Anten Wi-Fi

Khi thit b Wi-Fi ngy cng "trn ngp" th trng, lm th no chn c thit b p ng nhu cu ca bn: tc nhanh, tnh nng phong ph v hu ch, vng ph sng rng... la chn mt thit b mng Wi-Fi (access point, router...) "va " ngoi yu t thng hiu, cc tnh nng cn thit v chun Wi-Fi ph hp, bn cn phi quan tm thm n cc thng s k thut ca anten. Mt anten c cc thng s ph hp s mang n cho bn mt khng gian lm vic "rng ri" hn. Bn nn tham kho k nhng thng s sau trong ti liu i km sn phm khi chn anten.

li (gain)

li l mt thut ng m t s tng bin ca tn hiu v tuyn, n v o l decibel (dB) hay dBi ch li ca anten ng hng (isotropic) v dBd ch li ca anten dipole na bc sng (half-wave dipole). Mt dB bng log10 ca cng sut u ra chia cho cng sut u vo. V d, cng sut u vo l 30mW v cng sut u ra l 60mW th li tnh c l 3dB. Theo cng thc ny, c tng 3dB th cng sut tnh theo mW s tng gp i. Chng hn, AP c cng sut 50mw v s dng anten 3dB (loi Rubber Duck) th cng sut pht thc s ca AP l 50*2 (3dB bng gp i cng sut) = 100mw. Anten c li cng cao th khong cch sng di cng xa. Vic tp trung cng sut pht ca chng cht ch hn lm cho nhiu nng lng c truyn n ch hn, khong cch xa hn.

Cch tnh gia dB v Watt: 0db = 1mW

C tng 3dB th cng sut tnh theo mW s c tng gp i:

3dB = 2mW

6dB = 4mW...30dB = 1000mW = 1W

Cc loi anten v hng nh rubber hay omni c li t 2-12dBi do chng phi pht 360 theo chiu ngang (anten ng hng li cng cao th kch thc cng ln v phn cc dc cng nh). Cc loi anten nh hng nh flat, sector thng thng c li t 8-20dBi, gc pht theo chiu ngang khong 10 - 120 (anten nh hng c li cng cao, kch thc cng ln v bp sng cng nh). Ln hn na l cc loi anten cho (Grid anten) c li ln, c khi ln n 30dBi hoc cao hn.

S phn cc (polarization)

S phn cc ca sng l hnh nh li bi u mt ca vect trng khi c quan st dc theo chiu truyn sng. S phn cc ca anten c th c phn loi nh tuyn tnh, trn hay ellip. Sng v tuyn thc cht c to bi 2 trng: in trng v t trng. Hai trng ny nm trn 2 mt phng vung gc vi nhau.

Tng ca 2 trng c gi l trng in t. Mt phng song song vi thnh phn anten c gi l E-plane, mt phng vung gc vi thnh phn anten c gi l H-plane. Chng ta ch quan tm ch yu n in trng v v tr v hng ca n trong mi tng quan n b mt tri t s quyt nh s phn cc ca sng. S phn cc l hung vt l ca anten theo phng ngang (horizotal) hay dc (vertical). in trng l song song vi thnh phn bc x ca anten v th nu anten nm dc th cc ca anten l dc hay cn gi l phn cc dc (in trng vung gc vi mt t). Phn cc dc thng c s dng trong mng Wi-Fi (WLAN) l vung gc vi mt phng ca tri t. Phn cc ngang l song song vi mt t.

Bp sng (beamwidth)

Vic lm hp hay tp trung cc bp sng ca anten s lm tng li ca anten. Bp sng l rng ca tia tn hiu RF m anten pht ra. Bp sng dc c o theo v vung gc vi mt t, cn bp sng ngang cng c o theo v song song vi mt t. ng vi mi kiu anten khc nhau s c bp sng khc nhau.

Vic chn la anten c bp sng rng hay hp thch hp l vic lm quan trng t c hnh dng vng ph sng mong mun. Bp sng cng hp th li cng cao.

