Lecture 20
More on Waveguides and
Transmission Lines
20.1 Circular Waveguides, Contd.
As in the rectangular waveguide case, the guidance of the wave in a circular waveguide can
be viewed as bouncing waves in the radial direction. But these bouncing waves give rise
to standing waves expressible in terms of Bessel functions. The scalar potential (or pilot
potential) for the modes in the waveguide is expressible as
Ψαs(ρ, φ) = AJn(βsρ)e±jnφ (20.1.1)
where α = h for TE waves and α = e for TM waves. The Bessel function or wave is expressible
in terms of Hankel functions as in (19.2.5). Since Hankel functions are traveling waves, Bessel
functions represent standing waves. Therefore, the Bessel waves can be thought of as bouncing
traveling waves as in the rectangular waveguide case. In the azimuthal direction, one can
express e±jnφ as traveling waves in the φ direction, or they can be expressed as cos(nφ) and
sin(nφ) which are standing waves in the φ direction.
20.1.1 An Application of Circular Waveguide
When a real-world waveguide is made, the wall of the metal waveguide is not made of perfect
electric conductor, but with some metal of finite conductivity. Hence, tangential E field is
not zero on the wall, and energy can dissipate into the waveguide wall. It turns out that due
to symmetry, the TE01 mode of a circular waveguide has the lowest loss of all the waveguide
modes including rectangular waveguide modes. Hence, this waveguide mode is of interest to
astronomers who are interested in building low-loss and low-noise systems.1
The TE01 mode has electric field given by E = ˆφEφ. Furthermore, looking at the magnetic
field, the current is mainly circumferential flowing in the φ direction. Moreover, by looking
1Low-loss systems are also low-noise due to energy conservation and the fluctuation dissipation theorem
[103, 104, 109].
189
190 Electromagnetic Field Theory
at a bouncing wave picture of the guided waveguide mode, this mode has a small component
of tangential magnetic field on a waveguide wall: It becomes increasingly smaller as the
frequency increases (see Figure 20.1).
Figure 20.1: Bouncing wave picture of the Bessel wave inside a circular waveguide for the
TE01 mode.
The tangential magnetic field needs to be supported by a surface current on the waveguide
wall. This implies that the surface current on the waveguide wall becomes smaller as the
frequency increases. The wall loss (or copper loss or eddy current loss) of the waveguide,
hence, becomes smaller for higher frequencies. In fact, for high frequencies, the TE01 mode
has the smallest copper loss of the waveguide modes: It becomes the mode of choice (see
Figure 20.2). Waveguides supporting the TE01 modes are used to connect the antennas of
the very large array (VLA) for detecting extra-terrestrial signals in radio astronomy [110] as
shown in Figure 20.3.
More on Waveguides and Transmission Lines 191
Figure 20.2: Losses of different modes in a circular waveguide . It is seen that at high
frequencies, the TE01 mode has the lowest loss (courtesy of [111]).
192 Electromagnetic Field Theory
Figure 20.3: Picture of the Very Large Array (courtesy of [110]).
Figure 20.4 shows two ways of engineering a circular waveguide so that the TE01 mode
is enhanced: (i) by using a mode filter that discourages the guidance of other modes but
not the TE01 mode, and (ii), by designing ridged waveguide wall to discourage the flow of
axial current and hence, the propagation of the non-TE01 mode. More details of circular
waveguides can be found in [111]. Typical loss of a circular waveguide can be as low as 2
dB/km.
As shall be learnt later, an open circular waveguide can be made into an aperture antenna
quite easily, because the fields of the aperture are axially symmetric. Such antenna is called a
horn antenna. Because of this, the radiation pattern of such an antenna is axially symmetric,
which can be used to produce axially symmetric circularly polarized (CP) waves. Ways to
enhance the TE01 mode are also desirable [112] as shown in Figure 20.5.
More on Waveguides and Transmission Lines 193
Figure 20.4: Ways to enhance the TE01 mode in a circular waveguide. Such waveguide is
used in astronomy such as designing the communication between antennas in a very large
array (VLA [110]), or it is used in a circular horn antenna [112].
194 Electromagnetic Field Theory
Figure 20.5: Picture of a circular horn antenna where corrugated wall is used to enhance the
TE01 mode (courtesy of [113]).
20.2 Remarks on Quasi-TEM Modes, Hybrid Modes,
and Surface Plasmonic Modes
We have analyzed some simple structures where closed form solutions are available. These
solutions offer physical insight into how waves are guided, and how they are cutoff from
guidance. As has been shown, for some simple waveguides, the modes can be divided into
TEM, TE, and TM modes. However, most waveguides are not simple. We will remark on
various complexities that arise in real world applications.
