng thng tr khng khng nht thit phi ph hp vi cc tiu chun bng thng tnh theo cc c trng khc ca anten. Cc nghin cu gn y ch ra rng phn cc xin (cross-polarization) l thnh phn chnh hn ch bng thng ca cc anten vi di bng rng.


26




2.2.2. nh hng ca cc tham s cht nn ti bng thng Bng thng tr khng ca anten vi di t l nghch vi h s phm cht Q ca anten (xem (2.2)). Do , ta c th thay i cc tham s ca cht nn chng hn nh hng s in mi r v dy h t c h s Q mong mun nhm tng bng thng tr khng. H s Q ca anten cng hng c nh ngha nh sau: Nng lng c tch tr (Energy stored) (2.3) Q= Nng lng mt mt (Power lost) Hnh 2.2 biu din nh hng ca dy cht nn ti bng thng tr khng v hiu sut vi 2 gi tr hng s in mi l 2.2 v 10. Ta thy rng bng thng tng n iu theo dy. Khi hng s in mi r gim th bng thng tng. iu ny c th c gii thch t vic thay i gi tr Q.

Hnh 2.2. nh hng ca hng s in mi v dy cht nn ti bng thng tr khng (VSWR < 2) v hiu sut bc x [7]

Hnh 2.3. S bin i ca h s Q ca anten vi di c patch hnh ch nht theo hng s in mi cht nn. Trong h=1.59 mm, W=0.9L, f=3 GHz [7] H s Q ca anten vi di vi patch hnh ch nht l hm ca r v h c v trong hnh 2.3 v 2.4. Hnh 2.3 ch ra rng, h s Q hu nh tng tuyn tnh khi tng r. Thnh phn bc x (patch) hnh ch nht c m hnh ho nh mt t c mt mt (lossy capacitor), vic tng h s Q l do nng lng c tch tr (energy strored) tng v nng lng bc x gim khi tng r (theo cng thc 2.3). Tng t, khi dy cht Sinh vin: Lu Vn Hoan 27 Lp K49 Thng tin v tuyn



nn tng ln, nng lng c tch tr gim lm gim h s Q nh thy trong hnh 2.4. V mt vt l, iu ny xy ra l do trng vin (fringing field) tng khi tng h v gim r. Kt lun, tng h v gim r s lm tng bng thng tr khng ca anten. Tuy nhin, phng php ny ch c th p dng khi h < 0.02. C nhiu nhc im do s dng cc cht nn c hng s in mi cao v dy ln, bao gm: Nng lng sng mt tng ln, dn ti kt qu l hiu sut bc x km (xem hnh 2.2). Bc x t cc sng mt c th lm mo gin bc x gn u cui ng tip in vi di. Cc cht nn c dy ln, khi tip in ti cnh ca thnh phn bc x s lm tng bc x gi (spurious radiation) t ng vi di ti nhng ch thay i v rng (step-in-width) v nhng ch bt lin tc trn ng vi di. S bc x t u tip in (probe feed) cng s tng. Cc cht nn dy hn 0.110 v c r=2.2 c tr khng ti im tip in cho anten tng, dn ti cc vn v phi hp tr khng. Cc mode bc cao hn chy dc theo chiu dy cht nn c th tng, iu ny lm mo cc gin bc x v thay i tr khng c tnh. y l mt tham s hn ch vic t c bng thng ln hn.

Hnh 2.4. S bin i ca h s Q ca anten vi di c patch hnh ch nht theo dy cht nn. Trong r=2.2, W=0.9L, f=3 GHz [7] 2.2.3. La chn hnh dng thnh phn bc x thch hp Ngi ta nhn thy rng: mt vi hnh dng thnh phn bc x (patch) c h s Q thp hn so vi nhng hnh dng khc. Do bng thng ca chng s cao hn. Cc hnh dng thnh phn bc x ny bao gm: vnh khuyn (annular ring), vnh hnh ch nht/hnh vung (rectangular/square ring), patch phn t bc sng (c ngn mch) v mt s hnh dng khc. Anten c patch vnh khuyn trn vi b = 2a khi hot ng mode TM12 cho bng thng gp 5 ln anten patch hnh ch nht vi L = 1.5W. Tng t, anten c patch vnh hnh ch nht/hnh vung vi chu vi trung bnh 1 g c th c s dng.


28




Bng thng ca anten c patch phn t sng ngn mch hay vnh khuyn c so snh vi anten c patch hnh trn hay hnh ch nht trong bng 2.1. Chng ta c th nhn thy t bng ny rng: bng thng ca anten patch hnh ch nht tng khi rng ca patch tng. Bng 2.1. So snh bng thng ti VSWR = 2 Hnh dng patch Kch thc patch Bng thng (%) Hnh ch nht hp L=4.924 cm, W=2.0 cm 0.7 Hnh ch nht rng L=4.79 cm, W=7.2 cm 1.6 Hnh vung L=W=4.82 cm 1.3 Hnh trn Bn knh a=2.78 cm 1.3 Hnh vnh khuyn b=8.9 cm, a=4.45 cm 3.8 Hnh ch nht bc sng L=2.462 cm, W=2.0 cm 1.05 r = 2.32, h = 1.59 mm, f = 2 GHz 2.2.4. La chn k thut tip in thch hp C nhiu k thut tip in khc nhau nh: tip in bng u d (probe feeding), tip in cnh patch (edge feeding), ghp gn patch vi mt ng truyn vi di (proximity coupling to a microstrip line), ghp khe patch vi mt ng tip in vi di (aperture coupling to a microstrip feed line). Trong phng php ghp khe (aperture coupling) c s dng cho cc anten bng rng trn cc cht nn dy. iu ny l do thc t rng: k thut tip in ny cho php mt lng ln tham s c th iu chnh nh di khe, rng khe v hnh dng khe, . Phng php ghp khe khi c iu chnh mt cch k lng c th lm tng bng thng mt cch ng k. Bng thng khong 70% c th t c khi s dng k thut tip in ny. Anten vi di ghp khe Anten vi di ghp khe c th hin trong hnh 2.5. N bao gm mt patch hnh ch nht c kch thc a x b c in trn cht nn c dy h v hng s in mi ra. Patch vi di c tip in bi ng vi di thng qua mt khe h hoc mt rnh rch trn mt phng t chung ca patch v ng tip in vi di nh trong hnh 2.5. Khe c cc kch thc l La x Wa v tm khe ti im (x0, y0). rng ca ng vi di l W v c in trn mt cht nn c dy t v hng s in mi rf. Tr khng c trng ca ng vi di c k hiu l Z0m v tr khng c trng ca khe c k hiu bi Z0s. Vic ghp khe vi gia patch v ng vi di xy ra bi v khe ph v s lin tc ca dng in chy theo dc theo patch. Phn tch anten vi di ghp khe cho thy: hu ht cc c tnh u tng t vi anten khe tip in bng ng vi di. K thut tip in ghp khe c gii thiu bi Pozar v n c nhiu u im so vi cc k thut tip in khc. Cc u im l: bo v c anten khi s bc x gi t phn tip in (spurious feed radiation), s dng cht nn cho c cu trc tip in v anten, s dng cht nn dy tng bng thng ca anten.


