# Intrinsic impedance inside a good conductor

I am currently studying the textbook Microwave Engineering, fourth edition, by David Pozar. Chapter 1.4 THE WAVE EQUATION AND BASIC PLANE WAVE SOLUTIONS says the following:

Plane Waves in a General Lossy Medium
Now consider the effect of a lossy medium. If the medium is conductive, with a conductivity $$\sigma$$, Maxwell's curl equations can be written, from (1.41a) and (1.20) as $$\nabla \times \bar{E} = - j \omega \mu \bar{H}, \tag{1.50a}$$ $$\nabla \times \bar{H} = j \omega \epsilon \bar{E} + \sigma \bar{E}. \tag{1.50b}$$ The resulting wave equation for $$\bar{E}$$ then becomes $$\nabla^2 \bar{E} + \omega^2 \mu \epsilon \left( 1 - j \dfrac{\sigma}{\omega \epsilon} \right) \bar{E} = 0, \tag{1.51}$$ where we see a similarity with (1.42), the wave equation for $$\bar{E}$$ in the lossless case. The difference is that the quantity $$k^2 = \omega^2 \mu \epsilon$$ of (1.42) is replaced by $$\omega^2 \mu \epsilon[1 - j(\sigma/\omega \epsilon)]$$ in (1.51). We then define a complex propagation constant for the medium as $$\gamma = \alpha + j \beta = j \omega \sqrt{\mu \epsilon} \sqrt{1 - j \dfrac{\sigma}{\omega \epsilon}}, \tag{1.52}$$ where $$\alpha$$ is the attenuation constant and $$\beta$$ is the phase constant. If we again assume an electric field with only an $$\hat{x}$$ component and uniform in $$x$$ and $$y$$, the wave equation of (1.51) reduces to $$\dfrac{\partial^2{E_x}}{\partial{z}^2} - \gamma^2 E_x = 0, \tag{1.53}$$ which has solutions $$E_x(z) = E^+ e^{- \gamma z} + E^- e^{\gamma z}. \tag{1.54}$$ The positive traveling wave then has a propagation factor of the form $$e^{-\gamma z} = e^{- \alpha z} e^{-j \beta z},$$ which in the time domain is of the form $$e^{- \alpha z} \cos(\omega t - \beta z).$$ We see that this represents a wave traveling in the $$+z$$ direction with a phase velocity $$v_p = \omega / \beta$$, a wavelength $$\lambda = 2\pi / \beta$$, and an exponential damping factor. The rate of decay with distance is given by the attenuation constant, $$\alpha$$. The negative traveling wave term of (1.54) is similarly damped along the $$-z$$ axis. If the loss is removed, $$\sigma = 0$$, and we have $$\gamma = jk$$ and $$\alpha = 0$$, $$\beta = k$$.
As discussed in Section 1.3, loss can also be treated through the use of a complex permittivity. From (1.52) and (1.20) with $$\sigma = 0$$ but $$\epsilon = \epsilon^\prime - j \epsilon^{\prime\prime}$$ complex, we have that $$\gamma = j \omega \sqrt{\mu \epsilon} = jk = j \omega \sqrt{\mu \epsilon^\prime (1 - j \tan \delta)}, \tag{1.55}$$ where $$\tan \delta = \epsilon^{\prime \prime}/\epsilon^\prime$$ is the loss tangent of the material.
The associated magnetic field can be calculated as $$H_y = \dfrac{j}{\omega \mu} \dfrac{\partial{E_x}}{\partial{z}} = \dfrac{-j \gamma}{\omega \mu} (E^+ e^{-\gamma z} - E^- e^{\gamma z}). \tag{1.56}$$ The intrinsic impedance of the conducting medium is now complex, $$\eta = \dfrac{j \omega \mu}{\gamma}, \tag{1.57}$$ but is still identified as the wave impedance, which expresses the ratio of electric to magnetic field components. This allows (1.56) to be rewritten as $$H_y = \dfrac{1}{\eta} (E^+ e^{- \gamma z} - E^- e^{\gamma z}). \tag{1.58}$$ Note that although $$\eta$$ of (1.57) is, in general, complex, it reduces to the lossless case of $$\eta = \sqrt{\mu/\epsilon}$$ when $$\gamma = jk = j \omega \sqrt{\mu \epsilon}$$.

