ULTRA-COMPACT WIDEBAND ANTENNA FOR PORTABLE
DEVICE APPLICATIONS
Ahmed Toaha Mobashsher1, Norbahiah Misran1, Mohammad Tariqul Islam2
1Dept. of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 2Institute of Space Science (ANGKASA), Universiti Kebangsaan Malaysia,
Bangi, Selangor, Malaysia
Key words: Compact antennas; portable device; wideband antenna.
Abstract: An ultra-compact broadband antenna easily integrable on portable devices is presented. The antenna is composed of a printed rectangular radiating element and a C-strip printed as the ground plane on the other side. The overall antenna occupies a compact area of 56.5 x 2 x 0.5 mm3, yet generating two resonant modes to form 39% (850MHz) wide impedance bandwidth from 1.75 to 2.6GHz with stable omni-directional radiation pattern with low cross-polarization.
Ultra kompaktna širokopasovna antena za uporabo v
prenosnih napravah
Kjučne besede: kompaktne antene, prenosne naprave, širokopasovna antena
Izvleček: Predstavljena je zelo kompaktna širokopasovna antena, ki se jo zlahka namesti v prenosne naprave. Sestavljena je iz tiskanega pravokotnega sevajočega elementa in ozemljitvijo v obliki tiskanega C-traku na drugi strani tiskanine. Celotna antena zavzema površino 56.5 x 2 x 0.5 mm3, deluje v dveh resonančnih načinih, s pasovno širino 39% (850MHz) na frekvenčnem pasu od 1.75 do 2.6GHz s stabilnim večsmernim vzorcem sevanja in nizko navzkrižno polarizacijo.
1. Introduction
In the era of modern wireless communication systems, wideband or multi-band antennas with omni directional radiation characteristics play a vital role. Increasing demand for smaller size in portable devices results in a need for effective antenna miniaturization which is a critically challenging problem due to the inevitable trade-off between the size and the performance of the antenna due to the fact that antenna performance is bound with the fundamental limits based on the size of the antenna. This is especially true in the field of radio communications, where reducing the size of an antenna leads to smaller and light-weight systems, thereby enhancing portability and minimizing electromagnetic interference with other electronic devices. Therefore it is very desirable to miniaturize the antenna in order to scale down the system size.
Conventional internal antennas for portable device applications are generally in the forms of the planar inverted-F patch antenna, very-low-profile printed or metal-plate monopole antenna, and so on [1-3]. But such internal mobile antennas usually excite large surface currents on the system ground plane of the mobile phone, which functions as an effective radiation portion. Owing to the large excited surface currents on the system ground plane, especially in the region near the internal antenna, an isolation distance of
about 7mm or larger between the antenna and the nearby conducting elements or electronic components in the mobile phone is usually required to avoid large degradation effects on the performances of the internal antenna[4,5]. This is a big limitation for the portable devices.
To eliminate the effect of large ground plane this paper presents an ultra-compact antenna for mobile applications in DCS-1900/PCS/PHS (1850-1990MHz), WCDMA/ IMT-2000 (1920-2170MHz), wireless broadband internet (WiBro) (2.3-2.39 GHz), WLAN-IEEE 802.11b (2400-2483MHz) bands. The antenna comprises a rectangular radiating element and a C-strip printed as the ground plane. The antenna has the advantages of microstrip feeding and lack of big ground plane makes it suitable for co-existing integration on printed circuit board (PCB). Details of the antenna design and results are presented and discussed.
2. Antenna configuration
The configuration of the compact planar antenna is illustrated in Figure 1. The design of the antenna is based on an RT/Duroid R04350B planar substrate that has a permittivity of 3.45 and a thickness of 20mils (0.5mm). The total dimension of the antennas is 56.5 x 2 x 0.5. It is also noticed that the antenna size at the lowest frequency
ranging from 1.75GHz to 2.6GHz, or about 39% with respect to the centre frequency at 2.18GHz.