29




Hnh 2.5. Anten vi di tip in bng ghp khe [7] 2.2.5. K thut kch thch a mode S dng nhiu mode cng hng l phng php rt hiu qu trong thit k cc anen vi di bng rng. tng c bn ca phng php ny xut pht t cc b cng hng (resonator) c ghp, trong th 2 b cng hng hoc nhiu hn c ghp vi nhau bao ph ton di tn mong mun. Phng php ny c p dng cho nhiu hnh dng patch khc nhau. C nhiu phng php thit k theo khi nim ny vi mc tiu tng bng thng tr khng (impedance bandwidth). Ta kho st mt phng php, trong dng 2 thnh phn cng hng hoc nhiu hn (tn s cng hng ca mi thnh phn khc nhau i cht), mi thnh phn cng hng ny c ghp st vi thnh phn cng hng khc. Vic ghp st c iu khin tng bng thng. Cc phng php thng thng nht m rng bng thng s dng phng php ghp k sinh (parasitic coupling) c m t ngay sau y. 2.2.5.1. M rng bng thng s dng nhiu thnh phn bc x xp chng Anten vi di vi 2 patch c xp chng in hnh c th hin trong hnh 2.6. nh hng ca vic xp chng cc patch c nghin cu u tin vo nm 1978 ngay khi cc nghin cu v anten vi di bt u. Patch bn di c th c tip in bng mt connector ng trc hoc bi mt ng vi di. Patch bn trn c ghp st vi patch bn di. Kch thc ca patch pha trn hi khc kch thc ca patch bn di thu c tn s cng hng hi khc mt cht. Ta c th thay i mt s tham s, v d: dy d1 v d2 ca cc cht nn; hng s in mi r1 v r2; lch gia tm ca cc patch; kch thc ca cc patch v v tr tip in. i vi cc anten c 2 di tn, kch thc ca 2 patch c quyt nh bi 2 tn s cng hng mong mun. i vi anten bng rng, cc tham s khc c ti u ha t c bng thng ln nht c th. Mt s dch chuyn nh cc patch theo cc hng x v y cng c nh hng ng k n bng thng tr khng v gin bc x.


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Hnh 2.6. Anten vi di vi patch xp chng tip in bng ghp khe [7] Cc nghin cu ch ra rng, vic xp lch cc patch c tc dng m rng bng thng, tnh khng i xng ca cu trc lm tng bc x xin (beam squint) trong mt phng E. Bng thng tr khng t 10% ti 29% t c vi cc patch xp chng tip in bng u ni (probe-fed), v bng thng tr khng t 18% ti 67% t c vi cc patch xp chng tip in bng ghp khe. Bng thng ln hn c th t c khi s dng cht nn dy hn. Anten vi patch xp chng c mt s c im hp dn nh sau: Vic xp chng cc patch khng lm tng din tch b mt ca anten, so vi phng php s dng cc thnh phn k sinh ng phng, cc patch xp chng c s dng trong anten mng m khng cn tng khong cch gia cc patch v gii quyt c vn bp sng ph. Gin bc x v tm pha ca anten patch xp chng duy tr tnh i xng trn ton di tn hot ng ca n. 2.2.5.2. M rng bng thng s dng cc thnh phn k sinh ng phng Mt phng php khc cng cho c tnh bng rng l ghp gn cc patch ng phng c tn s cng hng khc nhau cht t. Mt vi hnh dng c th hin trong hnh 2.7 v 2.8. Ch c patch gia (patch iu khin) c tip in v cc patch khc hoc l c ghp khe hoc l c ghp trc tip vi patch iu khin. Trong hnh 2.7(a): 2 dipole ghp vi mt patch bc x c a ra vo nm 1979. Khi 2 patch c ghp khe vi patch iu khin dc theo cc cnh bc x ca patch iu khin (xem hnh 2.7(b)), bng thng ln nht c th tng ti 5.1 ln so vi bng thng ca anten ch c 1 patch hnh ch nht. Kiu ghp k sinh dc theo cc cnh khng bc x (hnh 2.7(c)) cho bng thng ln hn gp 4 ln. Mt cu hnh tng t bao gm cc patch bc sng c ngn mch (xung mt phng t) ghp vi mt patch bc sng dc theo cc cnh bc x (hnh 2.7(d)) cho bng thng ln gp 5.35 ln. Mt v d khc, 2 patch ngn mch di /4 c ghp vi nhau dc theo cnh bc x (xem hnh 2.7(e)). Cu hnh 4 patch c ghp khe ti 4 cnh ca mt patch iu khin th hin trong hnh 2.7(f). N cho bng thng ti 6.7 ln bng thng ca anten ch c mt patch n. Sinh vin: Lu Vn Hoan 31 Lp K49 Thng tin v tuyn



Hnh 2.7. Mt vi anten vi di bng rng s dng cc patch ghp khe ng phng [7] (a). Mt patch iu khin c ghp khe vi 2 dipole dc theo cc cnh bc x (b). Mt patch iu khin c ghp khe vi 2 patch dc theo cc cnh bc x (c). Mt patch iu khin c ghp khe vi 2 patch dc theo cc cnh khng bc x (d). 1 patch iu khin ghp khe vi 2 patch bc sng dc theo cc cnh bc x (e). Mt patch bc sng (iu khin) c ghp khe vi mt patch bc sng (f). Mt patch iu khin c ghp khe vi 4 patch dc theo 4 cnh ca n (g). Anten mng vi di vi mt patch iu khin v cc patch k sinh ghp khe (h). Mng 7 dipole ghp khe (i). Anten nhiu patch ghp khe cng hng ti nhiu tn s (j) Mt hnh dng khc ca anten nhiu patch ghp khe cng hng ti nhiu tn s Sinh vin: Lu Vn Hoan 32 Lp K49 Thng tin v tuyn