Tr khng (Impedance)

S bc x hiu dng ca mt anten l t s ca tng cng sut pht ra bi anten so vi cng sut t trm pht (ni vi anten) c chp nhn bi anten. Anten bc x mt s cng sut dng nng lng in t. Tt c cc thit b RF, ng truyn (cp), anten u c tr khng, chnh l t s gia in p v dng in. Khi anten c kt ni vi mt on cp, nu tr khng u vo ca anten trng khp vi tr khng ca radio v ng truyn th tng cng sut c truyn t radio n anten l ti a. Tuy nhin, nu tr khng khng ging nhau th mt s nng lng s b phn x ngc tr li ngun v s cn li s c truyn i n anten.

T s sng ng in p (VSWR)

VSWR (Voltage Standing Wave Ratio) xut hin khi tr khng khng tng thch gia cc thit b trong h thng RF. VSWR c gy ra bi mt b tn hiu RF b phn x ti im tr khng khng tng thch trn ng truyn tn hiu. Nu nh khng c phn x th VSWR s bng mt. Khi VSWR tng ln th s phn x s cng nhiu. Nu VSWR cao v cng sut cao th c th gy ra tnh hung nguy him nh khi ta s dng in p cao trong ng truyn, trong trng hp ti t nht, n c th bn ra tia la in. Tuy nhin, tnh hung ny s khng xy ra nu bn s dng cng sut thp khi trin khai mng WLAN. Phng thc thay i VSWR bao gm vic s dng thit b thch hp, kt ni chc chn gia cp v u ni, s dng tr khng tng thch gia cc thit b phn cng v s dng cc thit b cht lng cao l cc phng thc tt chng li VSWR. T s ny thng l 1,5:1

T s sng ng in p (VSWR)

VSWR (Voltage Standing Wave Ratio) xut hin khi tr khng khng tng thch gia cc thit b trong h thng RF. VSWR c gy ra bi mt b tn hiu RF b phn x ti im tr khng khng tng thch trn ng truyn tn hiu. Nu nh khng c phn x th VSWR s bng mt. Khi VSWR tng ln th s phn x s cng nhiu. Nu VSWR cao v cng sut cao th c th gy ra tnh hung nguy him nh khi ta s dng in p cao trong ng truyn, trong trng hp ti t nht, n c th bn ra tia la in. Tuy nhin, tnh hung ny s khng xy ra nu bn s dng cng sut thp khi trin khai mng WLAN. Phng thc thay i VSWR bao gm vic s dng thit b thch hp, kt ni chc chn gia cp v u ni, s dng tr khng tng thch gia cc thit b phn cng v s dng cc thit b cht lng cao l cc phng thc tt chng li VSWR. T s ny thng l 1,5:1

Ngoi cc thng s trn, bn nn quan tm n chun Wi-Fi m thit b s dng chn anten ph hp. Nh chng ta bit, mng khng dy hin nay hot ng trn 2 tn s: 2,4 GHz (802.11b/g, 802.11n draft) v 5 GHz (802.11a, 802.11n draft) hot ng mi trng trong nh (indoor) hoc ngoi tri (outdoor). Do , la chn mt anten lm vic hiu qu, bn phi chn tn s anten ng vi tn s ca thit b thu/pht sng Wi-Fi. Ngoi ra, bn cn phi quan tm thm n chun u ni ca anten. Hin nay c cc chun u ni thng dng: N-Female, N-Male, RP-SMA, RP-TNC...

Cc loi anten v th bc x

Anten s dng trong cc thit b Wi-Fi c thit k theo 2 dng: Loi lp c nh hoc ri. Loi anten c nh thng thy nht l card mng tch hp trn cc MTXT hay AP s dng anten c nh. Vi nhng thit b c anten c nh ny, bn khng c la chn no tt hn l dng anten ca hng cung cp. i vi cc thit b s dng anten ri th vic thay th bng mt anten khc t c vng ph sng nh mong mun kh d dng. Vic thay anten ph hp gip tng vng ph sng v tc , gim s lng AP v chi ph lp t...

