Title UWB antenna using offset feeding and slotted ground plane foron-body communications

Author(s) Sun, Y; Lui, L; Cheung, SW; Yuk, TI

CitationThe 2013 International Workshop on Antenna Technology (iWAT2013), Karlsruhe, Germany, 4-6 March 2013. In ConferenceProceedings, 2013, p. 332-335

Issued Date 2013

URL http://hdl.handle.net/10722/189908

RightsInternational Workshop on Antenna Technology: SmallAntennas and Novel Metamaterials Proceedings. Copyright ©IEEE.

UWB Antenna using Offset feeding and Slotted Ground Plane for On-body communications

Y.Y. Sun, L. Liu, S.W. Cheung and T.I. Yuk

Dept. of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong

[yysun, liuli, swcheung, tiyuk]@eee.hku.hk

ABSTRACT: An offset-fed Ultra-wideband (UWB) antenna with a slotted ground plane designed for on-body communications is presented in this paper. The antenna consists of a nearly square-shaped radiator, a feed line slightly offset from the middle along the radiator side and a ground plane with several rectangular slots. The offset feed line improves the radiation pattern for on-body communications and the slots on the ground plane improve impedance matching. Simulated and measured results are in good agreement in terms of S11, gain, efficiency and radiation pattern. Measured results show that the antenna can achieve a wide bandwidth from 2.4 to 14.5 GHz, a constant peak gain and high efficiency, and a more omnidirectional radiation pattern in E-plane, making it a good candidate for UWB on-body communications. INTRODUCTION Since the Federal Communications Commission (FCC) allocated 7.5 GHz spectrum from 3.1 to 10.6 GHz for radio applications with low power emissions in 2002 [1], Ultra-wideband (UWB) systems have received much attention. However, the design of efficient and compact size antennas for wideband applications is still a major challenge. Many microstrip-fed and coplanar waveguide-fed antennas have been reported for UWB applications [2-8]. These antennas employed either the monopole configuration with different shapes (circular ring, ellipse, annual ring, triangle, pentagon or hexagon) [6, 7] or the dipole configuration (e.g. the bow-tie antennas) [5]. Different techniques can be used to increase the impedance bandwidths of planar monopole antennas, e.g. using slots on the ground plane [7] and slot antenna geometry [8, 9], increasing the elliptical ratio of ellipse-shaped monopole [10], adding stairs to the lower edge of the radiator [11], adding a bent stub to one side of the radiating element and stepping the ground plane [12].

For body-centric wireless communications systems, with the existence of human body, several fundamental requirements, such as wide impedance bandwidth, low profile, high front-to-back ratio and good radiation characteristics in the proximity of the body, are needed to be fulfilled in the design of UWB antennas [13-17]. Depending on the channel used for propagation of the signals, body-centric wireless communication systems can be divided into in-body, on-body and off-body communications [13]. In-body communication refers to communication between two or more devices through the human body, which is from inside of the body to the outside of the body. For off-body communication, it is communication between devices on body with other devices away from the body. An antenna for off-body communication should have radiation patterns directed away from the body. For on-body communications, it refers to communication between two or more devices which are mounted on the same human body. Thus it is desirable for the antenna to have an omnidirectional radiation pattern on the body surface to achieve good on-body propagation. Most UWB antennas proposed for on-body communications have made different kinds of compromise. In [14], a UWB monopole antenna was placed perpendicularly to the body in order to have an omnidirectional radiation pattern along the body. It was high profile and so inconvenient for on-body communications. Some other UWB monopole antennas [15-17] were placed in parallel to the body so that they were low profile. However, the radiation pattern along the body was not omnidirectional, so that it was not ideal for on-body communications. In this paper, an UWB monopole antenna employing a nearly square-shaped radiator, a feed line slightly offset from the middle along the side of the radiator and a ground plane with three rectangular slots is proposed for on-body communications. The antenna is fabricated using a Rogers substrate. Results show that the proposed antenna has a more omnidirectional radiation pattern in the E-plane compared with typical monopole antennas, which makes it a good candidate for UWB on-body communications.

