Investigations on Ultra-Wideband (UWB) Monopole Antennas for Spectrum Monitoring

Abstract

Electro Magnetic Spectrum (EMS) is a precious natural resource that is getting consumed fast and every Hertz of frequency is being grabbed. Recycling, harmonisation and allotment of whatever is left has become a management challenge for regulatory bodies around the world. Spectrum monitoring is a vital part of spectrum management to keep a check on interference, EIRP compliance and proper utilisation of this limited resource. Effective spectrum monitoring requires sophisticated equipment like Automated Frequency Management System (AFMS). The acquisition of AFMS is difficult to most of the developing countries due to the high cost and non-availability of technology. Thus, there is a compelling need to design an UWB antenna, light in weight with moderate gain to match up with commonly available spectrum analysers, to detect and monitor frequency spectrum. Microstrip antennas were first conceptualised by Deschamps in 1953 before Munson and Howell practically demonstrated it in 1970s. A microstrip patch antenna is fabricated by etching the antenna element pattern in metal trace, bonded to an insulating dielectric substrate with radiating elements and the feeding mechanism. A single UWB antenna would also avoid the requirement of deploying multiple antennas for the spectrum range. The onerous task of spectrum management can thus be facilitated to a great extent by the proposed single UWB microstrip antenna. The design feasibility of a microstrip patch antenna capable of operating from a few GHz to 40 GHz and above, with omni-directional radiation patterns and moderate gain to utilise it for spectrum management was explored. Ten (10) different UWB microstrip monopole patch antennas with different radiating patch shapes and sizes have been studied using the finite-element method (FEM) based high-frequency simulation software –HFSS V13. All the designs have been simulated and specially optimised for compact size and maximum bandwidth using the HFSS. While the ideal omni-directional monopole characteristics were recorded at lower frequencies of the spectrum, the radiation patterns gradually become directional at the higher frequencies (size of antenna becomes large relative to wavelength). Different techniques to improve the bandwidth have been used. These include, change in shapes of a radiating patch, multi-stepped feed network, narrow slot of different shapes into the radiating patch of an antenna and ground plane below the feed network. Comparative performance of the CPW-fed patch antenna and microstrip feed patch antenna was examined in terms of the bandwidth and the radiation patterns at high frequencies. The CPW-fed antenna performed better at higher frequencies despite limitations in copper-etching accuracy. Simulations were validated with actual measurements recorded. Variations in recorded measurements are attributable to the factors such as manufacturing tolerances, undercuts while etching copper at sharp corners, effect of SMA connector at different frequencies and the limitations of the simulation software. Research work carried out during the Ph.D. course has been presented in total of five chapters. Chapter 1 covers the objectives with a brief description of the thesis content. The literature survey carried out on design of printed monopole antennas with emphasis on achieving higher bandwidth has been laid out. Chapter 2 deals with the simulation and design of printed antennas operating up to 20 GHz covering the entire FCC-authorized frequency band. The simulated and measured returnloss and parametric analysis of the antennas under consideration are discussed in this chapter. Chapter 3 presents the design and development of printed monopole antennas able to operate satisfactorily from 3 to 30 GHz. In this chapter, design methodology, simulation and parametric analysis are discussed. Simulated and experimental results of a few prototype antennas are recorded. Chapter 4 elaborates the design of a few ultra-wideband printed antennas suitable for spectrum monitoring from 3 to 40 GHz and beyond along with recorded parameters. Chapter 5 concludes the investigation and research findings of the design of UWB antennas. It also outlines the scope for future research in this domain.