# Recent Developments and State of the Art in Flexible and Conformal Reconfigurable Antennas
## 2 Frequency-Reconfigurable Antennas
### Antennas Based on Electrical Switches  
Electrical switches are employed for the interconnecting of adjacent elements of the antenna patches [18; 19; 20; 21] or varying the length of the slots on the patch or ground plane [1; 22], modification on feeding networks [23] or switching between feeding ports [17] in order to realize frequency reconfigurability. Reconfigurability is limited to specific frequencies by employing a pin diode and MEMS switch, whereas continuous ranges of frequency reconfigurability are obtained by using varactor diodes. In some research, the frequency reconfigurable antennas performance is analyzed through simulation [19; 23] or it is verified through fabrication of prototypes that employ a 1 mm stub as an ideal diode. This is due to the challenges in the realization of the antenna with electrical switches such as placing diodes on unconventional materials, the need for a large RF bias network, and the effects of the nonlinearity of diodes and harmonics parameters. In other research, the conductive parts of the antenna where the diodes are placed are made of conventional materials to make the integration process easier [20; 22; 54]. In Reference [18], the microstrip patch antenna is placed on the Indigo blue jeans. A diode placed between two rectangles on the patch creates the reconfigurability between 2.4 GHz and 2.5 GHz for Bluetooth and WiMAX applications. In other research [23], two diodes are placed across a T-slot developed on the feed line of the rectangular patch antenna to reconfigure its frequency bands between 1.8 GHz, 2.3 GHz, and 2.4 GHz. In simulation analysis, ShieldIt Super is employed for the antenna conductive parts and it is placed on a Felt substrate with \(\varepsilon_{r}\) = 1.44. In Reference [19], dual-band reconfigurability is achieved through the connection of the two rectangular patches by eleven switches. An antenna is placed on the denim jeans textile and reconfiguring, between its two frequency bands 2.44 GHz (Wi-Fi) and 3.54 GHz (WiMAX). Further investigation has been done by fabricating designed flexible reconfigurable antennas, employing DC circuits, and the measurement. The diodes are placed on the antenna, and in some cases, DC circuits are integrated as well. In Reference [20], the proposed textile antenna reconfigures between 2.45 GHz and 5 GHz. The copper tape is used for the patch and ground plane, and a denim material is used for the substrate. The diode is integrated on the copper tapes and connects two adjacent patches to create these two frequency bands. For the proposed antenna, the DC is connected to the antenna by means of two wires. In Reference [22], the proposed antenna is placed on a flexible polyethylene terephthalate which reconfigures between two states: a single band structure at 2.42 GHz and dual-band with different polarizations at 2.36 GHz and 3.64 GHz. The structure is composed of a folded slot and the radiation characteristics of the stub is altered by a PIN diode placed on the stub. The antenna is fabricated with inkjet-printing technology. Further study has been done on this antenna in [54]. An artificial magnetic conductor (AMC) surface is added to this structure in order to improve its radiation performance and to reduce the specific absorption rate (SAR). The performance of both antennas has been measured for the flat and curved positions. To control the diodes, DC voltage is applied with the RF signal through the input connector to eliminate the need of complex DC biasing circuit, thus preserving the antennas flexibility. The same approach for biasing of the diode is employed in [24], but more challenges arise in the design process when unconventional materials are employed for the conductor and ground plane of the antenna. The proposed antenna in [24] is fabricated by embedding conductive fabric into the flexible polydimethylsiloxane (PDMS) polymer [55; 56; 57] and all the active and passive elements of the antenna are fully encapsulated in the structure (Figure 2).