# Wire-Grid and Sparse MoM Antennas: Past Evolution, Present Implementation, and Future Possibilities
## 4 Reconfigurable and Smart Antennas
### Reconfigurable and Smart Antennas: The Field of Use
In the work [244], an automatic antenna design method based on the shape-blending algorithm was proposed. The algorithm was used to construct the shape of a wide slit antenna powered (fed) by a coplanar waveguide (CPW). First, two basic shapes were chosen as the initial and target shapes. Then, a shape-blending process was applied to obtain a series of shapes that were used as the geometric structure of the wide slit. In this way, a series of CPW-fed broadband antennas was obtained. These had similar but gradually changing characteristics. Experimental results have shown that the resonant frequencies change with the change of the slit shape in a certain frequency band, and the proposed antennas can be used in C-band and X-band radar applications. In the work [245], a new three-zone unit cell with electromagnetic bandgap (EBG) of compact size was designed, fabricated, and tested. The proposed EBG unit cell was based on a square mushroom-shaped EBG (M-EBG) structure with an interdigital coplanar waveguide (ICPW). By this method, the size of the proposed ICPW-EBG structure was reduced from \(\uplambda/2\) to \(\uplambda/4\), compared to the size of the standard M-EBG unit cell. The ICPW-EBG reflector can improve the directivity of the dipole antenna. The developed reflector ICPW-EBG with the dipole antenna provides operation in three bands, a low profile, and high gain which are appropriate for today's wireless communication systems. In [246], three configurations of compact flat multi-loop antennas were proposed as candidates for the standard 5G frequency band. Each antenna consisted of the same feeder part, but with different designs of the dipole, director, and reflector parts. In [247], the classic Chinese window array structure was combined with a multiband microstrip antenna design that can be used in wireless mobile terminal equipment. The antenna radiator has a rectangular bending structure with four loops, which increases the effective current path of the antenna radiator in a limited space, so that the whole antenna becomes miniaturized. The branched phase set structure of four rings increases the current path of the antenna, making the antenna multiband. Test results show that such antennas can cover four bands and give out six main frequency points to cover different navigation systems. The application of antenna arrays in various designs and elements is also widely used to improve the design and performance of millimeter range antennas, to achieve the characteristics of high gain and fast-beam formation with the possibility of wide scanning [248]. In [249], for 5G applications, a compact substrate integrated waveguide (SIW) antenna array was proposed. This array consists of four SIWs fabricated on a single substrate layer. In each resonator there is a rhombic-shaped slot and a slot with a triangular split ring, which resonate on the TE101 and TE102 modes at 28 GHz and 38 GHz, respectively. Given these characteristics, including two bands, high gain, narrow beamwidth, miniaturization, and single layer, the proposed antenna array is a suitable candidate for 5G millimeter band communication systems with the flexibility to switch operating frequency bands based on channel-quality changes. In addition, a reconfigurable Vivaldi antenna array was designed using a power divider for 5G applications in [250]. The proposed antenna consists of an array of the Vivaldi antennas, which is fed through a power divider. The power divider splits the power equally into two branches of the Vivaldi antennas. By implanting a pin diode, the desired frequency bands are achieved. Changing the antenna shape to meet the requirements was used in the work [251] to increase the bandwidth of a slot resonator antenna using a triangular slot on a half-mode substrate of an integrated antenna with a waveguide structure. This was influenced by the position, length, and width of the triangular slot.