**5. Metamaterials in Antennas**

In modern era, the communication system is shifting from wired to wireless. In this regard, there is a need for antennas for wireless transmission of signals. Hence, the demand for antennas which are having large operating bandwidth and high gain has increased. The researchers are searching for new kind antennas which satisfy the demand of modern communication. In this concern, DRA (dielectric resonator antenna) attracted much attention owing to its novel features such as high radiation efficiency and broad bandwidth [211–213]. Initially, Long and his coworkers started the experimental investigations on DRAs, followed by many researchers, now the DRAs are available in different shapes like rectangular, gammadion cross, cylindrical, quadruple, spherical, hemispherical, and so on [214–217]. Amongst, rectangular DRA exhibits good attractive properties. However, limited bandwidth and manufacturing cost are the major drawbacks of DRAs. To reduce the fabrication cost, cost effective additive manufacturing (like 3D printing) can be used in the place of the costly conventional manufacturing. In addition to cost effectiveness 3D printing allows to develop complex structures with specific features of antennas for demanding applications. With the help of SRR, biodegradable PLA and cost-effective 3D printing technique Kumar et al. [218], recently demonstrated the design and development of star shaped dielectric resonator antenna (SDRA). They reported that a special type of polymer called PLA is fabricated by additive manufacturing technique using renewable energy sources. In addition, SDRA shows high bandwidth of 37% that can be attained by modifying the shape of rectangular DRA. Further, the circular shaped SRR can serve as a metamaterial which lead to enhancement in gain, the maximum gain attained by the antenna is of 82.7%. Moreover, within in the operating band the proposed antenna exhibits the average efficiency of 80.51% and is circularly polarized at 5.8 GHz. Therefore, the outcomes are evidenced that the proposed antenna fulfils the WLAN bandwidth requirements (5.15–5.35 GHz and 5.725–5.825 GHz). This is useful in communication, C-band (aeronautical and meteorological radio navigation and satellite navigation systems.

In addition, the metamaterials are widely used in reduce the size of the antennas to attain multiband frequency response [219]. Negative order and zeroth order resonances are useful to design miniaturized antennas which are very useful in wireless vehicular communication systems [220–223]. Mehdipour et al. [224] demonstrated the design and applications of monopole antennas that are loaded by complementary split ring resonators (CSRR) and zeroth order resonator units. The proposed antennas can be operated in three tunable frequency bands. They reported that, the miniaturization of the proposed antenna is achieved by loading the zeroth order resonators. In addition, they observed the good agreement between the simulated and experimental results. Hence, the multi-band tunable response and miniaturized structure of the proposed antenna evinced that it is a prominent candidate for vehicular communication system. Further, Elwi [225] introduced a novel cylindrical antenna with miniaturized structure for multi-input and multi output systems. The proposed antenna consists a cylindrical shaped Kodak photo paper substrate (with height λo/4.5 and diameter λo/4.5, where λ<sup>o</sup> is the wavelength of the free space at 2.25 GHz) on which four omega shaped monopoles (the separation between each monopole is of λo/29) are folded. In the frequency range 2 to 3 GHz there exists the maximum coupling between the monopoles. The reduction in coupling is done by mounting the SRRs between the monopoles. With the help of Ink-jet deposition process the silver nanoparticles are printed on the substrate. Finally, it is observed that the proposed array shows a gain of 2.5 dB with wide radiation patterns which is suitable for the applications in the systems having multi-inputs and multi outputs.
