VTU NOTES QUESTION PAPERS NEWS VTU RESULTS FORUM BOOKSPAR ANDROID APP UNIT-4

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1 NTENN & PROPGTION 6EC6 1EC6 UNIT- Unit : Loop and Hon ntenna: Intoduction, small loop, compaison of fa field of small loop and shot dipole, loop antenna geneal case, fa field pattens of cicula loop, adiation esistance, diectivity, Hon antennas. Intoduction: It is a simple antenna. it may take many diffeent foms such as squae, ectangle, o cicle. Loop antenna with electically small cicumfeence have small adiation esistance compae to thei loss esistance. Thei adiation is poo and aely used in adio communication. These antenna ae used in eceiving mode whee antenna efficiency is not vey impotant. Mainly the loop antennas ae used in diection finding. The Small Loop: Conside a cicula loop of adius a with a unifom in-phase cuent as shown in figue.1. The adius a is vey small compaed to its wavelength [a<<]. The cicula loop may be appoximated by a squae loop of length d with a unifom in-phase cuent. The length d is chosen such that the aea of the squae loop is the same as the aea of the cicula loop ie., d πa (.1) Thus the loop can be teated as fou shot linea dipole. If the loop is oiented as shown in figue., its fa field has only an E components. To find the fa field patten in the yz plane it is only necessay to conside two of the linea dipoles and only. The dipole 1 and 3 do not contibutes to the total field since thei field components ae exactly equal and opposite in phase at all point in the yz plane. Page 1

2 Since the individual small dipoles and ae non diectional in the yz plane, the fields patten of the loop in this plane is the same as that of two isotopic point souces as in figue.3. Thus the total fields E is given by E E E e jψ / + E e jψ / Electic field fom individual dipole (.) ψ Phase diffeence between the two field component πd sinθ d sinθ πd Whee d The facto ψ/ is used since phase is efeed with espect to cente. E E je [cosψ / + [sinψ / ] j sinψ / ] + E [cosψ / j sinψ / ] d je sin[ sinθ ] (.3) The facto j indicates that the total field E in phase quadatue with the field E of thei individual dipoles of d<< than the above equation educes to Page

3 E je [ d sin θ ] The fa field E o of the individual dipole is given by j6π [ I ] L E [ θ 9 sinθ 1 θ angle measued whee[ I ] etated cuent on the dipole dis tan ce fom the dipole substituting E in the above equation, it educes to fom dipole axis] E 6π [ I] Ld sinθ (.) Thelength L of the dipoleis sameas d. 1π [ I]sinθ E. whee d, ea of the loop H Ed π[ I]sinθ. 1π [ L d] and d πd (.5) Radiation Resistance of a small loop antenna To find the adiation esistance the total powe adiated is to be calculated the total powe adiated is obtained by integating the poynting vecto 1 1 E W E X H η W W π π θ π π θ (1) 3 6π I π I sin sin 3 θ θ dθ d η 1 1 1π sinθ dθ d 3 16π I W (π ) 3 16π I W (.6) If the adiation esistance of the loop is R then the total powe dissipatedi R / whee I is the cuent Page 3

4 I R 16π I R 3π 31 whenthe numbe of tuns is ' n' R Ω n 31 Ω (.7) Field Compaison of Electic Dipole and Small Loop pplications of Loop antenna: The adiation esistance and efficiency of the loop antenna could be inceased by inceasing its peimete o the numbe of tuns. nothe way to incease the adiation esistance is to inset the feite coe of high pemeability within the loop. This will aise the magnetic field intensity and hence the adiation esistance, such a loop is called feite loop. Feite loop antenna of few tuns wound ound a small feite od ae used as antenna in tansisto adios. Loop antennas ae widely used fo diection finding. Hon ntennas Flaed waveguides that poduce a nealy unifom phase font lage than the waveguide itself. Constucted in a vaiety of shapes such as sectoal E-plane, sectoal H-plane, pyamidal, conical, etc. as shown in figue.. Page

5 Figue:.: Diffeent types of Hon antenna Hon ntennas -pplication eas Used as a feed element fo lage adio astonomy, satellite tacking and communication dishes common element of phased aays Used in the calibation, othe high-gain antennas Used fo making electomagnetic intefeence measuements Rectangula Hon antenna: ectangula hon antenna is as shown in figue.6. This is an extension of ectangula wave guide. TE 1 mode is pefeed fo ectangula hons. Page 5

6 Fig.6: Rectangula Hon antenna Conside a ectangula hon as in figue.6(b). fom figue L length of the hon side lengthl+δ petue θflae angle Fom the geomety, we have also that a L δ << 8δ and θ L cos L + δ θ a sin θ tan Whee θ tan ( L) 1 ( L + δ ) a L a 1 cos L Optimum hon dimensions L L + δ L δ cos ( θ / ) L optimum δ Page 6

7 ( θ / ) ( θ / ) δ cos L optimum length 1 cos Table: Poblem: a) Detemine the length L, H-Plane apetue and flae angles θ E and θ H ( in the E and H planes, espectively) of a pyamidal hon fo which the E-plane apetue ae 1. The hon is fed by a ectangula waveguide with TE1 mode. Let δ. in the E plane and.375 in the H plane. b) What ae the beam widths? c) What is the diectivity? Solution: Taking δ /5 in the E plane, the equied hon length 1 L a 6.5 8δ 8 5 and the flae angle in the E plane θ E a L 1 15 tan tan 9.1 Taking δ 3/8 in the H plane, the flae angle in the H plane Page 7

8 θ H L L + δ cos cos 1.5 and the H-plane apetue a H L tan θ H 6.5 tan HPBW ( EPlane) a E HPBW ( HPlane) 13.7 a H p D 1log 1log Poblems: ( ) 3.1dBi The adius of a cicula loop ntenna is.. How many tuns of the antenna will give a adiation esistance of 35 Ω. Poblem: π (.) < o 1 π (.) <.1 ' n R 31 hence n 7.156<.1 35 The impedance of an infinitesimally thin / antenna is 73+j.5 Ω Find the teminal impedance of infinitesimally thin / slot antenna. ( L.5 and L / D ) ( L.5 and L / w ) 35,76 Z j.5 1 j 11Ω Page 8

9 Poblem: The impedance of an thin cylindical antenna is esistive and equal to 67 Ω. Find the teminal impedance of complementay slot antenna ( L.75 and L / D 1) ( w D L /1.1 ) Z jω 67 Page 9