# 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
A new design of periodic metasurface grating was proposed in [238] to increase the bandwidth and gain while maintaining a low-profile antenna scheme. The results show that the proposed antenna may be potentially attractive for applications in satellite communications in the Ku-band. The five-band metamaterial absorber proposed in [239] consists of a three-layer structure of an upper metal resonator, an intermediate dielectric layer, and a lower metal plane. Such an absorber has potential applications in multiband electromagnetic invisibles, bionic sensors, and instruments for measuring thermal radiation. The paper [240] presents the results of research and development of antenna complexes for MIMO systems based on the use of fractal radiators and chiral metamaterials. In particular, the paper considers complexes based on radiators in the form of Koch, Hilbert, or dipole Sierpinski fractal curves, and triangles with fractal partitions by medians. These antenna complexes with fractal radiators located on substrates of chiral metamaterials were considered for a 2 \(\times\) 2 MIMO system, in which the same antennas are used for receiving and transmitting. The gain in spectral efficiency of the MIMO system with such antennas was evaluated in comparison with antenna complexes in which fractal radiators were located on dielectric substrates. The results of the study of the impedance and spatial characteristics of these antennas showed their performance and the possibility of implementing them for multifrequency (multiband) systems. It is shown that such antenna complexes allow for the realization of increased spectral efficiency of MIMO systems in several frequency bands. In [241], a closely spaced 2 \(\times\) 2 dual-band MIMO antenna with high isolation based on a half-mode substrate integrated waveguide (HMSIW) is presented. Dual-band operation of the antenna element is achieved by loading the rectangular section outside the radiating aperture of the HMSIW resonator. This resonator is coaxially excited, whereas the rectangular section is excited via non-contact through the HMSIW aperture. Good isolation is achieved by using spatial and polarization separation. A decoupling gain of about 10 dB is achieved by inserting small neutralization lines (SNLs) in the center of the MIMO antenna between the antenna element radiators without increasing the size of the original MIMO antenna. With an edge-to-edge distance of only 0.036 \(\uplambda\)0, the measured decoupling in both lanes was above 35 dB. In addition, the separation characteristics of the proposed MIMO antenna are suitable for a high-quality MIMO antenna system. In [242], a dipole antenna based on a decoupling bandpass filter (BPF) was developed. The BPF uses two U-shaped resonators mounted on the left side of an open transmission line and two L-shaped loops to generate signals with equal amplitude and inverse phase. In this way, the BPF volume is halved and the distance between the two output ports is considerably reduced. The BPF is then integrated with the dipole. Instead of a traditional L-shaped line with a wide ground plane, two thin microstrip lines of 1 mm width are used to connect the dipole and the BPF. The antenna bandwidth is further extended by merging the resonance of the dipole and the BPF. As a result, the new BPF and the dipole based on it have great potential for 5G MIMO applications. In [243], the researchers present a four-layer broadband directional antenna with symmetrical double polarization and an equilateral triangular slot, as well as a CPW power supply suitable for wireless laboratory measurements. The antenna structure consists of two equilateral triangular slots with a symmetrical pair of stubs. There are four layers consisting of two antennas and two reflectors; each antenna is connected to a CPW port, and these antennas are perpendicular to each other. A proper choice of the length and width of the slots gives a much broader bandwidth in impedance, dual polarization, and high decoupling.