Khi nim v cc thnh phn ghp khe c m rng cho cc mng anten ghp khe. Trong cc mng ny, mt patch iu khin c ghp khe st (v khng gian) vi cc patch xung quanh, mt s khe c ghp xa hn, to ra mt mng phng (xem hnh 2.7(g)). Khe ghp gia cc patch lin st nhau thng rt nh, bng khong 0.1 ti 2 ln dy cht nn. Anten mng phng c kch thc 1.56 mm x 30 mm x 30 mm ti tn s 8.55 GHz cho bng thng 16%, tng ch 9 dB v cc thy bn ch 26dB. Mt anten mng gm 7 dipole c ghp dc theo cc cnh khng bc x, nhng vi rng v khong cch khe gia cc dipole c ti u ha t c bng thng ln hn 8 ln (xem hnh 2.7(h)). Mt cu hnh khc l mt s dipole c chiu di khc nhau c kch thch bi mt khe t gia to ra cu hnh xp so le (xem hnh 2.7(i) v (j)). Mt s thay i nho nh cu hnh ny l tt c cc dipole hot ng cng mt tn s v c ghp khe vi ng tip in vi di. Bng thng ca cu hnh vi 3 dipole i xng l khong 23% i vi anten c in mi dy 0.0830. Bng thng ny bng khong 1.5 ln bng thng ca anten patch hnh vung tng ng. Cui cng, khi nim patch cng hng c xp so le c th c m rng cho mng loga chu k c th hin trong hnh 2.8(a). Anten vi patch cng hng c ghp trc tip cng c nghin cu. Cc anten ny c th hin trong hnh 2.8(b d). Trong cc trng hp ny, cc patch pha ngoi l cc patch k sinh vo patch c tip in, nhng cc patch k sinh c ghp trc tip ti patch c tip in. Bng thng cho cc anten 2.8(b), (c), (d) tng t nh cc anten ghp khe hnh 2.7(b), (c) v (f). Anten ghp k sinh ng phng vi patch hnh trn cng c nghin cu. N bao gm mt a vi di c ghp khe vi mt vnh khuyn ngn mch trn cng b mt. Anten ny c th hin trong hnh 2.8(e). Anten ny cng c th c xem nh mt hc hnh tr (cylindrical cavity) ghp vi mt khe vnh khuyn (annular slot). Cc tn s cng hng ca a vi di v cavity gn bng nhau. Anten ny to ra mt gin bc x i xng trn vi rng chm 10dB l khong 1600, v n thch hp chiu x (illuminate) cho mt b phn x vi F/D = 3. Phn cc ngang cc i (maximum cross-polarization) vi rng chm 10dB c o l -21dB trn ton di 5GHz ti 5.44GHz. Ta c th t c bng thng tr khng 10% bng cch iu chnh kch thc khe ghp. Tt c cc anten vi di vi cc b cng hng k sinh ng phng c mt nhc im l din tch b mt tng ln. Hn na, gin bc x v tm pha bin i r rt theo tn s, c bit i vi cc thit k bng thng rng hn.


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Hnh 2.8. Anten vi di bng rng s dng cc patch ghp khe ng phng [7] (a). Cu hnh ca mt mng vi di loga chu k ghp khe bng rng (b). Patch iu khin c ghp trc tip vi 2 patch dc theo cc cnh bc x (c). Patch iu khin c ghp trc tip vi 2 patch dc theo cc cnh khng bc x (d). Patch iu khin c ghp trc tip vi 4 patch dc theo 4 cnh ca n (e). Anten vi a vi di c ghp khe vi mt vnh khuyn ngn mch Sinh vin: Lu Vn Hoan 34 Lp K49 Thng tin v tuyn



2.2.5.3. Cc k thut kch thch a mode khc Cc phng php trong phn ny u s dng 2 mode c lp c kch thch vo cng mt patch hoc vo patch v mng tip in. Mt patch gn vung c s dng thu c bng thng gn 3 ln bng cch kch thch 2 mode vi phn cc trc giao. Trong trng hp ny, kch thch c hai mode, ng tip in c t dc theo ng cho (xem hnh 2.9(a)). th tr khng vo cho anten ny c v trong hnh 2.9(b) vi L = 13.6 cm, W/L = 0.99, 0/L = 0.16 v h/0 = 0.0037.

Hnh 2.9. Anten vi di bng rng s dng 2 mode phn cc trc giao [7] (a). Patch gn vung (b). th tr khng vo Patch hnh ch nht vi mt khe U c ct trn n (xem hnh 2.10) cng l anten bng rng. Patch v khe U c thit k cho tn s cng hng gn nhau. Tn s cng hng v h s Q ti im cng hng c th c iu khin c lp bng cch iu chnh chiu di v rng ca chng (patch v khe U).

Hnh 2.10. Patch c rch khe U to ra 2 tn s cng hng v tng bng thng [7] Patch hnh ch nht vi mt khe H c ct trn n hnh 2.11. 2.2.6. Cc k thut m rng bng thng khc Trong cc phn trc, chng ta thay i cc tham s ca cht nn lm tng bng thng tr khng. Cc patch cng hng c t so le trong cu hnh patch xp chng v cu hnh ghp khe c m t trong phn trc. Vic s dng nhiu hn mt mode kch thch vo cng mt patch cng c m t. Trong phn ny, chng ta tho lun thm mt vi phng php tng bng thng tr khng.


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Hnh 2.11. Mt anten dipole cun trn kp vi bng thng rng [7] 2.2.6.1. Phi hp tr khng Mt trong cc k thut trc tip v thng thng nht c s dng ci thin bng thng tr khng l s dng mt mng phi hp tr khng. Ta c th dng cc nhnh v cc on vi di bc sng c th c s dng cho mc ch ny. Mng phi hp tr khng nn c t gn thnh phn bc x nht c th. Tuy nhin, cc ch khng lin tc trong mng phi hp tr khng cng c th bc x, iu lm gim st c tnh phn cc ngang (cross-polarization) ca anten. S phc tp v cc mt mt ca mng phi hp tr khng ni chung hn ch bng thng c th t c ch khong t 10% ti 30%. Pues v Van de Capelle t c bng thng khong 10% ti 12% s dng mt mng phi hp tr khng ng phng th ng. Cc k thut tng t c s dng bi Paschen a ra bng thng ln hn 25%, bng thng bao ph c di GPS ch vi mt thnh phn bc x n. Mt phng php kho lo khc l mt thnh phn chuyn tip hnh nn 3D (3D transition) c s dng tng bng thng tr khng ti khong 90%. Hnh dng ca anten ny c th hin trong hnh 2.12. Anten v thnh phn chuyn tip khng c in trn bt k mt vt liu in mi no. Patch c chng tm bi mt tr (kim loi hoc khng phi kim loi). Thnh phn chuyn tip c th l di kim loi rng khng thay i dng nghing xung ti mt phng t nh th hin trong hnh 2.12(a), hoc c th l mt di c rng thon dn vung gc vi mt phng t nh trong hnh 2.12(b). C hai phng php ny cho kt qu bng rng. u im ca anten ny l c mt bng thng VSWR rt ln, khong 90%, trnh c cc nh hng ca sng mt v nh hng ca phn b (dispersion) ca cht nn, hiu sut cao hn. Sinh vin: Lu Vn Hoan 36 Lp K49 Thng tin v tuyn