Anten c 2 loi chnh c s dng trong WLAN: ng hng hay v hng (Omni-directional) v nh hng hay c hng (Directional).

Anten ng hng truyn tn hiu RF theo tt c cc hng theo trc ngang (song song mt t) nhng b gii hn trc dc (vung gc vi mt t). Anten ny thng c dng trong cc thit b tch hp Wi-Fi thng dng hin nay: ADSL, Broadband router, access point. Anten ng hng c li trong khong 6dB, thng c dng trong cc ta nh cao tng. Anten ng hng cung cp vng ph sng rng nht, to nn vng ph sng hnh trn chng chp ca nhiu AP bao trm c mt ta nh. Hu ht cc AP u s dng anten ng hng c li thp. Vic s dng anten c li cao hn s tng vng ph sng, do c th gim s lng AP tit kim chi ph.

Loi anten ny thng s dng trong m hnh im-im hay im-a im hay c th dng lp trn xe. Anten nh hng s l anten chnh pht tn hiu n my tnh hay cc thit b Wi-Fi khc, chng hn my in khng dy, PDA... Khi s dng ngoi tri, nn t antenna omni-directional gia nh ca ta nh. V d, trong khun vin ca mt trng i hc th anten c th c t trung tm ca trng c vng bao ph ln nht. Khi s dng trong nh, antenna nn c t gia nh ( trn nh) hay gia vng bao ph mong mun c vng bao ph ti u. Loi anten ny c vng bao ph theo dng hnh trn nn kh thch hp cho mi trng nh nh kho, trung tm trin lm...

Cc loi anten ng hng: Rubber Duck, Omni-directional, Celing Dome, Small Desktop, Mobile Vertical, Ceiling Dome...

Anten Rubber Ducky (hay Rubber Duck hay Rubber Duckie) c sinh vin Richard B. Johnson ch to vo nm 1958. Hin nay, anten ny thng c s dng ph bin trn cc im truy cp (access point) hay cc b nh tuyn (router) do c cu to n gin, h tr phn cc ng hng (phn cc ngang gc 360 ).

Anten Omni-directional

Antenna omni-directional c li cao th vng ph sng theo chiu ngang ln v vng ph sng theo chiu dc nh. c im ny c th c xem nh l mt yu t quan trng khi lp t mt anten c li cao trong nh (trn trn nh). Nu nh trn nh qu cao th vng bao ph c th khng th ph n nn nh, ni m ngi dng thng hay lm vic.

Anten nh hng

Anten nh hng (directional) c hng pht sng rt hp, thit b thu sng cn nm chnh xc trong phm vi pht sng hp ny ca anten nh hng mi c th thu c sng pht t anten. th bc x tng t nh nh sng ca n pin, tc khi chng ta chiu sng gn th chm sng s rng cn khi chiu sng vt xa th chm sng rt nh, nh l mt tia sng. li anten cng cao th bp sng cng hp, gii hn khu vc ph sng ca anten. Anten nh hng c li ln hn anten ng hng, t 12dBi hoc cao hn. Vic thay i li chnh l to ra cc anten khc nhau, mc ch l to ra cc bp sng vi gc pht khc nhau, gc pht theo chiu dc (vertical beamwidth) hay chiu ngang (horizontal beamwidth) cng nh th bp sng cng hi t v c ly pht s xa. ... Cc loi anten nh hng ny thng c gc pht theo chiu ngang khong 10 - 120 nn c li ln hn nh 18dBi, 21dBi...

Anten nh hng c nhiu kiu dng v kch thc khc nhau, in hnh c cc loi anten: Yagi, Patch, Backfire, Dish... Cc loi anten nh hng ny rt l tng cho khong cch xa, kt ni khng dy im-im.

Anten Yagi

Anten Yagi l loi anten nh hng rt ph bin bi v chng d ch to. Cc anten nh hng nh Yagi thng s dng trong nhng khu vc kh ph sng hay nhng ni cn vng bao ph ln hn vng bao ph ca anten omni-directional.

