1

THIT K ANTEN CHO CC THIT B DI NG HOT NG

TRONG DI TN GSM, UTMS, WLAN

1. Gii thiu

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, 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, 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).

Trong bi, trnh by thit k cc mt anten vi di bng rng a di tn, s

dng cho cc thit b di ng hot ng trong di tn GSM, 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 2000MHz. 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

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2. Phn tch v thit k anten

2.1. M t m hnh anten

Trong bi bo co 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,

UTMS v WLAN vi h s sng ng VSWR nh hn 2.5.

2.1.1. Gii thiu

Trong bi trnh by, 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),

UTMS (Universal Mobile Telecommunication System, 1920 MHz 2170 MHz) v

WLAN (Wireless Local Area Network, 2400 MHz 2484 MHz).

Hnh 2.1: Hnh dng ca anten c thit k trong kha lun

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2.1.2. Thit k thnh phn bc x

Hnh dng tng th ca anten m kho lun thit k c th hin trong hnh 2.1

v thnh phn bc x c th hin trong hnh 2.2 di y.

Hnh 2.2: 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 2.1.

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

2.3.

Hnh 2.3: 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

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tr ny rt gn vi bc sng ti tn s 900MHz 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 phn 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 3 di tn

GSM/UTMS/WLAN.

2.1.3. Thit k thnh phn phi hp tr khng di rng

Trong thit k ny 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.

S bin i ca tr khng Z(z) theo z ca phi hp tr khng dng tam gic l:

Hnh 2.4: B phi hp tr khng dng tam gic

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Vi Z0 = 100 v ZL = 50 . p ng bin ca h s phn x c:

Hnh 2.5: 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 ()

2.1.4. Thit k ng truyn vi di 50

Thit k vi Ansoft Designer 2.0, 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 2.6:

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Hnh 2.6: 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 2.7: c lng rng W ca ng truyn vi di

2.2. M phng cu trc anten vi phn mm Ansoft HFSS

HFSS l vit tt ca Hight Frequency Structure Simulator. HFSS l phn mm m

phng trng in t theo phng php ton sng (full wave) m hnh ha bt k

thit b th ng 3D no. u im ni bt ca n l c giao din ngi dng ha.

N tch hp m phng, o ha, m hnh ha 3D v t ng ha (t ng tm li gii)

trong mt mi trng d dng hc, trong li gii cho cc bi ton in t 3D

thu c mt cch nhanh chng v chnh xc. Ansoft HFSS s dng phng php

phn t hu hn (Finite Element Method, FEM), k thut chia li thch nghi

(adaptive meshing) v k thut ha. Ansoft HFSS c th c s dng tnh

ton cc tham s chng hn nh: tham s S, tn s cng hng, gin trng, tham

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s , ...

HFSS l mt h thng m phng tng tc, trong phn t mt li c bn l

mt t

din. iu ny cho php bn c th tm li gii cho bt k vt th 3D no. c bit l

i vi cc cu trc c dng cong phc tp. Ansoft l cng ty tin phong s dng

phng php phn t hu hn (FEM) m phng trng in t bng cc k thut

nh: phn t hu hn, chia li thch nghi,

Ansoft HFSS cung cp mt giao din trc gic v d dng s dng pht

trin cc m hnh thit b RF th ng. Chu trnh thit k c minh ha trong hnh

4.1, bao gm cc bc sau:

1. V m hnh vi cc tham s cho trc: v m hnh thit b, cc iu kin

bin v ngun kch thch.

2. Thit t cc thng s phn tch: thc hin thit t cc thng s

tm li gii.

3. Chy m phng: qu trnh ny hon ton t ng.

4. Hin th kt qu: a ra cc bo co v th trng 2D.

Trong qu trnh thc hin phn tch, HFSS s chia ton b cu trc thnh cc

t din nh (gi l mt li). H thng mt li s lp kn ton b cu trc. Ti mi

bc thch nghi, HFSS s tnh gi tr ca tham s S cho tng mt li. Gia 2 bc

thch nghi lin tip, HFSS s tnh gia s Delta S vi cng thc nh sau:

Delta S = Maxij [mag (SN

ij S(N-1)

ij)]

Trong i v j l ch s ca phn t tung ng trong ma trn S v N l ch s

ca bc thch nghi. Delta S l gi tr ln nht ca gia s ca bin ca tham s S

tng ng. HFSS s so snh gi tr Delta S ny vi tiu chun hi t do ngi dng

nh ngha kt lun s hi t ca li gii.

