Sixth Semester B.E. Degree Examination, Dec.2014/Jan.2015

\, \ Antennas and Propagation 1"\(\.,...

Max. M r :100•••Note: Answer any FIVEfull questions, selecting

atleast TWO questions from each part.

a. With the-"el of Maxwell's equation, explain how radiationplace? /Z."

b. Explain the f01t#"wingterms as related to antenna system: .,i) Directivity; ii) ~BW; iii) Effective length; iv) Beamcefti,lency. (08 Marks)

c. Show that the direc(vity for unidirectional operation is 2 + 1) for an intensity variation ofu = Urncosne. (06 Marks)

a. With a neat diagram, obtain an expression fo maximum effective aperture of a 1../2dipole./ ~7Ma~~

b. Derive relationship between maximu effective aperture and directivity of an antenna.(08 Marks)

c. Find the maximum power received~a ",is ance of O.75km over free space 110 Mhz circuitconsisting of a transmitting anti' aAof 30dR gain and a receiving antenna of 25dB gain, ifthe power i/p to the transmitting ntenna is 12~~atts. (05 Marks)

Starting from fundamentals derive the equation for r diation resistance ofHertzian dipole.12; (08 Marks)b. A dipole antenna at; ength 5cm is operated at a frequency ot: OOMHz with terminal current,

10= 120mA. At t'l}e, t = 1 sec, e = 45° and r = 3m. Find: i) '11:(' "'1) Ee and iii) al>' (08 Marks)c. Calculate!h -adiation resistance of a dipole of length = 'A]:' ssume triangular current

distributidl\l'l\ '-") (04 Marks)

~~ ~~a. Der'v the far field expressions for small loop antenna. -0b. Denve an expression and draw the field pattern for an array of two isotro~'

with equal amplitude and opposite phase. Take d = 1../2.c. Find half power beam width and directivity of a linear broadside array of four is

sources of equal strength with d = A/2?

(08 Marks)point sources

(08 Marks)trepic point

(0 Marks)~

a. Write explanatory note on: i) Folded - dipole antenna; ii) Yagi-uda antenna. (10 Marks)

b. Find the length, L, H-plane aperture and flare angles eE and eH of a pyramidal horn forwhich E-plane aperture is lOA. Horn is fed by a rectangular waveguide with TEIO mode.Assume 0 = 0.21..in E-plane and 0.3751.. in H-plane. Also fmd E-plane, H-plane beam widthsand directivity. (06 Marks)

c. A dish antenna operating at a frequency of 1.43 GHz has a diameter of 64mts and is fed by adirectional antenna. Calculate HPBW, BWFN and gain with respect to 1../2 dipole with evenillumination. (04 Marks)

lof2

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a. Write short notes on: i) Parabolic reflectors; ii) Log - periodic antenna. (12 Marks)b. Determine the cut-off frequencies and bandpass of a log periodic dipole array with a design

factor of 0.7. Ten dipoles are used in the structure, the smallest having a dimension L/2equal to 0.3m. (08

a. Define a wave tilt of a surface wave propagation. Also, prove that

W'l -I En -I [l l]ave tl t, a = tan - = tan ~. [ ]1/4'Ev VEr l+x2

b. Derive-~~e expression for resultant field strength at a point due to space'w \ve propagation./ " ~ (05 Marks)

For a VNF/eornrnunication link, a 35 watt transmitter is operatin OMHz. Determine thedistance u tgl( hich LOS would be possible given that h~i v of the transmitting andreceiving antefi):l are 40m and 25m respectively. Evaluate leld strength at the receiving~point. C V (05 Marks)

C~) ~~a. Define the following: i) UF; ii) Critical frequency!""ii) Virtual height; iv) Skip distance.

,I (08 Marks)b. Calculate the value of the or} r fng frequencYi. die ionosphere layer specified by refractive

index of 0.85 and an electron §}IlSity5 x ((j5 electrons/m3. Calculate the critical frequencyand MUF of the system with 8j = 30°. (06 Marks)

c. Calculate the critical frequencies fL f and E layers, for which, the maximum ionicdensities are 2.3 x l06

, 3.5 x 106 an~ 1.7 x l06 elections/cm3 respectively. (06 Marks)

<.9 (~OMarks)

o~***** ~

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