Abstract
This chapter will analyze the properties of electromagnetic wave propagation in anisotropic media. Of particular interest are positive index, anisotropic, and magneto-dielectric media. Engineered anisotropic media provide unique electromagnetic properties including a higher effective refractive index, high permeability with relatively low magnetic loss tangent at microwave frequencies, and lower density and weight than traditional media. This chapter presents research including plane wave solutions to propagation in anisotropic media, a mathematical derivation of birefringence in anisotropic media, modal decomposition of rectangular waveguides filled with anisotropic media, and the full derivation of anisotropic transverse resonance in a partially loaded waveguide. These are fundamental theories in the area of electromagnetic wave propagation that have been reformulated for fully anisotropic magneto-dielectric media. The ensuing results will aide interested parties in understanding wave behavior for anisotropic media to enhance designs for radio frequency devices based on anisotropic and magnetic media.
Keywords
- anisotropic
- wave propagation
- dispersion
- birefringence
- waveguides
- transverse resonance
1. Introduction
Recently engineered materials have come to play a dominant role in the design and implementation of electromagnetic devices and especially antennas. Metamaterials, ferrites, and magneto-dielectrics have all come to play a crucial role in advances made both in the functionality and characterization of such devices. In fact, a movement towards utilizing customized material properties to replace the functionality of traditional radio frequency (RF) components such as broadband matching circuitry, ground planes, and directive elements is apparent in the literature and not just replacement of traditional substrates and superstrates with engineered structures. A firm theoretical understanding of the electromagnetic properties of these materials is necessary for both design and simulation of new and improved RF devices.
Inherently, many of these engineered materials have anisotropic properties. Previously, the study of anisotropy had been limited mostly to the realm of optical frequencies where the phenomenon occurs naturally in substances such as liquid crystals and plasmas. However, the recent development of the aforementioned engineered materials has encouraged the study of electromagnetic anisotropy for applications at megahertz (MHz) and gigahertz (GHz) frequencies.
For the purposes of this chapter, an anisotropic electromagnetic medium defines permittivity (
2. Applications of anisotropy in radio frequency devices
Traditionally, the study of anisotropic properties was limited to a narrow application space where traditional ferrites, which exhibit natural anisotropy were the enabling technology. These types of applications included isolators, absorbers, circulators and phase shifters [1]. Traditional ferrites are generally very heavy and very lossy at microwave frequencies which are the two main limiting factors narrowing their use in RF devices; however, propagation loss is an important asset to devices such as absorbers. Anisotropy itself leads to propagation of an RF signal in different directions, which is important in devices such as circulators and isolators [1]. For phase shifters and other control devices the microwave signal is controlled by changing the bias field across the ferrite [1, 2]. However, newer versions of some of these devices, utilizing FETs and diodes in the case of phase shifters, rely on isotropic media to enable higher efficiency devices.
As early as 1958, Collin showed that at microwave frequencies, where the wavelength is larger, it is possible to fabricate artificial dielectric media having anisotropic properties [3]. This has led some to investigate known theoretical solutions to typical RF problems, such as a microstrip patch antenna, and extend them utilizing anisotropic wave propagation in dielectric media [4, 5]. The anisotropic dielectric antenna shows interesting features of basic antenna applications featuring anisotropic substrates. While these solutions establish a framework for electromagnetic wave propagation in anisotropic media, they simplify the problem by necessarily setting
The concept of artificial media is also exemplified by the proliferation of metamaterials research over the last few decades. Metamaterials incorporate the use of artificial microstructures made of subwavelength inclusions that are usually implemented with periodic and/or multilayered structures known as unit cells [6]. These devices operate where the wavelength is much larger than the characteristic dimensions of the unit cell elements. One characteristic feature of some types of metamaterials is wave propagation anisotropy [7]. Anisotropic metamaterials are used in applications such as directive lensing [8, 9], cloaking [10], electronic beam steering [11], and metasurfaces [12] among others.
Finally, a class of engineered materials exists that exhibits positive refractive index, anisotropy, and magneto-dielectric properties with reduced propagation loss at microwave frequencies compared to traditional ferrites. These materials show the unique ability to provide broadband impedance matches for very low profile antennas by exploiting the inherent anisotropy to redirect surface waves thus improving the impedance match of the antenna when very close to a ground plane. Antenna profile on the orders of a twentieth and a fortieth of a wavelength have been demonstrated using these materials with over an octave of bandwidth and positive realized gain [13, 14].
3. Plane wave solutions in an anisotropic medium
The recent development of low loss anisotropic magneto-dielectrics greatly expands the current antenna design space. Here we present a rigorous derivation of the wave equation and dispersion relationships for anisotropic magneto-dielectric media. All results agree with those presented by Meng et al. [15, 16]. Furthermore, setting
3.1. Source free anisotropic wave equation
In order to solve for the propagation constants, we will need to formulate the dispersion relationship from the anisotropic wave equation. This allows us to solve for the propagation constant in the normal direction of the anisotropic medium. We start with the anisotropic, time harmonic form of Maxwell’s source free equations for the electric and magnetic fields
where
Applying Eqs. (3) and (4) to Eqs. (1) and (2) yields the following
Using the radiation condition, we assume a solution of
Assuming a solution of
Using (7)–(12) allows for the transverse field components of the electric and magnetic fields in terms of the derivatives of
The relationships for the transverse field components, applied to (1) and (2), yield the following solutions for
Taking the cross product of both sides and substituting (1) and (2) for the right hand side of (17) and (18) yields
Equations (19) and (20) represent the vector wave equations in an anisotropic medium [12].
