Performance Limits for Microstrip Patch Antennas

Abstract

Microstrip patch antennas have become essential in modern communication systems due to their compact size and ease of fabrication. However, their performance is often constrained by narrow bandwidth, low radiation efficiency, and low gain, especially in miniaturized designs. The performance limits of microstrip patch antennas are helpful in understanding and overcoming these challenges. These performance limits are determined for three key metrics in this thesis, Q-factor (which is inversely proportional to bandwidth), radiation efficiency, and gain. Limits on these metrics provide insights into the maximum achievable performance for these antennas, guiding the design process beyond traditional optimization methods.
The performance bounds are determined through a current optimization approach, utilizing a method of moment implementation. This implementation makes use of specific Green's functions for layered media, determined using Sommerfeld integrals. This approach ensures that only the currents within the region where the patch is designed are required. Additionally, the currents are restricted to only components parallel to the ground plane, further refining the optimization process.
The results demonstrate that these theoretical bounds closely match the performance of traditional microstrip patch antenna designs in terms of Q-factor, radiation efficiency, and gain. Additionally, the derived Q-factor bounds are shown to be orders of magnitude tighter than the classical Chu limit for microstrip patch antennas. A central focus of this thesis is the balance between the electrical and/or physical size of the design region and performance. It explores whether enhancing radiation efficiency in miniaturized antennas is more effectively achieved by increasing the dielectric substrate between the patch and ground plane's relative permittivity or by refining the patch's geometry.
To make the presented bounds more accessible to a larger part of the antenna design community, this thesis introduces simple scaling rules. These rules allow designers to approximate lower Q-factor bounds using standard simulation tools. Additionally, by utilizing a new connection between Q-factor and radiation efficiency, the scaling rules are extended to estimate bounds on radiation efficiency as well. This approach translates complex theoretical insights into easy-to-use practical guidelines. The established relationship between lower Q-factor bounds and maximum radiation efficiency opens up new possibilities in antenna design. In specific scenarios, this relationship can improve the design process and simplify measurements by allowing one parameter to be inferred from the other.
A crucial part of understanding the Q-factor is the concept of stored energy. This thesis connects the stored energy in radiating systems to derivatives of the reactive power and/or dissipated power with respect to background material properties. More specifically, the stored electric energy is linked to the derivative with respect to permittivity, and stored magnetic energy is linked to the derivative with respect to permeability. Through this, a link between material losses and stored energies in radiating systems is established. This provides physical insights into stored energies and how they affect the performance of microstrip patch antennas.
Finally, this thesis investigates the use of a metasurface combined with an infrared camera as a method for imaging radio frequency fields. The measurement setup relies on the heat dissipated on metasurface elements due to the field from the device under test. A better understanding of this design setup requires an investigation into the effect of the metasurface elements on the measurement. This is done by simulating different metasurface element designs and assessing how their dissipated power correlates with the field in the absence of the metasurface.

• author: Nel, Ben
• supervisor: Mats Gustafsson; Anja Skrivervik Favre; Johan Lundgren; Daniel Sjöberg
• opponent: Prof. Viikari, Ville, Aalto University, Finland.
• organization: Electromagnetics and Nanoelectronics; Electromagnetic Theory (research group); Department of Electrical and Information Technology
• publishing date: 2025-01-16
• type: Thesis
• publication status: published
• subject: Other Electrical Engineering, Electronic Engineering, Information Engineering
• keywords: microstrip patch antennas, method of moments (MoM), physical bounds, Q-factor, radiation efficiency, gain, stored energy, permeability, permittivity, metasurface, near-field imaging, IR radiation
• defense location: Lecture Hall E:1406, building E, Klas Anshelms väg 10, Faculty of Engineering LTH, Lund University, Lund. The dissertation will be live streamed, but part of the premises is to be excluded from the live stream.
• defense date: 2025-02-14 09:15:00
• ISBN: 978-91-8104-357-0
• ISBN: 978-91-8104-356-3
• language: English
• LU publication?: yes
• id: 6edcf8e0-84f3-4f8e-83a5-2a1ef44b030a
• date added to LUP: 2025-01-20 18:53:37
• date last changed: 2025-04-04 15:17:25