## 3 Multiple Reconfigurable Features in Antenna
### Frequency and Polarization Reconfigurable Antennas  
An antenna of this type is very useful in imaging, sensing, tracking and radar applications [69]. With frequency reconfiguration, the antenna system is able to tune and make use of the available spectrum. Polarization diversity can reduce the effects of multipath and enhance channel capacity. There is a growing interest in using this type of reconfiguration combination and a number of designs have been proposed [70]. One important method makes use of an active electromagnetic band gap (EBG) or metasurface structure along with switches on top of the antenna resonator [69; 70; 71; 72]. However, most common is the integration of methods for achieving frequency and polarization reconfigurability using active elements such as varactor diodes and PIN diodes, in addition to some other modifications [73; 74; 75; 76]. Different antennas with frequency and polarization reconfigurable functions are reviewed below.  
Two such designs were introduced in [69; 70]. The configuration of the EBG in [69] composed of rectangular patch arrays on both sides of the substrate. A coplanar waveguide (CPW)-fed monopole antenna with wide bandwidth was placed above the designed EBG surface. By tuning the employed varactors in the EBG structure, the antenna can generate circular polarization at different frequencies from 1.1-1.7 GHz, and can be switched between LHCP and RHCP. For the design proposed in [70], the main radiator is a dipole antenna which can work at four different states, thanks to the reconfigurable EBG structure. By changing the diode states, the antenna frequency can be switched between 4.4-4.6 GHz. The polarization can be dynamically tuned for RHCP, LHCP, and LP.  
Another design was introduced in [71]. Its structure consists of a planar slot antenna as the main radiator, and a metallic reflector with a metasurface. By adjusting the positions between the center of the slot radiator and the metasurface, frequencies from 8.0-11.2 GHz and polarization states LP, LHCP, and RHCP could be chosen. A square ring antenna patch antenna with two horizontal gaps [72] provided LP at 2.4 and 4.18 GHz, and LHPC and RHCP characteristics at 3.4 GHz. A square-ring patch radiator cut in the ground plane has a vertical gap across which two PIN diodes bridge. A stub-loaded reconfigurable patch antenna with frequency and polarization agility was proposed in [73]; see Figure 7a. Twelve loading stubs are sited at the four edges of the main radiator. As illustrated in Figure 7b, the frequency can be tuned from 2.4-3.4 GHz with diverse polarization, and so the antenna is suitable for multi-mode 4G/5G communication systems.  
A simple monopole antenna with different states of frequency and polarization diversity was introduced in [74]. It includes a monopole radiator, a reflector, and a modified ground plane with two slots. A pair of switches reconfigures the antenna, allowing four states covering 2.02-2.56 GHz, 2.32-2.95 GHz, 1.92-2.70 GHz, and 1.88-2.67 GHz. The radiation is linearly polarized in states 1 and 2, and LHCP and RHCP in states 3 and 4. Another patch antenna design with a corner segment providing frequency/polarization diversity was presented in [75]. A PIN diode between the patch radiator and the embedded corner segment switches the frequency between 5.6-5.8 GHz and the polarization between LHCP and RHCP. In [76], a reconfigurable patch antenna design with dual-probe coupler feed network was presented. The proposed antenna has two layers and an active element (varactor diode) installed between the inner and outer patch on the top layer. Polarization diversity is achieved using the probe feed network. Furthermore, the antenna can tune its operation frequency from 2.05-3.13 GHz by variation of the varactor diode capacitance between 12.33-1.30 pF.