3.2. Dispersion equation for Hz
We expand (19) in terms of (13)–(16)
Evaluating the remaining cross product of (21) yields the final form of the expanded wave equation
Taking the dot product of (22) with
By keeping in mind that
Combining the
3.3. Dispersion equation for Ez
Expanding the
Evaluating the remaining cross product of (29) gives the final form of the expanded wave equation
Taking the dot product of (30) with
Keeping in mind that
Combining the
3.4. Transmission and reflection from an anisotropic half-space
Birefringence is a characteristic of anisotropic media where a single incident wave entering the boundary of an anisotropic medium gives rise to two refracted waves as shown in Figure 1 or a single incident wave leaving gives rise to two reflected waves as shown in Figure 2. We call these two waves the ordinary wave and the extraordinary wave. To see how the anisotropy of a medium gives rise to the birefringence phenomenon, Eqs. (28) and (36) will yield a solution for
Equations (28) and (36) yield the following solutions in unbounded anisotropic media restricted by the radiation condition in all three dimensions
Plugging (37) into (36) (equivocally we could substitute (38) into (19)) allows the generation of a polynomial equation whose solutions give the values of
Dividing out the
Finally, multiplying out (40) yields a fourth order polynomial whose roots yield the four values of
Equation (41) is directly responsible for the existence of the extraordinary wave that is characteristic of the birefringence phenomenon. In an isotropic medium, the resulting polynomial for
4. Anisotropic rectangular waveguide
Electromagnetic wave behavior of waveguides is well understood in the literature. The mode within a waveguide that are based on the voltage and current distributions within the waveguide make up the basis for the electric and magnetic field calculations. This section derives similar formulations for a rectangular waveguide uniformly filled with an anisotropic medium as shown in Figure 3. Figure 3 shows propagation in the
4.1. Anisotropic mode functions
Assume source free Maxwell’s equations in the same form as (1) and (2). Then the transverse electromagnetic fields are defined
where
4.1.1. Incident TE mode
Assuming only a TE type mode in the waveguide sets
To solve for
Expanding the curl of (52)
Isolating the
and substituting (48) and (49) for
Assuming a solution of the form
which meets the boundary conditions at the PEC walls of the waveguide, then plugging (58) into (53) imposes the following restriction on the values of the tensors in (3) and (4)
Solving (62) for
Equations (62) and (63) provide the criteria for determining the cutoff frequency for the propagation of modes inside the waveguide. Plugging (59) into (46)–(49) yields the following equations for the TE mode vectors in (42) and (43)
4.1.2. Incident TM mode
Assuming only a TM type mode in the waveguide sets
Solving (66)–(69) for
Equation (72) represents the anisotropic wave equation for the time harmonic electric field. Expanding the curl of (72) and isolating the
Plugging (73) into (66)–(69) yields the following equations for the TM mode vectors in (42) and (43)
4.2. Anisotropic transverse resonance
This section describes the derivation of an anisotropic transverse resonance condition established between resonant walls of a rectangular waveguide. Assume an infinite rectangular waveguide partially loaded with an anisotropic medium, then
4.2.1. Electromagnetic fields in free space regions
Calculating the fields in the free space region of the waveguide begins with Maxwell’s source free Eqs. (1) and (2) and the equations for the individual vector components of the electromagnetic fields (13)–(16). Using the standard derivation of the wave equation for
Utilizing (52) and setting
Equation (80) is a standard differential equation with a known solution [17]
where
Accounting for the restrictions imposed by the transverse resonance conditions on
4.2.2. Electromagnetic fields in anisotropic region
Starting with (1) and (2) for the source free Maxwell’s equations in an anisotropic medium, the vector components (13)–(16) led to the derivation of the dispersion Eqs. (26) and (38) for
The cutoff frequency or resonance of a rectangular waveguide is determined when the propagation constant in the direction of resonance, in the case the
Solving (82) for
We can see from (13)–(16) that based on our resonance conditions on
4.2.3. Characteristic impedances of the two regions
The first boundary condition exists at the perfect electric conductor (PEC) boundary when
Plugging (87) into (80) and (81) yields
Similarly,
Equations (89) and (90) solve for the impedance of the free space region as
within the region 0 ≤ (
Plugging Eqs. (80) and (81) into (92) and (93) yields the following set of equations
Equations (95) and (96) give two equations to solve for three unknowns. Match equation (91) to the impedance in the anisotropic region at
where
Substituting (101) into (97) yields the last equation along with (95) and (96) to solve for
4.2.4. Anisotropic transverse resonance condition
To simplify the calculation, consider Figure 4 as slice of Figure 3 in only one direction that is partially filled with an anisotropic medium. Figure 4 represents a transmission line representation that allows for a solution to
Starting with the short located at
Now calculate
The symmetric transverse resonance condition simplifies (103) to
Plugging (98) and (101) into (104) yields the following equation for
where λ is wavelength. Importantly, the solution of (105) shows that the transverse resonance only depends on two of the six
4.2.5. Suppression of birefringence in a rectangular waveguide
Section 3.4 discusses the phenomenon of birefringence in an unbounded anisotropic half-space by deriving the existence of a fourth order polynomial for the wavenumber in the propagation direction. However, for low order resonances, a rectangular waveguide suppresses the birefringence inherent to anisotropic media by suppressing propagation in the vertical direction of the waveguide. In other words,
and results in the following second order polynomial for
The suppression of the
5. Conclusions
Recently engineered materials have come to play an important role in state of the art designs electromagnetic devices and especially antennas. Many of these engineered materials have inherent anisotropic properties. Anisotropic media yield characteristics such as conformal surfaces, focusing and refraction of electromagnetic waves as they propagate through a material, high impedance surfaces for artificial magnetic conductors as well as high index, low loss, and lightweight ferrite materials. This chapter analyzes the properties of electromagnetic wave propagation in anisotropic media, and presents research including plane wave solutions to propagation in anisotropic media, a mathematical derivation of birefringence in anisotropic media, modal decomposition of rectangular waveguides filled with anisotropic media, and the full derivation of anisotropic transverse resonance in a partially loaded waveguide.