20.2.1 Quasi-TEM Modes
Figure 20.6: Some examples of practical coaxial-like waveguides (left), and the optical fiber
(right). The environment of these waveguides is an inhomogeneous medium, and hence, a
pure TEM mode cannot propagate on these waveguides.
More on Waveguides and Transmission Lines 195
Many waveguides cannot support a pure TEM mode even when two conductors are present.
For example, two pieces of metal make a transmission line, and in the case of a circular coax,
a TEM mode can propagate in the waveguide. But most two-metal transmission lines do not
support a pure TEM mode: Instead, they support a quasi-TEM mode. In the optical fiber
case, when the index contrast of the fiber is very small, the mode is quasi-TEM as it has to
degenerate to the TEM case when the contrast is absent.
When a wave is TEM, it is necessary that the wave propagates with the phase velocity
of the medium. But when a uniform waveguide has inhomogeneity in between, as shown
in Figure 20.6, this is not possible anymore. We can prove this assertion by reductio ad
absurdum. From eq. (18.1.16) of the previous lecture, we have shown that for a TM mode,
Ez is given by
Ez = 1
jωεi
(β2
i − β2
z )Ψe (20.2.1)
The above derivation is valid in a piecewise homogeneous region. If this mode becomes TEM,
then Ez = 0 and this is possible only if βz = βi. In other words, the phase velocity of the
waveguide mode is the same as a plane TEM wave in the same medium.
Now assume that a TEM wave exists in both inhomogeneous regions of the microstrip line
or all three dielectric regions of the optical fiber in Figure 20.6. Then the phase velocities
in the z direction, determined by ω/βz of each region will be ω/βi of the respective region
where βi is the wavenumber of the i-th region. Hence, phase matching is not possible, and the
boundary condition cannot be satisfied at the dielectric interfaces. Nevertheless, the lumped
element circuit model of the transmission line is still a very good model for such a waveguide.
If the line capacitance and line inductances of such lines can be estimated, βz can still be
estimated. As shall be shown later, circuit theory is valid when the frequency is low, or the
wavelength is large compared to the size of the structures.
20.2.2 Hybrid Modes–Inhomogeneously-Filled Waveguides
For most inhomogeneously filled waveguides, the modes (eigenmodes or eigenfunctions) inside
are not cleanly classed into TE and TM modes, but with some modes that are the hybrid of
TE and TM modes. If the inhomogeneity is piecewise constant, some of the equations we have
derived before are still valid: In other words, in the homogeneous part (or constant part) of the
waveguide filled with piecewise constant inhomogeneity, the fields can still be decomposed into
TE and TM fields. But these fields are coupled to each other by the presence of inhomogeneity,
i.e., by the boundary conditions requisite at the interface between the piecewise homogeneous
regions. Or both TE and TM waves are coupled together and are present simultaneously, and
both Ez 6 = 0 and Hz 6 = 0. Some examples of inhomogeneously-filled waveguides where hybrid
modes exist are shown in Figure 20.7.
Sometimes, the hybrid modes are called EH or HE modes, as in an optical fiber. Never-
theless, the guidance is via a bouncing wave picture, where the bouncing waves are reflected
off the boundaries of the waveguides. In the case of an optical fiber or a dielectric waveguide,
the reflection is due to total internal reflection. But in the case of metalic waveguides, the
reflection is due to the metal walls.
196 Electromagnetic Field Theory
Figure 20.7: Some examples of inhomogeneously filled waveguides where hybrid modes exist:
(top-left) A general inhomogeneously filled waveguide, (top-right) slab-loaded rectangular
waveguides, and (bottom) an optical fiber with core and cladding.
20.2.3 Guidance of Modes
Propagation of a plane wave in free space is by the exchange of electric stored energy and
magnetic stored energy. So the same thing happens in a waveguide. For example. in the
transmission line, the guidance is by the exchange of electric and magnetic stored energy
via the coupling between the capacitance and the inductance of the line. In this case, the
waveguide size, like the cross-section of a coaxial cable, can be made much smaller than the
wavelength.
In the case of hollow waveguides, the E and H fields are coupled through their space and
time variations. Hence, the exchange of the energy stored is via the space that stores these
energies, like that of a plane wave. These waveguides work only when these plane waves can
“enter” the waveguide. Hence, the size of these waveguides has to be about half a wavelength.
