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Wave guides
Wave guides are used to carry electromagnetic waves.
Wave guide is a hollow metallic tube which can be used
to transmit the EM waves where in the transmission due
to the reflections from the inner sides of the tube.
The traveling distance of waves in wave
guides depends on the frequency of the
wave.
Higher frequency waves travel longer
distances through guides than lower ones.
i) Case of high frequency wave - high incident angle
ii) Case of medium frequency wave
iii) Case of low frequency wave
BUT……………….,Wave
guides will carry or propagate
signals above a certain frequency, known as CUT OFF
frequency, below which the wave transmission is not
possible.
….. no wave
By solving Maxwell's equations by subjected to appropriate boundary
conditions at the walls of the wave guides. Such solutions rise to a number of
field configurations. Each configuration is known as a mode.
When EM wave passes through the wave guide, one can
define the electromagnetic configurations with in the
wave guide.
The electromagnetic configurations would be
i) Hz = 0 and presence of Ez ----------called as TM mode
or ELECTRIC WAVES.
ii) Ez =0 but presence of Hz
MAGNETIC WAVES
------------ Transverse Electric modes or
What is TEM
mode?
TEM means transverse Electromagentic wave where in
both of Electric and magnetic fields will become zero at
one point.
Wave guides…
Shape only matters………..
Rectangular wave guides are the earliest
wave guides of transmission lines.
These are used in many applications such as
isolaters, detectors, attenuators for various
standard wave guide bands of 1GHz to
220GHz.
Rectangular wave guide can support TE and
TM modes but not TEM because one can not
define a unique voltage since there is only
one conductor in wave guide.
The cut-off frequency in Rectangular wave guide
is (TE 1,0)
The cut-off wave length in Rectangular wave guide is
At given operating frequency, only those
frequencies which are above cut-off
frequency will propagate. The below
frequencies will not propagate that means
they will decay exponentially and this
modes are called cut-off modes.
The dominant mode of propagation (TE11) in circular waveguide of
radius ‘a’
ƛc = 2πa/1.814
The dominant mode of propagation (TM01) in circular waveguide of
radius ‘a’
ƛc = 2πa/2.405
Applications
Optical fibers transmit light and signals for long distances
and with a high signal rate.
In a microwave oven a waveguide transfers power from the
magnetron, where waves are formed, to the cooking
chamber.
In a radar, a waveguide transfers radio frequency energy to
and from the antenna, where the impedance needs to be
matched for efficient power transmission.
A waveguide called a stripline can be created on a printed
circuit board, and is used to transmit microwave signals on
the board.
Waveguides are used in scientific instruments to measure
optical, acoustic and elastic properties of materials and
objects.