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Hnh 2.8: Chu trnh thc hin m phng vi HFSS

2.3. Thit t cc tham s trong HFSS

2.3.1. Solution Setup

Click phi vo Analysis v chn Add Solution Setup. Ca s Solution Setup s

hin th. Ti y ta s thit t Solution Frequency (hay Adaptive Frequency) v

Convergence Criteria (tiu chun hi t).

General Tab

Solution Frequency

Tn s ny c s dng bi thnh phn chia li thch nghi (adaptive mesher) t

ng chia mt li. Sau khi thit t Solution frequency, tt nhin s nhn c

mt gi tr bc sng tng ng, v HFSS s thit t vic tnh ton cc mt li

theo gi tr bc sng ny. Do , nu thit t tn s Solution frequency cao hn,

ta s nhn c bc sng ngn hn, mt li ta thu c cng nh hn. iu

cng c ngha rng, s lng mt li s ln hn. V l thuyt, kt qu m phng

nhn c s chnh xc hn. V vy, Solution frequency khng nht thit phi

trng vi tn s cng hng ca cu trc.

Tuy nhin, ta s gp mt vn khc l: ti nguyn ca my vi tnh (b nh) l c

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hn. Do , nu s lng mt li qu ln, s lm trn b nh my tnh. Gii php

cho vn ny c thc hin nh sau: chia di tn quan tm (500 MHz 3 GHz),

i vi anten c thit k trong kha lun ny, thnh 5 di tn nh hn v thc hin

phn tch vi thit t Solution frequency cho tng di nh nh sau:

La chn Solution frequency

Hnh 2.9: Thit t cc ty chn Solution Setup trong tab General

Solve Ports Only

Khi s dng ty chn ny, HFSS xc nh tn s hoc mode ban u c s

dng kch thch cu trc. Li gii ch p dng ti cc port c th c s dng

tnh ton gin trng ti port. Li gii ny hu dng khi cn xc nh s lng

mode, di port v/hoc thit lp port hp l trc khi chy Analysis tm ton

500 -- 1000 MHz 1000 MHz

1000 -- 1500 MHz 1500 MHz

1500 -- 2000 MHz 2000 MHz

2000 -- 2500 MHz 2500 MHz

2500 -- 3000 MHz 3000 MHz

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b li gii.

Adaptive Solutions

Maximum Number of Passes: gi tr ny iu khin s lng bc thch nghi ln

nht m th tc chia li thch nghi s thc hin khi n c gng tho mn tiu chun

hi t.

Maximum Delta S Per Pass: gi tr ny nh ngha tiu chun hi t cho qu trnh

chia li thch nghi. Thng thng gi tr ny c chn khong 0.02 hoc 0.01 l

.

Qu trnh chia li thch nghi c th s ngng khi s lng bc thch nghi cha t

n gi tr Maximum Number of Passes nu gi tr Delta S tha mn tiu chun

hi t Maximum Delta S Per Pass c thit lp trn.

Advanced Tab

Initial Mesh Options

Do Lambda Refinement: li khi to l mt h thng mt li c dng t din bt

k. Qu trnh Lambda Refinement s thao tc trn li khi to cho ti khi hu ht

cc di ca phn t mt li xp x bc sng trong khng kh v 1/3 bc

sng trong in mi. Bc sng c tnh ton t gi tr tn s c nhp vo trong

tu chn Solution Frequency trong tab

General

Use Free Space Lambda: tu chn ny s p qu trnh Lambda Refinement hng ti

kch thc mt li xp x bc sng trong khng kh. Cc c tnh vt liu ca

cu trc s b b qua. iu ny c th hu dng trong cc ng dng s dng cc in

mi c dn in cao.

