# Recent Developments and State of the Art in Flexible and Conformal Reconfigurable Antennas
## 2 Frequency-Reconfigurable Antennas
### Antennas Based on Electrical Switches
To realize the antenna reconfigurability, two varactors are placed between a rectangular patch and a parasitic patch (made of conductive fabric) and are biased with same DC voltage. The frequency resonance is tuned continuously between 2.3 GHz and 2.65 GHz. The antenna in [21] consists of a rectangular-grounded loop which is excited by a monopole antenna fed by a coplanar waveguide. A U-shaped slot is loaded to the monopole to achieve dual-band operation. This dual polarized antenna operates within 2.21 GHz to 2.69 GHz and 3.14 GHz to 3.55 GHz bands. The resonance of the higher band is tuned by switches placed within the U-slot. Another approach for designing a reconfigurable antenna is by switching between two modes of the antenna through two-port excitation [17]. The proposed graphene conductive ink printed textile-based microstrip antenna in [17] can switch between 3.03 GHz and 5.17 GHz. The frequency reconfigurability is achieved by exciting the two modes of TM\({}_{02}\) and TM\({}_{20}\) through two-port excitation. A challenge of the switching between these two ports is addressed by introducing a single pole single throw switch to the structure. DC circuits are integrated in different parts of antenna such as ground plane [1; 58] or within the structure. In References [25; 59], DC control circuits are integrated within commercial snap-on buttons. The snap-on reconfigurable wearable antennas are proposed to address the challenge of connection between rigid components to textile materials. This dual-band reconfigurable textile patch antenna is fabricated and the measurement results are reported showing a tuning range of 32.8% and 8.8% at 2.45 GHz and 5.8 GHz ISM bands, respectively [25]. The methods discussed so far have all been based on pin diodes and varactors. RF MEMS switches are also widely employed by researchers for realization of reconfigurability due to their low loss and distortion, small size, and high isolation [60]. The MEMS switch geometry and materials can be tailored to meet a specific size requirement and the desired actuation voltage, respectively [60; 61]. While in most published literature, MEMS switches are integrated on the rigid substrate and one switch topology is used, great effort in integrating MEMS switches on flexible substrate is demonstrated in [62]. A multiband Sierpinski fractal antenna is proposed in [60], which is realized on a liquid crystal polymer (LCP) substrate, with three sets of used RF MEMS switches. Each switch has a different actuation voltage. The antenna operates at four frequencies between 2.4 GHz and 18 GHz without the need of extra lines to bias the switches. As shown above, the main challenges for realizing frequency reconfigurable antennas with the electrical switches are based on the need for a DC-bias, and the adverse effects of rigid components on the antenna's flexibility. Moreover, in reconfigurable antennas based on electrical switches, antenna performance is degraded. The harmonics and intermodulation distortion is generated because of diode nonlinearities and gain compression.