Anten Yagi hay cn gi l anten Yagi-Uda (do 2 ngi Nht l Hidetsugu Yagi v Shintaro Uda ch to vo nm 1926) c bit n nh l mt anten nh hng cao c s dng trong truyn thng khng dy. Loi anten ny thng c s dng cho m hnh im- im v i khi cng dng trong m hnh im-a im. Anten Yagi-Uda c xy dng bng cch hnh thnh mt chui tuyn tnh cc anten dipole song song nhau (xem hnh).

Anten Patch c hnh thnh bng cch t 2 vt dn song song nhau v mt ming m gia chng. Vt dn pha trn l mt ming ni v c th c in trn bng mch in. Anten Patch thng rt hu ch bi v chng c hnh dng mng.

tm hiu v anten

Antenna la mt b chuyn i dong in di chuyn tn s cao thanh song in t, hoc ngc lai chuyn song in t thanh dong in xoay chiu. Antenna co th c dung bc xa nng lng ra khng gian, hoc nhn nng lng t khng gian.Antenna va ng dy dn (feeder) ong vai tro thit bi ghep gia cac mach in t va khng khi. Cac antenna tip xuc trc tip vi khng gian bn ngoai con feeder la b phn giao tip gia antenna vi mach in t. Ngo vao cua feeder phai phi hp tr khang vi may phat, con antenna phat nhn nng lng t may phat qua feeder va bc xa ra khng gian. Nng lng truyn trong khng gian vi vn tc anh sang (3 x108m/s). S bc xa nng lng c c trng bi tn s hoat ng hoc bc song cua no. Cng thc sau biu din s lin h gia tn s va bc song:

trong o: lamda: bc song co n vi la mf : tn s co n vi la Hzc: tc anh sang (300 x 106 m/s).2. Truyn song v tuynCac nguyn tc cua bc xa in t trong khng gian c bt ngun t ly thut v tinh cam ng cua trng in t. Trc tin, trng t bin thin tao ra trng in bin thin; va sau o, trng t bin thin nay lai tao ra dong in bin thin. Qua trinh c th lp lai tao thanh song in t bao gm hai phn: trng in (E) va trng t (H), chung phu thuc ln nhau. Cac trng E va H lun vung goc vi nhau va vung goc vi hng truyn cua song in t ra khng gian. Hinh 8-1 miu ta mi lin h gia cac trng E va H vi chiu cua song in t, ng thi minh hoa qui tc ban tay trai xac inh chiu cua cac song trong khng gian.

Click this bar to view the full image.

Hinh 8.1. Trng in t, qui tc ban tay trai

Hinh 8.2. S truyn cac song trc giao E va H

Cac thanh phn E va H cua song RF cung pha thi gian, nhng lch 900 v khng gian. Chiu cua E xac inh tinh phn cc cua song. Hinh 8-2 minh hoa song phn cc doc. Nu cac thanh phn E va H c xoay i 900, ta noi song c phn cc ngang. Cng song c xac inh bng cach o cng trng in E.3. Song mt t va song khng gianDipole co th c inh hng sau cho hai thanh song song vi b mt cua trai t, trng hp nay goi la dipole ngang, hoc con goi la antenna Hertz. Hinh 8-3 A la thi bc xa cua dipole vi cng sut tp trung mt phng ngang va khng co bc xa theo hng song song vi dipole. Hinh 8-3 B la thi bc xa 3 chiu cua dipole noi trn.Chiu dai tht s cua antenna nho hn khng ang k so vi dai tin toan. dai vt ly cua antenna xp xi bng 95% cua dai ly thuyt.

Hinh 8.3. Hinh anh hai chiu va ba chiu cua dipole

S phn phi in ap va dong in qua antenna c minh hoa trong hinh 8-4. Chu y rng tai tm cua dipole, dong in cc ai (co nghia in tr la nho nht) va tai u cui dipole in ap la ln nht (khi o in tr la ln nht).

