EC 2305 – TRANSMISSION LINES and WAVEGUIDES DEPARTMENT of ELECTRONICS and COMMUNICATION ENGINEERING THIRD YEAR / FIFTH SEMESTER Regulation 2008

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EC 2305 – TRANSMISSION LINES AND WAVEGUIDES DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING THIRD YEAR / FIFTH SEMESTER Regulation 2008

UNIVERSITY SOLVED TWO MARKS WITH ANSWERS

May/June 2012, Nov / Dec 2010, April / May 2011, Nov / Dec 2011, May / June 2007, Nov / Dec 2008,

Nov / Dec 2009, April /May 2010, Nov / Dec 2012, May / June 2013.

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MAY / JUNE 2012

1. Write Relationships between neper and decibel?

2. Write the disadvantages of constant K filter?  Ideally the attenuation should change sharply in the stop band. But in all proto type filters the attenuation changes gradually in the stop band. Hence frequencies near cut off frequency are passed through the filter.  In the pass band output of the filter should remain constant which means the characteristics impedance remains constant. But characteristic impedance varies with frequency from value R throughout the pass band. Hence the filter cannot be terminated properly. 3. Draw the equivalent circuit of a unit length of a transmission line.

4. What is meant by infinite line?  It is an imaginary line of infinite length having input impedance equal to the characteristic impedance of the transmission line.  A line of finite length, terminated in a load equivalent to its characteristic impedance appears the sending end as an infinite line. 5. Write the disadvantages of single stub matching. 

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  Single stub matching requires the stub to be placed at a definite point on the line.  it requires two adjustment to be made, these being the location and the length of the stub.  Single stub matching is adequate for open wire lines. But for coaxial lines this is not suitable because it is difficult to obtain the location of voltage minimum without slotted line section. That is coaxial line placement of a stub at exact point is difficult.

6. Write the relationship between SWR and reflection coefficient.

S= 1+ | k | / 1- | K| (OR) K= S-1/S+1.

7. Write Maxwell’s equations.

8. What is meant by dominant mode? What is the dominant mode for parallel plate wave guide?

The mode which has the lowest cutoff frequency is called dominant mode.TE10 is the dominant mode in TE waves. 9. Write the expression for the wave impedance and guide wavelength for TEM Mode. 10. What is the dominant mode of a rectangular waveguide? Why? The wave which has the lowest cutoff frequency is called the dominant wave. In rectangular wave guide, the lowest order TE wave, TE10 mode has the lowest cutoff frequency and so TE10 wave is the dominant wave.

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NOV / DEC 2010

1. A constant K T –Section high pass filter has a cutoff frequency of 10 KHz. The design impedance is 600 ohms. Determine the value of L.

L=Rk/4πfc L= 600/4π×10×10ᵌ L=4.77mh.

2. What are the advantage of m-derived filters?

  Attenuation rises sharply at cutoff frequency.  In the pass band output of the filter remains constant which means the characteristic impedance remains constant throughout the pass band

3. Define delay distortion? When a signal having many frequency components are transmitted along the line, all the frequencies will not have same time of transmission, some frequencies being delayed more than others. So the received end waveform will not be identical with the input waveform at the sending end because some frequency components will be delayed more than those of other frequencies. This type of distortion is called phase or delay distortion.

4. Write the expressions for the phase constant and velocity of propagation for telephone cable.

5. A lossless line has a characteristics impedance of 400 ohms. Determine the standing wave ratio if the receiving end impedance is 800+ j 0.0 ohms.

ANS: Z0=400ohm, Zr=800+j0.0ohms. K=ZR-Z0/ZR+Z0 =1/3, S= 1+|K|/1-|K| = 1+1/3 / 1- 1/3 =2. 6. Write the expressions for the input impedance of open and short circuited dissipationless line.

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Zoc = -j R0 cot (2π/lambda)s. Zsc = jR0 tan (2π/lambda)s.

7. Define the cutoff frequency for the guided waves. The frequency bellow which the wave is attenuated exponentially and above which the wave propagation occurs is called cutoff frequency. The frequency at which wave motion ceases is called critical frequency or cutoff frequency fc of a wave guide. At cutoff frequency fc.

