JOLRNAL OF S C I E N C E * TECHNOLOGY * > o . » J B - i U i i
A NOVEL CO.MPACT MICROSTRIP DIPOLE .ANTENNA FOR BLUETOOTH/WLAN MOBILE TERMINALS
MO HINH ANTEN L U O N G CUC VI DAI MOl
CHO CAC THIET BI DAU CUOI DI D O N G SU' DUNG CONG NGHE BLUETOOTH W LAN Nguyen Khac Kiem, Dao Ngoc Chien
Hanoi University of Science and Technology-
ABSTRACT
A novel compact microstrip d'ipole antenna operating at 2.45 GHz for Bluetooth/WLAN applications is presented in this paper Two techniques are applied to traditional microstrip dipole antenna model to Increase the electrical length, including the use of capacitive load at the terminals and the meandered-line on each arm of the dipole. As a result, the proposed antenna has size reduction up to 26% of total size In comparison with the traditional model, and the bandwidth of thii:
dipole antenna spreads from 2.37 to 2.53 GHz (6.53%), which covers the 2.402 - 2.480 GHz range of the ISM (Industrial, Scientific, and Medical) band. The antenna with optimal parameters is fabricated and measured. The results show a good agreement between simulation and measurement, and hence confirm the feasibility of the proposed antenna In practical applications.
TOM TAT
Mdt md hinh mdi ciia anten Iwong cwc vi dii kich thwdc nhd, boat ddng d viing tin sd 2,45 GHz cho dc thiit bi di ddng sir dung cdng nghe truyin din vd tuyin Bluetooth hay WLAN dwgc trinh biy trong bai bio nay. Hai ky thuit ting dd dii dien dwgc ip dung vio md hinh anten Iwong ci/c vi dii truyin thing bao gdm viec sir dung tal thuin khing tai diu cudi vi tao dwdng gip khiic trin tirng nhinh cua twang cwc. Kit qui li md hinh anten di xuit cd kich thwdc nho hon 26%> so vdi md hinh anten truyin thing, vdi bang thdng trii tir 2,37 din 2,53 GHz (twang dwong 6,53% tin si trung tim) bao triim bing tin khdng phii dang ky tir 2,402 din 2.480 GHz cho dc irng dung cdng nghiep, khoa hoc vi y ti Md hinh anten vdl cic thdng sd tdi wu dwgc chi tao vi do dac thwc nghiem. Cic kit qui tinh toin md phdng vi do dac thwc nghiem thi hien sw ding nhit, qua dd khing dinh tinh kha thi cua md hinh anten mdi khi triin khai vio dc irng dung thwc ti.
I. INTRODUCTION
Bluetooth'W L.*\N is an open wireless technology standard for exchanging data over short distances (using short length radio waves) from fixed and mobile dev ices, creating personal area networks (PANs) with high levels of security. Bluetooth uses a radio technology called frequency-hopping spread spectrum, vvhich chops up the data being sent and transmits chunks of it on up to 79 bands of I MHz width in the range from 2402 to 2480 MHz. This is in the globally unlicensed Industrial, Scientific and Medical (ISM) 2.4 GHz short-range radio frequency band. In recent vears, design of antennas that are suitable for mobile devices supporting Blueloolh, W I ..AN technology has been become the uruent demand.
To satisfy these demands, many antenna configurations have been proposed, such as the planar monopole antenna, microstrip patch antenna [1], microstrip dipole antenna [2-4], etc.
Among them, microstrip dipole antenna is more interested by advantages as compactness, light- weight, low cost, and isotropic radiation pattem.
Microstrip dipole antenna consists of two narrow microstrip conductors on one side of the substrate, vvhich are separated by a feeding gap.
These antennas have been applied in communication devices such as notebook computer and access point for WLAN operations [5.6].
In this paper, the techniques using capacitive load at terminals [7] and meandered- line on each arm of the dipole [8] have been applied to the traditional microstrip dipole antenna. By using the capacitive load at the 30
JULKNAL OF S C I E N C E * TECHNOLOGY * No. 8 3 B - 2 0 I I
terminals of the dipole, distribution of electric current can be adjusted, and hence the terminal impedance has a finite value and the terminal current will not equal to zero. This means makes the distribution of current of the dipole would like to be prolonged. With the second technique, the meandered-line is introduced by a number of slotted-pairs on each arm to increase electrical length of the dipole while still to maintain physical length. By combining these two techniques, the overall size of the antenna has been significantly reduced in comparison with the traditional dipole model, finally, an optimal design has fabricated and measured to validate the performance as well as the feasibility of the antenna in practical applications.
