Switchable Electromagnetic Bandgap Surface Wave Antenna

International Journal of Antennas and Propagation, Apr 2014

This paper presents a novel switchable electromagnetic bandgap surface wave antenna that can support both a surface wave and normal mode radiation for communications at 2.45 GHz. In the surface wave mode, the antenna has a monopole-like radiation pattern with a measured gain of 4.4 dBi at ±49° and a null on boresight. In the normal mode, the antenna operates like a back-fed microstrip patch antenna.

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Switchable Electromagnetic Bandgap Surface Wave Antenna

Switchable Electromagnetic Bandgap Surface Wave Antenna Qiang Bai, Kenneth L. Ford, and Richard J. Langley Department of Electrical and Electronic Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK Received 3 December 2013; Accepted 20 February 2014; Published 10 April 2014 Academic Editor: Giacomo Oliveri Copyright © 2014 Qiang Bai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract This paper presents a novel switchable electromagnetic bandgap surface wave antenna that can support both a surface wave and normal mode radiation for communications at 2.45 GHz. In the surface wave mode, the antenna has a monopole-like radiation pattern with a measured gain of 4.4 dBi at ±49° and a null on boresight. In the normal mode, the antenna operates like a back-fed microstrip patch antenna. 1. Introduction Many types of wearable antennas have been proposed in recent years designed for body area networks (BANs). Antennas may operate in either of two modes, on-body and off-body. On-body communication refers to the transmission of signals across the human body as a surface wave, which requires the wearable antenna to have maximum directivity along the body surface while, in the off-body (normal) mode, the communication occurs away from body to a node such as a base station, which demands that the antenna have the maximum radiation in the boresight direction. Therefore ideally such antennas have to be designed to support each mode of operation. For many antennas, it is useful if they are able to operate without being affected by their environment, for example, radiating into the human body or placing them on metal surfaces such as vehicles. Hence, a large ground plane or electromagnetic bandgap (EBG) structure may be also necessary to reduce the detuning effect and the backward radiation. Some antenna designs using EBGs as artificial magnetic conductors (AMCs) have been reported in [1–5] for off-body communication and in [6–8] for on-body communication. Only a small number of papers have studied a switchable system which can support both on- and off-body communications [9–11]. The mushroom-like EBG structure has been well studied and widely used in various designs to improve antenna performance [1, 12, 13]. Recent research also observed that the surface wave bandgap may disappear when the vertical vias are removed from the mushroom-like EBG structure. Hence instead of being suppressed, a strong surface wave can be exited and radiated on the via-less EBG material. Based on this feature, several surface wave antennas (SWA) have been designed to achieve a monopole-like radiation pattern on a thin, planar structure [14, 15]. However, in previous designs, the antenna could only support surface wave communication, and it was difficult to achieve the normal mode communication feature as the antenna was fully covered by the EBG cells. This paper presents a novel switchable surface wave antenna based on bandgap materials which keeps the planar structure and low thickness but can support both surface wave (on-body) and normal (off-body) modes of communication at 2.45 GHz. Although not designed on textile materials, this paper shows the techniques for generating a dual mode switchable antenna. The performance of the antennas is investigated based on numerical and experimental methods. CST Microwave Studio was used for the antenna simulations. The surface wave communication mode is further investigated by studying the use of an EBG to couple antennas/sensors together around the body. 2. Antenna Design The proposed switchable surface wave antenna comprises three layers. The radiating antenna is a 27 mm × 26.5 mm microstrip patch printed on a grounded slab and back-fed by a SMA connector (Figure 1). The feeding point is 6 mm offset from the patch centre. The top layer consists of a parasitic identical patch aligned above the lower patch antenna and an optimized EBG surface (Figure 1(a)). The EBG, with 25 mm × 25 mm unit cell, was designed, so that a surface wave was excited and propagated when the reflection phase of the EBG cell was approximately −90° (Figures 2(a) and 2(b)) [15]. There are six strips at the edges of the surface and six PIN diodes switching the EBG surface to the strips to give the radiation characteristics. Both layers are made of FR4 material with a total thickness of 3.2 mm and an overall size of 100 × 190 mm. The bias network can be printed on the back side of the top layer to actuate the PIN diodes. And also the PIN diodes can be replaced by varactor diodes to tune the surface response. Figure 1: Geometries of the switchable SWA: (a) top view, EBG based antenna layer with PIN diode switches, (b) side view. Figure 2: (a) Photograph of the fabricated model (without PIN diodes), (b) EBG reflection ph (...truncated)


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Qiang Bai, Kenneth L. Ford, Richard J. Langley. Switchable Electromagnetic Bandgap Surface Wave Antenna, International Journal of Antennas and Propagation, 2014, 2014, DOI: 10.1155/2014/693852