8. For a frequency of 6GHz and plane separation of 3 cm, find the group and phase velocities for the dominant mode. Ans : C/P=5.42ms, CP=1.659m/s.

9. Calculate the cutoff wavelength for the TM11 mode in a standard rectangular waveguide if a=4.5cm. Ans: m=1 n=1

Lamdanot=4.02.

10. Give the applications of cavity resonators.  Used in cavity meters.  Used in klystron amplifiers.  Used in Reflex klystron oscillators and cavity magnetrons.  Used in duplexers in radar systems.

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APRIL / MAY - 2011

1. For a symmetrical network, define propagation constant and characteristics impedance. The current ratio or voltage ratio is expressed in exponential term under the condition of Z0 termination. Characteristic impedance of a symmetrical network is the impedance measured at the input terminals of the first network in a chain of infinite networks in cascade and is denoted as Z0. 2. What are the disadvantages of m-derived filters?

 Attenuation rises sharply at cut off frequency.  In the pass band output of the filter remains constant which means the characteristic impedance remains constant throughout the pass band.

3. How can distortion be reduced in a transmission line?

A line in which there is no phase or frequency distortion is called a distortion less line. The condition for a distortion less line is given as RC=LG.

4. A transmission line has Z0=745 12degree ohms and is terminated is ZR=100ohm calculate the reflection loss in Db.

Answer: k=0.6475, 1/|k|=3.7751 dB 5. Express standingwave ratio in terms of a reflection coefficient.

S= 1+|K|/1-|K| (OR) |K|=S-1/S+1.

6. Mention the application of quarter waveline.

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A quarter wave line is used for impedance matching.(i.e) it matches the load with the sources and ensures that maximum power is being transferred to the load. Application: i). used as a transformer to match a load ZR ohms. ii). Used as impedance matching inverter. iii). Used as couple a transmission line to a resistive load such as an antenna. 7. The electric field in free space is given by E=50cos[10T+BX] V/M. find the direction of wave propagation and Beta. the wave is propagating along –a beta=w/c=1/3=0.333rad/m. 8. Define skin depth. Skin depth is a measure of depths to which an Em wave can penetrate the medium. 9. Compare transmission line and wave guide. Answer: TRASMISION LINE WAVE GUIDE 1. May operate from DC to It can operate only above a very high frequency. certain frequency called cutoff frequency. 2. Inefficient due to skin Larger bandwidth and lower effect and dielectric loss. attenuation.

10. An air filled resonant cavity with dimensions a= 5 cm, b= 4 cm, c= 10 cm is made of copper. Find the resonator frequency for lowest order mode.

Answer: f101 = v/2 = 3.335 GHz.

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Nov / Dec - 2011

1. What is the significance of propagation constant in symmetrical network? 2. Draw the equivalent electrical circuit for a piezoelectric crystal. 3. When does a finite line appear as an infinite line? 4. If a line is to have neither frequency nor delay distortion, how do your relate attenuation constant and velocity of propagation to frequency? 5. For the line of zero dissipation. What will be the values of attenuation constant and characteristic impedance? 6. Define standing wave ratio? 7. Write down the relationship between phase velocity and group velocity? 8. Write down the equations for characteristic impedance for TM, and TE waves. 9. A rectangular waveguide of cross section 5cm x 2cm is to propagate TM11 mode at 10GHz. Determine the cutoff wavelength. 10. What is the need for guide termination?

MAY/JUNE - 2007

1. What is frequency distortion? 2. Calculate the load reflection coefficient of an open and short circuited line. 3. Find the VSWR and reflection coefficient of a perfectly matched line with no reflection from load? 4. Name the few applications of half wave line. 5. What is the cut off frequency of TEM Waves? 6. Give the expression that relates phase, group, free space velocities. 7. Why the TE10 wave is called as dominant mode? 8. How the TE10 mode is launched?

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9. Define the quality factor of a cavity resonator. 10. What is cavity resonator?

TWO MARKS

Answers