2.4
Understand the method of propagation
mode/excitation in waveguides.
• TE/TM are the configuration of E and H fields. The two mode
consists of subscript ‘m’ and ‘n’ which will determine the field
patterns and it refers to whole / integer number.
m
n
 Integer number.
 Integer number.
 Denotes the number of half
wavelength of intensity or @ semi
sinusoidal wave pattern (λ / 2) at E
or H field intensity.
 Denotes the number of half
wavelength of intensity or @ semi
sinusoidal wave pattern (λ / 2) at E and
H field intensity.
 Refers to the width or dimension ‘a’
of the rectangular waveguide.
 Refers to the narrow dimension ‘b’ of
the rectangular waveguide.
• The propagation mode of TE and TM in the rectangular
waveguide depends on critical / cutoff method used.
• The characteristics used to identify the critical / cutoff method
are fc (f0) and λc ( λ 0).
• Different TE and TM modes all have different cutoff
wavelength and therefore encounter different characteristic
wave impedance. Eg. TM11, TM12 etc.
• The dimension of the waveguide and the propagation modes
used is affected by the cutoff wavelength / frequency (f0 and
λ 0).
• Only certain frequency / wavelength are being allowed to
propagate in the waveguide.
ModeTE m,n
•
•
•
•
•
•
The TE, or transverse electric, mode have electric fields in the transverse plane,
that is, only across the waveguide.
The magnetic field in the TE mode runs both transversely and axially along the
waveguide.
The electric field are transverse and the magnetic field are extend across the
cross section and along the waveguide itself.
The subscript after the TE or TM refers to the number of electric or magnetic field
variation across the height and width of the waveguide.
For example, in the TE0,1 mode, the electric field is uniform with no variations
across the height dimension, but has one variation across the width. The field
goes from zero at edge of the guide to a maximum at the center and back to zero
at the opposite end of the guide.
For TE 0,2 mode, the electric field still has no variations across the height of the
guide, but has two variations across its width. The field goes from zero at the one
edge of the guide, to a maximum, to zero, to a maximum with the opposite field
direction, and then back to zero at the other guide wall.
ModeTEm,n
• Refers to the existence of one half wavelength (λ / 2) across the guide
width (m) and narrow dimension (n).
• Needs modification of wave. They are used in practice as often as the TEmo
mode (except TE11 mode which does have some practical application.
• Eg. TE 11 (1,1 – has one λ / 2 at guide width and narrow dimension ).
λ/2
λ/2
Mode TM m,n
• The TM or transverse magnetic mode , have
magnetic fields that exist only in the
transverse and axial direction along the guide.
• H field across the propagation direction
• Loop can be seen at front view.
• Loop cannot be seen on top view.
• 1 loop = half wavelength, λ / 2.
• TM 11 –half wavelength (λ / 2) exist at guide
width & half of E field intensity exist at
narrow dimension.
• Side view – E field ends at 90°.
• TM modes are govern by relations identical to
those governing TEmn modes except that the
equation for characteristic wave impedance
Z0.
2.4.2 Explain how to excite the dominant modes for
propagation mode in TE m,n and TM m,n
• The cut off frequency can be shown graphically below
Single
mode
3 modes
4 modes
TE 11
Dominant
mode TE00
TE 20, TE 01
TE 10
f
Figure : Composite table of fc for different TE mode.
• The analysis can be carried out for TM modes,
yielding results which are similar to those obtained
previously with TE mode .
• Excersice :
The WG-16 waveguide recommended for X-band
(8-12GHz) transmission has dimension a=0.9”
and b=0.4”. Calculate the frequency range over
which the dominant mode propagates.
Waveguide Components
Waveguide components are:
a.
Connectors/Joint
b. Attenuators
c.
Coupler
d. Basic accessories (bends, corner, tapered,
twist)
e. Junction-T and Hybrid-T
Connectors/Joint
Attenuators
• Attenuator works by putting carbon vane or flap into the
waveguide
• Currents induced in the carbon cause loss
• Load is similar but at end of guide
Coupler
• Launches or receives power in only 1 direction
• Used to split some of power into a second guide
• Can use probes or holes
BASIC ACCESSORIES (Bends, corner,
tapered, twist)
• Bends
– Called E-plane or H-Plane bends depending on the
direction of bending
• Tees
– Also have E and H-plane varieties
– Hybrid or magic tee combines both and can be used
for isolation
• Tapered
• Twist
Junction-T
• Waveguide junctions are used when power in a
waveguide needs to be split or some extracted.
• There are a number of different types of waveguide
junction that can be use, each type having different
properties - the different types of waveguide
junction affect the energy contained within the
waveguide in different ways.
