Miniaturized Printed Circuit Board Antennas for Ultra-Wide Band Applications

Ntsanderh C. Azenui* and H.Y. David Yang

Department of Electrical and Computer Engineering University of Illinois at Chicago

I. Introduction

IEEE 802.15 standard utilizing the frequency spectrum from 3 to 10 GHz for

high-data rate ultra-wide band applications has attracted significant attention recently in wireless industry. High-performance miniaturized front-end printed circuit board antennas are essential and has received significant research interest recently. The circular or elliptical shape monopole antennas are found to have sufficient 10dB return-loss bandwidth for ultra wide band applications [1-3]. The antennas integrated on a printed circuit board can be viewed as a suspended monopole antenna collapsing on the substrate surface. Without conductor on the back to form a resonator, it maintains the low-Q feature of a suspended antenna and is broadband in nature. A 50Ω trace extends into antenna central arm and it requires a conductor backing to form a microstrip transmission line. The conductor on the backside is also necessary as the ground for the chips and board circuits. The dimensions of overall communication systems are fairly small. The board space left for an antenna is usually very limited. In this paper, a miniaturized elliptical-shaped patch antenna backed by FR4 substrate and fed by a microstrip line, and some of its derivatives serving for further size reduction are proposed and compared for ultra-wide band (3.1-10.6 Hz) radio applications. The antennas under investigation include elliptical, hollow elliptical and half elliptical monopole antennas. The simple, thin, very small and low-profile designs and especially the microstrip feeding are preferential properties of the proposed antennas from the point of view of easy integration with other parts of an ultra-wide band MMIC circuit for a transceiver or any other ultra-wide band system. The antennas show good performance characteristics of wide impedance bandwidth, consistent radiation pattern, low cross polarization and a substantial gain ( 5.2≥ dBi). Simulations and measurements results for the antennas are presented and discussed in this regard. Among the three antennas proposed and compared here, the half-sized elliptical patch antenna stands out because it achieves unprecedented miniaturization while exhibiting all the desired performance characteristics.

II. The Characteristics of Elliptical, Hollow Elliptical and Half Elliptical

Antennas

The microstrip-fed elliptical patch antenna (MFEPA) proposed is shown in Figure 1(a). It consists of an elliptical-shaped copper patch of major axis a = 27 mm, and minor axis b = 18 mm and, therefore, of ellipticity ratio a/b = 1.39. The patch is 0.002 mm thick and is printed on FR4 ( re = 4.2) material with no ground

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plane directly underneath it. The substrate has a thickness of 0.762 mm (= 30 mil). The antenna is fed by a (50Ω) microstrip line with width 1.524 mm (=60 mil) backed by a ground plane. The ground plane has dimensions W = 45 mm and L = 21.5 mm, and the length of the substrate is mm50Ls = . Notably, the transition from the microstrip line to the antenna patch is direct, without an extra length of conducting strip of the microstrip transmission line extending beyond the ground plane to the patch.

An examination of the current distribution on the elliptical patch antenna reveals that, for all frequencies in its operational bandwidth, surface current is mostly concentrated in the bottom periphery of the patch close to the feed, and there is very low surface current density around the center of the patch and above it. From this realization, a circular section of radius, r, 7.4 mm was cut out from the elliptical patch to eliminate the region of low current density. This yielded a modified MFEPA of smaller size and lighter weight, which we call MFEPA1 that is shown in Figure 1(b).

It would be desirable to have a design that spans less of the effective area on the circuit board than the MFEPA or MFEPA1. In this light, the design approach of removing regions of low current density from the patch was pushed a little further by subtracting the semi-elliptical part of relatively small surface current densities of the patch from the original MFEPA design resulting in a half-sized MFEPA. This antenna which will be called MFEPA2a is shown in Figure 1 (c). However, it was found that the in-band is in the 3.8-12.0 GHz frequency range. Therefore, to obtain a return loss performance similar to that of the MFEPA or MFEPA1, the half-sized MFEPA had to be redesigned. It was found that with dimensions of a’ = 30mm, b’/2 =12mm, and W = 55mm (a new design designated as MFEPA2b), the half-sized MFEPA achieved the desired operational bandwidth.

