Small Size CPW-Fed Chip Antenna for Integration with RF/Wireless Communication Systems *
Bedri A. Cetiner(1), Luis Jofre(2), G.P.Li(1), Franco De Flaviis(1)
(1) Electrical and Computer Eng. Dept.,University of California, Irvine, CA 92697.
[email protected] (2) Dept. of Signal Theory and communications, Technical University of Catalonia, Spain,
Abstract : A small size dual-band antenna for integration with 5-6 GHz RF/Wireless communication systems is presented. The size and structure of the antenna are designed for integration with a RF printed circuit board of a given size (thickness=1.36mm=0.045λ), metallization structure (4-level of metalization) and material properties (FR4, εr=3.78, loss tangent=0.01). It is capacitively fed by a coplanar waveguide (CPW) to allow easier integration with active and passive circuit components. The procedure used to develop this new antenna structure is described. Computational results showing the antenna’s impedance behavior and radiation characteristics are given. I. Introduction: New communication environments typically consist of a broad range of application classes of devices providing data, video, and audio services. Interoperation of these devices without mutual interference is required. In addition the proliferation of cheaper, smaller, more compact and higher performance mobile terminals has been fueling the effort to establish unified multi-service wireless local area networks (WLANs). The new standards, like the IEEE 802.11a, operate at higher frequency bands. The 5-6GHz range of IEEE 802.11a offers much higher speeds (up to 54 Mb/s) compare to the previous standards (eg. IEEE 802.11b) and allows multiple classes of devices to be interconnected in the same wireless network. As a consequence today’s communication systems are becoming smaller, more integrated and unified, both from network services as well as circuit components points of view. Antennas following this trend have to be integrated with mobile terminals which dictate design trade-offs with antenna design, while satisfying desired impedance behavior and radiation characteristics. The network adapter of a typical WLAN is available as a PC card that is installed in a mobile computer. The network adapter includes a transmitter, a receiver, an antenna and hardware that provides a data interface to the mobile computer. In this work, we design a small size antenna that can be integrated into a PCMCIA card in a laptop computer, a PCI card for a desktop or a RF chip module. The proposed antenna also satisfies the desired impedance behavior and the radiation characteristic which needs to be omni-directional in the azimuth plane. The structure of the antenna presented in this work evolved from previously designed, fabricated and characterized dime and q-dime antenna structures [1], [2]. Section II contains the design methodology for the antenna structure and the antenna characterization is presented by providing computational results. II. Design, Structure and Computational Results: A novel broadband miniature antenna, the dime antenna, has recently been reported [1]. The q-dime antenna was obtained by modifying the dime antenna structure for multi-element antenna wireless communication systems [2]. Both dime and q-dime antennas have three metalization levels and are fed by a coaxial cable. In the dime structure two vertical slots are formed
between the three metallization levels. In the q-dime the first slot is horizontal and the second vertical. The dual-band behavior is obtained by adjusting the horizontal slot dimension to the λl/2 resonant length corresponding to the lower frequency fl , and similarly, the vertical slot dimension is chosen to be the λu/2 resonant length corresponding to the upper frequency fu. The antenna structure proposed in this work has evolved from the dime and q-dime antennas. It is our aim to design a structure that can be integrated into a RF printed circuit board of given size and material characteristics (FR4 substrate, εr=3.78, loss tangent=0.01). The RF board originally has four metalization levels that are separated by specific dimensions. We have the flexibility to use the metallization levels as desired and eliminate others. When the small thickness of the substrate (h=1.36mm) is considered it is advantageous to use a horizontal slot rather than a vertical one in order to eliminate one of the metallization levels used in the dime and q-dime structures. Although the first q-dime slot is horizontal as opposed to the dime , there are still three metallization levels. From the q-dime structure one can eliminate one of the metallization levels by bending the vertical wall ,which surrounds the lower patch and stretches from ground plane to lower patch level, in an upward direction. The result is a structure which can be fed by a CPW line. The schematic of the proposed antenna geometry is shown in Fig.1 (for the sake of the illustration the upper patch is shown as separated from the other parts of the structure). In this new geometry we do not only eliminate one of the metallization levels but also replace the coaxial cable feed with a CPW feed that offers a number of advantages over other feed structures. For example, low radiation leakage that allows placement of circuit elements close together with minimum mutual coupling, planar construction without using via holes, and suitability for integration with active