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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
Low-SAR, Miniaturized Printed Antenna for Mobile, ISM, and WLAN Services K. S. Sultan, H. H. Abdullah, E. A. Abdallah, and E. A. Hashish
Abstract—Recently, the mobile handsets support both the mobile and wireless LAN services. Thus, our target is to introduce a new antenna that supports both services. The antenna operates for most of the mobile applications such as the GSM 850, GSM 900, DCS 1800, PCS 1900, UMTS 2100, and most of the LTE bands, especially the low-frequency LTE 700 band. The antenna also supports the WiMAX, wireless local area network (WLAN), and the Industrial, Scientific, and Medical (ISM) bands. The antenna not only has a compact size, but also it supports a low specific absorption rate (SAR) radiation at all the operating frequencies. It consists of a monopole, a meander line, and an electromagnetic band-gap (EBG) structure. In order to cover the low frequency bands, a meander line is utilized due to its compact size, but unfortunately it is a narrowband antenna that is difficult to cover both LTE700 and GSM 900 bands. The solution stems from the use of an EBG structure that widens the band to cover the two low frequency bands and to reduce the maximum SAR. The higher frequency bands are supported by both the monopole and the meander line since they acts as traveling-wave antennas at the high frequency bands. The antenna meets three challenging parameters: the compact size, the multiband operation including the low frequency bands, and the low SAR radiation. Good agreement is found between the experimental and the simulated results. Index Terms—Electromagnetic band-gap (EBG), Industrial, Scientific, and Medical (ISM), , Long Term Evolution (LTE), meander, monopole, specific absorption rate (SAR), wireless local area network (WLAN).
I. INTRODUCTION
W
ITH the rapid growth of communication technologies, mobile communications require a mobile phone to be operated in various communication services. This has led to a great demand for designing antennas with some desirable features such as multiband operation, low specific absorption rate (SAR), light weight, and low profile [1]–[9]. Nowadays, the fourth generation of mobile communications, the Long Term Evolution (LTE), is expected to deliver multimedia services anywhere, anytime. The LTE standard is scheduled to operate in different frequency bands that range from 400 MHz to 4 GHz with bandwidths of 1.4 and 20 MHz [1], [2]. The continuous growth of wireless mobile services has forced the Manuscript received July 13, 2013; accepted August 06, 2013. Date of publication September 05, 2013; date of current version September 19, 2013. This work was supported by the National Telecom. Regulatory Authority (NTRA), Ministry of Communications and Information Technology (MCIT). K. S. Sultan, H. H. Abdullah, and E. A. Abdallah are with the Electronics Research Institute, Giza, Egypt (e-mail:
[email protected]). E. A. Hashish is with the Electronic and Communication Department, Faculty of Engineering, Cairo University, Giza, Egypt. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2280955
worldwide mobile handset manufacturers to consider the mutual interactions between the mobile terminals and the human body. While part of the electromagnetic wave radiated by the antenna is absorbed by the human head, some mobile handset antenna characteristics, such as radiation pattern, radiation efficiency, bandwidth, and return loss are altered due to the proximity of the human head. The mutual effects of the human head and the antenna have been introduced by many research works [10]–[17]. Through the last years, different methods to reduce the SAR produced by a handset antenna were used—specifically, auxiliary antenna elements, ferrite loading, the electromagnetic band-gap (EBG)/artificial magnetic conductors (AMCs) surfaces, and metamaterials [15]. In [16], a combination of the main antenna and a director or a reflector was introduced to increase the effective radiation efficiency and to reduce the SAR. The disadvantage of the method is that a separate antenna element is needed, resulting in increasing the size and cost of the antenna. In [17], a ferrite sheet was proposed to be used as a protection attachment between the antenna and the head. The drawback of the technique is the use of an expensive ferrite material that has special properties of permittivity and permeability to achieve low SAR [15]. In [13]–[15], the EBG and metamaterial techniques are used due to their options. The EBG technique reduces the antenna SAR up to 75% [15]. On the other hand, extensive research efforts are exerted in minimizing mobile handset