# Flexible Antennas: A Review
## 5 Miniaturization of Flexible Antenna  
The desire to connect all electronic devices into the IoT, has accelerated the need for integrated smaller flexible antennas. As a result, research investigations for small antennas have been increasing. The major challenge for researchers in this field is reducing the size of the antenna in order to integrate with miniaturized devices without compromising the antenna performance parameters such as impedance matching, gain, bandwidth, radiation pattern, and efficiency. Although, it is a difficult and daunting task, researchers have found a number of creative approaches for shrinking antenna size. In the literature, there are various techniques proposed to reduce the antenna dimensions. Here, the focus is on the methods used for lowering the form factor of the flexible antennas. The applied methods can be mainly classified into three groups: material based miniaturization, topology-based miniaturization, and use of electromagnetic bandgap (EBG) structures. The first technique to reduce the flexible antenna dimension is utilizing high relative permittivity materials. The operating frequency relies on the dielectric environment of the antenna. The higher value of the dielectric constant (k), the smaller the size of the antenna. In an earlier study [187], the authors proposed a silver nanowire/nano paper composite based antenna that has a very high dielectric  
Figure 11: Flexible antennas for applications above 12 GHz. (**a**) Graphene antenna prototype for 5G applications [179], (**b**) millimeter-wave planar antenna prototype on PDMS substrate [185], and (**c**) electromagnetic bandgap (EBG) backed millimeter-wave multiple-input-multiple-output (MIMO) antenna for wearable use [186]. constant, k = 726.5 at 1.1 GHz. This k value was much higher than the typical flexible substrate used for flexible antenna such as polyethylene naphthalene (PEN) with k = 3.4, polyethylene terephthalalate (PET) with k = 3.1, and polyimide (PI) with k = 3.4. According to this research, using the proposed nanowire composite as a substrate downsized the antenna dimension by about a half. Bending and thermal tests were also conducted, showing the suitability of the composite for reducing the dimension of the flexible antenna. The same technique is applied in many other studies [188; 189; 190]. In another study [191], locally filled high permittivity substrate was used to reduce the antenna size. Antenna downsizing based on the change of topology is a popular approach. Altering the geometry, the current density distribution, and the electrical dimensions change the antenna properties. Optimization is needed to ensure the certain property of an antenna. There are different meandering lines used to increase the electrical length and miniaturize the flexible antenna. In an earlier study [192], asymmetric meander line was used to reduce the size of the flexible antenna and increase the gain. An earlier report [193] presented a compact dual-band flexible antenna that used a meander line to downsize the antenna and extract dual-band characteristics. The fractal antenna can provide characteristics like a larger antenna with a smaller dimension because of the efficient use of the area. By applying the Minkowski fractal geometry, miniaturization was achieved in a wearable electro textile antenna [194]. An ultra-thin flexible antenna that consisted of rectangular fractal patches with a stub was demonstrated in [195]. This rectangular fractal patch was shown to achieve 30% miniaturization compared to the traditional quadrilateral fractal patch. A defected ground plane was adopted as a method to control gain, radiation, and the dimension of an antenna. A compact wearable antenna with a double flexible substrate was designed with this concept [196]. Etching or printing slots on the flexible substrate is another way to manipulate the characteristics of the flexible antenna. [197; 198; 61] showed the compact, flexible antenna with slots in the patch.