Advances in reconfigurable liquid-based reconfigurable antennas are enabling new possibilities to fulfil the requirements of more advanced wireless communication systems. In this review, a comparative analysis of various state-of-the-art concepts and techniques for designing reconfigurable antennas using liquid is presented. First, the electrical properties of different liquids at room temperature commonly used in reconfigurable antennas are identified. This is followed by a discussion of various liquid actuation techniques in enabling high frequency reconfigurability. Next, the liquid-based reconfigurable antennas in literature used to achieve the different types of reconfiguration will be critically reviewed. These include frequency-, polarization-, radiation pattern-, and compound reconfigurability. The current concepts of liquid-based reconfigurable antennas can be classified broadly into three basic approaches: altering the physical (and electrical) dimensions of antennas using liquid; applying liquid-based sections as reactive loads; implementation of liquids as dielectric resonators. Each concept and their design approaches will be examined, outlining their benefits, limitations, and possible future improvements.
This work presents the design and optimization of an antenna with defected ground structure (DGS) using characteristic mode analysis (CMA) to enhance bandwidth. This DGS is integrated with a rectangular patch with circular meandered rings (RPCMR) in a wearable format fully using textiles for wireless body area network (WBAN) application. For this integration process, both CMA and the method of moments (MoM) were applied using the same electromagnetic simulation software. This work characterizes and estimates the final shape and dimensions of the DGS using the CMA method, aimed at enhancing antenna bandwidth. The optimization of the dimensions and shape of the DGS is simplified, as the influence of the substrates and excitation is first excluded. This optimizes the required time and resources in the design process, in contrast to the conventional optimization approaches made using full wave "trial and error" simulations on a complete antenna structure. To validate the performance of the antenna on the body, the specific absorption rate is studied. Simulated and measured results indicate that the proposed antenna meets the requirements of wideband on-body operation.
This paper presents the investigation of path loss variation for subject-specific on-body radio propagation channels, considering the effect of metallic spectacles and loop like metallic accessories. Adding metallic items may affect the operability of Body Centric Wireless Communications (BCWC). Measurements were carried out in an RF-shielded room lined with microwave absorbing sheets for strategically placed bodyworn antennas covering the upper front torso and the lower limbs. The path loss of the on-body radio channel was characterized explicitly taking into account the body size of the subjects. For metallic loop-like accessories, the results indicate that for underweight subjects, there was a slightly higher influence, up to 2%, compared to normal and overweight subjects. Our findings indicate that a noticeable effect exists on on-body channels for dynamic movements where the metallic watch acts as a local scatterer that affects the non-line-of-sight (NLOS) signal path between transmitter and receiver for underweight subjects in comparison to normal and overweight subjects. The path loss decreases when the receiving terminal was positioned very close to the metallic item. If a loop-like metallic accessory is not appropriately considered when designing the radio channel on a subject, the reliability of the body-centric wireless system may degrade.
This paper presents a compact circularly polarized (CP) antenna with an integrated higher order harmonic rejection filter. The proposed design operates within the ISM band of 2.32 GHz- 2.63 GHz and is suitable for example for wireless power transfer applications. Asymmetrical truncated edges on a square ring create a defected ground structure to excite the CP property, simultaneously realizing compactness. It offers a 50.5% reduced patch area compared to a conventional design. Novel stubs and slot shapes are integrated in the transmission line to reduce higher (up to the third) order harmonics. The proposed prototype yields a -10 dB reflection coefficient (S11) impedance bandwidth of 12.53%, a 3 dB axial ratio bandwidth of 3.27%, and a gain of 5.64 dBi. Measurements also show good agreement with simulations.
This work reviews design aspects of liquid metal antennas and their corresponding applications. In the age of modern wireless communication technologies, adaptability and versatility have become highly attractive features of any communication device. Compared to traditional conductors like copper, the flow property and lack of elasticity limit of conductive fluids, makes them an ideal alternative for applications demanding mechanically flexible antennas. These fluidic properties also allow innovative antenna fabrication techniques like 3D printing, injecting, or spraying the conductive fluid on rigid/flexible substrates. Such fluids can also be easily manipulated to implement reconfigurability in liquid antennas using methods like micro pumping or electrochemically controlled capillary action as compared to traditional approaches like high-frequency switching. In this work, we discuss attributes of widely used conductive fluids, their novel patterning/fabrication techniques, and their corresponding state-of-the-art applications.
In this paper, a compact textile ultrawideband (UWB) planar monopole antenna loaded with a metamaterial unit cell array (MTMUCA) structure with epsilon-negative (ENG) and near-zero refractive index (NZRI) properties is proposed. The proposed MTMUCA was constructed based on a combination of a rectangular- and a nonagonal-shaped unit cell. The size of the antenna was 0.825 λ0 × 0.75 λ0 × 0.075 λ0, whereas each MTMUCA was sized at 0.312 λ0 × 0.312 λ0, with respect to a free space wavelength of 7.5 GHz. The antenna was fabricated using viscose-wool felt due to its strong metal-polymer adhesion. A naturally available polymer, wool, and a human-made polymer, viscose, that was derived from regenerated cellulose fiber were used in the manufacturing of the adopted viscose-wool felt. The MTMUCA exhibits the characteristics of ENG, with a bandwidth (BW) of 11.68 GHz and an NZRI BW of 8.5 GHz. The MTMUCA was incorporated on the planar monopole to behave as a shunt LC resonator, and its working principles were described using an equivalent circuit. The results indicate a 10 dB impedance fractional bandwidth of 142% (from 2.55 to 15 GHz) in simulations, and 138.84% (from 2.63 to 14.57 GHz) in measurements obtained by the textile UWB antenna. A peak realized gain of 4.84 dBi and 4.4 dBi was achieved in simulations and measurements, respectively. A satisfactory agreement between simulations and experiments was achieved, indicating the potential of the proposed negative index metamaterial-based antenna for microwave applications.