Theory of Electromagnetic Waves in Anisotropic, Magneto-Dielectric, Antenna Substrates
Open Access
- Author:
- Talalai, Gregory A
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 31, 2018
- Committee Members:
- James Kenneth Breakall, Dissertation Advisor/Co-Advisor
James Kenneth Breakall, Committee Chair/Co-Chair
Victor P Pasko, Committee Member
Julio Urbina, Committee Member
Michael T Lanagan, Outside Member - Keywords:
- Magnetic Materials
Antennas
Electromagnetic Theory - Abstract:
- The unique electromagnetic properties of magneto-dielectric composite materials, including ferromagnetic particle composites, ferromagnetic film laminates, and crystal-oriented ferrites have yet to be extensively leveraged in the design of modern microwave devices and antennas. By surveying the relevant literature, we first compile the relevant information on the dynamic permeability of magneto-dielectric materials, focusing on the various formulations of Snoek’s laws, their achievable permeability spectra, and the sources of anisotropy in a passive material medium. In subsequent chapters, an electromagnetic theory is developed for the investigation of planar antennas printed on anisotropic, magneto-dielectric substrates. For the case of a substrate whose in-plane permittivity and permeability are isotropic, expressions are derived for the surface wave modes. Similar to an isotropic substrate, the anisotropic magneto-dielectric substrate admits of a finite series of surface wave modes, which exist only provided the frequency of operation is above their respective cutoff frequencies. In most cases, the primary TM mode has no cutoff frequency. However, contrary to an isotropic substrate, the primary TM surface wave mode is suppressed completely in the anisotropic substrate provided the in-plane permeability, and out-of-plane permittivity are equal to 1. The more general theory for a fully anisotropic substrate can be developed indirectly through an investigation of wave propagation and radiation along principal axes and planes of the material. TE and TM decompositions of the electromagnetic field may be given for waves propagating within principal planes, but are impossible for arbitrary directions of propagation. An eigenvector approach shows how to resolve the electromagnetic field solutions into types, but these classifications are not, in general, TE/TM. For planar antennas the anisotropic magnetic response of a magneto-dielectric substrate enables circularly polarized radiation, even for single-fed linear antennas. Furthermore, the cutoff frequencies for the surface wave modes become directionally dependent. Dyadic Green’s functions are derived for a grounded, infinite, anisotropic magneto-dielectric substrate using spectral domain techniques. Utilizing the dyadic Green’s functions, the surface wave excitations and radiation patterns of short electric dipole antennas are calculated. The principal plane patterns for a dipole over a substrate possessing in-plane anisotropy are also derived. A method of moments computer program is developed for the numerical investigation of the input impedance and efficiency of microstrip dipoles printed over an anisotropic magneto-dielectric substrate. Example results are given to illustrate the effect of the anisotropic properties of the magneto-dielectric substrate on the dipole’s resonant length, impedance, and efficiency. Notably, the results indicate that a higher efficiency is obtained if the permittivity of the substrate in the direction normal to the air-substrate interface is small, an effect attributable to the suppression of the primary TM surface wave mode. From these results, we also conclude that the permeability in the direction normal to the air-substrate interface is unimportant for thin substrates. Consequently, no penalty is paid for utilizing anisotropic magnetic materials such as the crystal-oriented ferrites, which only possess an in-plane permeability. The superior frequency-permeability Snoek product of the oriented magnetic materials compared to traditional isotropic materials, combined with the unimportance of the out-of-plane permeability, lead to the conclusion that anisotropic magneto-dielectric materials are potentially better-suited for antenna applications than their isotropic counterparts.