# 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
The reconfigurability is realized by the concept of light activation and the bending of the polymer sheet assembly toward the light and away from the second port of the microstrip transmission line. The antenna is fabricated using RT/duroid 5870 with \(\varepsilon_{r}\) = 2.33. The transformation from a planar structure to a 3-D structures for frequency reconfigurability is employed in [33; 38; 40; 74]. A frequency switchable monopole antenna which is designed on an origami magic cube is proposed in [40]. The antenna reconfigures between 850 MHz to 1000 MHz and 1.3 GHz to 1.6 GHz for the unfolded and folded states due to the change in the length of the antenna. The same approach is used in [74] to design a dual-band antenna operating at 1.57 GHz and 2.4 GHz in the folded state and 900 MHz and 2.3 GHz in the unfolded state. The proposed antenna consists of a meandered monopole that is loaded by a microstrip open-ended stub. The open-ended stub controls the second resonance of antenna by changing its length (Figure 3). In Reference [38], a frequency-reconfigurable dipole antenna using an origami flasher is proposed. In this structure, the dipole length is increased in the folded state into a cube. The antenna frequency is switched between 750-800 MHz (folded) and 1.19-1.26 GHz (unfolded). The transformation from a dipole structure to a 3D conical spiral structure is made in a proposed morphing Nojima origami antenna in [33]. The antenna operates at 0.48 GHz and 2.5 GHz in unfolded (dipole antenna) and fully folded state (conical spiral antenna), respectively. Most origami-based antennas are designed using paper which creates poor durability and performance stability [75]. Moreover, to accurately control the change in the antenna dimensions and shape is not easy due to the high flexibility nature of paper. This also limits the range of the frequency reconfiguration to a couple of points. Therefore, in [75], a dipole antenna based on multi-material 3D printing technology is proposed, which can provide a continuous frequency reconfigurability from 0.95 GHz to 1.6 GHz through a precise structure manipulation by a robot. In Reference [76] a frequency reconfigurable monopole antenna based on kirigami technique is presented. A three-story tower kirigami is used for this antenna. This technique provides a more stable structure compared to a free-standing structure, as well as a volume reduction. In this antenna, a horizontal monopole antenna is designed on a two dimensional flat PET film with a frequency of 0.42 GHz, which can be stretched to the three-storey tower to operate at a frequency of 0.53 GHz. Most of the origami-based antennas use paper in their structure due to its high foldability, high thermal stability and lightweight. Despite the advantages of using paper in origami antennas, the roughness on the surface of paper creates some electrical properties of the conductive patterns. This can be addressed by coating the paper with polymers, but the folding of antenna will be affected. Therefore, in [77], a nonopaper approach with higher transparency and a smoother surface is presented for a foldable frequency reconfigurable antenna. The frequency of V-shaped silver nanowire antennas on nonopaper is switched between 2.3 GHz to 3.0 GHz in unfolded and folded states, respectively. One of the challenges of this type of antenna is its limited durability. The conductive parts of these antennas such as copper tape and conductive inks are prone to delamination and deterioration in  
Figure 3: The frequency reconfigurable antenna based on changing the antenna shape (Origami Magic Cube): (**a**) unfolded; (**b**) folded [74]. conductivity due to the extensive mechanical stress. In addition, substrates such as paper and memory polymers are not robust to extensive folding [37]. Although origami-based antennas are not practical for communication systems yet, they have the potential to be used with advances in material and actuator technologies [38]. In Table 1, the presented flexible frequency-reconfigurable antennas are listed.