References
- 1.
Pozar DM. Microwave Engineering. 3rd ed. New York: John Wiley and Sons; 2005. pp. 106-117 - 2.
Ince WJ, Stern E. Mint: Non-reciprocal remanence phase shifters in rectangular waveguide. IEEE Transactions on Microwave Theory and Techniques. 1967; MTT-15 (2):87-95 - 3.
Collin R. Mint: A simple artificial anisotropic medium. IRE Transactions on Microwave Theory and Techniques. 1958; 6 :206-209 - 4.
Pozar D. Mint: Radiation and scattering from a microstrip patch on a uniaxial substrate. IEEE Transactions on Antennas and Propagation. 1987; AP-35 (6):613-621 - 5.
Graham J. Arbitrarily Oriented Biaxlly Anisotropic Media: Wave Behvaior and Microstrip Antennas [thesis]. Syracuse: University of Syracuse; 2012 - 6.
Torrent D, Sanchez-Dehesa J. Radial wave crystals: Radially periodic structures from anisotropic metamaterials for engineering acoustic or electromagnetic waves. Physics Review Letters. 2009; 103 - 7.
Sanchez-Dehesa J, Torrent D, Carbonell J. Anisotropic metamaterials as sensing devices in acoustics and electromagnetism. In: The Proceedings of the International Society for Optics and Photonics (SPIE). 2012; San Diego, California. Washington: SPIE - 8.
Ma YG, Wang P, Chen X, Ong CK. Mint: Near-field plane-wave-like beam emitting antenna fabricated by anisotropic metamaterial. Applied Physics Letters. 2009; 94 - 9.
Cheng Q. Directive radiation of electromagnetic waves based on anisotropic metamaterials. In: The Proceedings of IEEE Asia-Pacific Conference on Antennas and Propagation. Singapore. New York: IEEE; 27–29 August 2012 - 10.
Schurig D, Mock JJ, Justice BJ, Cummer SA, Pendry JB, Starr AF, Smith DR. Metamaterial electromagnetic cloak at microwave frequencies. Science. 2006; 314 :977-980 - 11.
Wong J, Balmain K. A beam-steerable antenna based on the spatial filtering property of hyperbolically anisotropic metamaterials. In: The Proceedings of IEEE International Symposium of the Antennas and Propagation Society. Honolulu. New York: IEEE; 9–15 June 2007 - 12.
Cai T, Wang GM. Polarization-controlled bifunctional antenna based on 2-D anisotropic gradient metasurface. In: The Proceedings of IEEE Conference on Microwave and Millimeter Wave Technology. Beijing. New York: IEEE; 5–8 June 2016 - 13.
Mitchell G, Weiss S. An overview of ARL's low profile antenna work utilizing anisotropic metaferrites. In: The Proceedings of the IEEE International Symposium on Phased Array Systems and Technology. Waltham. New York: IEEE; 18–21 October 2016 - 14.
Mitchell G, Wasylkiwskyj W. Mint: Theoretical anisotropic resonance technique for the design of low-profile wideband antennas. IET Microwaves, Antennas & Propagation. 2016; 10 :487-493 - 15.
Meng FY, Wu Q, Li LW. Mint: Transmission characteristics of wave modes in a rectangular waveguide filled with anisotropic Metamaterial. Applied Physics A: Materials Science and Processing. 2009; 94 :747-753 - 16.
Meng FY, Wu Q, Fu JH. Mint: Miniaturized rectangular cavity resonator based on anisotropic metamaterials bilayer. Microwave and Optical Technology Letters. 2008; 50 :2016-2020 - 17.
Boyce WE, DiPrima RC. Elementary Differential Equations. 7th ed. New York: John Wiley and Sons; 2001