antennas in size and cost in conjunction with increasing the services provided by the antenna [11]–[15]. Quad-band antennas that cover the GSM 900, DCS 1800, PCS 1900, and the Industrial, Scientific, and Medical (ISM) 2450 bands were introduced [3], while Ciais et al. [4] replace the ISM 2450 band covered in [3] by the UMTS 2100 band at 6-dB bandwidth. This antenna has a size double that of the antenna in [3]. Tzortzakakis et al. [5], Ku et al. [6], and Tang et al. [7] introduced two different compact antennas that cover the quad band with a noticeable reduction in size compared to [3]. Zhang et al. [8] introduced another type of antenna that consists of a folded loop inverted-F that covers the heptaband: GSM850, GSM900, GSM1800, GSM1900, UMTS, GPS, and wireless local area network (WLAN) (at 6-dB bandwidth) with slight size increase of the proposed antenna in [7]. With the advent of the LTE services, Bhatti et al. [2] introduced a compact size antenna that covers the LTE 700 band. Recently, Young et al. [9] introduced an octaband antenna with more mm . The antenna operating bands compact size of are the LTE 700, GSM 850, GSM 900, DCS 1800, PCS 1900, WCDMA 2100, LTE 2300, and the LTE 2500 bands. Although the antenna proposed by Young et al. [9] has a compact size
1536-1225 © 2013 IEEE
SULTAN et al.: LOW-SAR, MINIATURIZED PRINTED ANTENNA FOR MOBILE, ISM, AND WLAN SERVICES
and covers octabands of the operating frequencies, still many bands need to be covered. In addition, it consists of multilayers that complicate the fabrication process. In this letter, a novel internal antenna consisting of a monopole with a meander line and an EBG structure embedded on the bottom layer of the substrate to reduce SAR and to cover multibands including the LTE bands is introduced. The proposed antenna at 6 dB has a bandwidth extending from 587 to 977 MHz and from 1.67 to 8.63 GHz, which means that it supports the following operating bands: GSM 850, GSM 900, DCS 1800, PCS 1900, UMTS 2100, ISM 2450, most LTE bands, WiMAX (2.3–2.4, 2.5–2.69, 3.3–3.8, 3.4–3.6, and 5.1–5.8 GHz), and WLAN (2.4–2.5, 4.8–5,4.825–5.515, 5.425–5875, and 5.125–5.875 GHz), with a size of mm . The letter is organized as follows. Section II explains the antenna design without EBG and describes the antenna performance. Section III shows the antenna design with EBG and describes the antenna performance together with a comparison between the simulated and the experimental results. In Section IV, the SAR results are introduced. Finally, Section V presents the conclusions for this research.
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Fig. 1. Geometry of the proposed antenna with EBG. (a) Front view. (b) Back view. (c) Unit cell of EBG.
II. ANTENNA DESIGN WITHOUT EBG The proposed antenna is a planar printed antenna with compact dimensions of mm before applying the EBG structure. The antenna is designed over FR4 substrate with 0.8 mm thickness and loss tangent of 0.025. The proposed antenna is composed of a planar monopole and a planar meander line. The monopole antenna is an inverted-L shape. The electrical length of the monopole is a quarter-wavelength at 2350 MHz. The monopole operating bands are (1700–3000) and (4600–5500) MHz. The dimensions of the monopole antenna are mm . In addition, the meander line increases the path over which the surface current flows, and that eventually results in lowering the resonant frequency. The electrical length of the meander line is optimized to resonate at 900 MHz (860–1020 MHz). The optimized length of the meander line is 111 mm. It is worth noting that the meander line operates also at higher frequencies as a traveling wave antenna. The combination of the monopole and the meander line contributes to open the higher bands to operate from 1.675 up to 8.15 GHz. The separation between the monopole and the meander line is 1 mm, which controls the matching condition. A ground plane of mm area is chosen to be coplanar with the radiating elements. It is worth mentioning that the length of the ground plane has a negligible effect on the performance. III. ANTENNA DESIGN WITH EBG When EBG structures interact with electromagnetic waves, they show amazing properties such as frequency passbands, stopbands, or band-gaps. The characteristics of the EBG structure shown in Fig. 1(b) and (c) are tested by a microstrip line over one column of the EBG unit with a 0.5-mm gap. The fabricated antenna is shown in Fig. 2. As shown in Fig. 3, the proposed EBG configuration reveals stopbands at most of the mobile applications bands. This means that it has high surface impedance within these bands, where the tangential magnetic
Fig. 2. Photograph of the fabricated antenna. (a) Front view. (b) Back view.
