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Chapter: Transmission Lines and Waveguides : Waveguides and Cavity Resonators

Important Short Questions and Answers: Circular Waveguides and Cavity Resonators

Transmission Lines and Waveguides - Waveguides and Cavity Resonators - Important Short Questions and Answers: Circular Waveguides and Cavity Resonators

CIRCULAR WAVEGUIDES AND CAVITY RESONATORS

95.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

 

96.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.

For the same operating frequency, circular waveguide is bigger in size than a rectangular waveguide.

 

97.Mention the applications of circular waveguide.

 

Circular waveguides are used as attenuators and phase-shifters

 

98.Which mode in a circular waveguide has attenuation effect decreasing with increase in frequency?

TE01

 

99.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

 

100.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

 

101.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.

 

102.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

 

103.       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

 

104.       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

105.       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.

106.       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.

 

107. 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

108.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.

 

109.What are the performance parameters of microwave resonator?

 

The performance parameters of microwave resonator are:

 

(i) Resonant frequency

 

(ii)Quality factor

 

(iii)           Input impedance

 

110.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.

 

111.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 = π W/ P

Where W is the maximum stored energy P is the average power loss

112.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.

 

113.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.

 

114.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 .

 

115.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

 

116.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.

 

117.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

 

118.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 .

 

119.What are the types of cavity resonators?

 

There are two types of cavity resonators. They are: a ) Rectangular cavity resonator

 

b ) Circular cavity resonator

 

120.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.

 

121.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

 

122.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

 

123.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

 

124.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

 

125.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.


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