Hinh 8.4. S phn /2(b dong va ap,

4. in tr antennain tr tai im .(cp in (c goi la in tr bc xa) cua dipole vao khoang 73 in tr bc xa cua antenna la mt ai lng ly thuyt. in tr bc xa cua antenna c tinh bng cng thc sau:

Trong o:P: la cng sut cua trng in t (cng sut bc xa).I : Dong antenna (gia tri hiu dung).Rr : in tr antenna.in tr vao cua antenna con c goi la tr khang (Zi) antenna. Tr khang ngo vao cua antenna phu thuc vao nhiu yu t, nh la cao antenna, vi tri im cp in, va loai antenna. Antenna hoat ng ging nh mach cng hng ma tr khang ngo vao tai tn s cng hng la in tr thun. Trn tn s cng hng, antenna co tinh cam. Di tn s cng hng, antenna co tinh dung. Tr khang ngo vao cua antenna (Zi) phu thuc vao tn s hoat ng va dai antenna. Do o, tr khang ngo vao khng th bit , phu(chinh xac, tr khang trung binh cua dipole vao khoang 73-80 thuc vao tn s hoat ng.

IEEE nh ngha anten l phn ca h thng truyn hay nhn c thit k bc x hay nhn sng in t. Ni cch khc, anten ly tn hiu RF (c sinh ra bi radio) v bc x n vo trong khng kh hay anten c th nhn sng in t cho radio.

Mt khi nim quan trng cn phi bit l anten ng hng (hay bc x ng hng - isotropic). Theo phng din ton hc, y l trng hp l tng khi mt anten khng b suy hao (lossless anten) pht ra tn hiu theo mi hng mt cch bng nhau. Xt mt hnh cu v b bc x ng hng nm trung tm ca hnh cu, lc trng in t s bng nhau tt c cc im trn b mt hnh cu. Anten ng hng l mt im tham chiu hu ch khi chng ta xem xt cc loi anten khc nhau.

1. Nhng thuc tnh ca anten c th chn la c anten ng n th iu quan trng l bn phi hiu c mt s thuc tnh m t v anten. Chng bao gm dng bc x ca anten, hng tnh ca anten, li, tr khng u vo, s phn cc, v bandwidth.

Hng tnh ca anten (directivity of anten)Hng tnh ca anten m t cng ca mt bc x theo mt hng xc nh tng ng vi cng bc x trung bnh hay ni cch khc, n cho bit mt cng sut bc x tng ng vi cng sut bc x c phn tn mt cch ng dng.

li (gain)Gain cng din t cng mt khi nim nh directivity nhng n cn bao gm c s mt mt (v cng sut) ca chnh bn thn anten. Bn c th nh ngha bc x hiu dng c s dng m rng directivity gip xc nh c li; mt b bc x hon ho s c bc x hiu dng bng 1.

n v dng biu din li c th l dBi ( li tnh theo dB ca anten ng hng) hay dBd ( li dB ca anten half-wave dipole). chuyn i gia dBd v dBi th ta ch cn cng thm 2.2 vo li dBd c c li dBi. Vic ghi nh quy c ny l quan trng bi v mc d hu ht cc nh sn xut u biu din li theo dBi nhng mt s khc li biu din theo dBd. Hnh di minh ha mt dng bc x ca anten nh hng. Lu l s bc x ny tn ti trong khng gian 3 chiu

Dng bc x (radiation pattern)Dng bc x ca anten m t s khc nhau v gc bc x mt khong cch c nh t anten. N thng c din t bng thut ng hng (directivity) hay li (gain) ca anten.

Anten thng c main lobe hay beam (vng bc x), chnh l hng c li ln nht, v minor lobe m c th hn l side lobe hay back lobe ty thuc vo hng ca minor lobe so vi main lobe. Cc nh sn xut thng m t anten bng li hay main lobe, h cng thng xc nh thm beamwidth ( rng ca vng bc x) ca anten.

Nguyn l half-power beamwidth c nh ngha bi IEEE nh sau: trong mt radiation pattern ta ct phn c cha main lobe, gc gia 2 hng ct trong cng bc x ch bng mt na ca gi tr ti a. V d, ta ly dng bc x ca anten trong hnh trn v i n im c 2 pha ca main lobe ni m li thp hn 3 dB (gim mt na) so vi im ln nht ca lobe, im ny chnh l half-power point. Khi , gc gia chng chnh l half-power beamwidth. Hnh di minh ha iu ny.