The remainder of this paper is organized as follow. In Section II, the detailed design of the microstrip dipole antenna is presented.
Simulation and measurement results are shown and discussed in Section III while conclusions are presented in Section IV.
II. ANTENNA DESIGN
(a)
(b) Figure 1. (a) Traditional microstrip dipole antenna, lb) Field transition from TFM mode to dipole antenna.
The configuration of the tradition microstrip dipole antenna is shown in Fig. 1(a).
I he antenna is printed on a ER4 substrate with a dielectric constant e, = 4.4, a thickness h = 1.6 mm, and a tangential loss tanS = 0.025. Two narrow rectangular microstrip conductors are copper and are printed on the top of the substrate. Each of them is connected with a
microstrip bend. The width of gap between Uvo arms is g. The dipole is fed through a feeding line with length of l_f by a 50-n SMA connector, in which one arm is connected to inner pin and the other is connected to outer shell. Fig. 1(b) represents field transition from TEM mode inside coax cable to the dipole antenna. By adjusting the size of microstrip bend w_c, impedance matching can be obtained.
The microstrip dipole antenna, as shown in Fig. 1(a), can be designed for the lowest resonant frequency using transmission line model. It is well-known that the effective dielectric constant (Ecfr) of a microstrip line is given bv [9]
Eeff •
Ep H- 1 Er 1
2 7 1 4- 12h/W (1) So, the length of microstrip line L can be calculated through
l.o=-
fjE, (2)
eff
where c is velocity of light, f is resonant frequency, and X^ is effective wavelength. The length of each arm is about quarter-wavelength, so the total length of the dipole antenna is about half-wavelength. With the parameters f = 2.45 GHz. E, = 4.4, h = 1.6 mm, so W, g, If, d, and vv_c are chosen of 2.5 mm, 1.22 mm, 5 mm, I mm, and 1.5 mm, respectively. In this case, the length of rectangular arm L is ofXJA ~ 18 mm.
Fig. 2 shows (a) the dipole antenna structure having capacitive loads at the terminals of the dipole, and (b) the equivalent model.
In this case, the distribution of current on the dipole can be determined by an approximate method, in vvhich the dipole is modeled as parallel wires with capacitive loads at the temiinals. If the function of current is given bv
l(z) = IcCos(kz)-l-i —sin(kz) (3) P
where L and Uc represent current and voltage at the tenninal. respectively, while p is the characteristic impedance of the dipole. Here,
U , : ioC (4)
JOL RNAL OF SCIENCE* TECHNOLOGY * > o . » J B - . : u i l
So that. (3) can be rewritten in follow form l(z) = I(.(cos(kz) H -sin(kz) ) (5)
V pcoC /
here C represents capacitance of the load.
If vve define f Asinv|/ = 1
(6)
(7) Acosvi/ = ——
fflCp
So I(z) = cAsin(kz -F \y)
' • • - ' "
T~^
1. •
'[^
\
t: *
L
" r ''•'ll
W I'-''
t
i
(a)(b) Figure 2. Dipole antenna hcn-ing capacitive loads at terminals tai and equivalent model (b).
From (7), the distribution of current on the dipole using capacitive loads at the terminals has the sine-shaped, but the nodes of current are not at the terminals of the dipole (z
= 0). It is shifted to a position that is determined bv kz = - y . This means that the capacitive load can be replaced by a long line equivalent having electrical length kf, = if. As a result, physical length of the dipole can be shortened while still can maintain the characteristics of radiation of the antenna.
Fig. 3 shows the optimal design of the microstrip dipole antenna, which adopts two techniques of using the capacitive loads at the temiinals and the meandered-line introduced by a number of slotted-pairs on each arm of the dipole to significantly reduce the overall size of the antenna.
Figure 3. Novel microstrip dipole antenna using capacitive load and meandered-line.
Table 1. Design specifications of different dipole antenna structures.
Antenna tvpe
TDA
DAC
DAM
Design parameters W sub = 50. L sub = 15. w = 2.5.
