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Figure (4.9a) shows the propagation of the TEM wave in the x‐direction of an unbounded medium. Figure (4.9b) further shows that for the TEM wave, electric and magnetic field components, i.e. the pairs (E_y, H_z), or (E_z, H_y), are transverse to the direction of propagation, i.e. ± x ‐ direction. For the field pair (E_y, H_z), the wave is y‐polarized; and for the field pair(E_z, H_y), it is the z‐polarized. The polarization of an EM‐wave is determined by the direction of the vector. The propagating wave is called the uniform plane wave, as amplitudes of electric and magnetic fields are constant over the equiphase surfaces. Figure (4.9c) shows that the phases of the E_y field at any instant of time, over the equiphase surfaces, are either 0° or 180°. The (y‐z)‐plane is the equiphase surface.

The field components of a uniform plane wave do not change with y and z coordinates, i.e. ∂/∂y(field E or H) = ∂/∂z(field E or H) = 0. The field components are a function of x only. So, the field components of the EM‐wave propagating in the x‐direction can be written as follows:

(4.5.20)

In the above expressions, the (−) sign shows the wave propagation in the positive x‐direction, whereas the (+) sign is for the wave propagation in the negative x‐direction. The wave propagation in the positive x‐direction is discussed below.

Figure 4.9 TEM mode wave in an unbounded medium.

The above expressions related to a uniform plane wave, in an external source‐free lossless medium, can be applied to the Maxwell equations (4.4.1). In the present case, the del operator is replaced by a derivative with respect to x, i.e. as a derivative with respect to y and z are zero. Maxwell first curl equation is reduced to a simpler form:

(4.5.21)

On separating each component of the fields, the following expressions are obtained:

(4.5.22)

Likewise, the following expressions are obtained from Maxwell's second curl expression (4.4.1b):

(4.5.23)

It is seen from the above equations that the E_x and H_x components, in the direction of propagation, are time‐independent, i.e. constant. They do not play any role in the wave propagation and can be assumed to be zero, without affecting the wave propagation [B.3]. Only transverse field components play a role in wave propagation. The time‐varying H_y component generates the E_z, whereas the time‐varying E_y component generates the H_z. It is also true for another time‐varying pair (H_z, E_z). Maxwell divergence relations also show ∂E_x/∂x = ∂H_x/∂x = 0. Again, E_x and H_x components do not show any variation along the direction of propagation that is essential for wave propagation. So, in the TEM mode propagation, the longitudinal field components are zero, i.e. E_x = H_x = 0.

By using equation (4.5.20) with the above equations, the time‐harmonic fields are rewritten as follows:

(4.5.24)

On using equation (4.5.20) with the above equations, the following algebraic expressions are obtained for the EM‐wave propagating in the positive x‐direction:

(4.5.25)

The wave impedance in free space, or in homogeneous unbounded medium, is defined in a plane normal to the direction of propagation [B.2]. For instance, Fig. (4.9a) shows the direction of propagation is along the x‐axis, and wave impedance is defined in the (y‐z) plane. It is also called the intrinsic impedance η₀ of free space and intrinsic impedance η of material filled homogeneous space. The following expression is obtained for the wave impedance η, for y‐polarized waves, from equation (4.5.25a):

(4.5.26)

Equation (4.5.25b) provides the following wave impedance for the z‐polarized waves:

(4.5.27)

The positive wave impedance of the (E_y, H_z) or (−E_z, H_y) fields corresponds to a wave traveling in the +x direction. However, for the (E_z, H_y) fields, the wave impedance is negative showing the wave propagation in the negative x‐direction. Under certain conditions, a medium can have an imaginary value of propagation constant, i.e. β_x = − jp. In this case, the wave impedance becomes reactive, and there is no wave propagation. Again, the wave equation (4.5.20) is reduced to E_i = E_0ie^−px e^jωt and H_i = H_0i e^−px e^jωt for the wave propagation in the positive x‐direction. These are decaying nonpropagating evanescent mode waves. These are only decaying oscillations.

The wave equations (4.5.13a) and (4.5.13b), for the E_y and H_z field components, are solved to get the total solution as a superposition of two waves traveling in opposite directions:

(4.5.28)

In the above expression, is the intrinsic or characteristic impedance, of the uniform plane in free space. The power movement is obtained from the Poynting vector . The power of the forward wave travels in the positive x‐direction. The direction of the power movement is the direction of the group velocity. In the x‐direction, the direction of the phase velocity is associated with the direction of the propagation vector, i.e. the wavenumber .

Figure (4.9d) shows the propagating EM‐wave in an arbitrary direction of the wavevector The wavevector is normal to the equiphase surface. The position vector of a point P at the equiphase surface is . The following expressions describe the propagating wave as a solution to the wave equations (4.5.12a) and (4.5.12c):

(4.5.29)

where . Equation (4.5.29d) is the dispersion equation. In the above equations, field quantities show time dependence, i.e. temporal dependence, through factor e^jωt and space dependence, i.e. spatial dependence, through factor . It also shows the lagging phase of the propagating wave in the positive direction. This is the convention adopted by engineers. On several occasions, physicist prefers an alternative sign convention, i.e. e^−jωt and . It leads to a leading phase for propagating waves in a positive direction. A reader must be careful while reading literature from several sources. For y‐polarized wave propagating in the x‐direction with k_y = k_z = 0 and k_x = β_x − jα, the solution of the wave equation could be written as follows:

(4.5.30)

The second terms of the above equations show wave propagation in the negative x‐direction. The wave equations show the decaying propagating waves. For the case α = 0, the above equations are the same as that of equations (4.5.28).