Hnh 2.12. Anten vi patch n bng rng s dng thnh phn chuyn tip 3D [7] (a). Patch l lng vi mt thnh phn chuyn tip 3D nghing (b). Patch l lng vi mt thnh phn chuyn tip 3D dng ng 2.2.6.2. Mc ti in tr Theo Pozar, bng thng tr khng ca anten vi di c th tng ln bng cch hn ch cc tham s mt mt, nhng b li hiu sut bc x gim. Mt mt gy ra do vt liu in mi, lp ng dn in, do chnh ti in tr mc vo. Hnh 2.13 th hin mt anten vi di c gn thm in tr 1 gn cnh ca patch.

Hnh 2.13. Anten vi di bng rng c mc thm ti in tr l = 0.79L [7] Ti Return Loss = 10 dB, bng thng ca anten ny bng 4.9 ln bng thng khi khng mc thm ti in tr. Thm na, kch thc patch gim i 0.39 ln so vi khi khng mc thm ti in tr. Tuy nhin, h s tng ch li gim. 2.3. Anten vi di nhiu bng tn 2.3.1. Anten vi di 2 tn s cng hng Anten vi di cng hng kp c th hot ng ti 2 tn s cng hng trn cc cu trc n hoc a bc x. V l thuyt, cc anten vi di cng hng kp phi c cng cc c tnh bc x v phi hp tr khng ti c hai tn s cng hng. thc hin iu ny bng cch s dng cng ngh phng l mt vn phc tp. C mt s phng php thit lp anten cng hng kp v c phn thnh 3 loi chnh: Kch thch anten bng 2 mode. S dng nhiu patch bc x cho anten. Mc ti hn hp. Sinh vin: Lu Vn Hoan 37 Lp K49 Thng tin v tuyn



K thut kch thch bng 2 mode c thc hin bng cch kch thch 2 mode cng hng khc nhau ln mt patch vi di n. i vi cu trc s dng nhiu patch bc x, cng hng kp t c bng cch s dng nhiu patch bc x, mi patch cng hng ti mt tn s nht nh. Cu trc ny c th c thc hin bng cch s dng nhiu patch ng phng hoc xp chng nhiu patch, trong cc patch bc x c hnh dng ging nhau hoc khc nhau. Cc anten ny hot ng hot ng ti 2 tn s vi cng phn cc hoc 2 phn cc khc nhau. Khi anten vi di c yu cu l phi hot ng ti 2 tn s khc bit, k thut c s dng ph bin nht l mc ti hn hp hoc mc ti in khng cho 1 patch n. Mt s kiu mc ti thng s dng nh: nhnh, khe hnh V, ngn mch, s dng t in v khe. Anten vi di 2 tn s c th hnh thnh bng cch xp chng 2 phn t mch di, mi phn t c tn s cng hng ring, v c tip in ni tip nh trn hnh 2.14. c tnh quan trng nht cn lu y l khi mt trong 2 phn t mch di khng cng hng vi tn s lm vic ca phn t kia th n ging nh mt phin kim loi b on mch. iu ny cho php mi phn t mch di hot ng c lp ti tn s khong cng hng ca phn t mch di kia. iu ny c th t c khi khong cch tn s nh hn 10%, hoc khi cc tn s l hi nhau.

Hnh 2.14. Hnh dng anten mch di 2 bng tn [1] (a). Tip in ni tip cho tng phn t (b). Cu trc tng ng ti tn s f1 (c). Cu trc tng ng ti tn s f2 2.3.2. Anten vi di nhiu hn 2 tn s cng hng S dng k thut kch thch a mode trn ta cng c th thit k cc anten vi di nhiu tn s cng hng. Bng cch chn cc tn s cng hng nm trong di tn mong mun. Ta c th xp chng nhiu phn t v tip in ni tip to thnh mt h anten mch di nhiu tn s. iu quan trng l phi tm im ph hp ca mi phn t Sinh vin: Lu Vn Hoan 38 Lp K49 Thng tin v tuyn



c th t ngun nui vo vi tr khng u vo l 50 . Kch thc chnh xc ca phn t tip in ni tip v cc im ph hp t ngun nui tt nht l c xc nh bng thc nghim. 2.4. Phi hp tr khng di rng Nh trn trnh by, c nhiu phng php thit k anten mch di bng rng, phn ny xin trnh by chi tit phng php phi hp tr khng di rng. 2.4.1. ngha ca vic phi hp tr khng Ni dung ca phi hp tr khng c minh ho hnh 2.15 trong s dng mt mch phi hp t gia ti v ng truyn dn sng. Mch phi hp thng l mt mch khng tn hao trnh lm gim cng sut v c thit k sao cho tr khng vo nhn t ng truyn c gi tr bng tr khng sng Zo ca ng truyn. Khi y s phn x sng pha tri ca mch phi hp v pha ng truyn dn s khng cn na, ch cn trong phm vi gii hn gia ti v mch phi hp, cng c th l phn x qua li nhiu ln. Qu trnh phi hp cng c coi l qu trnh iu chnh.

Hnh 2.15. Mch phi hp tr khng khng tn hao gia tr khng ti bt k v ng truyn dn sng c tr khng c trng Z0 [2] Khi thc hin phi hp tr khng cng sut truyn cho ti s t c cc i, cn tn tht trn ng truyn l cc tiu. Phi hp tr khng s gip ci thin t s tn hiu/tp nhiu ca h thng khc trong h thng s dng cc phn t nhy cm nh anten, b khuch i tp m thp, . Khi ti v ng truyn c phi hp s khng c sng phn x trn ng truyn, do cng sut truyn vo ti t cc i, bng vi cng sut a vo. Khi khng m bo vic phi hp tr khng s xut hin sng ng trn ng truyn. Nu gi tr Vmax ti im bng in p trn ng truyn t ti hoc vt qu gii hn cho php Vx s xy ra nh la, iu ny c th dn ti vic ph hu ng truyn. 2.4.2. Phi hp tr khng di rng Khi kho st b phi hp tr khng dng on bin i /4, ta thy nu cc tr khng cn phi hp c chnh lch cng cao th di tn ca thit b phi hp cng hp. V vy mun m rng di tn phi dng nhiu on bin i mc ni tip nhau thnh chui mi on ch phi hp vi mt t s tr khng thp m thi. chnh l tng ca b bin i tr khng nhiu cp hay nhiu phn on. B bin i gm N on dy truyn sng (hnh 2.16) c di ging nhau nhng khc nhau v tr khng c tnh. Hy rt ra cng thc gn ng tnh h s phn x tng . Sinh vin: Lu Vn Hoan 39 Lp K49 Thng tin v tuyn



