# Recent Developments and State of the Art in Flexible and Conformal Reconfigurable Antennas
## 2 Frequency-Reconfigurable Antennas
### Mechanically Frequency-Reconfigurable Antennas
#### 2.2.3 Kirigami and Origami-Based Antennas  
In the class of mechanically reconfigurable structures, Kirigami and Origami-based antennas demonstrate RF characteristics that can be self-tuned by shape reconfiguration. These antennas with their unique features are able to change the shapes and sizes of the 3-D structures or transform 2-D shapes into other 2-D and 3-D shapes by employing flexible materials [70; 71]. This leads to the changing of the effective lengths [30], size [31] and type of antenna [32; 33]. Various types of frequency reconfigurable origami-based antennas such as helix antennas [31; 34; 35], microstrip [32], yagi [36], dipole [37; 38; 39] and monopole [40] antennas have been presented in the literature. In most of the existing 3-D origami helical antenna [41; 42; 43; 44; 45; 46; 47; 48; 49], accordion antenna [51], and spring structure [50], the frequency reconfigurability is achieved by changing the antenna's height. In [49], the proposed helical antenna is fabricated by copper foil on a paper, which switches between 1.82 and 2.14 GHz in folded state (H = 38 mm), and 860 MHz in the unfolded state (H = 255 mm). The same approach is presented in [43], in which origami quadrifilar helical antenna reconfigures between the normal mode at frequencies of 0.83 GHz, 1.17 GHz, or 1.5 GHz with H = 371 mm and axial mode at frequency of 1.23 GHz with circular polarization with H = 200 mm. In Reference [31], the frequency is tuned in the 3-D origami bifilar helical antenna by applying a force at its top and controlling its height. The antenna is made of copper tape on a sketching paper substrate with \(\varepsilon_{r}\) = 3.2. The frequency can be reconfigured among seven frequencies from 0.86 GHz to 3 GHz for three heights of 273 mm, 154 mm, and 25 mm. Another origami quadrifilar helical antenna is presented in [44]. The resonance frequency is tuned between 2.07 GHz, 3 GHz and 4.45 GHz by varying the height of antenna to 120 mm, 75 mm and 105 mm. Unlike the previously presented origami helical antenna, an origami reflector is used around this antenna to enhance its gain. This design is modified and an origami reconfigurable quadrifilar helical with circular polarization for higher frequency bands is presented [72]. The proposed antenna reconfigures between K, Ka, and extremely high-frequency bands by varying the height of antenna to 364 mm, 345 mm and 181 mm, respectively. Beside the helical origami structures, in spring antenna, the frequency can be tuned by changing the height of structure. In Reference [50], a tunable origami spring antenna with continuous frequency shifting from 1.1 GHz to 1.4 GHz is presented. The antenna is folded and unfolded with the help of an actuation system to the various heights from 60 mm to 145 mm. The actuation system is also employed in multi-radii monodilar helical antenna [73]. The antenna reconfigures between three bands of 1.22-1.84 GHz and 1.78-3.54 GHz and 3.64-4.04 GHz, with circular polarization. The antenna consists of a combination of two helical antennas with different radii and is fabricated from Kapton and copper trace. Frequency reconfigurability based on converting 2D structure into 3D shape is presented in [30; 37; 30]. In References [30; 37], reconfigurable origami-based dipole antennas with mechanically robust and durable structures have been introduced. In Reference [37], an E-textile accordion-dipole is fabricated on an Organza fabric. The frequency is reconfigured from 760 MHz to 1015 MHz range through changing the effective lengths by placing the antenna on a series of four Styrofoam fixtures which provide the changing angles of inclination from 0 to 60 degrees. A self-folding origami antenna that converts 2D structure into 3D shape is proposed in [32]. The antenna is a polystyrene subassembly that reconfigures between a microstrip patch antennas with a bandwidth of 1.5 GHz to 3.5 GHz to a monopole antenna with a bandwidth of 2.0 GHzto 2.3 GHz.