L= 18.6.g= 1.221,d= 1, w c = 1.5. l_f= 5
W sub = 43. L sub = 15, vv = 2.5, L= 13.6,g= 1.221,d= 1, u c = 1.5. 1 f = 5. 11 = 1, 12 = 10, wl =0.5, w2 =0.5
W sub = 37, L sub = 15, w = 2.5, L-- 11.5,g= 1.221,d= 1, w c = 1.5, 1 f = 5, 11 - 1, 12 = 10, ul =0.5, w2 = 0.5, s = a = 0.5 III. RESULTS AND DISCUSSIONS
Fig. 4 presents simulated and measured results of input return loss (RL) (Sn is always referred as RL) of the TDA. It can be seen that the bandwidth of this antenna (Sil < -10 dB) covers the ISM band (2.402-2.480 GHz) well.
The TDA is then modified by adding two vertical strips with length of 12 at the end of dipole's arms to produce a DAC. In order to investigate the effects of capacitive load on performance of the antenna, the length of the dipole L is fixed at 18.6 mm while that of vertical strips 12 takes different values of 4 mm, 6 mm, 8 mm. and 10 mm. Simulated results of RL are shown in Fig. 5. of which the results of bandwidths are shown in Table 2. It can be seen that the longer length of 12 produces the lowei resonant frequency. Also from this simulation, the bandwidth of the antenna does not alter so much, ranging from 11.64 to 12.63 percent of center frequency.
• - , 1
JOLRNAL OF SCIENCE* TECHNOLOGY * No. 83B-201I
^ ^ ^
. -
' Simulation Measured
(
\ ^ V / '
V / ' \» / ' \» / '
V / 'T f -4
-6
3 . 1 0 •
7 "2
-u
- 1 6 •
-18 -20
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Frequency (GHz|
Figure 4. Return loss of traditional dipole antenna with L = 1H.6 mm.
NvN
'. \ \ \
• • . \ \ \ / •
• \ V Y / /
•.vV\7 •..v»,\/
^ ^ ^ 12 = 4 m m
— — • 12 = 6 m m 12 •- « m m "
• • 1 2 - 1 0 m m •
iTCtiucncy ((^11/)
Figure 5. Return loss of DAC antenna with different 12 and fixed L ^= IS.6 mm.
Table 2. Results of DAC antenna with different 12.
12 (mm)
4 6 8 10
Resonant frequency (GHz)
2.258 2.1')2 2.123 2.063
Bandwidth
(%)
12.62 11.98 11.66 11.64
Fig. 6 presents the results of the DAC antenna for different 12 and L. As can be seen, the antenna with 12 = 10 mm and L = 13.6 mm has good enough bandwidth of 10.41% and small si/e. and hence to satisfv the goal of size reduction. 12 and L are chosen as 10 mm and 1 ' 6 mm. respectively, for the next processes.
In order to further increase electrical length oflhe D.AC, a number of pair of slits N i-. etched in each arm of the dipole. as shown in I Ig. 3. \ slit is with width s and length W a .
I he antenna is now named D.AM. I ig. 7 shows input RI. oflhe I),A\I antenna with different N.
- - 1 0 - 1 2
. -
•
-
— ™
— — ^^—
112 = 4 m 12 = 6 m 12 = Sm P - lOn
,
T 1
Js
Tiand L = 16 4 mm Tl and L = 15 5 mm T i a n i l L = 14 6 mm m and L = 13 6 mm
^
\ ^
<^
V.7
' 1
•
;
^ ^ ^ •
.
1
frequency (GHz)
Figure 6. Results of input RL of D.AC antenna with different 12 and L.
As can be seen, the increase of number of pair of slits allows moving the resonant frequency down to lower frequency range, i.e., the electrical length of the antenna is increased while the physical size of the antenna does not change.
Results of input RL of the DAM antenna with different length L and number of pair of slits N. vvhich are designed to achieve resonant frequency at 2.45 GHz, are shown in Fig. 8. As can be seen, although the electrical length of the arm of dipole almost does not change by shortening its physical size but increasing number of pair of slits, the bandwidth of the D.A\1 antenna is reduced due to narrow arms.
2.2 2.4 Frequency (GHz)
Figure Input RL of the DAM antenna with different number of pair of slots for 12 = 10 mm and L = 13.6 mm.
Fig. 9 presents a prototyped DA.\1 antenna with L = 11.5 mm, 12 = 10 mm, and N
= 3, whose simulated and measured results of input RL are shown in Fig. 10. From Fig. 10, it is seen that the bandwidth of DAM antenna spreads from 2,37 GHz to 2.53 GHz. covering the frequency range from 2.402 to 2.480 GHz for ISM applications.
