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TWO MARK QUESTIONS AND ANSWERS
UNIT V
RECTANGULAR WAVEGUIDE
- Why is circular or rectangular form used as waveguide?
Waveguides usually take the form of rectangular or circular cylinders because of its simpler forms in use and less expensive to manufacture.
- What is an evanescent mode?
When the operating frequency is lower than the cut-off frequency, the propagation constant becomes real i.e., γ= α . The wave cannot be propagated. This non- propagating mode is known as evanescent mode.
- What is the dominant mode for the TE waves in the rectangular waveguide?
The lowest mode for TE wave is TE10 (m=1 , n=0)
- What is the dominant mode for the TM waves in the rectangular waveguide?
The lowest mode for TM wave is TM11(m=1 , n=1)
- What is the dominant mode for the rectangular waveguide?
The lowest mode for TE wave is TE10 (m=1 , n=0) whereas the lowest mode for TM wave is TM11(m=1 , n=1). The TE10 wave have the lowest cut off frequency compared to the TM11 mode. Hence the TE10 (m=1 , n=0) is the dominant mode of a rectangular waveguide.Because the TE10 mode has the lowest attenuation of all modes in a rectangular waveguide and its electric field is definitely polarized in one direction everywhere.
- Which are the non-zero field components for the for the TM11 mode in a
rectangular waveguide?
Hx, Hy ,Ey. and Ez.
- Define characteristic impedance in a waveguide
The characteristic impedance Zo can be defined in terms of the voltage-current ratio or in terms of power transmitted for a given voltage or a given current.
Zo (V,I) = V/I
- Why TEM mode is not possible in a rectangular waveguide?
Since TEM wave do not have axial component of either E or H ,it cannot
propagate within a single conductor waveguide
- Explain why TM01 and TM10 modes in a rectangular waveguide do not exist.
For TM modes in rectangular waveguides, neither m or n can be zero because all the field equations vanish ( i.e., Hx, Hy ,Ey. and Ez.=0). If m=0,n=1 or m=1,n=0 no fields are present. Hence TM01 and TM10 modes in a rectangular waveguide do not exist.
- What are degenerate modes in a rectangular waveguide?
Some of the higher order modes, having the same cut off frequency , are
called degenerate modes. In a rectangular waveguide , TEmn and TMmn modes ( both m _ 0 and n _ 0) are always degenerate.
CIRCULAR WAVEGUIDES AND CAVITY RESONATORS
- What is a circular waveguide?
A circular waveguide is a hollow metallic tube with circular crosssection
for propagating the electromagnetic waves by continuous reflections from
the surfaces or walls of the guide
- Why circular waveguides are not preferred over rectangular waveguides?
The circular waveguides are avoided because of the following reasons:
a) The frequency difference between the lowest frequency on the dominant
mode and the next mode is smaller than in a rectangular waveguide, with
b/a= 0.5
b) The circular symmetry of the waveguide may reflect on the possibility of
the wave not maintaining its polarization throughout the length of the
guide.
c) For the same operating frequency, circular waveguide is bigger in size
than a rectangular waveguide.
- Mention the applications of circular waveguide.
Circular waveguides are used as attenuators and phase-shifters
- Which mode in a circular waveguide has attenuation effect decreasing with
increase in frequency?
TE01
- What are the possible modes for TM waves in a circular waveguide?
The possible TM modes in a circular waveguide are : TM01 , TM02 ,
TM11, TM12
- What are the root values for the TM modes?
The root values for the TM modes are:
(ha)01 = 2.405 for TM01
(ha)02 = 5.53 for TM02
(ha)11 = 3.85 for TM11
(ha)12 = 7.02 for TM12
- Define dominant mode for a circular waveguide.
The dominant mode for a circular waveguide is defined as the lowest
order mode having the lowest root value.
- What are the possible modes for TE waves in a circular waveguide?
The possible TE modes in a circular waveguide are : TE01 , TE02 ,
TE11, TE12
- What are the root values for the TE modes?
The root values for the TE modes are:
(ha)01 = 3.85 for TE01
(ha)02 = 7.02 for TE02
(ha)11 = 1.841 for TE11
(ha)12 = 5.53 for TE12
- What is the dominant mode for TE waves in a circular waveguide
The dominant mode for TE waves in a circular waveguide is the
TE11 because it has the lowest root value of 1.841
- What is the dominant mode for TM waves in a circular waveguide
The dominant mode for TM waves in a circular waveguide is the
TM01 because it has the lowest root value of 2.405.
