Propagation constant

The term "propagation constant" is somewhat of a misnomer as it usually varies strongly with ω. It is probably the most widely used term but there are a large variety of alternative names used by various authors for this quantity. These include transmission parameter, transmission function, propagation parameter, propagation coefficient and transmission constant. If the plural is used, it suggests that α and β are being referenced separately but collectively as in transmission parameters, propagation parameters, etc. In transmission line theory, α and β are counted among the "secondary coefficients", the term secondary being used to contrast to the primary line coefficients. The primary coefficients are the physical properties of the line, namely R,C,L and G, from which the secondary coefficients may be derived using the telegrapher's equation. Note that in the field of transmission lines, the term transmission coefficient has a different meaning despite the similarity of name: it is the companion of the reflection coefficient.

The propagation constant, symbol ${\displaystyle {\gamma }}$, for a given system is defined by the ratio of the complex amplitude at the source of the wave to the complex amplitude at some distance x, such that,

${\displaystyle {\frac {A_{0}}{A_{x}}}=e^{\gamma x}}$

Since the propagation constant is a complex quantity we can write:

where

• α, the real part, is called the attenuation constant
• β, the imaginary part, is called the phase constant

That β does indeed represent phase can be seen from Euler's formula:

${\displaystyle e^{i\theta }=\cos {\theta }+i\sin {\theta }\,\!}$

which is a sinusoid which varies in phase as θ varies but does not vary in amplitude because

${\displaystyle \left|e^{i\theta }\right|={\sqrt {\cos ^{2}{\theta }+\sin ^{2}{\theta }}}=1}$

The reason for the use of base e is also now made clear. The imaginary phase constant, iβ, can be added directly to the attenuation constant, α, to form a single complex number that can be handled in one mathematical operation provided they are to the same base. Angles measured in radians require base e, so the attenuation is likewise in base e.

The propagation constant for copper (or any other conductor) lines can be calculated from the primary line coefficients by means of the relationship

${\displaystyle \gamma ={\sqrt {ZY}}}$

where

${\displaystyle Z=R+i\omega L\,\!}$, the series impedance of the line per unit length and,

${\displaystyle Y=G+i\omega C\,\!}$, the shunt admittance of the line per unit length.

In telecommunications, the term attenuation constant, also called attenuation parameter or attenuation coefficient, is the attenuation of an electromagnetic wave propagating through a medium per unit distance from the source. It is the real part of the propagation constant and is measured in nepers per metre. A neper is approximately 8.7 dB. Attenuation constant can be defined by the amplitude ratio

${\displaystyle \left|{\frac {A_{0}}{A_{x}}}\right|=e^{\alpha x}}$

The propagation constant per unit length is defined as the natural logarithm of the ratio of the sending end current or voltage to the receiving end current or voltage.

In electromagnetic theory, the phase constant, also called phase change constant, parameter or coefficient is the imaginary component of the propagation constant for a plane wave. It represents the change in phase per unit length along the path travelled by the wave at any instant and is equal to the real part of the angular wavenumber of the wave. It is represented by the symbol β and is measured in units of radians per unit length.

From the definition of (angular) wavenumber for TEM waves:

${\displaystyle k={\frac {2\pi }{\lambda }}=\beta }$

For a transmission line, the Heaviside condition of the telegrapher's equation tells us that the wavenumber must be proportional to frequency for the transmission of the wave to be undistorted in the time domain. This includes, but is not limited to, the ideal case of a lossless line. The reason for this condition can be seen by considering that a useful signal is composed of many different wavelengths in the frequency domain. For there to be no distortion of the waveform, all these waves must travel at the same velocity so that they arrive at the far end of the line at the same time as a group. Since wave phase velocity is given by

${\displaystyle v_{p}={\frac {\lambda }{T}}={\frac {f}{\tilde {\nu }}}={\frac {\omega }{\beta }}}$

it is proved that β is required to be proportional to ω. In terms of primary coefficients of the line, this yields from the telegrapher's equation for a distortionless line the condition

${\displaystyle \beta =\omega {\sqrt {LC}}}$

However, practical lines can only be expected to approximately meet this condition over a limited frequency band.