ANTENNA DESIGN The geometry of the proposed monopole antenna used for studied is shown in Fig. 1. The antenna had a nearly square-shaped radiator with an area of WP×LP and was microstrip-fed. The microstrip-feed line had a width of wf to achieve 50-Ω characteristics impedance. The radiator and feed line were printed on one side of the substrate, and the feed line

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2013 International Workshop on Antenna Technology (iWAT)

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was offset from the centre of the radiator and the ground plane. The ground plane had a height of hg and was printed on the other side of the substrate. Three rectangular stubs with the dimensions 3.5×1.5 mm2 were added to the top of the ground plane, making it a slotted the ground plane. The antenna was fabricated on a substrate, Rogers RO4350, with a thickness of 0.8 mm, a relative permittivity of 3.5 and a total area of W × L. A nearly square-shaped radiator is capable of supporting multiple resonant modes, just like the other radiator shapes such as triangle, circle, ellipse and etc., and so can provide a wide impedance bandwidth for an antenna. In our design of Fig. 1, we used a nearly square-shaped radiator so that we could the stubs on the ground plane to improve impedance matching. The two stubs on the right of the feed line were used to improve matching mainly at low frequencies and the stub on the left of the feed line was for improving match at high frequencies. The position of each stub on the ground plane had a significant effect on impedance matching. The parameters of the antenna were optimized using computer simulation and listed in Table 1.

TABLE I (unit:mm)

W L Wp Lp D d wf 40 33 22 21.75 10 4.3 1.7 hf hg l1 l2 l3 l4

10.875 9.375 6.6 6.8 3.2 12.9

(a) Top view (b) Bottom view (c) Photos of the antenna

Fig. 1 Proposed antenna

RESULTS AND DISCUSSIONS The geometry of proposed antenna shown in Figs. 1(a) and (b) has been optimized using the EM simulation tool, CST MS 2009. The antenna has also been prototyped as shown in Fig. 1(c) for verification of the simulated results. The simulated and measured S11 of the antenna are shown in Fig. 2. The antenna had a measured impedance bandwidth (S11 < -10 dB) from 2.4 to 14.5 GHz. The discrepancy between the simulated and measured results was mainly due to soldering, inaccuracies during fabrication and the SMA connector. The peak gain and efficiency of the antenna in the frequency range from 1 to 16 GHz are shown in Fig. 3. The antenna had a maximum measured gain of 5.9 dBi at 12.7 GHz with an average gain of 4.42 dBi from 2.4 to 14.5 GHz. The antenna had a maximum measured efficiency of 95.3% at 3 GHz with an average of 86.6% through the whole bandwidth from 2.4 to 14.5 GHz. The simulated and measured efficiencies did not agree well at high frequencies from about 15 to 16 GHz, which was due to the quality of our SMA connector. The simulated and measured radiation patterns of the proposed antenna at 4, 10 and 14 GHz are shown in Fig. 4. It can be seen that the radiation patterns of the antenna in the xy-plane were not omnidirectional which was due to the offset-feed line as explained as follows. Simulated results have showed that if the antenna with stubs on the ground plane was not offset fed, then the radiation patterns were just like those of a typical monopole. However, if the antenna with no stub on the ground plane was offset fed, the radiation patterns were similar to those of the proposed antenna. The radiation patterns in the yz-plane at 4 and 10 GHz did not have obvious null in the +ve and –ve z-directions (on top and bottom) of the antenna which were very different from a typical monopole antenna. This was again due to the antenna being offset fed. The radiation pattern is more omnidirectional in the E-plane, making the antenna suitable for on-body communications. Since the proposed antenna will be placed in parallel with the body, it is low profile and convenient for on-body communications.

Y

Z

X

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Fig. 2 Simulated and measured S11 of the antenna.

(a) (b)

Fig. 3 Simulated and measured (a) peak gains and (b) efficiencies

(a) 4 GHz (b) 10 GHz (c) 14 GHz

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(d) 4 GHz (e) 10 GHz (f) 14 GHz

Fig. 4 Simulated and measured radiation patterns in (a), (b) & (c) y-z plane and (d), (e) & (f) x-y plane

CONCLUSIONS An offset-fed UWB antenna with a slotted ground plane has been designed and proposed for on-body communications. The antenna has a size of 40×33 mm2. The use of rectangular stubs on the top side of the ground plane improves the impedance matching. The offset feed line improves the radiation characteristics in the E-plane for on-body communications compared with typical monopole antennas. The proposed antenna can achieve an impedance bandwidth from 2.4 to 14.5 GHz. The wide impedance bandwidth, constant peak gain, high efficiency and a more omnidirectional radiation pattern in the E-plane make the proposed antenna suitable for UWB on-body communications.

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