- What is the dominant mode in a circular waveguide
The dominant mode for TM waves in a circular waveguide is the
TM01 because it has the root value of 2.405. The dominant mode for TE waves in
a circular waveguide is the TE11 because it has the root value of 1.841 .Since the
root value of TE11 is lower than TM01 , TE11 is the dominant or the lowest order
mode for a circular waveguide.
- Mention the dominant modes in rectangular and circular waveguides
For a rectangular waveguide,
the dominant mode is TE01
For a circular waveguide,
the dominant mode is TE11
- Why is TM01 mode preferred to the TE01 mode in a circular waveguide?
TM01 mode is preferred to the TE01 mode in a circular waveguide, since
it requires a smaller diameter for the same cut off wavelength.
- What are the performance parameters of microwave resonator?
The performance parameters of microwave resonator are:
(i) Resonant frequency
(ii) Quality factor
(iii) Input impedance
- What is resonant frequency of microwave resonator?
Resonant frequency of microwave resonator is the frequency at which the energy in the resonator attains maximum value. i.e., twice the electric energy or magnetic energy.
- Define quality factor of a resonator.
The quality factor Q is a measure of frequency selectivity of the resonator.
It is defined as
Q = 2 π x Maximum energy stored / Energy dissipated per cycle
= ω W/ P
Where W is the maximum stored energy
P is the average power loss
- What is a resonator?
Resonator is a tuned circuit which resonates at a particular frequency at
which the energy stored in the electric field is equal to the energy stored in the magnetic field.
- How the resonator is constructed at low frequencies?
At low frequencies upto VHF ( 300 MHz) , the resonator is made up of
the reactive elements or the lumped elements like the capacitance and the inductance.
- What are the disadvantages if the resonator is made using lumped elements at high frequencies?
1) The inductance and the capacitance values are too small as the
frequency is increased beyond the VHF range and hence difficult to
realize .
- What are the methods used for constructing a resonator?
The resonators are built by
a) using lumped elements like L and C
b) using distributed elements like sections of coaxial lines
c) using rectangular or circular waveguide
32. What is a transmission line resonator or coaxial resonator?
Transmission line resonator can be built using distributed elements like
sections of coaxial lines. The coaxial lines are either opened or shunted at the end
sections thus confining the electromagnetic energy within the section and acts as the resonant circuit having a natural resonant frequency.
- Why transmission line resonator is not usually used as microwave resonator?
At very high frequencies transmission line resonator does not give very
high quality factor Q due to skin effect and radiation loss. So, transmission line resonator is not used as microwave resonator.
- What are cavity resonators?
Cavity resonators are formed by placing the perfectly conducting sheets on
the rectangular or circular waveguide on the two end sections and hence all the sides are surrounded by the conducting walls thus forming a cavity. The electromagnetic energy is confined within this metallic enclosure and they acts as resonant circuits .
- What are the types of cavity resonators?
There are two types of cavity resonators. They are:
a ) Rectangular cavity resonator
b ) Circular cavity resonator
- Why rectangular or circular cavities can be used as microwave resonators?
Rectangular or circular cavities can be used as microwave resonators because they have natural resonant frequency and behave like a LCR circuit.
- How the cavity resonator can be represented by a LCR circuit?
The electromagnetic energy is stored in the entire volume of the cavity in
the form of electric and magnetic fields. The presence of electric field gives rise to a capacitance value and the presence of magnetic field gives rise to a inductance value and the finite conductivity in the walls gives rise to loss along the walls giving rise to a resistance value. Thus the cavity resonator can be represented by a equivalent LCR circuit and have a natural resonant frequency.
- Name the three basic configurations of coaxial resonators.
The basic configurations of coaxial resonators are:
d) Quarter wave coaxial cavity
e) Half wave coaxial cavity
f) Capacitance end coaxial cavity
39. What is the dominant mode for rectangular resonator?
The dominant mode of a rectangular resonator depends on the
dimensions of the cavity.
For b<a<d, the dominant mode is TE101.
- What is the dominant mode for circular resonator?
The dominant mode of a circular resonator depends on the dimensions
of the cavity. Ford< 2a, the dominant mode is TM010
- When a medium is said to be free- space.
A free-space medium is one in which there are no conduction currents and no charges.
- Which are the non-zero field components for the for the TE10 mode in a rectangular waveguide?
Hx, Hz and Ey.
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