Reconfigurable antennas play important roles in smart and adaptive systems and are the subject of many research studies. They offer several advantages such as multifunctional capabilities, minimized volume requirements, low front-end processing efforts with no need for a filtering element, good isolation, and sufficient out-of-band rejection; these make them well suited for use in wireless applications such as fourth generation (4G) and fifth generation (5G) mobile terminals. With the use of active materials such as microelectromechanical systems (MEMS), varactor or p-i-n (PIN) diodes, an antenna’s characteristics can be changed through altering the current flow on the antenna structure. If an antenna is to be reconfigurable into many different states, it needs to have an adequate number of active elements. However, a large number of high-quality active elements increases cost, and necessitates complex biasing networks and control circuitry. We review some recently proposed reconfigurable antenna designs suitable for use in wireless communications such as cognitive-ratio (CR), multiple-input multiple-output (MIMO), ultra-wideband (UWB), and 4G/5G mobile terminals. Several examples of antennas with different reconfigurability functions are analyzed and their performances are compared. Characteristics and fundamental properties of reconfigurable antennas with single and multiple reconfigurability modes are investigated.

https://www.mdpi.com/2079-9292/8/2/128/pdf

Recent Developments of Reconfigurable Antennas for Current and Future Wireless Communication Systems

In this paper, a flexible antenna with a simple structure, small size (15mm×17mm×1mm), and light weight for medical wearable applications is proposed. The antenna consists of slot radiation elements fed by a coplanar waveguide structure and a floating ground. Flexible conductive cloth MKKTN260 is used to fabricate the antenna, while felt with good insulation is used as the substrate. The measured impedance matching bandwidth (|S
 11
|less than -10dB) of the antenna is from 5.578 to 5.898GHz, which can cover the whole ISM 5.8GHz band (5.725-5.825GHz) and perform good wearable radiation characteristics. The floating ground structure and the interaction between slots make the antenna radiated efficiently (the measured peak realized gain is 4.85dBi at 5.8GHz) to the +Z direction, which is away from the human body. Meanwhile, without any specially designed reflecting structures, the specific absorption rate value is below the criterion set by the FCC and ICNIPR. The bending configurations are also studied for this antenna.

/pdf/cpw-fed-slot-antenna-for-medical-wearable-applications-1vbreswn56.pdf

CPW-Fed Slot Antenna for Medical Wearable Applications

In this paper, a low-profile wearable antenna with metamaterial (MM) for wireless body area network (WBAN) applications is presented. The designed antenna with MM operates in the ultra-wideband (UWB) between 4.55 and 13 GHz and it has a thickness of 4.68 mm. To the best of our knowledge, it is the lowest thickness reported in the literature for UWB antennas with MM. The proposed is designed and manufactured using fully flexible textiles. The designed antenna was simulated in free space and on the human body model. Simulation results show that gain, directionality, and front-to-back ratio of the antenna increase considerably with the placement of the MM. Also, in simulations, it is found that the specific absorption rate (SAR) values for the designed antenna reduce by 98.3% when MM is used. These SAR values calculated for the designed antenna with MM are well below the limits defined in European standards. The designed antenna and metamaterials were manufactured, too, and scattering parameters were measured. Measurement results are in good agreement with the results found in the simulations. It shows that the proposed antenna is very suitable for use in WBAN applications due to its low thickness, having low SAR, and UWB operation.

Design and analysis of low profile and low SAR full-textile UWB wearable antenna with metamaterial for WBAN applications

The paper presents a highly efficient, low cost, ultra-wideband, microstrip monopole antenna for microwave imaging and wireless communications applications. A new structure (z-shape, ultra-wideband (UWB) monopole) is designed, which consists of stepped meander lines to achieve super-wide bandwidth and high efficiency. Three steps are used to design the proposed structure for the purpose to achieve high efficiency and wide bandwidth. The antenna bandwidth is enhanced by varying the length of meander line slots, optimization of the feeding line and with the miniaturization of the ground width. The simulated and measured frequency bands are 2.7–22.5 GHz and 2.8–22.7 GHz (156% fractional bandwidth), respectively. The dimensions of the antenna are 38 mm × 35 mm × 1.57 mm, and its corresponding electrical size is 2.41 λg × 2.22 λg × 0.09 λg, where guided wavelength λg is at the center frequency (12.75 GHz). This antenna achieved a high bandwidth ratio (8.33:1). The realized gain is varying from 1.6–6.4 dBi, while that of efficiency is 70% to 93% for the whole band. Radiation patterns are measured at four operating frequencies. It has an acceptable group delay, fidelity factor, and phase variation results that satisfy the limit of ultra-wideband in the form of the time domain.

https://www.mdpi.com/1424-8220/20/1/115/pdf

High Efficient and Ultra Wide Band Monopole Antenna for Microwave Imaging and Communication Applications.

Wearable antennas have received a great deal of popularity in recent years owing to their enticing characteristics and opportunities to realize lightweight, compact, low-cost, and versatile wireless communications and environments. These antennas must be conformal, and they must be built using lightweight materials and constructed in a low-profile configuration when mounted on various areas of the human body. These antennas ought to be able to function close to the human body with limited deterioration. These criteria render the layout of wearable antennas demanding, particularly when considering factors such as investigating the usability of textile substrates, high conductive materials during fabrication processes, and the effect of body binding scenarios on the performance of the design. Although there are minor differences in magnitude based on the implementations, several of these problems occur in the body-worn deployment sense. This study addresses the numerous problems and obstacles in the production of wearable antennas, their variety of materials, and the techniques of manufacturing alongside with bending scheme. This is accompanied by a summary of creative features and their respective approaches to address these problems recently raised by work in this area by the science community. Â© 2020. All Rights Reserved.