Trong khi nim radiation pattern ca anten th t s front-to-back ca anten s so snh li ln nht ca anten trn main lobe vi li ln nht trn back lobe. Trong v d trn th t s front-to-back l 20 dB v main lobe l 15 dBi, back lobe l -5 dBi; S khc nhau gia 15 dBi (-5 dBi) = 20 dBi chnh l t s front-to-back

Cng sut bc x (radiated power)By gi chng ta s kim tra cng sut tht s c pht ra bi radio ni vi anten. Cng sut bc x hiu dng (ERP = Effective Radiated Power) c tnh bng cch ly li ca anten tnh theo dBd (tng ng vi half-wave dipole) v nhn n vi cng sut m transmitter cung cp cho anten. Tuy nhin, hu nh bn phi thng xuyn thc hin nhng tnh ton ny bng cc hm log, dB, iu ny c ngha l bn cng thm li ca anten vo cng sut t transmitter. Thng th li ca anten c biu din theo n v dBi, mt thut ng khc thng c s dng cho cng sut bc x l Cng sut bc x ng hng hiu dng (EIRP = Effective Iotropic Radiated Power; c sch gi l Equivalent Isotropic Radiated Power), n hon ton tng t vi ERP nhng li ca anten c biu din tng ng vi b bc x ng hng.

S phn cc (polarization)Sng in t c pht ra bi anten c th to ra nhng dng khc nhau nh hng n s qung b. Cc hnh dng ny s ty thuc vo s phn cc ca anten, c th l phn cc tuyn tnh (linearly) hay phn cc vng (circularly).

Hu ht cc anten trn th trng WLAN u s dng phn cc tuyn tnh, c th theo chiu ngang (phn cc ngang) hoc theo chiu dc (phn cc dc). Nu theo chiu ngang th vector trng in s nm trn mt mt phng thng ng, nu theo chiu dc th vector trng in nm trn mt phng nm ngang. Phn cc dc l ph bin hn mc d i khi phn cc ngang li hot ng tt hn. Mc d s l khng ph hp nu bn s dng anten phn cc vng cho kt ni trong nh, nhng nu bn s dng wireless bridge th bn c th dng anten phn cc vng. Cng ging nh anten phn cc tuyn tnh, anten phn cc vng cng c 2 trng hp: Phn cc tay tri v phn cc tay phi. Nu nh vector trng in quay theo chiu kim ng h khi n tin gn n bn th c gi l phn cc tay tri. Tng t, nu vector quay ngc kim ng h th gi l phn cc tay phi. Anten phn cc vng l bt bin (hoc l phn cc tri, hoc l phn cc phi) khi n quay trong khi anten phn cc tuyn tnh c th chuyn t phn cc ngang thnh phn cc dc khi n quay. Ni chung, i vi kt ni LOS (Line Of Sight) th bn nn s dng cng cc c 2 u kt ni.

Tr khng (impedance)S bc x hiu dng ca mt anten l t s ca tng cng sut pht ra bi anten so vi cng sut t transmitter (ni vi anten) c chp nhn bi anten. Anten bc x mt s cng sut dng nng lng in t. Tt c cc thit b RF radio, ng truyn (cable), anten u c tr khng, chnh l t s gia in p v dng in. Khi anten c kt ni vi mt on cable, nu tr khng u vo ca anten trng khp vi tr khng ca radio v ng truyn th tng cng sut c truyn t radio n anten l ti a. Tuy nhin, nu tr khng khng ging nhau th mt s nng lng s b phn x ngc tr li ngun v s cn li s c truyn i n anten. T s sng ng in p (VSWR = Voltage Standing Wave Ratio) m t s phn x ny. Nu nh khng c phn x th VSWR s bng 1. Khi VSWR tng ln th s phn x s cng nhiu. Nu VSWR cao v cng sut cao th c th gy ra tnh hung nguy him nh khi ta s dng in p cao trong ng truyn, trong trng hp ti t nht, n c th bn ra tia la in. Tuy nhin, tnh hung ny s khng xy ra nu bn s dng cng sut thp khi trin khai mng WLAN.