Fwi (a)
(b)
W (c)
Fig. 1:
Schematic Diagrams of proposed antenna (a) top view, (b) side view
(1.75GHz) is 0.33 1o x 0.012 1o x 0.003 1o. In one side of the antenna the rectangular radiating patch fed by a microstrip line of 0.4mm is designed. On the other side a C-strip acting as the ground plane. The feeding microstrip line is positioned at the middle of the antenna, while the rectangular patch is fed at the lower corner, which makes it only 1.5mm wide. The compactness of the planar antenna is attributed to the slim structure of the radiation element and the folded configuration of the C-strip, while the broad band performance is a result of the mutual coupling between the rectangular radiating patch and C-strip. However the tapped pad is added to enhance the lower limit of the operating frequency. The geometrical parameters of the antenna are shown in mm in the Table 1.
Table 1: Geometrical parameters of the slot-type antenna
L1	25	h	0.5
L2	31.5	W	2
L3	16.5	Wp	1.2
Fw1	1	Fw2	0.4
3. Results & discussions
Generally, a 6-dBreturn loss is acceptable for the mobile phone applications [2]. The return loss of the antenna is exhibited in Figure 2. From the curve it is apparent that the antenna achieved -6dB return loss bandwidth of 850MHz,
Fig. 2: Return loss of the designed antenna
The E and H plane radiation patterns of the proposed antenna at 1.85GHz and 2.4GHz has been shown in Figure 3. It can be realized that the antenna produces omnidirectional symmetrical radiation pattern in both the resonating frequencies. Moreover, a 30dB cross-polarization is observed in both E-and H-planes.
(a)
(b)
Fig. 3: Simulated radiation pattern of the proposed antenna at a) 1.85GHz, b) 2.4GHz
Fig. 4: Antenna gain of the proposed antenna (a) lower band and (b) upper band
Figure 4 depicts the peak gain and antenna efficiency of the proposed antenna. With a maximum gain of 1.9dBi, the antenna provides >0dBi over the whole resonating band of the antenna. The antenna efficiency takes into account the reflection losses, that is to say na = nr ^ (1-|S„ |2), where nr is the radiation efficiency. A conductivity of a = 5.8 x 107 S/m for copper and a loss tangent of tanS = 0.0036 for the low loss Duroide dielectric substrate were used in the simulation. A good antenna efficiency of 80% over operating band is observed for the antenna
4. Conclusion
The design of a low-profile microstrip-fed printed antenna with extremely small width, operating in the DCS-1900/ PCS/PHS, WCDMA/IMT-2000, WiBro, WLAN-IEEE 802.11b bands, has been proposed in this paper. The design is simple and compact with very few design parameters. The proposed antenna has small dimensions of 56.5 x 2 x 0.5 mm3 suitable for integration with application specific circuits. The present design stands out as a potential candidate for portable applications.
References
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/3./ H.-W. Hsieh, Y.-C. Lee, K.-K. Tiong, and J.-S. Sun, "Design of a multiband antenna for mobile handset operations," IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 200-203, 2009.
/4./ K. L. Wong, S. W. Su, C. L. Tang, and S. H. Yeh, "Internal shorted patch antenna for a UMTS folder-type mobile phone," IEEE Trans. Antennas Propag., vol. 53, pp. 3391-3394, Oct. 2005.
/5./ S. W. Su, K. L. Wong, C. L. Tang, and S. H. Yeh, "Wideband monopole antenna integrated within the front-end module package," IEEE Trans. Antennas Propag., vol. 54, pp. 1888-1891, Jun. 2006.
Ahmed Toaha Mobashsher1, Norbahiah Misran2 Dept. of Electrical, Electronic and Systems Engineering Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 1i_toaha@yahoo.com, 2bahiah@vlsi.eng.ukm.my
Mohammad Tariqul Islam3 Institute of Space Science (ANGKASA) Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 3titareq@yahoo.com
Prispelo: 07.09.2010
Sprejeto: 23.08.2011