Broadband lossy impedance matching of antennas

Open Access
Li, Kaiming
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
January 15, 2016
Committee Members:
  • James Kenneth Breakall, Dissertation Advisor
  • James Kenneth Breakall, Committee Chair
  • Ram Mohan Narayanan, Committee Member
  • Shizhuo Yin, Committee Member
  • Michael T Lanagan, Committee Member
  • Broadband antennas
  • impedance matching
  • multi-objective optimization
In RF applications such as transmitters, amplifiers, receivers, and antennas, a task of vital importance is the design of an impedance matching network, one that can transfer the most power from the source to the load. Lossless matching networks at a single frequency have been well studied, while the broadband impedance matching problem was only defined 70 years ago. This dissertation provides a thorough background of the theory basis and design approaches in the history of broadband impedance matching. Lossless impedance matching optimization using the MATLAB Global Optimization Toolbox is discussed, and an approach combining brute-force techniques and the Real Frequency Technique is proposed. The bandwidth of a candidate 80-meter high-frequency dipole antenna has been increased from 4.1% to at least 15.0% after the optimization, with a Voltage Standing Wave Ratio (VSWR) of 2:1. In order to match the source and load over a wide band, a tradeoff is forced between the antenna gain and its bandwidth. The lossy impedance matching problem is investigated in the dissertation as a multi-objective optimization problem. Multiple optimization algorithms are used to find the Pareto front for a given lossy network topology. With equal weight on the objectives, the bandwidth of the dipole is further increased to 16.9%. This approach was applied to conformal antennas such as a low-profile bow-tie antenna close to a ground plane and compared with the new approach of using metamaterial inserted between the antenna and the ground plane. There is considerable interest and a main goal of this dissertation to find if another approach such as this lossy matching method could compete with the metamaterial technique. With lossy matching, the unnecessarily high gain of the antenna is traded in for a decrease in the reflection, increasing the bandwidth of the bow-tie to more than 70.7%, or [200MHz, 400MHz]. Considering the high cost of manufacturing metamaterial to achieve similar performance, the approach found for the first time in this dissertation is a iv significant breakthrough in the design and practicality of making future conformal and other types of antennas. The unique and innovative techniques utilized in this dissertation include performing lossless and lossy impedance matching optimization using results from more numerical platforms like FEKO and GNEC for various antenna configurations. In this dissertation topologies have been thoroughly explored including circuits with various numbers of elements and those including the insertion of lossy components into the lossless networks. Additionally, the latest nature-inspired optimization algorithms were applied to the lossy impedance matching problem and compared with the traditional algorithms.