Reconfigurable Antennas: Theory and Techniques – A Survey

Introduction

Antennas are the front end elements and one of the most critical components of nowadays wireless communication systems and technologies. There are different types of antennas to utilize for different applications, but reconfigurability in antennas provides an additional layer of functionality to the system. In this technological growth, the need for adaptive wireless communication systems is more present than ever before. To satisfy this need, a robust communication system - which can be benefitted by the antenna’s reconfigurable properties is desired. In this scenario, multi-functional reconfigurable antennas serve gainfully, by adding interoperable communication between users. Multi-functional reconfigurable antennas are antennas which can dynamically be reconfigured in polarization, beam direction, frequency, etc. Recent trends reveal that modern mobile handsets are required to operate at multiple frequency bands and to be able to cover various commercial communication services.

Generally, though broadband and multiband antenna designers have reached important milestones on the path to achieve the above goal, the number of wireless communication systems is increasing dramatically, meaning that adjacent interfering signals between those systems for the broadband and/or multiband antennas are also increasing. It is desired that the antenna can be tunable in active ways to meet the requirements of covering multiple bands individually or simultaneously while being able to minimize or eliminate unwanted radio frequency interferences. Given the progress in electronic circuit packaging, embedded components for antennas have been studied widely for tunability and reconfigurability. It is common ground that a single smart-designed antenna having efficient bandwidth and enough tuning range can be used for different operating frequency bands.

This chapter investigates the concept of reconfigurability of an antenna's properties, such as frequency, radiation pattern, and polarization, for different types of antenna systems. In scientific literature, several techniques are proposed for tunable and reconfigurable antennas such as RF switches, micro - electromechanical systems (MEMS) switches, PIN diodes, varactor diodes, and screws. We review the advantages, disadvantages and tuning/reconfiguring mechanisms of all proposed techniques. Amidst them, we realize that variable reactive loading technique is very promising for designing antennas tunable over wide frequency ranges thanks to its excellent DC voltage controlled reactance and continuity properties.

Most types of antennas such as linear, loop, slot and even microstrip are designed with respect to resonance. The frequency, the bandwidth and the radiation pattern of the structure are defined by the effective electrical length of the antenna. In the case of a linear dipole, resonance is observed when the operating frequency is such that its electrical length is half a wavelength long. Under these conditions the dipole has an omnidirectional radiation pattern centered on its axis. To migrate to another operating frequency, just modifying its length to half of the matching wavelength is enough. The radiation pattern of the modified dipole is identical to the original which is a direct result of the current distribution remaining unaltered relative to the wavelength. Loop, slot and microstrip antennas behave in a similar manner.

MIMO (Multiple Input –Multiple Output) and diversity-related applications literature has recently been overwhelmed with several proposals for antennas with radiation pattern reconfigurability. Even though the majority of these antennas are planar and very compact in size, in order to be used in mobile applications, a great number of them still remain in proof of concept stages, just for demonstration capabilities only. The use of PIN diodes in order to adjust the flow of the electrical current within a structure, is a technique proposed quite often. Altering the states of the PIN diodes in the structure leads to a different current distribution which in turn results in another radiation pattern effectively creating multiple modes of operation for the structure. Moreover, the electrical length of the structure’s elements is altered with the inclusion of PIN diodes as well as the mutual coupling between elements in proximity. This causes the radiation pattern to alter as well (Yee, 2013).