Hnh 2.16. B bin i nhiu phn on [2] Ti mi ch ni, ta xc nh c cc h s phn x ring nh sau:0 = Z1 Z 0 Z1 + Z 0 Z n +1 Z n Z n+1 + Z n ZL ZN ZL + ZN

(2.4a)

n =

(2.4b)

N =

(2.4c)

Ta cng gi thit l cc Zn s tng hoc gim n iu dc theo chiu di ca b phi hp tr khng, ng thi coi ZL l thc. iu c ngha cc n l thc v c cng du. Ta thy: n > 0 khi ZL > Z0 v n < 0 khi ZL < Z0. H s phn x tng: ( ) = 0 + 1 e 2 i + 2 e 4 i + ... + N e 2 iN (2.5) Ta gi thit tip l b bin i c ch to i xng, sao cho: 0 = N , 1 = N 1 , 2 = N 2 , Biu thc (2.5) c vit li nh sau:

( ) = eiN 0 eiN + eiN + 1 ei( N 2) + ei( N 2) + ...

{ [

] [

] }

(2.6)

N 1 i e + e i . Nu N chn, s hng cui s l N . 2 2 Phng trnh (2.6) c dng mt chui Fourier cosine hu hn theo , nh sau: i vi N chn 1 ( ) = 2e iN 0 cos N + 1 cos( N 2) + ... + n cos( N 2n ) + ... + N 2 (2.7) 2 i vi N l

Nu N l, s hng cui s l

(

)

( ) = 2e iN 0 cos N + 1 cos( N 2 ) + ... + n cos( N 2n ) + ... + N 1 cos 2

(2.8)

ngha quan trng ca cc kt qu trn l ch trn thc t ta c th tng hp c h s phn x tng di dng mt hm s theo tn s (hay ) bng cch la chn thch hp cc n v s dng s lng va cc phn on (s N). iu l hin nhin v nh ta bit, mt chui Fourier cosine c th c biu th nh mt hm trn tu Sinh vin: Lu Vn Hoan 40 Lp K49 Thng tin v tuyn



nu a vo mt s s hng va . Ni dung ny c gi l l thuyt cc phn x nh. trn ta tho lun cch thc mt tr khng ti thc bt k c th c phi hp vi mt ng truyn trn ton mt bng thng mong mun bng cch s dng b phi hp nhiu phn on. Khi s lng cc phn on ri rc (N) tng ln th s khc bit v tr khng c tnh gia cc phn on s gim i. Khi N tin n v cng, chng ta c mt cu trc m s bin i tr khng c trng l lin tc (hnh 2.17).

Hnh 2.17. Phi hp tr khng bng di lin tc v m hnh tng s lng phn on N ln tin ti v cng [2] (a). B phi hp tr khng lin tc (b). B phi hp nhiu phn on Tt nhin, trong thc t, mt b phi hp tr khng phi c chiu di hu hn, v thng khng di hn mt vi phn on. Nhng thay v cc phn on ri rc, b phi hp tr khng c th l on lin tc nh trong hnh 2.17. Sau , bng cch thay i hnh dng ca b phi hp tr khng, chng ta c th t c cc c tnh di truyn khc nhau. Ta s rt ra l thuyt gn ng v b bin i lin tc, da trn l thuyt cc phn x nh biu th s ph thuc ca h s phn x theo s bin i ca tr khng c tnh Z(z). Hnh 2.17a m t mt on ng truyn lin tc ging nh c ghp bi nhiu vi phn on z m tr khng c tnh ca cc vi phn on chnh nhau mt lng Z (z ) . Khi gia s ca h s phn x ti z s l: =

(Z + Z ) Z (Z + Z ) + Z

Z 1 Z = 2Z 2 Z

(2.9)

Cho z 0 , ta nhn c: 1 dZ 1 d (ln Z Z 0 ) d = dz = dz 2 Z 2


41


Kha lun tt nghip i hc Lu rng:


d (ln f ( z )) 1 df (z ) df ( z ) d (ln f ( z )) dz = = dz f ( z ) dz f ( z) dz p dng l thuyt cc phn x nh, h s phn x tng ti z = 0 c th tm c bng cch ly tng tt c cc h s phn x ring, vi gc dch pha tng ng: L 1 d Z ln dz ( ) = e 2iz trong = 2l . (2.10) 2 z =0 dz Z 0 Nu bit Z(z) th c th tm hm ( ) l hm ph thuc tn s, ngc li nu cho trc ( ) th v nguyn tc c th xc nh hm Z(z), tuy nhin y l mt th tc rt kh v trong thc th ngi ta thng trnh thc thin.2.4.3. Mt s b phi hp tr khng di rng Chng ta s xem xt mt vi trng hp c bit ca Z(z) v a ra cc nh gi v p ng ca ( ) . 2.4.3.1. B phi hp tr khng lin tc dng hm m Chng ta hy xem xt b phi hp tr khng lin tc dng hm m, trong : Z ( z ) = Z 0 ez , trong 0 < z < L (2.11) Hm Z(z) trong (2.11) c v trong hnh 2.19a.

Hnh 2.18. B phi hp tr khng dng hm m Ti z = 0, ta mong mun t c: Z(0) = Z0. Vi biu thc ca Z(z) chn trn, ta s c: Z(0) = Z0e0 = Z0 (ng nh iu ta mong mun) Ti z = L, chng ta mong mun t c: Z(L) = ZL = Z0 eL = 1 ZL ln L Z0

(2.12)

By gi chng ta s tm h s phn x tng () bng cch s dng (2.12) v (2.11) thay vo (2.10), chng ta c:1 d ln ez dz ( ) = e 2 jz 20 dz ln Z L / Z 0 2 jz ( ) = e dz 2L 0 ( ) = ln Z L / Z 0 jL sin L e 2 LL L

(

)

(2.13)


42




(a)

