## 3 Multiple Reconfigurable Features in Antenna
### Radiation Pattern and Polarization Reconfigurable Antennas  
Radiation pattern and polarization reconfigurable antennas offer benefits such as providing pattern diversity and supporting different polarizations on a single element antenna radiator [77]. They also can improve the capacity of communication systems, increase signal power, improve radiation coverage, and avoid polarization mismatching. Radiation pattern reconfigurability can steer the antenna pattern to satisfy system needs [78], and can improve channel capacity without increasing a radiator's volume [79; 80]. Many such antennas have been proposed recently [77; 78; 79; 80; 81; 82; 83; 84]. An omnidirectional microstrip-fed patch antenna with a switchable polarization and radiation pattern was introduced in [77]. A pair of back-to-back coupled patches fed at a corner, and a shared ground plane. A phase shifting technique provides a high degree of polarization and radiation pattern reconfigurability. An antenna with switchable radiation patterns and polarization diversity was proposed in [78]. The design concept is to place reconfigurable parasitic elements around the main radiator. Each parasitic element includes a dipole radiator with a PIN diode, whereby the radiation pattern can be switched into different directions by changing the diode states. Furthermore, the antenna can be dynamically reconfigured into three different polarizations. In [79], a PIFA was proposed with radiation pattern and polarization diversity; the main radiator has an inverted F-shaped parasitic structure. Using only one diode, the antenna can switch the antenna pattern and polarization properties in the frequency range of 2.357-2.562 GHz. In [80], a reconfigurable antenna design with switchable radiation pattern and polarization diversity for the band of 2.45-2.65 GHz was introduced. The antenna includes three circular substrate disks for radiator, feeding and switching along with four plastic screws to support the substrates, with an annular slot structure etched in the circular patch radiator. The reconfigurable feed network with one input and four output ports and with L-shaped feeding probes are placed in the middle layer which includes three power dividers. The antenna can provide switchable radiation beams for broadside and conical circular polarized modes. In [81], a design of a reconfigurable antenna with a metasurface structure was proposed. The overall structure has three layers. The metasurface, to improve the antenna gain, was on the top layer, with the ground plane and the microstrip feed-line on the middle and bottom layers respectively. The antenna is designed to work at 5 GHz, with switchable polarization using the PIN  
Figure 7: **(a)** Configuration and **(b)** performance of the frequency and polarization reconfigurable antenna in [73]. Reproduced with permission from [73], Copyright IEEE, 2015.  
diodes in the ground plane. In addition, the antenna radiation pattern can be steered between \(\pm 20^{\circ}\) by controlling the PIN switched in the metasurface. A 0.9 GHz reconfigurable quadrifilar helical antenna with a switchable feeding network was proposed in [82]. Figure 8 shows the cuboid radiator, reconfigurable feeding-network, biasing network, and the fabricated prototype. By switching the diode states, the antenna can provide four different modes with polarization and radiation pattern reconfiguration. It has a low profile and provides wide bandwidth with broad axial ratio characteristics and LHCP/RHCP. Another pattern and polarization reconfigurable antenna was presented in [83]. The radiator is a square patch with four coupled parasitic structures fed by a coaxial cable. Using PIN diodes, the antenna provides three different modes for polarization and pattern diversity at one frequency. A multi-layer wideband reconfigurable circular polarized (CP) antenna with four radiating arms was proposed in [84]. The radiating arms are connected to a reconfigurable feeding network with PIN diodes to provide wideband CP waves in RHCP and LHCP modes.