# 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  
Using the MoM, a novel frequency reconfigurable H-shaped patch antenna was designed in [231]. The reconfigurability in frequency is obtained by connecting two switches in a basic antenna by using MEMS. By controlling the switches, the antenna can be operated at three different frequencies. The effects of antenna length, size, substrate, thickness, and shape have been evaluated and produced good and acceptable 2D radiation patterns in elevation plane, gain, and efficiency. The work [232] proposed a novel concept of a beam-reconfigurable antenna, which rotates the beam of a two-element Yagi antenna without mechanical rotation. The antenna was based on an arrangement of six wires extending radially from the apex of a support. A switch circuit was used to combine two wires into the driven dipole and two wires into the parasitic dipole. Using this concept, improvementsare expected from the use of horizontal dipoles or folded dipoles and additional radiator types can be configured, such as single-dipole or vertical top-loaded radiator antennas. Realizations of the concept seem to be feasible up to the low microwave frequency range by suitable miniaturization of the switching circuit, and such reconfigurable antennas may provide diversity gain in communication systems. Later in [233], the authors presented a conventional single-band microstrip antenna in the shape of a pentagon. The 1 \(\times\) 2 and 1 \(\times\) 4 matrices of the conventional patch antenna were designed to increase its gain and directivity. The conventional patch antenna was converted into a dual-band antenna using the fractal geometry of the Durer pentagon and varactor diodes. The proposed antenna can be used for satellite communications and radar applications. In the work [234], a square Koch fractal slot antenna for the UHF band is presented, which has FR4-G10 and Cuclad 250 substrates. The contribution of this work is in the use of fractal geometry to create an inexpensive slot antenna operating at UHF frequencies on a limited CubeSat surface area, and to optimize it for possible use with solar cells. In the work [235], a printed annular reconfigurable metasurface (ARM) comprising concentric ring slots etched in a metal screen loaded by stubs and varactor diodes was proposed to realize two-dimensional leaky-wave antennas (LWAs) operating on a pair of azimuthally independent TM and TE cylindrical leaky modes. The ad hoc method of moments approach was used for the calculation of the antenna's far-field patterns. In the work [236], a novel design method of electronically controlled beam-reconfigurable antennas using a liquid crystal metasurface was proposed. The metasurface is a two-dimensional plane composed of a series of sub-wavelength liquid crystal unit cells. The beam in desired radiation direction can be achieved by varying the impedance distribution of the liquid crystal metasurface according to a microwave holographic technique. Such a beam-reconfigurable antenna with a low-profile and smoothly adjustable beam has great value in the practical applications of mobile communications. An antenna in [237] is based on the metamaterials concept and operates in the terahertz (THz) band for integrated applications. The proposed antenna consists of five layers of polyimide and aluminum as top and bottom substrates, a radiation section, a grounding layer, and a power line. To achieve high-operating parameters without increasing the physical dimensions of the antenna, the waveguide properties integrated into the metamaterial and substrate were applied to the antenna design. This was achieved by making linear tapered slots on the top surface and metal through-holes throughout the middle ground plane connecting the top and bottom substrates. The results confirmed that the proposed low-profile antenna, with a compact size, a wide bandwidth in the terahertz range, and an economical and easy-to-fabricate configuration, could be suitable for integrated circuits in the terahertz range.