Waveguide junction types
• There are a number of different types of waveguide junction. The
major types are listed below:
– H-type T Junction: This type of waveguide junction gains its name
because top of the "T" in the T junction is parallel to the plane of the
magnetic field, H lines in the waveguide.
– E-Type T Junction: This form of waveguide junction gains its name as
an E- type T junction because the tope of the "T" extends from the
main waveguide in the same plane as the electric field in the
waveguide.
– Magic T waveguide junction: The magic T waveguide junction is
effectively a combination of the E-type and H-type waveguide
junctions.
– Hybrid Ring Waveguide Junction: This form of waveguide junction is
another form of waveguide junction that is more complicated than
either the basic E-type or H-type waveguide junction.
E-type waveguide junction
• It is called an E-type T junction because the junction arm, i.e.
the top of the "T" extends from the main waveguide in the
same direction as the E field. It is characterized by the fact
that the outputs of this form of waveguide junction are 180°
out of phase with each other.
H-type waveguide junction
• This type of waveguide junction is called an H-type T junction
because the long axis of the main top of the "T" arm is parallel
to the plane of the magnetic lines of force in the waveguide. It
is characterized by the fact that the two outputs from the top
of the "T" section in the waveguide are in phase with each
other.
Magic T Hybrid
• It is a combination of the H-type and E-type T
junctions. The most common application of
this type of junction is as the mixer section for
microwave radar receivers.
•
•
When a signal is applied into the "E plane"
arm, it will divide into two out of phase
components as it passes into the leg consisting
of the "a" and "b" arms. However no signal will
enter the "E plane" arm as a result of the fact
that a zero potential exists there - this occurs
because of the conditions needed to create
the signals in the "a" and "b" arms. In this way,
when a signal is applied to the H plane arm, no
signal appears at the "E plane" arm and the
two signals appearing at the "a" and "b" arms
are 180° out of phase with each other.
When a signal enters the "a" or "b" arm of the
magic t waveguide junction, then a signal
appears at the E and H plane ports but not at
the other "b" or "a" arm as shown.
Hybrid ring
• This form of waveguide junction overcomes the power limitation of
the magic-T waveguide junction.
• A hybrid ring is constructed from a circular ring of rectangular
waveguide - a bit like an annulus. The ports are then joined to the
annulus at the required points. Again, if signal enters one port, it
does not appear at all the others.
• The hybrid ring is used primarily in high-power radar and
communications systems where it acts as a duplexer - allowing the
same antenna to be used for transmit and receive functions.
• During the transmit period, the hybrid ring waveguide junction
couples microwave energy from the transmitter to the antenna
while blocking energy from the receiver input. Then as the receive
cycle starts, the hybrid ring waveguide junction couples energy
from the antenna to the receiver. During this period it prevents
energy from reaching the transmitter.
2.5.1 Explain the application of waveguide
components
a.
b.
c.
d.
Slotted section
Isolator
Circulator
Mixers
Slotted section
• A slotted is a transmission line configuration
(usually a waveguide or coaxial line) that
allows the sampling of the electric field
amplitude of a standing wave on a terminated
line.
• With this device the SWR and the distance of
the first voltage minimum from the load can
be measured, and from these data the load
impedance can be determined.
Circulator and Isolator
• Both use the unique properties of ferrites in a magnetic field
• Isolator passes signals in one direction, attenuates in the other
• Circulator passes input from each port to the next around the circle, not to
any other port
Circulator Top View
PRECESSION
Mixers
• The mixer takes two signals and combines them creating new signals. It
can be used to translate microwave signals into much lower
frequencies that an inexpensive radio receiver can tune. It can even
reverse the effect taking low frequencies and translating them back
into the microwave range.
• Mixers are needed in most microwave systems because the RF signal is
way too high to process its information (for example, looking for a
Doppler shift in an X-band radar application, you won't find many A/D
converters than can handle 10 GHz!)
Mixer ports
• There are three ports on a mixer, the radio frequency (RF) port, the
local oscillator port (LO), and the intermediate frequency port (IF).
• The RF port is where the high frequency signal is applied that you
want to down convert it, or where the high-frequency signal is
output in an up converter.
• The local oscillator (LO) port is where the "power" for the mixer is
injected. In this case, the power that is applied is RF, not DC like it
would be in an amplifier. The LO signal is the strongest signal, and is
used to turn the diodes on and off in a switching mixer (which is
nine out of ten mixers). The switching action effectively reverses
the path of the RF to the IF.
• The IF port is where the RF signal that was modified by the LO
signal is passed, and its waveform is filtered to become the IF signal
Sources of attenuation in waveguide
component