## RF Modelling and Antennas - May 2012

### Electronics & Telecomm. (Semester 5)

TOTAL MARKS: 80

TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.

(2) Attempt any **three** from the remaining questions.

(3) Assume data if required.

(4) Figures to the right indicate full marks.
**1 (a)** Describe the concept of near field and far field radiation.(5 marks)
**1 (b)** Explain the principle of pattern multiplication with suitable example.(5 marks)
**1 (c)** Explain Radiation pattern of short dipole.(5 marks)
**1 (d)** Differentiate between broadside and end fire array.(5 marks)
**2 (a)** Describe parabolic reflector and its use of microwave frequencies. Discuss Cassegrain method of feeding the parabolic reflectors.(10 marks)
**2 (b)** Derive the expression for vector potential wave equation.(10 marks)
**3 (a)** Show that the radiation resistance of a wire dipole is given by R_{rad}=80π^{2} (dl/λ)^{2} ,where dl is the small length of wire dipole. Also if operating frequency of a half wave is 400 MHz, find its effective area?(12 marks)
**3 (b)** Explain in brief the working of helical antenna with one application.(8 marks)
**4 (a)** What is line of sight propagation? Obtain the expression for range of line of sight for spacewave propagation in terms of antenna's transmitting and receiving heights.(10 marks)
**4 (b)** Explain the structure of microstrip antenna. Discuss its feed mechanisms and application.(10 marks)
**5 (a)** Explain ducting effect. Under what conditions this effect takes place.(10 marks)
**5 (b)** State and give the application of reciprocity theorem. Show that the transmitting and receiving radiation patterns of antennas are equal.(10 marks)
**6 (a)** State and derive FRIIS transmission equation.(10 marks)
**6 (b)** A uniform linear array is required to produce an end fire beam when it is operated at 12 GHz. It contains 50 radiations and are spaced 0.5λ. Find the progressive phase shift to produce the end fire beam. Also find the array length.(10 marks)
**7 (a)** Ionospheric propagation.(5 marks)
**7 (b)** Ground interference effects.(5 marks)
**7 (c)** Maxwell's equations for harmonic variations.(5 marks)
**7 (d)** Log periodic antenna.(5 marks)