The surface plasmonic waveguide is an exception in that the exchange is between the
electric field energy stored with the kinetic energy stored in the moving electrons in the
plasma instead of magnetic energy stored. This form of energy stored is sometimes referred
to as coming from kinetic inductance. Therefore, the dimension of the waveguide can be very
small compared to wavelength, and yet the surface plasmonic mode can be guided.
20.3 Homomorphism of Waveguides and Transmission
Lines
Previously, we have demonstrated mathematical homomorphism between plane waves in lay-
ered medium and transmission lines. Such homomorphism can be further extended to waveg-
More on Waveguides and Transmission Lines 197
uides and transmission lines. We can show this first for TE modes in a hallow waveguide,
and the case for TM modes can be established by invoking duality principle.2
20.3.1 TE Case
For this case, Ez = 0, and from Maxwell’s equations
∇ × H = jωεE (20.3.1)
By letting ∇ = ∇s + ∇z , H = Hs + Hz where ∇z = ˆz ∂
∂z , and Hz = ˆzHz , and the subscript
s implies transverse to z components, then
(∇s + ∇z ) × (Hs + Hz ) = ∇s × Hs + ∇z × Hs + ∇s × Hz (20.3.2)
where it is understood that ∇z × Hz = 0. Notice that the first term on the right-hand side
of the above is pointing in the z direction. Therefore, by letting E = Es + Ez , and equating
transverse components in (20.3.1), we have3
∇z × Hs + ∇s × Hz = jωεEs (20.3.3)
To simplify the above equation, we shall remove Hz from above. Next, from Faraday’s law,
we have
∇ × E = −jωμH (20.3.4)
Again, by letting E = Es + Ez , we can show that (20.3.4) can be written as
∇s × Es + ∇z × Es + ∇s × Ez = −jωμ(Hs + Hz ) (20.3.5)
Equating z components of the above, we have
∇s × Es = −jωμHz (20.3.6)
Using (20.3.6), Eq.(20.3.3) can be rewritten as
∇z × Hs + ∇s × 1
−jωμ ∇s × Es = +jωεEs (20.3.7)
The above can be further simplified by noting that
∇s × ∇s × Es = ∇s(∇s · Es) − ∇s · ∇sEs (20.3.8)
But since ∇ · E = 0, and Ez = 0 for TE modes, it implies that ∇s · Es = 0. Also, from
Maxwell’s equations, we have previously shown that for a homogeneous source-free medium,
(∇2 + β2)E = 0 (20.3.9)
2I have not seen exposition of such mathematical homomorphism elsewhere except in very simple cases [31].
3And from the above, it is obvious that ∇s × Hs = jωεEz , but this equation will not be used in the
subsequent derivation.
198 Electromagnetic Field Theory
or that
(∇2 + β2)Es = 0 (20.3.10)
Assuming that we have a guided mode, then
Es ∼ e∓jβz z , ∂2
∂z2 Es = −βz 2Es (20.3.11)
Therefore, (20.3.10) becomes
(∇s2 + β2 − βz 2)Es = 0 (20.3.12)
or that
(∇s2 + βs2)Es = 0 (20.3.13)
where β2
s = β2 − β2
z is the transverse wave number. Consequently, from (20.3.8)
∇s × ∇s × Es = −∇2
sEs = β2
s Es (20.3.14)
As such, (20.3.7) becomes
∇z × Hs = jωεEs + 1
jωμ ∇s × ∇s × Es
= jωεEs + 1
jωμ βs2Es
= jωε
(
1 − βs2
β2
)
= jωε βz 2
β2 Es (20.3.15)
Letting βz = β cos θ, then the above can be written as
∇z × Hs = jωε cos2 θEs (20.3.16)
The above now resembles one of the two telegrapher’s equations that we seek. Now looking
at (20.3.4) again, assuming Ez = 0, equating transverse components, we have
∇z × Es = −jωμHs (20.3.17)
More explicitly, we can rewrite (20.3.16) and (20.3.17) in the above as
∂
∂z ˆz × Hs = jωε cos2 θEs (20.3.18)
∂
∂z ˆz × Es = −jωμHs (20.3.19)
More on Waveguides and Transmission Lines 199
The above now resembles the telegrapher’s equations. We can multiply (20.3.19) by ˆz× to
get
∂
∂z Es = jωμˆz × Hs (20.3.20)
Now (20.3.18) and (20.3.20) look even more like the telegrapher’s equations. We can have
Es → V , ˆz × Hs → −I. μ → L, ε cos2 θ → C, and the above resembles the telegrapher’s
equations, or that the waveguide problem is homomorphic to the transmission line problem.