Plane Waves in a Good Conductor
Many problems of practical interest involve loss or attenuation due to good (but not perfect) conductors. A good conductor is a special case of the preceding analysis, where the conductive current is much greater than the displacement current, which means that $$\sigma \gg \omega \epsilon$$. Most metals can be categorized as good conductors. In terms of a complex $$\epsilon$$, rather than conductivity, this condition is equivalent to $$\epsilon^{\prime\prime} \gg \epsilon^\prime$$. The propagation constant of (1.52) can then be adequately approximated by ignoring the displacement current term, to give $$\gamma = \alpha + j \beta \simeq j \omega \sqrt{\mu \epsilon} \sqrt{\dfrac{\sigma}{j \omega \epsilon}} = (1 + j) \sqrt{\dfrac{\omega \mu \sigma}{2}}. \tag{1.59}$$ The skin depth, or characteristic depth of penetration, is defined as $$\delta_s = \dfrac{1}{\alpha} = \sqrt{\dfrac{2}{\omega \mu \sigma}}. \tag{1.60}$$ Thus the amplitude of the fields in the conductor will decay by an amount $$1/e$$, or $$36.8\%$$, after traveling a distance of one skin depth, because $$e^{-\alpha z} = e^{-\alpha \delta_s} = e^{-1}$$. At microwave frequencies, for a good conductor, this distance is very small. The practical importance of this result is that only a thin plating of a good conductor (e.g., silver or gold) is necessary for low-loss microwave components. The intrinsic impedance inside a good conductor can be obtained from (1.57) and (1.59). The result is $$\eta = \dfrac{j \omega \mu}{\gamma} \simeq (1 + j) \sqrt{\dfrac{\omega \mu}{2 \sigma}} = (1 + j) \dfrac{1}{\sigma \delta_s} \tag{1.61}$$ Notice that the phase angle of this impedance is $$45^\circ$$, a characteristic of good conductors. The phase angle of the impedance for a lossless material is $$0^\circ$$, and the phase angle of the impedance of an arbitrary lossy medium is somewhere between $$0^\circ$$ and $$45^\circ$$.
Table 1.1 summarizes the results for plane wave propagation in lossless and lossy homogeneous media.

The intrinsic impedance inside a good conductor can be obtained from (1.57) and (1.59). The result is $$\eta = \dfrac{j \omega \mu}{\gamma} \simeq (1 + j) \sqrt{\dfrac{\omega \mu}{2 \sigma}} = (1 + j) \dfrac{1}{\sigma \delta_s} \tag{1.61}$$ Notice that the phase angle of this impedance is $$45^\circ$$, a characteristic of good conductors. The phase angle of the impedance for a lossless material is $$0^\circ$$, and the phase angle of the impedance of an arbitrary lossy medium is somewhere between $$0^\circ$$ and $$45^\circ$$.

How do we get $$\eta = \dfrac{j \omega \mu}{\gamma} \simeq (1 + j) \sqrt{\dfrac{\omega \mu}{2 \sigma}} = (1 + j) \dfrac{1}{\sigma \delta_s}$$? Furthermore, what does the author mean when they say that "the phase angle of this impedance is $$45^\circ$$, a characteristic of good conductors"? This isn't clear to me.

• Would like to just state that the complex impedance is a mathematical tool. The Impedance of a real wave is a real number May 30, 2022 at 15:37
• @jensenpaull Thanks for the clarification. May 31, 2022 at 14:19
• The complex impedance assumes the wave is in the form $\vec{E}_{0} e^{i(\vec{k}\cdot \vec{r}-\omega t)}$ which is obviously not a real wave. Complex waves are simpler to analyse/derive May 31, 2022 at 14:26

I think, it's just algebra.

First:

$$\frac{1}{1+j} = \frac{1-j}{(1+j)(1-j)} = (1-j)/2$$

Second : $$\gamma =(1+j) \sqrt{\frac{\omega\mu\sigma}{2}} \quad$$ then

$$\frac{1}{\gamma }= \frac{1-j}{2}\sqrt{\frac{2}{\omega\mu\sigma}}$$

So:

$$\eta = j\frac{\omega\mu}{\gamma} = (j-j^2) \omega\mu \sqrt{\frac{1}{2\omega\mu\sigma}}= (j+1) \sqrt{\frac{\omega^2\mu^2}{2\omega\mu\sigma}} =(1+j) \sqrt{\frac{\omega\mu}{2\sigma}}$$

The following equal sign is based on the definition of the skin depth which is:

$$\delta_s = \sqrt{\frac{2}{\mu\omega\sigma}}$$

So $$(\sigma\delta_s)^{-1}$$ is :

$$(1+j) \frac{1}{\sigma\delta_s} = (1+j)\sqrt{\frac{\mu\omega\sigma}{2\sigma^2}} = (1+j)\sqrt{\frac{\mu\omega}{2\sigma}}$$

The $$45^\circ$$ is related with the fact that

$$\frac{1+j}{\sqrt{2}} = \cos\left( \dfrac{\pi}{4} \right) + j \sin \left( \dfrac{\pi}{4} \right) = \exp\left( j\dfrac{\pi}{4} \right)$$

and that in radians $$\pi/4 \equiv 45^\circ$$. So one can write:

$$\eta = \lvert \eta \rvert e^{i\phi}$$ with $$\lvert \eta \rvert = \sqrt{\dfrac{\mu\omega}{\sigma}}$$ and $$\phi = \dfrac{\pi}{4}\equiv 45^\circ$$.

• Oh no, that's wrong. I have fixed it. Thank you for the remark. May 31, 2022 at 14:44