In particular, the phase constant ${\displaystyle \beta }$ is not always equivalent to the wavenumber ${\displaystyle k}$. Generally speaking, the following relation

${\displaystyle \beta =k}$

is tenable to the TEM wave (transverse electromagnetic wave) which travels in free space or TEM-devices such as the coaxial cable and two parallel wires transmission lines. Nevertheless, it is invalid to the TE wave (transverse electric wave) and TM wave (transverse magnetic wave). For example,[2] in a hollow waveguide where the TEM wave cannot exist but TE and TM waves can propagate,

${\displaystyle k={\frac {\omega }{c}}}$

${\displaystyle \beta =k{\sqrt {1-{\frac {\omega _{c}^{2}}{\omega ^{2}}}}}}$

Here ${\displaystyle \omega _{c}}$ is the cutoff frequency. In a rectangular waveguide, the cutoff frequency is

${\displaystyle \omega _{c}=c{\sqrt {\left({\frac {n\pi }{a}}\right)^{2}+\left({\frac {m\pi }{b}}\right)^{2}}},}$

where the integers ${\displaystyle n,m\geq 0}$ are the mode numbers, and a and b the lengths of the sides of the rectangle. For TE modes, ${\displaystyle n,m\geq 0}$ (but ${\displaystyle n=m=0}$ is not allowed), while for TM modes ${\displaystyle n,m\geq 1}$. The phase velocity equals

${\displaystyle v_{p}={\frac {\omega }{\beta }}={\frac {c}{\sqrt {1-{\frac {\omega _{\mathrm {c} }^{2}}{\omega ^{2}}}}}}>c}$

The phase constant is also an important concept in quantum mechanics because the momentum ${\displaystyle p}$ of a quantum is directly proportional to it,[3][4] i.e.

${\displaystyle p=\hbar \beta }$

where ħ is called the reduced Planck constant (pronounced "h-bar"). It is equal to the Planck constant divided by 2π.

The term propagation constant or propagation function is applied to filters and other two-port networks used for signal processing. In these cases, however, the attenuation and phase coefficients are expressed in terms of nepers and radians per network section rather than per unit length. Some authors[5] make a distinction between per unit length measures (for which "constant" is used) and per section measures (for which "function" is used).

The propagation constant is a useful concept in filter design which invariably uses a cascaded section topology. In a cascaded topology, the propagation constant, attenuation constant and phase constant of individual sections may be simply added to find the total propagation constant etc.

Cascaded networks

Three networks with arbitrary propagation constants and impedances connected in cascade. The Z_i terms represent image impedance and it is assumed that connections are between matching image impedances.

The ratio of output to input voltage for each network is given by[6]

${\displaystyle {\frac {V_{1}}{V_{2}}}={\sqrt {\frac {Z_{I1}}{Z_{I2}}}}e^{\gamma _{1}}}$

${\displaystyle {\frac {V_{2}}{V_{3}}}={\sqrt {\frac {Z_{I2}}{Z_{I3}}}}e^{\gamma _{2}}}$

${\displaystyle {\frac {V_{3}}{V_{4}}}={\sqrt {\frac {Z_{I3}}{Z_{I4}}}}e^{\gamma _{3}}}$

The terms ${\displaystyle {\sqrt {\frac {Z_{In}}{Z_{Im}}}}}$ are impedance scaling terms[7] and their use is explained in the image impedance article.

The overall voltage ratio is given by

${\displaystyle {\frac {V_{1}}{V_{4}}}={\frac {V_{1}}{V_{2}}}\cdot {\frac {V_{2}}{V_{3}}}\cdot {\frac {V_{3}}{V_{4}}}={\sqrt {\frac {Z_{I1}}{Z_{I4}}}}e^{\gamma _{1}+\gamma _{2}+\gamma _{3}}}$

Thus for n cascaded sections all having matching impedances facing each other, the overall propagation constant is given by

${\displaystyle \gamma _{\mathrm {total} }=\gamma _{1}+\gamma _{2}+\gamma _{3}+\cdots +\gamma _{n}}$

The concept of penetration depth is one of many ways to describe the absorption of electromagnetic waves. For the others, and their interrelationships, see the article: Mathematical descriptions of opacity.

• Propagation speed

• "Propagation constant". Microwave Encyclopedia. 2011. Archived from the original (Online) on July 14, 2014. Retrieved February 2, 2011.
• Paschotta, Dr. Rüdiger (2011). "Propagation Constant" (Online). Encyclopedia of Laser Physics and Technology. Retrieved 2 February 2011.
• Janezic, Michael D.; Jeffrey A. Jargon (February 1999). "Complex Permittivity determination from Propagation Constant measurements" (PDF). IEEE Microwave and Guided Wave Letters. 9 (2): 76–78. doi:10.1109/75.755052. Retrieved 2 February 2011. Free PDF download is available. There is an updated version dated August 6, 2002.