Bandwidth Bandwidth ca anten nh ngha vng tn s m anten cung cp hiu nng c th chp nhn c, thng thng c nh ngha bi tn s gii hn trn hay tn s ti a v tn s gii hn di hay tn s ti thiu. Trong trng hp ny, hiu nng c th chp nhn c c ngha l cc c im ca anten nh dng bc x v tr khng u vo khng b thay i khi hot ng trong dy tn s . Mt s anten c xem nh l broadband (bng rng) trong t s gia tn s ln nht v tn s nh nht l ln hn 2. Tuy nhin, bi v anten bng rng thng c hiu nng km v bi v s phn b tn s 802.11 hin ti khng yu cu anten bng rng nn trng hp duy nht m bn c th s dng anten bng rng chnh l khi bn mun s dng c 2 bng tn 2.4 Ghz ISM v 5 Ghz UNII trong 1 anten duy nht.

Khi chn la anten bn s thy c nhiu thuc tnh ca anten c lin quan n vi nhau, v th mc d dng nh l ti u khi chng ta chn anten c tt c thuc tnh u ti a hoc ti thiu, iu ny thng l khng th. V d, Nu bn chn beamwidth rt rng th bn phi hy sinh li. Nu bn chn anten bng rng th bn c th thy rng dng bc x ca chng l rt khc nhau. V th, iu cn thit l bn phi xc nh c thuc tnh no l quan trng cho vic trin khai ca bn.

2. Kiu AntenHnh di minh ha nhiu loi anten khc nhau

Vi anten half-wave dipole th chiu di t u cui n u cui s bng na bc sng tn s . Anten omni-directional cung cp li bng nhau theo mi hng trn mt mt phng, thng l mt phng ngang. Anten dipole thng l omni-directional. Anten Omni-directional thng c s dng khi trin khai mng WLAN bi v chng cung cp vng bao ph theo mi hng. Anten Yagi-Uda c xy dng bng cch hnh thnh mt chui (array) tuyn tnh cc anten dipole song song nhau.

Anten Yagi l loi anten nh hng rt ph bin bi v chng kh d ch to. Cc anten nh hng nh yagi thng cung cp vng bao ph nhng vng kh vi ti hay nhng ni cn vng bao ph ln hn vng bao ph ca anten omni-directional.

Mt loi anten nh hng khc l anten Patch c hnh thnh bng cch t 2 vt dn (conductor) song song nhau v mt ming m (substrate) gia chng. Vt dn pha trn l mt ming ni (patch) v c th c in trn bng mch in. Anten Patch thng rt hu ch bi v chng c hnh dng mng. Ngoi ra cn c nhiu loi anten khc nh broadside anten (c main beam song song vi mt phng ca anten); fire anten (main beam nm trong mt phng ca anten); v pencil beam anten (cung cp 1 tn hiu duy nht, rt hp (narrow) v li cao cho anten.

*ti sao angten lai co th bt nhiu song cung luc?

1 Anten c th cng 1 lc bt c nhiu sng v :

Cc tn hiu sng m anten nhn c s truyn n b phn tch sng .Ni y ch yu l gm cc mch cng hng L-C .Mi cp L-C ch cho 1 tn s sng xc nh qua n mnh nht ,cc tn sng khc th khng cho qua ( thcc t th cng qua c nhng rt yu )

V d d thy nht l ci Radio .Khi bn ang d i thc cht l bn ang iu chnh t in C .Vic thay in dung C dn n vic thay i tn s cng hng ca my .Khi bn c i mong mun th tn s i l tn sng cng hng ca Radio m n cho php qua vo b phn bin i cao tn thnh m tn to m thanh n tai bn .