(b) Hnh 2.19. B phi hp tr khng lin tc dng hm m [4] (a). S bin i ca tr khng theo z (b). p ng bin ca h s phn x () Ta nhn xt rng, php ly o hm ny tha nhn rng (h s lan truyn ca b phi hp tr khng kiu lin tc) khng l hm ca z. Gi nh ny ni chung ch hp l i vi cc ng truyn TEM. Bin ca h s phn x tng () trong (2.13) c v trong hnh 2.19b; ch rng cc nh ca || gim khi tng chiu di L ca b phi hp tr khng. V chiu di L nn chn ln hn /2 (L > ) nhm mc ch ti thiu ho s mt phi hp tr khng tn s thp.2.4.3.2. B phi hp tr khng lin tc dng tam gic Tip theo chng ta xem xt b phi hp tr khng lin tc dng tam gic i vi thnh phn [dln(Z/Z0)]/dz. Trong , Z(z) c dng:

Z 0 e 2 ( z / L ) ln Z L / Z 0 Z ( z) = 2 2 Z 0 e ( 4 z / L 2 z / L 1) ln Z L / Z 0 2

vi 0 z L / 2 vi L / 2 z L

(2.14)

Hnh 2.20. B phi hp tr khng dng tam gic Sinh vin: Lu Vn Hoan 43 Lp K49 Thng tin v tuyn

Kha lun tt nghip i hc T biu thc (2.14) ta xc nh c: d (ln Z / Z 0 ) (4 z / L2 ) ln(Z L / Z 0 ) = 2 dz (4 / L 4 z / L ) ln(Z L / Z 0 )2

Ngnh: Thng tin v tuynvi L / 2 z L vi 0 z L / 2

(2.15)

th ca Z(z) c v trong hnh 2.21a. Thay (2.15) tr li (2.10) ta c:Z 1 ( ) = e jL ln L Z 2 0 sin( L / 2) L / 2

(2.16)

(a)

(b)

Hnh 2.21. B phi hp tr khng lin tc dng tam gic [4] (a). S bin i ca tr khng theo z (b). p ng bin ca h s phn x () Bin ca l || c v trong hnh 2.21b. Ch rng, vi L > 2, cc nh ca || ca b phi hp tr khng lin tc dng tam gic nh hn cc nh tng ng ca || ca b phi hp tr khng lin tc dng hm m. Nhng im khng (ca ||) u tin i vi b phi hp tr khng lin tc dng tam gic xy ra ti L = 2, trong khi im khng (ca ||) u tin i vi b phi hp tr khng lin tc dng hm m xy ra L = .2.4.3.3. B phi hp tr khng lin tc Klopfenstein Hy xem xt mt thc t rng, c rt nhiu hnh dng c th chn cho b phi hp tr khng lin tc. Do , ta c th t ra cu hi l Hnh dng no ph hp nht i vi b phi hp tr khng lin tc?. Vi mt chiu di cho trc ca b phi hp tr khng lin tc (ln hn mt gi tr ti hn), b phi hp tr khng lin tc


44




Klopfenstein c cho l ti u phng din h s phn x nh nht trn ton di truyn. Nu xt c im h s phn x cc i trong di truyn, th b phi hp tr khng Klopfenstein cho chiu di phn phi hp tr khng ngn nht. B phi hp tr khng lin tc Klopfenstein nhn c t b phi hp di rng nhiu on Chebyshev khi ta cho s lng cc on tin ti v cng, iu ny cng tng t nh phn b Taylor ca l thuyt mng anten. Chng ta s khng th hin chi tit cc bin i ny, m ch a ra cc kt qu thit k cc b phi hp tr khng lin tc Klopfenstein.

Hnh 2.22. B phi hp tr khng Klopfenstein Loga ca tr khng c trng ca b phi hp tr khng Klopfenstein c cho bi: 0 1 A2 (2 z / L 1, A) ln Z ( z ) = ln Z 0 Z L + (2.17) vi 0 z L 2 cosh A Trong , hm ( x , A) c nh ngha nh sau:

( x, A) = ( x, A) = 0

x

I1 ( A 1 y 2 ) A 1 y2

dy

vi x 1

(2.18)

Trong , I1(x) l hm Bessel sa i. Hm ny nhn cc gi tr c bit di y: ( 0, A ) = 0 ( x ,0 ) = x / 2 (1, A) =cosh A 1 A2

Nhng nu khng th hm I1(x) phi c tnh ton theo phng php s. H s phn x tng c cho bi:( ) = 0 e jL

cos ( L) 2 A 2 cosh A

vi L > AA 2 (L) 2 .

(2.19)

Nu L < A, th ta c cos ( L ) 2 A 2 tr thnh cosh

Trong (2.17) v (2.19), 0 l h s phn x ti tn s 0 Hz, v c cho bi: Z Z0 1 Z L ln 0 = L Z L + Z0 2 Z0

(2.20)

Di truyn c nh ngha vi L A , v gi tr gn cc i trong di truyn l:Sinh vin: Lu Vn Hoan 45 Lp K49 Thng tin v tuyn

Kha lun tt nghip i hcm =


0 (2.21) cosh A Bi v ( ) dao ng trong khong (0 / cosh A,+ 0 / cosh A) vi L > A.

Ch rng, hm Z(z) trong (2.17) c cc bc nhy ti z = 0 v z = L (hai u ca b phi hp tr khng). Ging nh b phi hp tr khng nhiu on Chebyshev, p ng ca b phi hp tr khng lin tc Klopfenstein c cc nh (phn gn) bng nhau trn ton di truyn ca n.

(a)

(b)

Hnh 2.23. B phi hp tr khng lin tc Klopfenstein [4] (a). S bin i ca tr khng theo z (b). p ng bin ca h s phn x ()2.4.4. Tiu chun Bode Fano Chng ta s gii hn cc tho lun i vi mch trong hnh 2.1, mng phi hp tr khng khng tn hao c s dng phi hp mt ti phc bt k trn ton bng thng ca n. Chng ta c th t ra my cu hi sau y: Chng ta c th t c phi hp tr khng hon ho khng (h s phn x bng 0) trn ton bng thng xc nh?


46




Nu khng t c phi hp tr khng hon ho, th chng ta c th phi hp tr khng tt ti mc no? u l s tho hip gia m, s phn x cc i c th cho php trong di truyn v bng thng? Mng phi hp tr khng phi phc tp n mc no tho mn cc tiu chun cho trc? Cc cu hi ny c th c tr li bi tiu chun Bode Fano (ch ph hp vi mt s loi tr khng ti nht nh). Tiu chun ny ch ra: gii hn nh nht (v l thuyt) bin ca h s phn x c th t c i vi mt mng phi hp tr khng bt k. Do , tiu chun Bode Fano th hin kt qu ti u c th t c mt cch l tng, v d kt qu nh vy trong thc t ch l xp x. Tuy nhin, cc kt qu ti u nh vy lun quan trng, bi v chng a ra gii hn trn v hiu sut v cung cp mt im chun m cc thit k thc t c th so snh.


