Design of Compact Antennas With Metasurface for Wideband and Wireless Applications

Abstract

An antenna is one of the essential elements in a wireless system, that converts the guided waves in an electronic circuit to unguided waves in the air and vice versa. They are often designed according to the specifications of the underlying system. Compact antennas are required in miniaturised systems such as those used in an aircraft. They are designed by modifying or appending the antenna with additional structures or circuit elements without degrading its responses. In this thesis, the design of compact antennas is investigated with metasurface for two unique purposes - i. wideband applications for detection/sensing application, and ii. spatial modulation to communicate a multipath environment.

For wideband applications, a spiral antenna is considered a primary radiator due to its wideband impedance matching and circular polarization (CP) response with simple and planar geometry. It has a bidirectional radiation pattern on either side of the structure, along the axis of the antenna. But in many practical applications, a single-sided radiation pattern is extracted by placing it above a metallic body of a ship or aircraft, which disturbs the freestanding radiation response of the antenna. A conductor placed more than half a wavelength away from the spiral reduces the boresight gain significantly at high frequency, whereas the same placed too close to the antenna degrades the matching and polarization performance at low frequency. These issues have been addressed over years with different techniques, but the design of compact spiral still possesses significant challenges especially when a frequency band of 1-18~GHz is considered.

As this research work begins, the spiral is placed at different heights above a metallic conductor and the effects are observed over the considered frequency range. It is followed by an investigation with profiled metallic geometries to combine the benefits of varying antenna heights at different frequencies. Based on these observations, a compact spiral antenna is designed by placing it above a modified conical conductive backing to radiate a CP wave over a wide frequency band.

In the next part of this thesis, some of the challenges at low frequencies are addressed using different absorber techniques when the spiral is kept extremely close to a conductor. A hybrid technique consisting of absorbing material and resistors is proposed to design such a compact spiral antenna for wideband application. To improve the performance below 2~GHz, a wideband metasurface absorber is investigated with the spiral. The metasurface possesses significant electromagnetic absorption at low frequency and has been used to design a spiral antenna for 1-18~GHz with an extremely low profile.

Another work with a compact spiral antenna approaches to tilt its main beam over a wide frequency range. This investigation is required to compensate for the shift in the antenna main beam due to the supporting structure or to tilt the antenna main beam in a given direction for different purposes. A semicircular lens made of lossy dielectric material is placed above a compact spiral to fulfil this requirement. Effects of different material properties and lens profiles are investigated to arrive at the final design. Since placing the lens along the spiral affects the compactness of the antenna and disturbs the planar profile required in a flush mounting configuration, a sectoral metasurface is designed and printed on the backside of the antenna substrate. The metasurface possesses effective material properties to tilt the antenna main beam at a consistent angle.

For all cases, numerical investigations were carried out to optimize the antenna geometries followed by prototyping and characterization of some of these structures. The measured results are compared with the simulated outcomes and the numerical predictions have been verified. This required the design and realization of a wideband balun and appropriate fixtures to integrate various parts of this antenna in a flush-mount arrangement.

For a unique wireless application with a compact antenna, a digitally reconfigurable metasurface in the vicinity of a patch antenna is proposed, to realize for the first time a modulator for a spatial modulation technique known as media-based modulation (MBM). MBM facilitates a fast, secure, and multiuser wireless link in a multipath environment (e.g., indoor or office environments) by exploiting the multipath components of the channel. The metasurface works as an electromagnetic window as the power flowing through the unit cell can be electronically controlled by switching a PIN diode embedded within. A significant difference in transmission coefficient is observed between the two switching states of the unit cell. A meander geometry is used to make it compact and the diode is placed between the meander and one of the two contiguous strips that provides the necessary biasing to the diode. Numerical investigations are carried out to characterize the unit cell, and to optimize the array dimensions and the gap between metasurface and antenna. A prototype of the array is fabricated with the necessary control circuitry and a complete wireless link is set up to communicate in a real-time environment. Experiments are carried out in different scatter free and scattering environments in line of sight and non-line of sight configurations to validate the theoretical predictions of MBM. The effects of multipath as a factor that improves communication performance are also validated. In the end, data transmission over a wireless link is also demonstrated using this scheme.