(a) MFEPA (b) MFEPA1 (c) MFEPA2 Figure 1: Structure of three microstrip-fed elliptical-patch based UWB antennas.

The MFEPA antennas were simulated using Zeland IE3D simulator and some parametric studies were performed on each of them to optimize their return loss characteristics while prioritizing the small size. The parameters studied include the ellipticity ratio and the distance of the patch from the end of the ground plane of the microstrip line. Return-loss and pattern measurements of the designed

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antennas were also carried out. The simulated return loss versus frequency plots for the antennas with optimum return loss characteristics, whose structures correspond to the ones described earlier, are shown in Figure 2 (a). Table 1 below compares the structure and impedance bandwidth characteristics of the antennas. In the table, 1L , 1W , and 1A represent the length, width, and area, respectively, of the smallest rectangle that encloses the horizontal cross-section of the patch. Also, BW represents the 10 dB return loss bandwidth. From Table 1, it is observed that half-elliptical antenna occupies the smallest antenna area.

(a) Return loss characteristics of MFEPA’s (b) Gain characteristics of MFEPA’s Figure 2. Return Loss and Gain characteristics of MFEPA’s

Table 1. Comparison between MFEPA’s Patch Shape

1L ( cm )

1W ( cm )

1A ( 2cm )

BW (GHz)

elliptical 1.8 2.7 4.86 2.8-12 Crescent-

shaped 1.8 2.7 4.86 2.8-12

Half-sized elliptical

1.2 3.0 3.60 3.0-12

The simulated gain was found to vary between 2.5 to 10.4 dBi within the operational bandwidth of the antennas and increase with frequency. Figure 2 (b) displays the gain characteristics of the antennas. Studies of the radiation pattern characteristics of the optimized MFEPA reveal that the antenna radiates mostly in the broadside ( 0,0 == φθ ) direction for lower band frequencies from 3.0-6.0 GHz. However, as the frequency increases beyond 6.0 GHz, its main beam splits, and tilts away from the broadside direction and is more towards the direction 1501206030,6030 00000 ≤≤∪≤≤≤≤ φφθ . Its E- and H- plane radiation patterns for 3, 6, 9, and 10 GHz are shown in Figure 3. Overall, the variation in its radiation pattern with frequency is not drastic; it is actually minimal. Thus, the MFEPA antenna can be said to show a more or less consistent radiation pattern in its operational bandwidth. In addition, the radiation pattern of MFEPA1 and MFEPA2b were found to vary only slightly from that of the

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original MFEPA. The MFEPA1 and MFEPA2b therefore offer reduced-size alternatives for the original MFEPA without much trade-off in performance characteristics. These findings demonstrate that the semi-elliptical microstrip-fed patch, MFEPA2b, provides the best option of all three designs for ultra-wide band (2.8-12 GHz) performance and miniature size. Indeed, the MFEPA2b represents a 26% size reduction compared to the original MFEPA.

f = 3.0 GHz f = 6.0 GHz f = 10.0 GHz Figure 3. Radiation patterns of the original MFEPA.

III. Conclusions This paper presents three cases of ultra-wide band printed-circuit board

antennas namely the microstrip-fed elliptical patch antenna (MFEPA), its crescent-shaped variant (MFEPA1), and the semi-elliptical microstrip-fed patch (MFEPA2). These antennas have been shown to possess the desirable characteristics for ultra-wide band antennas. The comparison shows that semi-elliptical patch antenna offers smallest area with about the same antenna performance.

References [1]. Agrawall N.P., Kumar G., Ray K.P., "Wide-band Planar Monopole Antennas", IEEE Trans. on Antennas and Propagation, vol. 46, No. 2, pp. 29-295, Feb. 1998 [2]. Ying Chen and Y.P. Zhang, “A planar antenna in LTCC for single-package ultrawide-band radio”, IEEE Trans. on Antennas and Propagation, vol. 53, No. 9, pp. 3083-3093, September 2005. [3]. Jianxin Liang; Chiau, C.C.; Xiaodong Chen; Parini, C.G. “Study of a printed circular disc monopole antenna for UWB systems,” IEEE Trans. on Antennas and Propagation, vol. 53, No. 2, pp. 3500-3504, November 2005.

E-plane H-plane

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