or passive elements of the RF board. The antenna structure only requires the usage of the top and bottom metallization levels of the substrate where the cylindrical vertical slot is created. The thickness of the vertical slot is dictated by the substrate thickness of h=1.36mm. The length of this slot, Su, is determined by the length of the two circular vertical walls, Lu, such that Su=πr-2Lu≈ λu/2 (See Fig.1). The bottom layer of the structure, which includes bottom circular patch, is connected to the upper circular patch with a radius of r, through the symmetric vertical walls. A horizontal slot with width wb is etched onto bottom metallization layer (See Fig.1a). Two symmetrical sectoral slits with θs being defined as the slit angle are also etched in this layer within the region occupied by bottom circular patch. The CPW feed network is on the bottom metallization layer and has a width of w and gap spacing of g between the signal line and the ground plane, forming a 50Ω characteristic impedance. The CPW signal line protrudes a short distance of a inside the lower patch layer and a distance b separates it from the bottom patch. The horizontal and vertical slots are simultaneously fed by a capacitively coupled CPW feed. The influence of different antenna components (design variables), which are introduced above, on antenna performance was studied to characterize the antenna performance. Ansoft HFSS7 was used as the characterization tool. The antenna design is part of our ongoing effort to design and fabricate smart RF chip modules including integration of MEMS switches within antenna structure[3]. At this phase of this work we are providing computational results only. The antenna has dual-frequency behavior determined by the vertical slot length Su and horizontal slot width wb. The location of these two frequencies can easily be controlled by varying Su and wb. The return loss as a function of frequency for different lengths of the vertical slot Su is graphed in Fig.2a. It should be noted here that Su is changed by varying the length of the vertical walls, Lu. The curves show that
the higher resonant frequency is controlled by Su, while the lower resonant frequency remains unaffected. In Fig.2b the influence of the horizontal slot width wb on return loss is shown. As expected wb plays the same role on the lower resonant frequency as does Su on the higher resonant frequency. The input matching with a 50 Ω CPW for the two operating frequencies is achieved by proper choice of a and b. The other parameter that affects input matching level is the slit angle θs. These slits force the input current to reach the ground plane by traveling a longer path(≈λ/2 at the central resonant frequency), starting from the feed point and going along the slits. The slits allow us to reduce the size of the antenna while controlling the input matching level. In Fig.2c the return loss is shown for different dimensions of a and b while θs is fixed at 53o. Radiation characteristics for the proposed antenna are also studied. Fig 3a and 3b show the co-polar and cross-polar radiation patterns in x-z and y-z plane, respectively. In accordance with its geometry and size, the antenna exhibits a co-polar orientation parallel to the field orientation existing on the slots, Eθ on the x-z plane and Eφ on the y-z plane. It has a quite uniform pattern close to elemental dipole behavior, which makes it very suitable for broad coverage applications. A numerical gain of 2.4dBi suggests a good compromise between size and gain. III. Conclusions: A novel compact CPW-fed dual-band antenna possessing attractive impedance and radiation characteristics has been proposed for integration with the next generation of RF/Wireless communication networks operating at 5-6GHz band. An extensive numerical study has been performed to identify the effects of key antenna elements on its performance. The size and structure of the antenna have been optimized so that it can be integrated with the given RF chip size, its metallization structure, and material properties while satisfiying required impedance behavior and radiation characteristics. References [1] B.A.Cetiner, L. Jofre, F. De Flaviis, “A Miniature Broadband Antenna for Portable Communications Terminals ”, IEEE AP-S International Symposium, Boston, July 8-13, 2001 [2] L. Jofre, B.A.Cetiner, F. De Flaviis, “Miniature Multielement Antenna for Wireless Communications”, IEEE trans. on AP special issue on Wireless Communications, March 2002 [3] B.A.Cetiner, L. Jofre, F. De Flaviis, “Reconfigurable Miniature Multi-element Antenna for Wireless Networking” RAWCON 2001, Boston, Massachusetts, August 19-22, 2001
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(b) (a) Fig.1 Geometry of the proposed dual-frequency CPW-fed antenna. (a) Top view of the bottom layer, (b) 3-dimensional schematic. h=1.36mm, r=5mm, w=2.3mm, g=0.2mm
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Fig.2 Return loss of the antenna as a function of (a) vertical slot length, Su : a=0.6mm, b=0.3mm, wb=0.55mm, θs=53o (b)horizontal slot width, wb: a=0.4mm, b=0.2mm, Su=9.3mm, θs=55o (c) feed location, a and b: Su=10.2mm, wb=0.55mm, θs=53o.
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Cross-pol Fig.3. Radiation patterns of the proposed antenna at 5.2 GHz with the dimensions of Su=8.7 a=0.6mm, b=0.3mm, wb=0.55mm, θs=53o (a) in E-plane (x-z plane) (b) in H-plane (y-z plane)
