**1. Introduction**

A reconfigurable antenna is an antenna having a capability to modify its properties dynamically, in a controlled and reversible manner. The need for multifunctional (e.g., direction finding, beem steering, radar, control, and command), highperformance and cost-effective devices within a confined volume places a greater burden on today's transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. Reconfiguring an antenna is achieved through deliberately changing its frequency, polarization, or radiation characteristics. Many techniques are there to achieve this change by redistributing antenna currents and thus altering the electromagnetic fields of the antenna's effective aperture, thereby adapting to changes in environmental conditions or system requirements (i.e., enhanced bandwidth, changes in operating frequency, polarization, and radiation pattern) [1, 2]. This concept can significantly reduce the number of components, hardware complexity, and cost of the communication systems. The first patent on reconfigurable antennas appeared in 1983 by Schaubert [3]. Excellent overview of reconfigurable antennas, with many examples, is given in [4].

The type of reconfiguration and the technique to achieve need to be addressed before designing an antenna. There are four fundamental reconfigurable properties of antenna, i.e., frequency of operation, radiation pattern, polarization, or a combination of any of these properties [4].

These antennas are those antenna systems that are capable of modifying their properties such as frequency, pattern, and polarization dynamically, within a controlled strategy, and with permissible reversibility. The reconfigurability of the antenna array is based on the modification of the geometry of system or behavior of its elements with regular maintenance of the efficiency of the antenna against changes on its environment and mission objectives. Reconfigurable antennas are usually result in low cost, lightweight, low volume but with complexity of maintenance and proper repairing due to being able to offer the same functionalities as multiple conventional antennas [5]. Moreover, the modification of the antenna elements in terms of electrical characteristics causes strong complexity in design of feeding networks results [6–8], and it is more worse if extra controlling elements are added. Due to this reason, the use of arrays technology by introducing parasitic elements in antenna is very common. Because of these parasitic elements, extra current is induced due to nearfield effects without any need of a feeding network in its design, creating beam pattern reconfiguration with a noticeable very low complexity [9, 10]. The most common antennas that use parasitic elements based on linear dipole arrays are known as Yagi-Uda antenna arrays [11]. In such antenna geometry, one more element is added along with parasitic linear dipole known as driven, used in front of those to get reconfiguration in radiation pattern. Also, linear and planar arrays are being used to achieve same goal by use of switch on and off condition, one by one in the same elements [9]. One more antenna that is printed dipole antenna having single parasitic element is used for wide scanning at the cost of little deviation in the gain of antenna, known as beam scanning antenna [12]. As of interest in beam pattern reconfiguration for the antenna, few dielectric materials can also be responsible for reconfiguration by changing its value due to environment behavior and surrounding, giving impressive results for so many applications. As relative dielectric constant of medium, which is gaseous, particularly air is the main ingredient having constant near unity, is basically changed due to change in temperature, pressure, and humidity that can easily be checked during hygrometry measurements using Yagi-Uda antennas [13–15]. So, the environment effect on dielectric constant is widely used to achieve reconfiguration. Later, this constant can also be changed in the presence of airborne particles and due to pollution in industrial and urban area such as droplets in clouds, which is relevant in today's antenna design [16]. One more aspect is important during the design of reconfigurable array antenna, which is dynamic range ratio that should be controlled in the amplitude of excitation, which is one main constraint. This dynamic range ratio is the ratio between the maximum and the minimum excitation amplitude of the array elements in antenna, which allows the practical realization of feeding networks, under which noncomplex design is offered with less number of power dividers or the design of micro stripline should become simple. There are so many methods given to work together with pattern and feeding network, which are mainly to reduce the dynamic range ratio, specified value for each excitation amplitude, or controlling phase. Reconfigurability in array antennas is second significant capability that may be requested nowadays from antenna designer to get wide applications. In current scenario, communication systems might in fact have to accomplish multitasking missions, in which the pattern must be reshaped by keeping the constant the value of excitation amplitudes of the elements and also modifying the sole excitation phases. Getting frequency reconfiguration and polarization reconfiguration without changing the parameters of antenna is also very promising and difficult task.
