Universal Journal of Electrical and Electronic Engineering 6(4): 239-258, 2019 http://www.hrpub.org DOI: 10.13189/ujeee.2019.060406 Reconfigurable Antennas and Their Applications Harish Chandra Mohanta1,2,*, Abbas Z. Kouzani1, Sushanta K. Mandal3 1School of Engineering, Deakin University, Australia 2Department of Electronics and Communication Engineering, Centurion University of Technology and Management, India 3Department of Electronics and Communication Engineering, Sharda University, India Received August 10, 2019; Revised September 12, 2019; Accepted September 21, 2019 Copyright©2019 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract Reconfigurable antennas are capable of impedance bandwidth, polarization, and radiation pattern dynamically altering their frequency, polarization, and as per the operating requirements of the host system. They radiation properties in a controlled and reversible manner. can radiate multiple patterns at different frequencies and They modify their geometry and behaviour to maximize polarizations. Obtaining the desired functionality for a the antenna performance in response to changes in their reconfigurable antenna and integrating it into a complete surrounding conditions. To implement a dynamical system to achieve an efficient and cost-effective solution is response, they employ different mechanisms such as PIN a challenging task for antenna designers. diodes, varactors, radio-frequency microelectromechanical Converting an antenna into a reconfigurable device by systems (RF-MEMS), field effect transistors (FETs), applying different techniques to change the antenna’s parasitic pixel layers, photoconductive elements, internal structure has been challenging. Multiple factors mechanical actuators, metamaterials, ferrites, and liquid need to be considered such as achieving a good gain, good crystals. These mechanisms enable intentional distribution efficiency, stable radiation pattern, and a good impedance of current on the antenna surface producing reversible match throughout all the antenna’s operation states. modification of their properties. This paper presents the To achieve a good gain, stable radiation pattern, and a design process and applications of reconfigurable antennas. good impedance throughout the operation states, the The activation mechanisms of reconfigurable antennas, reconfigurable antenna designers must focus on the and their design and operation optimization are discussed. following questions: (i) Which antenna property (e.g. The latest advances on reconfigurable metamaterial frequency, radiation pattern, or polarization) must be engineering, and the current trends and future directions modified? (ii) How are the radiating elements of the relating to reconfigurable antennas are reviewed. Finally, antenna structure reconfigured to achieve the required the applications of reconfigurable antennas in cognitive property? (iii) Which reconfiguration technique can radio, multi-input multi-output (MIMO) systems, satellite minimize the negative effects on the antenna communications, and biomedical devices are highlighted. performances? A reconfigurable antenna provides the same Keywords Radiation Pattern, Polarizations, Radio functionality as that given by multiple single-purpose Frequency Microelectromechanical System, PIN Diodes, antennas. This offers saving in costs, weight, volume, and Varactors, Metasurface, Optical Switches, Reconfigurable maintenance/repair resources. The following subsections Antennas present the definition of the critical parameters for antenna development. 1.1. Antenna Parameters 1. Introduction Antenna parameters can be classified into four Due to the rapid growth of wireless communications, categories: physical parameters, circuit parameters, and the high demand for the integration of multiple transition parameters, and space parameters. The physical wireless standards into a single platform, it is highly parameters include size, material, temperature, wind desirable that the operating frequency, radiation pattern, pressure, supporting structure, and coating. The circuit and polarizations of antennas can be reconfigurable [1]. parameters include input impedance, bandwidth, radiation Reconfigurable antennas modify their operating frequency, resistance, self and mutual inductance and gain. The 240 Reconfigurable Antennas and Their Applications transition parameters include loss, attenuation and mismatch. Finally, the space parameters are radiation pattern shape, beam width, directivity, radiation intensity, Over the years, antennas have been designed with input lobes and polarizations. impedance 50 Ω which perfectly match with 50 Ω The input impedance over frequency (typically called characteristics transmission line system, leads to reflection the frequency response) and the radiation (or radiation coefficient of zero, a VSWR of one and an RL of infinity. pattern) are two basic parameters which are used to Smith chart describes about the input impedance, reflection characterize an antenna. The frequency response is used for coefficient, and frequency behaviour that may be either the input impedance matching. Without the proper input inductive or capacitive. Smith chart provides the impedance match, the transmitting system will suffer from information about the antenna’s resonant behaviour as the component failure and power wastage due to severe antenna is resonant at frequencies at pure and convenient reflections, while the receiving system will suffer from input impedance. reduction in sensitivity and require additional amplification. The antenna’s frequency response determines its radiation 1.3. Radiation Characteristics pattern. Radiation pattern is one of the most important characteristics of antennas. It is a graphical representation 1.2. Frequency Response of antenna’s far-field radiation properties. It also describes Frequency response is the input impedance over the the ability of an antenna to transmit/receive signals in a frequency range. The complex input impedance can be direction. It can be specified by the unit decibels (dB). represented as: Polarization of an antenna’s radiation pattern can be defined as the change in the electric field vector with zin (ω) = R(ω) + jX (ω) (1) respect to the direction of propagation. Polarization is an important parameter in an antenna. It acts as a filter for where Zin( ω ) is the complex input impedance, R(ω ) is undesired signals. There are three types of polarizations resistance, X( ω ) is reactance and ω = 2πf is the which are linear, elliptical, and circular polarization [2]. In frequency in radian. The complex input impedance case of polarization, the antenna radiation pattern describes describes the antenna as a circuit element. The antenna’s the fundamental shape and characteristics of the antenna. input impedance determines the reflection coefficient ( Γ ), Beam width is one of the parameter which can be described return loss (RL), VSWR (voltage standing wave ratio). in a two-dimensional plane in the radiating volume of the ω − antenna. The half-power beam width of the antenna can be zin ( ) z0 defined as the parameter which measures the angle Γ = (2) Zin (ω) + z0 surrounding the direction of maximum radiation. It describes the antenna’s normalized radiation intensity is where Zin(ω ) is input impedance of the antenna and Z0 is greater than 0.5. the characteristics impedance of the transmission line. Γ The first null beam width can be defined as it is the angle between the first pattern nulls adjacent to the main lobe of is always a negative number. We can also use S11 to represent . the pattern. The two parameters which are half-power Γ beam width and full-null beam width describe the shape Return Loss=RL= -20 log Γ (3) and the extent of the main beam of the antenna. Directivity and gain describe the antenna efficiency. The The return loss is always a positive number. The possible antenna’s directivity can be defined as the ratio of the values of return loss range from 0 dB to ∞ dB. The voltage radiation intensity in a given direction to the average standing wave ratio (VSWR) is defined as: radiation intensity in overall direction. Gain can be defined +Γ as the ratio of the radiation intensity of a test antenna in a Vmax 1 VSWR = = (4) given direction, to the radiation intensity of an isotropic Vmin 1−Γ antenna. If a direction is not particularly specified for directivity Where Vmax is the maximum voltage amplitude of the or gain, the direction of the maximum radiation intensity is standing wave and Vmin is the minimum voltage amplitude implied which describes the loss as the loss does not arise of the standing wave. The possible VSWR values from 1 to from the impedance and polarization mismatch. If the ∞. antenna is ideal and there is no power dissipation, then the VSWR=1 means the transmission line is perfectly gain and directivity of an antenna can be interchanged. matched with antenna input impedance, and VSWR=∞ Antenna efficiency is an antenna parameter which captures means the transmission line is completely unmatched with the antenna’s losses and describes a relation related to the antenna input impedance. gain and directivity by the relation: Universal Journal of Electrical and Electronic Engineering 6(4): 239-258, 2019 241 G = ηD (5) 2. Classification of Reconfigurable where G is the gain of an antenna is the directivity D of an Antennas antenna