AMIETE – ET (NEW SCHEME) - Code: AE72

Subject: MICROWAVE THEORY AND TECHNIQUES

Time: 3 Hours Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q.1 must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions, answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1 Choose the correct or the best alternative in the following: (210)

a. If the spacing between the transmission lines is smaller than the wavelength of the transmitted signal, the transmission line must be analysed as___________.

(A) a coaxial transmission line (B) a waveguide

(C) a twisted pair (D) a two open wire line

b. In a standing wave pattern of the voltage wave, the distance between any two successive maxima of minima is__________.

(A) one wavelength (B) one fourth wavelength

(C) one half wavelength (D) n wavelength

c. The dominant mode in a particular wave guide is the mode having ___________ cut-off frequency.

(A) lowest (B) low

(C) high (D) highest

d. The tunnel diode is a _________ resistance semiconductor p-n junction diode.

(A) nil (B) positive

(C) negative (D) high resistance

e. A parametric device is one that uses___________.

(A) linear reactance (B) time invariant reactance

(C) non linear reactance (D) time varying resistance

f. In practice, the electronic efficiency of a Klystron amplifier is in the range of__________.

(A) 60 to 65% (B) 0 to 15%

(C) 85 to 100% (D) 15 to 30%

g. The crossed-field (CFA) is used in space communication because the CFA is characterised by________.

(A) moderate power and BW

(B) high efficiency and saturated amplification

(C) small size and low weight

(D) all given in (A), (B) & (C).

h. The parameters of a rectangular micro stripe are w = 10 mils and t = 2.8 mils. On transformation in to a circular conductor, the d, the diameter of the wire over ground is

(A) 7.236 mils (B) 6.7 mils

(C) 10 mils (D) 2.8 mils

i. The packaging density of a typical monolithic microwave IC (MMIC) is _________ compared to conventional ICs.

(A) the same (B) quite low

(C) quite high (D) high

j. In free space the propagation-delay time is________.

(A) 3.333 ns/m (B) 3.333

(C) 3.333 ms/m (D) 3.333 s/m

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2 a. Express the propagation constant in terms of the transmission line parameters R, L, G and C. (2)

b. Derive the following expressions for transmission line:

(i) attenuation constant (ii) phase constant

(iii) characteristic impedance (iv) phase velocity (6)

c. Show that for air insulated conductors, the phase velocity is approximately equal to velocity of light in vacuum. (2)

d. A transmission line has the following per unit length parameters: , , and . Calculate the propagation constant and characteristic impedance of this line at 500 MHz Recalculate these quantities in the absence of loss (R=G=0). (6)

Q.3 a. Derive the total solution of the Helmholtz equation in rectangular coordinates

(8)

b. Explain the methods of exciting various modes in rectangular waveguides. (3)

c. An air-filled rectangular waveguide of inside dimensions operates in the dominant mode. Determine:

(i) the cut-off frequency

(ii) find the phase velocity in the guide at frequency of 4GHz

(iii) find the guided wavelength at the same frequency (5)

Q.4 a. Derive the S-matrix of a directional coupler. Also reduce the S-matrix in the terms of p, the transmission factor and q, the coupling factor. (6)

b. A 2 W power source is connected to the input of a directional coupler with C = 20 dB, D = 25 dB, and an insertion loss of 0.7 dB. Find the output powers (in dBm) at the through, coupled, and isolated ports. Assume all ports are to be matched. (6)

c. Explain briefly Faraday-rotation isolator. (4)

Q.5 a. Draw the equivalent circuit for a parametric amplifier. Clearly mark all the parameters and list out the two properties of a parametric up-converter. (4)

b. Explain the J-E characteristics of Gunn diode. (5)

c. Explain each term for parametric up-converter:

(i) Power gain (ii) Noise figure and (iii) Bandwidth

and calculate the same for the following given parameters

Ratio of output frequency over signal frequency:

Figure of merit:

Factor of merit figure:

Diode temperature:

(4+3)

Q.6 a. Explain the operating principle of Helix Travelling wavetubes. (4)

b. Explain the limitations of conventional vacuum tubes and how it can be overcome? (6)

c. What do you mean by velocity modulation in Klystron and derive the expression for it? (6)

Q.7 a. List the three types of magnetrons. (3)

b. Give the Hull cut-off voltage and Hull cut-off magnetic flux density for a linear magnetron. (7)

c. A linear magnetron has the following operating parameters:

Anode voltage:

Anode current:

Magnetic flux density:

Distance between cathode and anode: d = 6 cm

Compute

(i) The Hull cut-off voltage for a fixed .

(ii) The Hull cut-off magnetic flux density for a fixed (6)

Q.8 a. Explain the difference between microstripline and stripline (4)

b. List the parameters namely Inductance, L, Capacitance, C, Characteristic impedance & phase velocity, for a lossless parallel strip line. (8)

c. A gold parallel strip line has the following parameters:

Relative dielectric constant of polyethylene:

Strip width: w = 25 mm

Separation distance: d = 5 mm

Calculate the

(i) Characteristic impedance of the strip line

(ii) Strip-line capacitance

(iii) Strip-line inductance

(iv) Phase velocity. (4)

Q.9 a. List the advantages of MMIC circuits over discrete circuits. (3)

b. List the basic characteristics required for ideal substrate materials. (3)

c. Describe a thin film planar resistor and express the resistance in terms of its parameters. (5)

d. Explain the planar capacitor film development in MMICs. (5)