Thus, it is easy to deduce that a linear (horizontal) array of dihedrons with 908 of angular aperture (with a vertical edge of flat intersection) placed on a warped surface is a very good alternative to current commercial catadioptric devices for retro- directive applications in cars. Obviously, it is also possible to make the same assertion about invisibility applications, using dihedrons of 1208 if the observer movement is also restricted to the horizontal plane; even more so in this type of applications, due to the dispersive characteristics of both types of dihedrons (908 and 1208), when sight lines are placed outside the horizon- tal plane. 6. CONCLUSION This document has shown that inverted cone structures of 908 and 1208 of aperture can be used as retrodirective and invisible devices with a broad margin of angular aperture (>508). It has also been demonstrated that an array of several inverted cones provides similar behavior, but this structure adds the advantages of a reduced thickness and the possibility to be warped without serious loss of performance. Consequently, it allows shield structures to be formed around any target indepen- dently of its size and geometry for either kind of applications (retrodirectivity and invisibility). In the case of retrodirectivity, it has also been demonstrated that a laboratory prototype of an array structure of inverted cones with 908 of angular aperture presents better and more sta- ble behavior than a commercial device used in vehicles as a catadioptric element. Moreover, the novelty of this structural geometry should be highlighted, which, in the case of retrodirectivity applications, could be a good alternative to commercial devices based on similar physical concepts. The structure of inverted cones with 1208 of angular aperture could provide a good alternative to cur- rent technology of shielded paints for invisibility applications, and could provide a good alternative to obtain better signal dis- tribution within buildings as a dispersive element for Wi-Fi and Li-Fi technologies. ACKNOWLEDGMENTS The authors thank Julio Guti errez R  ıos of the Faculty of Infor- mation Technology of the Polytechnic University of Madrid (UPM) for the comparative measurements carried out with its fluorescence sensor. The work at the Complutense University of Madrid (UCM) has been partially sponsored by the Spanish Ministry of Econo- my and Competitiveness and by the European Regional Devel- opment Fund (ERDF) under the project MTM2014-54141-P “Algebra-geometric constructions: fundamentals, algorithms and applications”. The work at the University of Cantabria (UC) has been spon- sored by the Spanish Ministry of Economy and Competitiveness through the project TEC2014-58341-C4-1-R. The work at the Spanish Council for Scientific Research (CSIC) has been internally sponsored by the Group of Antenna Technology of the “Leonardo Torres Quevedo” Institute of Physics Technology (ITEFI). REFERENCES 1. L.C. Van Atta, Electromagnetic reflector, US Patent 2,908,002, October 1959. 2. N. Merc e, H. Palacios, A. Pedreira, P. Tejedor, and J. Vassal’lo, Pla- nar retrodirective reflectors, International Conference on Electromag- netics in Advanced Applications (ICEAA’99), Turin, Italy, 1999. 3. L. Cabria, J.A. Garc  ıa, T. Aballo, and Z. Popovic, Polar Phase- Conjugating Active Arrays for Spectrally-Efficient Linear Wireless Links, 2010 IEEE MTT-S International Microwave Symposium Digest, Anaheim, CA, 2010, pp. 77–80. 4. M. Fairouz and M. Saed, A retrodirective array with reduced surface waves for wireless power transfer applications, Progress in Electro- magnetics Research C 55 (2014), 179–186. 5. M.G.J. Minnaert, Light and Colour in the Outdoors, Springer-Verlag Edit., ISBN 978-1-4612-2722-9, 1993. 6. R. Greenler, Rainbows, Halos and Glories, Cambridge University Press, ISBN 978-0521388658, January 26, 1990. 7. J. Fischer, T. Ergin, and M. Wegener, Three-dimensional polarization-independent visible-frequency carpet invisibility cloak, Optics Letters 36 (2011). 8. F. L. Metamateriales, Y. L. Xu, W. S. Fegadolli, M. H. Lu, J. E. B. Oliveira, V. R. Almeida, Y. F. Chen, and A. Scherer, Experimental demonstration of a unidirectional reflectionless parity-time metama- terial at optical frequencies, Nature Materials 12 (2013), 108–113. 9. J.A. Garc  ıa, R. Gonzalo, J. Guti errez-R  ıos, A. Taz on, and J. Vassal’lo, Contribution to the improvement of flat reflectors, 2nd Meeting of the COST IC 0603, Bonn (Germany), 2007. 10. A. Pedreira and J. Vassal’lo, Anechoic Reflector, 30th European Microwave Conference (EuMC 2000), Paris, France, 2000. 11. B. Nair and V. F. Fusco, Planar printed self-channelling scatterer, Electronic Letters v39 (2003). 12. J. Guti errez-R  ıos and J. Vassal’lo, Electromagnetic reflector without echo in the incident direction of the signal, ES 2.374.125 B1 Spanish Patent, date of priority 01/06/2009, published by the OEPM in the date 19/12/2012. 13. H. Haral, Wireless data from every light bulb, TED Global, Edin- burgh, 2011. 14. Y. Wang, S. Videv, and H. Haas, Dynamic Load Balancing with Handover in Hybrid Li-Fi and Wi.Fi Networks, 2014 IEEE 25th International Symposium on Personal, Indoor and Mobile Radio Communications, 2014, pp. 574–579. 15. N. Al Abdulsalam, R. Al Hajri, Z. Al Abri, Z. Al Lawati, and M. Bait-Suwailam Mohammed, Design and Implementation of a Vehicle to Vehicle Communication System Using Li-Fi Technology, 2015 International Conference on Information and Communication Tech- nology Research (ICTRC 2015), 2015, pp. 136–139. 16. J. Guti  errez-R  ıos, J. Vassal’lo-Sanz, J. Vassal’lo-Saco, I. Soto, E. Gallego, P. Maraver, A.E. Orobio, and A. Medrano, Sensor of fluo- rescent substances, U201100703 Spanish Patent, date of priority 23/ 05/2008, published by the OEPM (17/04/2012), BOPI-T2 page 8415, ISSN 1889-1292 V C 2016 Wiley Periodicals, Inc. METAMATERIAL-INSPIRED MICROWAVE SENSOR FOR MEASUREMENT OF COMPLEX PERMITTIVITY OF MATERIALS Aakriti Raj, 1 Abhishek Kumar Jha, 2 M. Arif Hussain Ansari, 1 M. Jaleel Akhtar, 1,2 and Sidhartha Panda 1 1 Materials Science Programme, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India 2 Department of Electrical Engineering, Indian Institute of Technology Kanpur, Uttar Pradesh 208016, India; Corresponding Author: akjha.rf@gmail.com Received 22 March 2016 ABSTRACT: This article presents the design and development of a metamaterial inspired planar microwave sensor for the measurement of complex permittivity of the solid and liquid sample under test (SUT). A hexagonal complementary split ring resonator (CSSR) is etched on the ground plane of a 50 X microstrip line, which is excited by the electric DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 58, No. 11, November 2016 2577