Adaptive Options

Refinement Per Pass: s lng mt li tng ln sau mi bc thch nghi v c

iu khin bi tu chn Refinement Per Pass, tham s ny tnh theo phn trm. iu

ny m bo rng gia mi bc thch nghi, mt li xo trn v m bo rng

bn s khng nhn c s hi t sai.

Minimum Number of Passes: mt phn tch thch nghi s khng dng cho ti khi s

lng bc ti thiu m bn xc nh trong ty chn ny hon thnh, thm ch

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ngay c khi tiu chun hi t tho mn.

Minumum Converged Passes: mt phn tch thch nghi s khng dng tr khi s

lng bc hi t ti thiu m bn xc nh hon thnh. Tiu chun hi t phi

ph hp vi s lng bc ti thiu ny trc khi cc phn tch thch nghi dng.

Thng thng cc ty chn trong tab ny u mc nh l . Nu mun kt qu

chnh xc hn, ta c th thay i cc ty chn ny.

Hnh 2.10: Thit t cc ty chn Solution Setup trong tab Advanced

2.3.2. Mesh Operations

Click phi vo Mesh Operations, chn Assign, chn On Selection, chn Length

BasedCa s Element Length Based Refinement xut hin. Ti y cho php bn

thit t cc ty chn cho qu trnh chia li thch nghi

Maximum Length of Elements

Kinh nghim thc t t m phng cho ta mt nhn xt rng: thit t ty chn

Maximum Length of Elements vi gi tr bng 1/30 gi tr bc sng trong chn

khng tn s cao nht trong di tn thc hin phn tch s cho kt qu chnh

xc.

V d trong di 500 MHz 1000 MHz th tn s 1000 MHz l tn s cao nht trong

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di, bc sng trong chn khng tng ng vi tn s l 300 mm, do 1/30 ca

bc sng ny s l 10 mm.

Gi tr Maximum Length of Elements c thit t cho 5 di tn trong m phng

cho anten trong kha lun nh sau:

500 -- 1000 MHz 10 mm

1000 -- 1500 MHz 7 mm

1500 -- 2000 MHz 5 mm

2000 -- 2500 MHz 4 mm

2500 -- 3000 MHz 3 mm

Hnh 2.11: Thit t ty chn Mesh Operations

2.3.3. Radiation Boundary

Bin bc x (radiation boundary), cng cn gi l bin hp th (absorbing

boundary). Sng in t bc x ra ngoi cu trc anten v i thng ti bin bc x.

H thng s hp th cc sng bc x ny ti bin bc x. Cc bin bc x cng c

th c t tng i gn vi cu trc v c th c hnh dng bt k. Vi cu trc

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c thit lp bin bc x, cc tham s S c tnh ton c tnh n c cc nh

hng ca mt mt do bc x. Khi bin bc x c thit lp trong mt cu trc,

trng xa c tnh ton da trn sng hp th thu c ti bin bc x.

n gin, ta thng v bin bc x l mt hnh hp ch nht, v di cnh ca

n thng c chn bng bc sng trong chn khng ca tn s thp nht trong

di tn quan tm. Tuy nhin, i khi qu trnh m phng vi thit t bin bc x

nh vy din ra qu chm, ta c th thit t bin bc x bng ch gi tr bc

sng trong chn khng ca tn s thp nht. Trong kha lun ny, tn s thp nht

trong di 500 MHz 3000 MHz l 500 MHz, do ta v bin bc x l mt hnh

hp ch nht (hnh 3.1) vi di cnh l 300 mm (1/2 bc sng ti tn s 500

MHz).

Hnh 2.12: Bin bc x cho anten trong kha lun

3. Kt qu m phng vi HFSS 13.0

Vi tiu chun hi t c thit t trn l: s thay i cc i ca bin ca

tham s S phi nh hn 0.02 (gi tr mc nh), HFSS cn 6 bc thch nghi tha

mn tiu chun hi t ny. Hnh 3.1 th hin qu trnh hi t ca li gii

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Hnh 3.1: S hi t ca li gii trong HFSS

th h s phn x S11 (Return Loss) theo tn s cho anten c m phng th

hin trong hnh 3.2.

