15 August 2013
Researchers at Pennsylvania State University in the US have designed and manufactured a frequency-selective surface (FSS) with both wideband and wide-angle characteristics. Their design may find use in a wide range of applications, from reflector antennas and satellite systems to radar and stealth technology.
Frequency-selective surfaces, also known as spatial filters, pass certain desired frequency bands and block others, such as those that interfere. Typically, these surfaces are doubly-periodic structures, comprising elements with adjustable resonances to provide the desired frequency response.
Dr Chiara Pelletti, one of the authors of the research, explained that FSSs are typically narrowband “since the typical FSS design relies upon resonance behaviours of the periodic array of elements that make up the screen.” She also explained that their angular response is limited to a relatively narrow range, and so “any attempt to widen this range by shaping the elements differently, by meandering for example, exacerbates the frequency bandwidth problem even further.” However, for a whole host of applications, an FSS covering a wide frequency band and a wide-angle coverage is necessary, “and yet,” said Pelletti, “such wideband and wide-angle FSSs have been quite elusive, despite the fact that researchers have pursued a variety of approaches to achieve their goal of designing such FSSs.
“We believe,” Pelletti continued, “that this is the first time we have cracked this barrier, which has existed for a very long time, and has prevented us in the past from designing this 'holy grail' of FSSs, namely wideband and wide-angle screens.” By following a novel design strategy they have completely departed from the conventional approaches to FSS design that inherently lead to narrowband and/or narrow angular coverage. This design strategy is both versatile and robust, and it permits systematic control of the pass-band as well as the stop-band. Furthermore, by following the proposed design, they achieve sharper roll-off from skirts than has been possible in some of the existing designs for similar FSSs.
FSS screens are thin and their thickness is generally a small fraction of the wavelength. To overcome this, previous work has involved stacking a number of screens on top of each other, but what separates the design of the Pennsylvania group is that they have achieved the specified performance characteristics by inserting a guided-wave structure. This structure is inserted between the top and the bottom layers, with radiating elements that have a relatively wide bandwidth as well as wide-angle response.
Pelletti expanded on this principle, explaining that “the waveguide, which contains a four-conductor transmission line at its centre, transports the energy from the top to the bottom layer, above the cut-off frequency of the waveguide, of course, which determines the lower edge of the passband, while the upper edge is determined by the length of the transmission line. To the best of our knowledge, the type of configuration we have proposed in this paper has never been employed in the past for thick FSS designs, which provides the response of the type we are seeking.”
While their FSS structure has been built and is ready for real-world application, the team are still investigating the possibility of further improving the angular response of the structure and to reduce the thickness of the FSS.
This development forms part of a wide range of projects for the group, as Pelletti explained: “Our group focuses on many different research aspects in electromagnetics, such as developing techniques in computational electromagnetics for solving problems involving billions of unknowns; design and optimisation of FSS; high performance radomes; wideband antenna and array designs; stealth technology and clocking; EMI/EMC problems in electronic packages; Body Area Networks; RF and optical sensors; geophysical exploration; modeling complex systems for estimating EMI effects, as well as coupling crosstalk and biological hazards.”
This article is based on the Letter: Frequency selective surface with wideband quasi-elliptic bandpass response (new window).
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