47




THIT K ANTEN DT CU TRC XON, TIPCHNG 3

IN DNG NG TRUYN VI DI

Tm tt Kha lun tp trung thit k mt anten n cc phng ph hp cho ng dng trong cc thit b cm tay di ng (mobile hanset). Anten bao gm mt b pht x hnh ch nht b x bi cc rnh un khc to thnh 3 nhnh, trong 2 nhnh cng hng v 1 nhnh iu chnh. Anten c in trn cht nn FR4 v c tip in bi mt ng vi di 50 . Anten ny c th hot ng trong cc di tn GSM, DCS, PCS, UTMS v WLAN vi h s sng ng VSWR nh hn 2.5.

Chng ny s trnh by chi tit phng php thit k tng thnh phn ca anten nh: thnh phn bc x, b phi hp tr khng di rng, ng vi di 50 Ohm.3.1. Gii thiu S pht trin nhanh cng ca cc h thng truyn thng khng dy hin i ko theo xu hng thit k cc anten nhiu bng tn v anten bng rng hoc cc anten c c hai tnh cht ny. C rt nhiu hnh dng anten khc nhau c nghin cu dnh cho cc thit b cm tay di ng, chng hn nh: anten hnh F ngc phng (PIFA, Planar Inverted-F Antenna), anten dy phng v anten n cc phng. PIFA c kch thc tng i nh, nhng bng thng ca n kh hp, v chiu cao t anten ti mt phng t phi th mi t c hiu sut c th chp nhn c. Anten dy phng cho bng thng ln hn, nhng n li c kch thc ln nn khng ph hp cho cc ng dng di ng. Cc anten n cc phng c cc u im hn nh: kch thc nh, bng thng ln cho cc ng dng di ng v tha mn yu cu v gin bc x. Tuy nhin, cu trc ca cc anten n cc li l cu trc 3D thay v 2D, iu ny lm tng s kh khn trong sn xut v gi thnh.

Trong kha lun ny, mt anten n cc phng vi cu trc 2D c thit k. C cu trc v cc tham s ca cu trc u c iu chnh mt cch cn thn t c yu cu cng hng nhiu tn s (a cng hng), bng thng v convenient profile. Anten c 3 nhnh v c in trn mt tm in mi. Trc tin, 2 nhnh c thit k cng hng 2 tn s nht nh, v sau nhnh th 3 c thm vo iu chnh tn s cng hng cho ph hp vi cc di tn mong mun. Vi din tch nh 36 x 15 mm2, anten p ng yu cu ca cc chun truyn thng sau: GSM (Global System for Mobile communications, 890 MHz 960 MHz), DCS (Digital Communication System, 1710 MHz 1880 MHz), PCS (Personal Communication Services, 1850 MHz 1990 MHz), UTMS (Universal Mobile Telecommunication System, 1920 MHz 2170 MHz) v WLAN (Wireless Local Area Network, 2400 MHz 2484 MHz).


48




Hnh 3.1. Hnh dng ca anten c thit k trong kha lun3.2. Thit k thnh phn bc x Hnh dng tng th ca anten m kho lun thit k c th hin trong hnh 3.1 v thnh phn bc x c th hin trong hnh 3.2 di y.

Hnh 3.2. Thnh phn bc x ca anten Thnh phn bc x n cc phng chim din tch l 36 x 15 mm2, v c in trn cht nn FR4 dy 0.8 mm (hng s in mi tng i l 4.4). Tm in mi ny c dng ph bin lm cc mch PCB cho in thoi di ng. Cht nn (lp in mi) rng 36 mm v di 75 mm. mt sau ca tm in mi, mt phng t c in c chiu rng 36 mm v chiu di 60 mm. Thnh phn bc x n cc c tip in bi mt ng vi di 50 nh c ch ra trong hnh 3.1. Sinh vin: Lu Vn Hoan 49 Lp K49 Thng tin v tuyn



Thnh phn bc x chnh (patch) ban u c dng hnh ch nht. Bng cch x mt rnh un khc trn thnh phn bc x ban u to ra 3 nhnh, trong nhnh cng hng th nht l nhnh di hn, nhnh cng hng th hai l nhnh ngn hn v nhnh iu chnh (nhnh th ba) vi cc kch thc chi tit c ch ra trong hnh 3.3.

Hnh 3.3. Kch thc chi tit thnh phn bc x ca anten Ta mong mun anten hot ng ti 2 di tn (di th nht cho GSM v di th hai gm 4 di gn nhau l DCS, PCS, UTMS v WLAN), do thit k ban u ch c 2 nhnh cng hng (khng c nhnh th ba). Chiu di ca nhnh di hn tnh t im tip in ti u cui ca nhnh cng hng th nht l khong 75 mm. Gi tr ny rt gn vi bc sng ti tn s 900 MHz trong khng gian t do. Cng nn ch rng, tn s cng hng ph thuc c vo chiu di ca nhnh v chiu rng ca u cui. Theo cch tng t, chiu di ca nhnh cng hng th hai tnh t im tip in ti u cui ca n l khong 35 mm, xp x bc sng ti tn s 2 GHz. di 2 nhnh cng hng c chn ngn hn so vi bc sng cng hng c chn. L do chnh l mt s tn ti trong thc t ca cht nn s thu ngn bc sng cng hng. Anten vi ch 2 nhnh cng hng 1 v 2 c kh nng hot ng 2 di tn. Tuy nhin, bng thng li cha bao ph tt c 5 di tn c lit k trn, c bit l di WLAN (kt qu m phng c th hin trong chng sau). Do , nhnh iu chnh (nhnh th ba) c thm vo ti mt v tr thch hp trn nhnh cng hng th nht. Cc kt qu m phng ch ra rng, bng cch iu chnh cn thn cc kch thc ca nhnh th ba, cc mode cng hng c bn v bc cao hn ca nhnh cng hng th nht c th c iu chnh ti tn s mong mun. Theo d liu m phng, tn s cng hng ca mode c bn c gim t 900 MHz xung 870 MHz. i vi mode bc cao hn, tn s cng hng thay i t ln hn 3 GHz xung khong 2.3 GHz. Do , anten khi c 3 nhnh c th hot ng c 5 di tn GSM/DCS/PCS/UTMS/WLAN.3.3. Thit k thnh phn phi hp tr khng di rng 3.3.1. So snh mt s b phi hp tr khng di rng Trong phn ny, ta a ra mt v d v thit k b phi hp tr khng di rng. Sau v th bin i ca Z(z) v ( ) so snh.