Hnh 3.2: th S11 cho anten

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Hnh 3.3: th VSWR cho anten

Vi mt mt do phn x (Return Loss) S11 = -8 dB (tng ng vi h s sng

ng VSWR=2.5), ta thy anten khng c nhnh th 3 cng hng gn cc tn s

900MHz v 2200MHz.

Tip theo ta xem xt cc gin trng bc x c a ra bi HFSS. y,

ta ch quan tm ti gin bc x trng xa trong cc mt phng ta XOY, XOZ

v YOZ.

Hnh 3.4: Gin bc x trng xa trong mt phng XOY

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T cc hnh 3.4, 3.5 v 3.6 ta thy, ti tn s 870 MHz, anten bc x c

hng trong mt phng XOY v XOZ, bc x v hng trong mt phng YOZ. Tuy

nhin, ti cc tn s cng hng cao hn th gin bc x trong c ba mt phng b

mo dn so vi ti tn s cng hng 870 MHz.

Hnh 3.5: Gin bc x trng xa trong mt phng XOZ

Hnh 3.6: Gin bc x trng xa trong mt phng YOZ

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Hnh 3.7: Gin bc x 3D trng xa trong h ta cc ti tn s 870 MHz

Hnh 3.8: Gin bc x 3D trng xa trong h ta cc ti tn s 2160 MHz

Hnh 3.9: Gin bc x 3D trng xa trong h ta cc ti tn s 2380 MHz

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T cc gin bc x trn ta thy, khi tn s tng ln th gin bc x ca anten b

bp mo dn, do nh hng ca bc x ca mt phng t, cng nh bc x do

ng tip in vi di, cng nh s lch phi hp tr khng tng ln.

4. Kt lun

Kha lun l bc m u trong nghin cu, thit k v ch to anten mch di

bng rng c kh nng hot ng ti nhiu bng tn. Tuy nhin trong iu kin c s

vt cht cn kh khn, em thc s c gng t c mt s kt qu thit thc

nht nh.

Hng pht trin tip theo ca kha lun gm nhng vn sau:

Ti u ha cc thit t tham s trong phn mm m phng Ansoft HFSS

13.0 thu c kt qu chnh xc hn na (Chi tit trnh by trong phn

ph lc B). Mt s tham s quan trng trong l:

o Mesh Operations

o Chia di tn cn quan st thnh cc di nh hn, thc hin phn tch

tng di vi tham s Solution frequency c chn ph hp cho tng

di.

Lm tng bng thng thm na. Tp trung vo vic iu chnh kch thc ca

nhnh cng hng th 1, v tr ca im tip in, v nghin cu chi tit cc

nh hng ca nhnh iu chnh (nhnh th 3).

La chn b phi hp tr khng di rng khc c c tnh tt hn. C th l

b phi hp tr khng lin tc Klopfenstein (nh chng 2 phn tch).

S dng cc thit b chuyn dng ch to anten nhm thc hin chnh xc

cc kch thc nh thit k.

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TI LIU THAM KHO

1. Constantine A. Balanis, Antenna Theory Analysis and Design, John

Willey & Son, INC, Second Editon

2. David M. Pozar, Microwave Engineering, John Willey & Son, INC,

Second Editon

3. Y. J. Wang, C. K. Lee, Design of Dual-Frequency Microstip Patch

Antennas and Application for IMT-2000 Mobile Handsets, Nanyang

Technological University, Nanyang Avenue, Singapore

4. Xu Jing, Zhengwei Du and Ke Gong, Compact Planar Monopole Antenna

for Multi-band Mobile Phones, Tsinghua University, Beijing, Peoples

Republic of China

5. Ramesh Garg, Prakash Bhartia, Inder Bahl, Apisak Ittipiboon, Microstrip

Antenna Design Hanbook, Artech House

6. U.S. Marine Corps, Field Antenna Handbook

7. Chin Liong Yeo, Active Microstrip Array Antennas, Submitted for the

degree of Bachelor of Engineering, University of Queensland