--------------------------------------------------------------------------------

mnh gii thiu cu to s lc 1 trm BTS l th no ta hnh dung

Trc ht ta xem hnh dng thc t ca n:

ta thy trong hnh c cc tm panel mu trng phn ra 3 hng khc nhau gi l sector ca anten .anten sector cng l anten, ni chnh xc th n l anten nh hng. Vi loi BTS dng 3 sector th mt anten ph 120 , nu l BTS dng 6 sector th mt anten ph 60 .ni chung tu thuc vo vng ph ca anten m lp t hay nhu sector

ta thy tng u l anten dng cho GSM bng tn 900MHZ

tng th 2 anten dng cho 3G (UTMS)

hnh nh ca 1 sector:

cu to bn trong ca 1 sector

hoc l theo kiu ny:

cc khi c dng hnh vung l cc chn t theo nhu dng khc nhau nh trong hnh

cch thc truyn ca 1 trm BTS

cc thng s k thut ca 1 nh sn xut anten BTS (kathrein)

phonglan_den86 nh thy Nguyn Quc Bnh bn vntelecom dch ra nh sau:

Frequency range : 824 960 MHz Di tn cng tc ca ng-ten (y l ng-ten BTS cho GSM di 900 MHz)

Polarization :Vertical Phn cc sng pht ra t ng-ten, l phn cc ng (vc-t in trng E vung gc vi b mt tri t nu ng-ten dng ng v khng c chc in)

Gain :15 dBi Tng ch ca ng-ten, 15 dB so vi ng-ten isotropic (l ng-ten gi nh khng c tht, c radiation pattern - tc l th phng hng - l hnh cu hon ho)

Half-power beam width H-plane: 65 rng bp sng na cng sut trn mt phng ngang (mt phng vc-t t trng, k hiu l H) l 65 . y l gc vi tm l tm pha ca ng-ten, gii hn bi cc tia vi tng ch gim 3 dB (hai ln) so vi tia chnh gia hng pht xE-plane: 15 rng bp sng na cng sut trn mt phng ng (mt phng vc-t in trng E) l 15 .Dng bp sng pht x t ng-ten nh vy ging nh mt tri chui

Electrical downtilt :315, adjustable in 1 steps Gc chc ng-ten iu khin theo phng php in (iu chnh pha sng a ti cc chn t ng-ten bn trong tm ng-ten mt cch thch hp s c th quay bp sng theo hng mong mun - nghch ln hay cp/chc xung) l t 3 n 15 , c th iu chnh c theo cc bc iu chnh tng mt

Side lobe suppression : > 12 dB (0... 20 above horizon) trit bp sng ph: ln hn 12 dB (bc x bp sng ph s c lm gim i hn 12 dB, tc l 16 ln, vi gc tnh t ng gii hn 3 dB ca rng bp sng na cng sut ln/xung so vi phng nm ngang t 0 n 20

Front-to-back ratio : > 25 dB T s phn cch hng pht sng v hng sau lng l 25 dB (l chnh lch mc cng sut gia hng pha trc v sau lng ng-ten)

Impedance :50 Tr khng 50 m, dy phi- 1/2 inches (ci ny ti cng khng chc lm)

VSWR : < 1.4 T s sng ng in p (Voltage Standing Wave Ratio) nh hn 1.4

Intermodulation : IM3 < 150 dBc (2 x 43 dBm carrier) T s xuyn iu ch bc ba (3rd order InterModulation) nh hn -150 dBc, tc l chnh lch mc cng sut hi bc ba gy ra do tnh phi tuyn ca ng-ten so vi cng sut thnh phn tn hiu hu ch (l cng sut sng mang - ch c trong dBc l vit tt ca carrier, l sng mang) nh hn 150 dB, vi ng-ten c cp 2 sng mang c cng sut mi sng mang l 43 dBm = 20 Watts

Max. power : 400 Watt (at 50 C ambient temperature) Cng sut ln nht cho php cp cho ng-ten l 400 Watts ti nhit mi trng 50 C

Input : 7-16 female Connector u vo ng-ten l connector ci

Connector position : Bottom V tr ca connector u phi vo ng-ten l di y ng-ten

Height/width/depth : 1294 / 258 / 103 mm Kch thc ng-ten tnh theo mm Cao/rng/su.