Fig. 3. Transmission characteristic of ground EBG.
field is small, even with a large electric field along the surface. The EBG structure is positioned perpendicular to the two antennas, the monopole, and the meander line. With the existence of the EBG, the EBG structure acts as an artificial magnetic conductor, AMC, within its stopbands. The AMC enhances the radiation in the direction opposite to the position of the human body. Thus, it lowers the SAR absorbed within the human tissues. The radiation pattern in the absence and in the presence
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Fig. 4. Radiation pattern in the -plane.
IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013
- and
-planes. The antenna is in the
Fig. 6. Simulated and measured return loss of the proposed antenna.
PARAMETERS
VALUES
Fig. 5. Radiation pattern in the -plane.
and
-planes. The antenna is in the
OF THE
OF THE
TABLE I PROPOSED ANTENNA (ALL DIMENSIONS MILLIMETERS)
TABLE II GAIN AND THE RADIATION EFFICIENCY PROPOSED ANTENNA
IN
OF THE
the antenna covers all the aforementioned mobile and wireless applications bands. Taking the 6-dB return-loss reference, the antenna operates in the two bands (587–977 MHz) and (1.67–8.63 GHz). Table II shows the gain and the radiation efficiency of the proposed antenna. IV. SAR CALCULATIONS
of the EBG is shown in Fig. 4 at 1.8 GHz. It is evident that the EBG enhances the radiation in the elevation plan. The 1.8 GHz is chosen as a sample, but the same notice is observed for all the mobile bands. The EBG not only lowers the SAR, but it opens the operating bands to include the LTE 700 band. Fig. 5 shows the measured and simulated radiation patterns at frequencies 0.9, 1.8, and 2.1 GHz. Radiation pattern measurements were carried out using SATIMO Anechoic antenna chamber where the available frequency range starts from 0.8 GHz. By simulating the antenna with the EBG in a coplanar position and in the bottom layer of the substrate, the results reveal good performance when positioned in the bottom layer. The antenna dimensions are tabulated in Table I with and without EBG. Fig. 6 shows the comparison between the simulated and measured results of the return loss of antenna with and without EBG. The simulated and the experimental results ensure that
As the use of the mobile phone is increased, the research on the health risk due to the electromagnetic (EM) fields generated from wireless terminals is widely in progress. Many factors may affect the EM interaction while using a cellular handset in close proximity to the head and hand. The SAR is a defined figure of merit to evaluate the power absorbed by biological tissues. The SAR limit specified in IEEE C95.1: 2005 has been updated to 2 W/kg over any 10 g of tissue [19], which is comparable to the limit specified in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines [20]. In designing antennas for mobile communications, it is important to investigate the SAR value produced by the radiation from the mobile handsets. In this letter, the reference power of the cellular phone is set to 500 mW. Fig. 7 shows the antenna structure in the vicinity of the human head model (Hugo Voxel model) [21]. The SAR values are calculated according to the 10-g standard
SULTAN et al.: LOW-SAR, MINIATURIZED PRINTED ANTENNA FOR MOBILE, ISM, AND WLAN SERVICES
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REFERENCES
Fig. 7. Antenna structure with the human head model (Hugo Voxel model).
TABLE III SAR VALUES OF THE PROPOSED ANTENNA
of the human tissue mass. The SAR calculations are done using the CST 2012 commercial package with Hugo model CST Microwave Studio [21]; the tissues that are contained have relative permittivities and conductivities, according to [22]. The tissues’ frequency dispersive properties are taken into consideration. Table III shows the averaged 10-g SAR at the aforementioned operating frequencies when the antenna is in close proximity to the body. It is worth mentioning that the antenna does not support the LTE 700 without EBG, so the SAR value is missing in Table III at 0.7 GHz without the EBG structure. It is noticed that the antenna fulfills the IEEE C95.1: 2005 and the ICNIRP standards. The other important concern is the effect of the human body on the antenna performance. Table III shows the performance of the antenna in the presence of a human body . It is noticed that is slightly reduced, but still acceptable results are obtained. V. CONCLUSION A new compact planar antenna design that supports all of the operating mobile services, ISM applications, and wireless communication services is introduced. The use of the EBG structure miniaturizes the size, widens the bands, and reduces the SAR values. The SAR values of the antenna satisfy the standard safety guidelines. The effect of the human body on the antenna performance was also taken into consideration. The antenna has more compact size when compared to other published antennas. The antenna was simulated using the CST simulator and fabricated using the photolithographic technique. Very good agreement is obtained between the simulated and the experimental results.
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