V d: Thit k b phi hp tr khng lin tc Thit k b phi hp tr khng lin tc dng tam gic, dng hm m v dng Klopfenstein (vi m = 0.02) phi hp ti 50 vi ng truyn 100 . V s bin i tr khng Z(z) theo z v bin ca h s phn x theo L. Sinh vin: Lu Vn Hoan 50 Lp K49 Thng tin v tuyn

Kha lun tt nghip i hc GiiB phi hp tr khng dng tam gic: T (2.14), s bin i ca tr khng Z(z) theo z l:


Z 0 e 2 ( z / L ) ln Z L / Z 0 Z ( z) = ( 4 z / L 2 z 2 / L2 1) ln Z L / Z 0 Z 0 e 2

vi 0 z L / 2 vi L / 2 z L2

Vi Z0 = 100 v ZL = 50 . p ng bin ca h s phn x c cho bi (2.16):1 Z | ( ) |= ln L 2 Z0 sin( L / 2) L / 2

B phi hp tr khng dng hm m: T (2.11) ta c: Z ( z ) = Z 0 e z , trong 0 < z < L vi = (1 / L) ln Z L / Z 0 = 0.693 / L . p ng bin ca h s phn x c cho bi (2.13): ln Z L / Z 0 sin L | ( ) |= L 2 B phi hp tr khng Klopfenstein: S dng (2.20) cho 0: Z Z0 1 Z L ln = 0.346 0 = L Z L + Z0 2 Z0

V s dng (2.21) cho A ta c: 0 0.346 m = A = cosh 1 0 = cosh1 = 3.543 cosh A 0.002 m Biu thc ca Z(z) c th tnh c theo (2.17). p ng bin ca h s phn x c cho bi (2.19): cos ( L) 2 A2 | ( ) |= 0cosh A

Di truyn i vi b phi hp tr khng Klopfenstein c nh ngha vi: L > A = 3.543 = 1.13. Hnh 3.4.a,b th hin s bin i tr khng (theo z/L) v p ng bin ca h s phn x (theo L) i vi 3 loi b phi hp tr khng trn.


51




(a)

(b)

Hnh 3.4. th cho v d trn [4] (a). S bin i ca Z(z) ca b phi hp dng tam gic (T), hm m (E) v Klop (K) (b). p ng bin - tn s ca h s phn x cho 3 loi b phi trn Ta d dng nhn thy rng, b phi hp tr khng Klopfenstein cho p ng mong mun || = 1.13 thp hn so vi 2 b phi hp tr khng kia. Trong di thng ca b phi hp tr khng Klopfenstein, p ng bin c cc nh bng nhau.3.3.2. La chn b phi hp tr khng di rng Trong kha lun ny ta la chn b phi hp tr khng dng tam gic. Do hnh dng ca n d dng thc hin c bng phng php th cng.


52




3.4. Thit k ng truyn vi di 50 3.4.1. Thit k vi Ansoft Designer 2.0 Chng ta s xc nh rng (W) ca mt ng truyn vi di c tr khng c trng Z0 = 50 , hng s in mi cht nn r = 4.4 (FR4-epoxy), chiu cao cht nn h = 0.8 mm, dy lp ng l t = 0.034 mm.

Cc bc thit k ln lt nh sau: 1. Khi ng Ansoft Designer 2.0 2. T menu Project, chn Insert Planar EM Design Sau ca s Choose Layout Technology xut hin cho php bn chn Layout. 3. Ta chn MS-FR4(Er=4.4) 0.060 inch, 0.5 oz copper. Sau nhn Open. 4. Ca s thit k xut hin, cho php bn thc hin cc thao tc thit k. T menu Layout, ta chn Layers, chn tab Stackup sa i cc thng s ca ng truyn nh hnh 3.5 sau:

Hnh 3.5. Thit lp cc thng s ca ng truyn 5. Ta s c lng rng (W) ca ng truyn vi di. T menu Planar EM, ta chn Estimate. Ca s Estimate xut hin, vi tab mc nh l Tline.

Hnh 3.6. c lng rng W ca ng truyn vi di Sinh vin: Lu Vn Hoan 53 Lp K49 Thng tin v tuyn



Nhp tr khng c trng ca ng truyn vo phn Estimate Parameters v nhn Calculate. Ansoft Designer cho kt qu W = 1.526 mm nh trong hnh 3.6.3.4.2. Thit k da vo l thuyt ng truyn vi di ng truyn phng (planar transmission line) l thnh phn to nn mng tip in vi di v cc anten mch in. Anten c th c tip in bi nhiu loi ng truyn khc nhau, nh l cc b chia cng sut, b dch pha, b suy gim,

C nhiu phng php s c gii thiu m hnh ha ng truyn vi di, v cc phng php ny s dng mt s lng ln php tnh ton hc. y, ta a ra cc biu thc c chnh xc ph hp vi cc loi sai s khc nhau, chng hn nh: dung sai trong cc tham s vi di (r, h v W) v sai s do php o. Cc biu thc s c th hin bao gm: tr khng c trng, bc sng, mt mt trn ng truyn, cng sut cho php (power handling capability) v h s phm cht Q. Mode chim u th trn ng truyn vi di l mode quasi-TEM. Ta s dng php phn tch quasi-static cho mt cu trc truyn dn xc nh dung khng C trn mi n v chiu di ng truyn. Sau , gi tr ny c s dng xc nh tr khng c trng (Z0) v hng s in mi hiu dng ca ng truyn (re) bng cch s dng cc quan h di y: Z 0 = (c CC a ) 1 (3.1) (3.2) re = C/Ca Trong , Ca l dung khng trn mi n v chiu di ca ng truyn vi di khi lp in mi c thay th bng khng kh v c l vn tc nh sng trong chn khng.3.4.2.1. Tr khng c trng Z0 Gi s dy ca lp ng dn in t = 0. Tr khng c trng ca mt ng truyn vi di c xc nh nh sau: 0 (3.3) Z0 = ln( F1 / u + 1 + 4 / u 2 ) 2 re Trong : F1 = 6 + (2 6) exp{(30.666 / u ) 0.7528 } 0 = 120 u =W /h

Hng s in mi hiu dng: +1 r 1 10 + (1 + ) ab re = r2 2 u

(3.4)

Trong :3 1 u 4 + (u / 52) 2 1 u a = 1 + ln 4 ln 1 + + 49 u + 0.432 18.7 18.1

0. 9 b = 0.564 r 0 .3 r

0.053


54




Hnh 3.7. Hnh dng ng truyn vi di v m hnh dn sng phng tng ng y, W l chiu rng ca di dn in, h l dy ca cht nn in mi, v re l gi tr hng s in mi hiu dng ti tn s zero ca ng vi di. Sai s tng i ln nht ca Z0 theo cng thc (3.3) nh hn 1%. chnh xc ca biu thc cho re nh hn 0.2% i vi cc cht nn c re

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