Introduction To Modern Planar Transmission Lines. Anand K. Verma
significant role in the modern microwave and antenna technologies. Some electromagnetic characteristics of the engineered composite materials, with negative permittivity and permeability, are also presented in the present chapter. These artificially structured materials are known as metamaterials. The realization, circuit modeling, and some applications of the metamaterials and metasurfaces are further presented in chapters 21 and 22, respectively. The properties of natural and artificial dielectrics are further examined in chapter 6.
Objectives
To discuss the normal and oblique incidence of the EM‐waves at the interface of two media.
To present the transmission line model of the normal and oblique incidence of the EM‐waves.
To obtain the dispersion diagrams of refracted waves in the isotropic and anisotropic media.
To obtain characteristics of Brewster and the critical angles of incidence.
To discuss the general properties of metamaterials media and their classifications.
To obtain some characteristics of the EM‐wave propagation in the metamaterials.
To obtain the circuit models of metamaterials.
To discuss the possibility of obtaining the flat lens, superlens, and hyperlens beyond the diffraction limit.
To discuss the nature of the Doppler effect and Cerenkov radiation in the metamaterials.
To discuss metamaterials as thin microwave absorbers.
5.1 EM‐Waves at Interface of Two Different Media
The EM‐waves can strike the interface of two media with different electrical characteristics, either normally or obliquely. If the second medium is a dielectric medium, the waves undergo both reflection and transmission, whereas if the second medium is a perfect conductor, the reflection occurs. The obliquely incident plane waves follow the well‐known Snell's law (also called Snell-Descartes law) of reflection and refraction. There are important applications of obliquely incident EM‐waves.
5.1.1 Normal Incidence of Plane Waves
Figure (5.1a) shows an interface of two media #1 and #2. The media are electrically characterized by the primary parameters εri, μri, σi; i = 1, 2 and also by the secondary parameters, such as the refractive index ni, and intrinsic or wave impedance ηi. Initially, both media are considered lossless dielectric media. Next, the second medium is treated as a PEC.
The incident wave in the medium #1, propagating in the x‐direction with propagation constant k1, is y‐polarized with an electric field component
(5.1.1)
where η1 and η2 are the intrinsic impedance of medium #1 and medium #2, respectively. The total tangential components of the electric and magnetic fields in both media are continuous across the interface at x = 0:
(5.1.2)
On solving the above last two equations, the reflection coefficient Γ, and the transmission coefficient τ are computed at the interface:
For a lossy medium, the intrinsic impedance is a complex quantity, and reflection and transmission coefficients are also complex quantities. However, the magnitude of the reflection coefficient of a passive medium is always equal to or less than unity, i.e. |Γ| ≤ 1. The magnitude and phase of the reflected and transmitted waves are different from that of the incident waves. For a lossless composite medium, the matching is obtained, i.e. no reflection Γ = 0, for an incident wave at the interface with η1 = η2. It could be realized in two ways:
In equations (5.1.4a,b), the first case is the usual one, as both media are identical. However, the second case is interesting; as the matching is possible even for dissimilar media. It may not be a practical one with natural materials. However, for the artificially engineered metamaterials, both μr and εr can be tuned independently to achieve the matching condition, It is discussed in subsection (5.5.8). The impedance‐matched Huygens' metasurface has also been developed by tuning of electric and magnetic inclusions discussed in subsection (22.5.2) of chapter 22.
Sometimes, the concept of the reflectance (reflectivity) R and transmittance (transmissivity) T are used to show the portion of the reflected and transmitted powers:
The total field in medium #1 is a combination of the incident and reflected waves causing interference of waves. The field is uniform in the x‐direction. However, it creates partly a standing wave, and partly the traveling wave moving in the x‐direction:
For Γ > 0, i.e. for η2 > η1, the above expression can be decomposed as follows:
In equation