1 August 2013
Researchers in Australia have developed a circularly polarised slot antenna with high front-to-back ratio that is only around a tenth of a wavelength thick by incorporating a metasurface. The low-cost, compact design could be of interest for WiFi and satellite as well as wireless sensor network applications.
Circularly polarised (CP) antennas have many performance advantages over their linearly polarised counterparts, particularly in minimising loss due to polarisation mismatch and multipath fading. CP antennas have long been the solution of choice for outdoor non-line-of-sight applications, such as satellite navigation and aerospace communications.
With the rapid growth in high speed and high data-rate wireless communications, the wideband characteristics of slot antennas have received a lot of attention for low-profile, miniature front-end systems. With breakthroughs in low cost designs and production, CP slot antennas mounted on metallic ground planes have become popular for both outdoor and indoor wireless communication systems.
However, CP slot antennas have a major disadvantage in terms of their inherently bidirectional radiation, which means that half of the input power is focused away from the desired direction. In many applications, CP antennas need to be mounted on metallic platforms or integrated with microwave electronic devices in layered structures. Hence, the low forward directivity and backward radiation have limited the applications of CP slot antennas.
Over the decades, many techniques have been developed to alleviate the adverse effects of back radiation in slot antennas. The most common methods used are: the lossy cavity-backed method; the adding of a metallic reflector beneath the slot aperture at a distance of quarter wavelength; the use of high-impedance substrate; and, more recently, the artificial magnetic conductor ground plane. These methods have improved matters but also introduced other problems such as low front-to-back ratio (<15dB), high profile geometry, narrow bandwidth and low directivity.
In their Letter, the authors from the Institute for Infrastructure Engineering (IIE) at the University of Western Sydney present a new technique to reduce the backward radiation of a CP slot antenna to achieve high front-to-back ratio with a structure only about a tenth of a wavelength in thickness.
Their antenna is a tri-plate sandwich structure with a metasurface formed by 36 square rings in the top-layer. It also includes a perturbed square ring-slot source antenna fed by a metamaterial-line based parallel-feed network. The feed network consists of a metamaterial wideband quadrature phase-shifter and is key to giving the antenna a wide axial-ratio (AR) bandwidth as it avoids the frequency dependency problem of a conventional microstrip-line, which would otherwise act to narrow the AR bandwidth.
The key element for reducing the backradiation problem, whilst keeping the structure low-profile, is the metasurface included in the top layer, 5 mm above the slot aperture. This is what allows the metallic reflector, used to deal with the backradiation, to be placed much closer than a quarter wavelength away from the source antenna. It can in fact be positioned just 5 mm (about 1/22nd of a wavelength) from the source and feed without sacrificing impedance matching.
The result is a low-cost antenna that is around a tenth of a wavelength thick with a front-to-back ratio of more than 23 dB at 2.7 GHz; also exhibiting a wide impedance bandwidth of 73.3%, broadside gain of 7 dBic and a 3-dB AR bandwidth of 29.1%.
The Sydney team now plans to realise an even more compact structure, as Dr Kwok Chung explained: “A further optimisation on the dual-feed network is being undertaken, with the aim of obtaining further size reduction. We believe that by conjoining a smaller feed-network and slot aperture implemented on a high-permittivity dielectric laminate, an even smaller wideband CP slot antenna with high front-to-back ratio can be achieved. The first devices that might use such antennas could be indoor WiFi or satellite navigation base-station antennas mounted on metallic platforms.”
The authors' work is part of the IIE’s Infrastructure Health Monitoring research program. This program includes material characterisation, novel sensor development and measurement of infrastructure system integrity using sensor technologies for global and local structural health monitoring. The program also uses imaging and signal processing techniques, smart structures and wireless sensor networks.
It is in this latter area that the present work sits: “Such small antennas can be used to extend wireless sensor network applications. We believe that many novel metasurfaces with slot antennas that achieve higher gain and wider bandwidth will be developed and applied in the coming years,” said Chung.
This article is based on the Letter: Metasurface loaded circularly polarised slot antenna with high front-to-back ratio (new window).
Institute for Infrastructure Engineering: http://www.uws.edu.au/iie/home (new window)
A PDF version (new window) of this feature article is also available.
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