## 3 Multiple Reconfigurable Features in Antenna
### Frequency and Radiation-Pattern Reconfigurable Antennas  
In this category, operation frequency or radiation pattern characteristics can be reconfigured [56]. The antenna radiation pattern can also be reconfigured between broad-side, end-fire, and omni-directional modes [57]; radiation pattern reconfiguration can enhance system performance, suppress noise, and save energy by improving signal directions. Frequency tuning is very useful to suppress interference from other wireless systems and also to reduce the number of the antennas required [58]. Due to this, a great number of antenna designs of this type have been reported [59; 60; 61; 62; 63; 64; 65; 66; 67]. The usual method in most designs is to switch the frequency operation of the antenna by using active elements, namely varactors or PIN diodes [59; 60; 61; 62; 63]. Pattern reconfiguration can be achieved using a modified ground plane with slot or slit structures to change the current distribution of the antenna substrate in order to steer the direction of the antenna radiation pattern [64; 65], or can use switchable directors at different sides of the main antenna radiator [66; 67]. In [58], a patch antenna design with dual-pattern frequency-reconfigurable characteristics was proposed. The antenna provides radiation patterns with monopolar and broadside modes and its frequency band can be switched using four varactors. A single-fed double-element array design with frequency and radiation pattern reconfigurability was introduced in [59]. The frequency operation of the antenna is tunable between 2.15-2.38 GHz. Beam scanning between \(\pm 23^{\circ}\) was obtained using a tuning mechanism. Another frequency and pattern reconfigurable antenna with combined monopole and patch antennas was proposed in [60]. A microstrip-fed patch to resonate at a lower frequency is printed on the upper layer and the monopole to operate at a higher frequency is placed in the bottom ground of a Rogers RO4350 substrate. Five PIN diodes, divided into group A (D\({}_{1}\), D\({}_{2}\), and D\({}_{3}\)) and group B (D\({}_{4}\) and D\({}_{5}\)), were employed. By changing the states of these two groups, the antenna can be switched into different modes. With group A ON, the antenna acts as an omnidirectional monopole radiator operating at 2.4 GHz. When group B is ON, the design performs as a broad-side patch antenna working at 5.5 GHz. Having both groups OFF, the monopole mode and patch mode are active. In [61], a reconfigurable slot antenna with a modified ground plane was reported. Two rectangular slots with six PIN diodes in the ground plane provide frequency reconfiguration with different radiation modes. The antenna can operate at eleven different frequencies of S and C band in the frequency range of 2.2-6.5 GHz. A wideband reconfigurable slot antenna for long-term evolution (4G-LTE) and C-band applications was presented in [62]. Its design contains a pair of symmetric slots fed by a fork-shaped feed-line. Pairs of PIN diodes (D\({}_{1}\) and D\({}_{2}\) and D\({}_{3}\) and D\({}_{4}\)) are loaded in the slotsand feed line to produce frequency and radiation pattern reconfiguration, respectively. Another design of a slot antenna with frequency and pattern reconfigurable properties was introduced in [63], where the antenna, fed by a coaxial port, is composed of four slits with three switches for tuning the scanning angle between \(\pm 15^{\circ}\). Two switches placed in the main slot radiator tune between three different narrow bands at 1.82 GHz, 1.93 GHz, and 2.10 GHz. BAR50-02V PIN diodes were used as switches. A flexible frequency and radiation pattern antenna design was proposed in [64]. Its schematic contains two symmetric hexagonal split rings with monopole branches and eight PIN diodes. By changing the diode states, the antenna frequency can switch between 1.9-2.4 GHz with pattern steering capability in two directions. In [65], a back-to-back F (BTBF) semi-circular antenna was presented (see Figure 6a), composed of a patch radiator with four identical elements.