# Flexible Antennas for a Sub-6 GHz 5G Band:
## 4 Flexible Antennas for a Sub-6 GHz 5G Band
### Single Element Flexible Antennas for the Sub-6 GHz 5G Band
#### 4.1.3 Metamaterial-Based Flexible Antenna  
Metamaterials (MTMs) are artificial structures that exhibit unique electromagnetic properties that cannot be seen in natural materials. These materials provide a low resonant frequency, compared to the resonant frequency of the antenna, resulting in the antenna miniaturization and also helping in enhancing the performance characteristics of the antenna including the gain, bandwidth, etc. The miniaturization of the antenna, based on MTMs, can be of various types, such as a metamaterial loading, a metamaterial inspired antenna, a meta surface loading, and a composite right/left hand material(CRLH) [54]. The following section gives a review of the metamaterial-based flexible antenna in the literature that is suitable for sub-6 GHz 5G applications.  
A low-profile and a dual-band CPW-fed elliptical ring with a split-triangular patch antenna with an enhanced gain, is proposed in [55]. The antenna is designed on a flexible polyimide material and resonates at 2.60 GHz with a bandwidth ranging from 2.52-2.62 GHz and at 3.48 GHz with a bandwidth ranging from 3.31-3.64 GHz, observed at two frequency bands. The elliptical complementary split ring resonator (ECSRR) improved the gain of the antenna to 2.39 dBi and 1.75 dBi, respectively, for 2.60 GHz and 3.48 GHz. The triangular slot as well as the rectangular slit increased the electrical length for tuning and thus making the antenna work for the dual-band operation. To check the conformal nature of the antenna, a convex bending across the different radii of 30, 15, and 10 mm is performed and it is found that the performance of the antenna is not significantly changed with the bending effect. The radiation pattern of the antenna at 2.6 and 3.48 GHz gives a directive radiation pattern across the E-plane and an omnidirectional radiation pattern in the H-plane.  
In another approach in [56], a circularly polarized metamaterial-loaded flexible antenna is fabricated on a flexible liquid crystal polymer substrate. Using metamaterial comprising of split ring resonators and a Complementary Split-ring Resonator (CSRR) on either side of the feedline and ground, helped in achieving a circular polarization. The antenna provides a bandwidth ranging from 3.4 to 3.78 GHz and 5.7 to 6.9 GHz. The boresight axial ratio is found to be less than 3 dB in both bands. The bending test of the antenna is carried out at different bending angles of 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 120\({}^{\circ}\). The impact of bending is mostly seen in the second band. The 30\({}^{\circ}\) angle shows a triple band variation. However, there is no significant variation for other bending angles compared to the planar antenna. The highest axial ratio is found at 30\({}^{\circ}\) further than the others.  
An artificial magnetic conductor is another kind of meta surface loading that is seen for the miniaturization of the antenna, as well as improving the performance of the antenna. A wearable high gain antenna that operated at 3.5 GHz, as well as 5.8 GHz with a low SAR is proposed in [57]. The antenna, which is placed over an AMC array, are fabricated on flexible Rogers ULTRALAM 3850 and RO3003 substrates, respectively. The proposed antenna exhibits a dual band resonance and acceptable gain at a distance of 15 mm from the human body model. In-order to reduce such separation and to enhance the gain and reduce the SAR, an AMC array is utilized. Cutting the edges of the conventional square patch to a triangular-shaped patch, cutting slots in the CPW ground plane, and cutting slots and slits in the radiator, help in achieving the single band, dual-band, and lowering the second frequency, respectively. To check the SAR, the antenna is placed at different distances over a human body model with skin, fat, and muscle tissues of thicknesses of 2, 8, and 23 mm, respectively. The SAR is measured as 0.0.683 W/kg at 3.5 GHz and 0.333 W/kg at 5.8 GHz for 1 g of tissue and 0.0226 W/kg at 3.5 GHz and 0.0814 W/kg at 5.8 GHz.