parallel plate summary sheet
TRANSCRIPT
-
8/18/2019 Parallel Plate Summary Sheet
1/2
Parallel Plate Waveguide Summary Sheet
Parallel Plate Geometry and Boundary Conditions
TEM (TM0) Mode
TEm (Transverse Electric) Modes
TMm (Transverse Magnetic) Modes
Cutoff
Frequency
At the plate
boundaries (x=
0 and x = a)
E tangential = 0
H normal = 0
This is
because the
plates are
perfect
conductors
0
4
4
=
=
=
−
−
z
t j z
x
t j z
y
E
eeC j
E
eeC H
ω γ
ω γ
ωε
γ
The TEM mode is the lowest order
mode and represents a plane wave
with E in the x direction going in
between the plates “without
reflections”
C 4 is a constant that will depend
on the field amplitudes. The TEM
mode is only mode without a
cutoff frequency.
The TE modes are field
configurations where the electric
field is the y direction only.
TEm m = 1, 2, 3, …. The value of
m represents the mode number. C 1
is a constant that will depend on
the field amplitudes.
t j z
x
t j z
z
t j z
y
ee xa
mC
j H
ee xa
mC
a j
m H
ee xa
mC E
ω γ
ω γ
ω γ
π
ωμ
γ
π
ωμ
π
π
−
−
−
⎟ ⎠
⎞⎜⎝
⎛ −=
⎟ ⎠
⎞⎜⎝
⎛ −=
⎟ ⎠
⎞⎜⎝
⎛ =
sin
cos
sin
1
1
1
The TM modes are field
configurations where the magnetic
field is the y direction only.
TMm m = 1, 2, 3, …. The value of
m represents the mode number. C 4
is a constant that will depend on
the field amplitudes.
t j z
z
t j z
x
t j z
y
ee xa
mC
a
jm E
ee xa
mC
j E
ee xa
mC H
ω γ
ω γ
ω γ
π
ωε
π
π
ωε
γ
π
−
−
−
⎟ ⎠
⎞⎜⎝
⎛ =
⎟ ⎠
⎞⎜⎝
⎛ =
⎟ ⎠
⎞⎜⎝
⎛ =
sin
cos
cos
4
4
4
με
με
π ω ω
a
m f f
a
m
c
c
2=>
=>
In the above equations, the
propagation term in the z direction is:
με ω π
γ 22
−⎟ ⎠
⎞⎜⎝
⎛ =
a
m
This term must be imaginary for
propagation and not attenuation and
puts a restriction on the cutoff
frequency for each mode m
For propagation of mode m
(applies to both TE and TM
modes)
We look for solutions
propagating in the z direction
with spatial variation: ze γ −
where
με ω π γ 22
−⎟ ⎠ ⎞⎜
⎝ ⎛ =
am
-
8/18/2019 Parallel Plate Summary Sheet
2/2
TE and TM Modes as Superpositions of Reflections of Plane Wave TEM Modes
It is often easier to think of the higher TE and TM modes as a superposition of multiple TEM modes. In order for the superposition to
be able to satisfy the boundary conditions, the null of one TEM ray must coincide with the crest of the other TEM ray at the
boundary so that they cancel out. In the picture above the nulls and crests of only one of the rays have been drawn in. For the
geometry to work out:
Cutoff Wavelength
The analysis above leads us to calculate a maximum wavelength for the given mode, this cutoff wavelength will correspond to the
cutoff frequency of the mode. From the formula above we see that the maximum λwill occur for θ= 0.
The physical interpretation of the cutoff frequency (or wavelength) from the geometric picture is that at this frequency the
geometry is such that θ= 0, making the two rays are propagating up and down and not down the waveguide anymore.
Guide Wavelength, Group Velocity, Phase Velocity
λ
null crest null
a
θm
acos(θm)
Crest (from bottom ray) and null
(from top ray) cancel out at this point
2)cos( λ θ ma m =
This is the
cutoff
wavelength
m
ac
2=λ με ω
λ
π β ==
2
με
π
με λ
π ω
a
m
c
c ==2
m = 1, 2, 3, …. The value
of m represents
the
same
mode number for both TE
and TM modes as
discussed above.
using
Which is the same value as we
derived from the analytical approach
before.
We can solve
for the cutoff
frequency
μελ θ
af
m
a
mm
22)cos( ==
We can also use this
relationship to
calculate
the reflection angle of
each mode as a function
of frequency and mode
number
λ
The reflecting waves create a repetitive interference pattern in
the z direction that repeats on a scale of λ , note that this is
different from λ , the free propagation wavelength
2
21
sin⎟ ⎠
⎞⎜⎝
⎛ −
==
a
mm λ
λ
θ
λ
λ
The phase velocity ( phv ) is how fast the superposed λ
pattern moves down the waveguide, the group velocity ( gv )is
how fast the individual rays move down the guide. Energy and
information travel at the group velocity.
m
ph
f f v
θ
λ λ
sin==
mg f v θ λ sin=