JOLRNAL OF SCIENCE* TECHNOLOGY * No.83B-201I
-2 -4 -6 -8 10 12 14 16
~ ~ '•
~
L - L = L -
13,2 12.6 11,5
mm mm mm
1
and \ and N and N
^ Y
- 1
= 7
• 5
\
Vk
\\
- \ l -
1 1 I 1
/
'i'i -Iff-
, '
•
'
. '
2.0 2.2 2.4 2.6 Frequency (C;Hz)
Figure 8. Input RL of DAM antenna with different length L and number of pair of slits N.
Figure 9. Prototyped DAM antenna with L=II.5 mm. 12 = 10 mm, and N = 3.
Simulated radiation patterns of the final DAM antenna in E-plane and H-plane at 2.45 GHz are plotted in Fig. 11 (a) and (b), respectively. From the figures, it is observed that the DAM antenna has omni-directional radiation property. Simulated results of peak gain of the DAM antenna are shown in Fig. 12.
A relative fiat-gain property can be observed from Fig. 12.
^^ ^^^
•
X"'/
•
^ ^ Simulation
^ ^ ^ ~ Measured
.
•
•
2.(1 2.1 2.2 2 J 2.4 2.5 2.6 2.7 2.8 2.9 3.0 Frequency (GHz)
Figure 10. Input RL of the optimal DAM antenna shown in Fig. 9.
\\. CONCLUSIONS
By implementing the capacitive loads at the temiinals and the meandered-line on each arm of traditional microstrip dipole antenna, a novel compact microstrip dipole antenna for
Bluetooth,'W LAN mobile applications operating at 2.45 GHz frequency band, has beei proposed. The physical size of the antenna ha:
been significantly reduced up to 26% ii comparison with the traditional inicrostrii dipole antenna.
From the simulated and measured results it is apparent that the proposed antenna can be integrated in small size mobile phones as wel as other mobile devices such as notebooks.
PDA, etc.
(a)
(b) Figure II. Radiation patterns of the final DAM antenna in fa) E-plane and (b) H-plane.
F r e q u e n c y ( G l l ^ )
Figure 12. Simulated peak gain of the DAh antenna.
34
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REFERENCES
1. L. Lu and J. C. Coetzee, "Reduced-size microstrip patch antenna for Bluetooth applications,' Electronicsl.ett., vol. 41,no. 17, pp. 944-945, Aug. 2005.
2. M. H. Jamaluddin, M. K. A. Rahim M. Z. A. Abd. .Aziz, A. Asrokin, "Microstrip dipole antenna for
\\ LAN application,'" 1st Intemational Conference on Communications and Signal Processing, 2005.
3. R. Grag, P. Bhartia, I. Bahl, and A. Itlipiboon. Microstrip Antenna Design Handbook, Artech House Inc., Norwood, USA (2002).
4. Zhijun Zhang. M. F. Iskander, J. C. Langer, J. Mathews, "Wideband dipole antenna for WLAN,"
IEEE Antennas and Propagation Society Symposium 2004, vol. 2, pp. 20-25, Jun. 2004.
5. C.-M. Su, H. Twuchen, and L-L Wong, "Printed dual-band dipole antenna with U-slotted arms for 2.4/5.2 GHz WLAN operation," Electron. Lett., vol, 38, pp. 1308-1309, May 2002.
6. H.-R. Ch., Liang-Chen Luo, Chi-Chang Lin and Wen-Tzu Chen, "A 2.4 GHz polarization diversity planar printed dipole antenna for WLAN and \\ ireless Communication Applications,'' Microwave Journal, vol. 45, no. 6, pp. 50-62, Jun. 2002.
7. Phan Anh, Antenna Theory and Techniques, Science and Technology Publishing House, Hanoi, Vietnam.
8. L. H. Truong and D. N. Chien, "A novel dual-band antenna for WLAN Applications," in Proc. of International Conference on Communications and Electronics 2006, vol. I, pp. 434-436, October 2006 (Hanoi, VN).
9. David M. Pozar, Microwave Engineering, John Wiley & Sons Inc. 1998.
Author's address: Dao Ngoc Chien - Tel: (-1-844) 3869.2242, Email: chiendn-fet(gmail.hut.edu.vn Hanoi University of Science and Technology
No. 1, Dai Co Viet Str., Hanoi, Vietnam