The characteristic impedance of this line is then
Z0 =
√ L
C =
√ μ
ε cos2 θ =
√ μ
ε
1
cos θ = ωμ
βz
(20.3.21)
Therefore, the TE modes of a waveguide can be mapped into a transmission problem. This
can be done, for instance, for the TEmn mode of a rectangular waveguide. Then, in the above
βz =
√
β2 −
( mπ
a
)2
−
( nπ
b
)2
(20.3.22)
Therefore, each TEmn mode will be represented by a different characteristic impedance Z0,
since βz is different for different TEmn modes.
20.3.2 TM Case
This case can be derived using duality principle. Invoking duality, and after some algebra,
then the equivalence of (20.3.18) and (20.3.20) become
∂
∂z Es = jωμ cos2 θˆz × Hs (20.3.23)
∂
∂z ˆz × Hs = jωεEs (20.3.24)
To keep the dimensions commensurate, we can let Es → V , ˆz × Hs → −I, μ cos2 θ → L,
ε → C, then the above resembles the telegrapher’s equations. We can thus let
Z0 =
√ L
C =
√ μ cos2 θ
ε =
√ μ
ε cos θ = βz
ωε (20.3.25)
Please note that (20.3.21) and (20.3.25) are very similar to that for the plane wave case, which
are the wave impedance for the TE and TM modes, respectively.
200 Electromagnetic Field Theory
Figure 20.8: A waveguide filled with layered medium is mathematically homomorphic to a
multi-section transmission line problem. Hence, transmission-line methods can be used to
solve this problem.
The above implies that if we have a waveguide of arbitrary cross section filled with layered
media, the problem can be mapped to a multi-section transmission line problem, and solved
with transmission line methods. When V and I are continuous at a transmission line junction,
Es and Hs will also be continuous. Hence, the transmission line solution would also imply
continuous E and H field solutions.
Figure 20.9: A multi-section waveguide is not exactly homormorphic to a multi-section trans-
mission line problem, circuit elements can be added at the junction to capture the physics at
the waveguide junctions as shown in the next figure.
20.3.3 Mode Conversion
In the waveguide shown in Figure 20.8, there is no mode conversion at the junction interface.
Assuming a rectangular waveguide as an example, what this means is that if we send at TE10
into the waveguide, this same mode will propagate throughout the length of the waveguide.
The reason is that only this mode alone is sufficient to satisfy the boundary condition at the
junction interface. To elaborate further, from our prior knowledge, the transverse fields of
the waveguide, e.g., for the TM mode, can be derived to be
Hs = ∇ × ˆzΨes(rs)e∓jβz z (20.3.26)
Es = ∓βz
ωε ∇sΨes(rs)e∓jβz z (20.3.27)
More on Waveguides and Transmission Lines 201
In the above, β2
s and Ψes(rs) are eigenvalue and eigenfunction, respectively, that depend
only on the geometrical shape of the waveguide, but not the materials filling the waveguide.
These eigenfunctions are the same throughout different sections of the waveguide. Therefore,
boundary conditions can be easily satisfied at the junctions.
However, for a multi-junction waveguide show in Figure 20.9, tangential E and H con-
tinuous condition cannot be satisfied by a single mode in each waveguide alone: V and I
continuous at a transmission line junction will not guarantee the continuity of tangential E
and tangential H fields at the waveguide junction. Multi-modes have to be assumed in each
section in order to match boundary conditions at the junction. Moreover, mode matching
method for multiple modes has to be used at each junction. Typically, a single mode inci-
dent at a junction will give rise to multiple modes reflected and multiple modes transmitted.
The multiple modes give rise to the phenomenon of mode conversion at a junction. Hence,
the waveguide may need to be modeled with multiple transmission lines where each mode is
modeled by a different transmission line with different characteristic impedances.
However, the operating frequency can be chosen so that only one mode is propagating at
each section of the waveguide, and the other modes are cutoff or evanescent. In this case, the
multiple modes at a junction give rise to localized energy storage at a junction. These energies
can be either inductive or capacitive. The junction effect may be modeled by a simple circuit
model as shown in Figure 20.10. These junction elements also account for the physics that
the currents and voltages are not continuous anymore across the junction. Moreover, these
junction lumped circuit elements account for the stored electric and magnetic energies at the
junction.
Figure 20.10: Junction circuit elements are used to account for stored electric and magnetic
energy at the junction. They also account for that the currents and voltages are not continuous
across the junctions anymore.
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