/pdf/recent-advances-in-wearable-antenna-technologies-a-review-54qzusyl6a.pdf

Recent Advances in Wearable Antenna Technologies: A Review

A compact frequency-agile multiband filtenna is presented in this letter. The proposed reconfigurable filtering antenna performs independent switching between four operating bands viz. 1.8 GHz (GSM), 2.4 GHz (Bluetooth), 3.5 GHz (WiMAX), and 5.2 GHz (WLAN). This switching allows the unused bands to be utilized by the secondary user, making the antenna suitable for cognitive radio applications. An elliptical wideband monopole antenna is excited through a reconfigurable bandpass filter (BPF) integrated onto the 50 Ω transmission line. The BPF comprises four distinct structures to tune to four different frequencies using p-i-n diodes. The proposed filtenna has the reflection coefficient less than −20 dB for all the desired operating bands. Furthermore, this multiband filtenna offers gain of about 1.1, 2.6, 3, and 3.4 dBi at the four frequency bands. The prototype filtenna is fabricated, and the simulation results are verified experimentally.

Multiband Reconfigurable Filtering Monopole Antenna for Cognitive Radio Applications

This paper presents a bandwidth enhanced, compact planar ultra-wideband antenna design for wireless personal area communication (WPAN) applications. The proposed antenna has fractal based geometry and is constructed using several iterations of a pentagon slot inside a circular metallic structure. The partial ground plane of the basic radiator is tapered, defected and a U slit is etched out from the microstrip feed to improve the −10 dB |S11| bandwidth. The proposed fractal based antenna has an impedance bandwidth from 2.9 GHz to 15 GHz with low profile configuration and is fabricated on FR4 substrate with dimensions of 32 mm × 32 mm × 1.6 mm. To authenticate the designed prototype, the antenna is fabricated and tested for impedance and radiation characteristics. The designed antenna has stable radiation characteristics in the operating band. Furthermore, the antenna is validated for its applicability in WPAN, by calculating fidelity factor through time domain analysis along with the transmission coefficient and group delay measurements.

Fractal based ultra-wideband antenna development for wireless personal area communication applications

In this paper, flexible, low profile and robust planar monopole antennas are designed and analyzed for its performance over conformal surfaces, intended for Ultra-Wideband (UWB) and Body Centric Wireless Communications (BCWC). By the aid of alike shape (modified rectangular shaped) radiator and truncated ground plane, two compact microstrip fed antennas are designed and fabricated on flexible substrates – Polyamide and Teslin (environmentally friendly, flexible and green-electronics). The proposed polyamide antenna with a size of 50 mm × 40 mm × 0.8 mm, radiates over frequency range from 1.8 to 13.3 GHz and teslin antenna with a dimension of 50 mm × 40 mm × 0.712 mm, covers the frequency range from 1.4 to 13.4 GHz. To validate the designed prototypes, the antennas are fabricated and the impact performance are analyzed, while flexing the antenna from its nominal straight arrangement (bending through its length, width, different bending radius and angles). Further, time domain analysis is carried out in free space and in various bent scenarios with pulse excitation to validate the antennas for UWB applications. The transmission coefficient characteristics (S21) and group delay for various bending conditions are measured and the fidelity factor for all the scenarios are greater than 0.82, which make the antennas good candidate for UWB-BCWC.

On the bending and time domain analysis of compact wideband flexible monopole antennas

This paper exhibits the design and implementation of a compact dual-band bandpass filter on paper substrate. The proposed filter which works on the principle of signal interference technique has a parallel combination of short-circuited coupled line and an H-shaped network of transmission lines with open stubs. Due to the superposition of signals of the two transmission paths, a second order dual-band bandpass response with four transmission zeroes is achieved. To validate the proposed design, theoretical analysis using the lossless transmission line model is carried out and acknowledged. The developed dual-band filter prototype with center frequencies 1.17 GHz and 4.06 GHz has 3-dB fractional bandwidths of 82.56% (0.69–1.656 GHz) and 20.59% (3.645–4.481 GHz), respectively. The simulated and experimental results are in good congruence with each other. The proposed dual-band bandpass filter can be used in various wireless applications, for example, Global System for Mobile Communications (880–915 MHz), Aircraft surveillance (960–1215 MHz), Indian Regional Navigation Satellite System (1.164–1.188 GHz), Amateur radio (1.24–1.30 GHz), Global Positioning System (1.559–1.61 GHz), Satellite phones (1.616–1.626 GHz), and Satellite communication (3.7–4.2 GHz).

Rama Rao Thipparaju

Papers

Multiband Reconfigurable Filtering Monopole Antenna for Cognitive Radio Applications

Fractal based ultra-wideband antenna development for wireless personal area communication applications

On the bending and time domain analysis of compact wideband flexible monopole antennas

Implementation of a compact dual-band bandpass filter using signal interference technique on paper substrate

Interdigitated photoconductive terahertz antenna for future wireless communications