A metamaterial approach to wideband radar cross section reduction that is thin, low-cost and doesn't dictate object shaping is presented in work from Iran. The approach uses principles potentially applicable to infrared and visible light as well as radar. The wideband RCS reducing surface is produced on a commercial high-frequency circuit substrate using conventional printed circuit manufacturing techniques Reducing an object's detectability to radar systems is now a well established priority for military applications, most famously in aircraft applications. ‘Stealth’ aircraft that were once well kept secrets are now a familiar concept to the general public and even fly in air shows. The radar cross section (RCS) of an object is essentially a measure of how detectable an object is to radar systems and so reducing an object's RCS is the basic goal of radar focused ‘stealth’ technologies. There are three main approaches to reducing RCS; radar absorbing material (RAM), object shaping and reflected wave cancelling by object coating. RAM materials, as the name suggests, are designed to reduce radar reflections back to the detector by absorbing the electromagnetic energy transmitted by the radar system, converting it into heat. However, they can be bulky and high maintenance. Object shaping works by shaping the surface of the object to minimise reflections of the radar waves back to the radar system, instead reflecting them in other directions. The obvious disadvantage to this approach is that it dictates the shape of the object, which can compromise the performance of the object for its primary purpose. For example, above all an aircraft must be shaped to enable it to fly, an aircraft that is very hard to detect but cannot take off is of no use to anyone. The work reported in this issue, by researchers at Iran University of Science and Technology (IUST) uses the third approach, in which a surface coating reflects the radar waves in such a way as to reduce the amount of radar energy that returns to the detector. This approach is low profile, low bulk and doesn't dictate the shape of the object. In the IUST team's work it can also be cheaply and simply manufactured using printed circuit board manufacturing techniques. Their proposed structure consist of a chessboard arrangement of two different artificial magnetic conductor (AMC) unit cell patterns, each of which reflects the radar waves differently. The key difference in the reflections is a 180° phase shift, which causes the reflections from the two different AMC cell types to interfere destructively and so significantly reduces the useful energy returned to the radar system's detector. “Since radar systems usually send out a short pulse signal in the time domain, wide bandwidth of the RCS reducer is necessary. The proposed AMC structure achieves a RCS reduction over a very wide bandwidth, about 85%,” said Dr Seid Hasan Sedighy, one of the IUST researchers on this project. The wide bandwidth achieved is possible because the critical 180° phase difference in the reflections is produced by the AMC unit cells over a wide frequency range, from 9.4 GHz to 23.28 GHz, providing 10 dB RCS reduction in that range. The peak RCS reduction is in excess of 40 dB, at 15 GHz. Printed circuit manufacturing techniques were used to create the AMC patterns on a commercial high-frequency circuit substrate, providing a low-cost manufacturing method. As flexible circuit board technologies are now available and continue to develop, this AMC-based, effective and relatively cheap approach to RCS reductions could be applied to arbitrary shapes. AMCs are metamaterial structures that behave as perfect magnetic conductors in a limited range of frequencies. The two unit cell AMC patterns used in this work are an arrangement of ‘E’ shapes and saltire arrows. The patterns were optimised to achieve the desired 180° phase difference in their reflections by simulation before being confirmed by experiment. The RCS reducer consists of a chessboard pattern of AMC patches using two different AMC unit cell designs, which produce reflections 180° out of phase with each other. The resulting destructive interference provides the intended RCS reduction The principles of AMCs and using destructive interference to reduce reflected energy are of course applicable to other frequencies of the EM spectrum, not just the band used for radar. This means that similar effects could be achievable in very different frequencies if the structures can be manufactured. Dr Sedighy expects this to lead to developments closer to photonic frequencies “It seems that this design approach will develop in the other frequency bands such as visible and infrared frequencies, which have many civil and military applications.”
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