7 jotennas & Wave Propagation 5-2 GATE ACADEMY PUBLICATIONS™ 3.1 Introduction Im order to have an orderly study of the present day practical antennas, it becomes secessary to dassify them in some way, as they are so diverse in type The firs factor in design of an antenna aystem or antenna is the frequency or a band of frequencies at which the tntenna has to operate. Obviously, then the classification of antennas by frequency of operation i perhaps the most convenient way. The accepted classification of radio frequencies ismentioned in below table eg. VLF, LF, ME, HF, VHF, UHF, SHF and EHP. Sandy ‘VLE (3-30 kHz) TF @0-300 KHz) ‘Long distance point to point service, marine and navigation aids [Baie] __MF 600-3000 KHz) _| Broadcasting, Navigation, Harbour, Telephones etc. (rae ‘HF (3-30 MHz) ‘Beamed communication services e.g. moderate and re Jong distance communication of all types, Short wave broadcasting to distant places. ‘Short distance comaunication, Television, Frequency ‘modulation, Radar relay, Telephony. Uli (6003000 Miz) | Short distance comanunication radar relay system, (Above 200 MHz are called | landing, Television. ¢ as microwaves to.) ‘HIF (6000 ~30,000 Miz) _| Radar, Radio and television relay Tinks, satelite (Atso called microwaves) | communication (8400 - 8500 MHz). "EHF (60,000 — 300,000 Miz) | Experimental, Amateur, for Government purpose. VHF (90300 Miz) 6.2 Effects of Ground on Antenna Performance ame 404 grounded veri te del vera ates ow Bt | Tadiating eystem « There is one ' GATE ACADEMY PUBLICATIONS > 5-8 Practical Antennas str Reflection Rays Image Fig, (b) Voltage current distribution in 04 antenna ‘The Marconi antenna has one important advantage over the ungrounded (or Hertz fantcnina) that only half height of the antanna is new required in comparison to ‘ungrounded antenna (Hertz antenna) to produce the same radiation pattern. ‘Ungrounded Antennas (Ground assumed to be a perfect conductors) : Ifa source of radiation is put near a reflecting surface, the radiation received at any distance point {s the vector sum of the direct and indirect (Le. reflected from the ground) radiations. Using the principle of image simplify the situation to a great extent. [tis assumed that fan image antentia is existing below the ground and is true mirror image of the actual antenna as shown in Fig. (). Fig. (a) Actual and image antenna Fig. Fig @® Fig. dh) Fig. (i) Effects of imperfect earth on. the pattern of un-grounded antennaat k=2 Now the resulting radiation received at a distant point ‘P is considered to be coming from the actual antenna and image antenna. The matters are simplified, if ground is, assumed to be perfect conductor ie. a perfect reflector. This assumption of perfect conductor is often justified.Antennas & Wavo Propagation, 5-4 __ GATE ACADEMY PUBLICATIONS “The currents lowing in the image antenna now have the same magnitude as that of actual antenna, The overall radiation pattem is calculated on the bass of being “used by an array of two neazby antennas. These are identical in length having equal 1 ‘irrents in magritudes and ae spaced by twice the height i) of actual antenna above {qround. The direction of curent flow in vertical antenas are same as that of actual Snferna while in cae of horizontal antenna i is in oppesite phase as shown in ig. (d) and (e). ‘Some typical radiation pattems for varying heights are shown for ungrounded antennas in Fig, (, (8) 2nd In practice the ground isan imperfect conductor and hence reflection coefficient of the ground is an important factor. Fora perfect condictor, reflection coefficient of cart is unity, The effect of imperfect ground when the antenna is ata height A above earths illustrated in Fig. (. 5.3 Effects of Antenna Height ‘Question 2. should be at least ‘quarter wavelength, above ground to make it resonant but when its possible then its effective height should coresponds to 4. When an antenga i nt of resonant length for given operating frequency its input impedance containsla reactive component. If the antenna height is between 2/4and 2/2, its input impedance contains an inductive reactance component. Such system may be made resonant by adding lumped ' reactance (inductive or capacitive) of suitable magnitude and size as shown in fig. (0) and (. a igs Fb setstoteg ae moo Ea Fig (2) Use of series loading coil Fig, (b) Various form of top loading with or ‘capacitance to attain the ‘current distribution (dotted lines) resonant length N/4 ‘Thus the grounded vertical antenna can be lengthened by adding inductance and Shortened by ang series capacitance. The series inductive or capacitive reactance ‘made equal to that of capacitive and inductive reactance presented by the vertical ) ‘adiator so that the impedance is resistive only atthe resonant frequency. However, radiation efficiency is sacrificed by this way as series inductance or capacitance adds to loss resistance and there is no improvement in the radiation resistance. Antesuia tadiation efficiency can be increased by increasing radiation resistance of the system. gare, ACADEMY PUBLICATIONS ™ 5-5 Practical Antarnay “Top loading is a method to increase the effective capacitance at the top ofthe anew This is accomplished by mounting one or more horizontal conductors atthe atera top as shown in Fig, 0). Top loading may assume the form of an inverted Lor Tx hut as shown in figure (). By efecting top loading the current at the base of he antenna is increased and current distribution become more uniform. “The current inthe horizontal portion is smaller than in vertical portion, thers antenna still works as vertically polarized antenna, The resulting current distoton, along the vertical portion i similar to that as if extra vertical section is added. Thus te tcurent along the upper portion of the vertical antenna increase and £0 abo the radiation resistance and radiation efficiency. ‘Lumped reactance may also be used to maintain a resonant condition in an antenna system ifthe associated transmitter is operated under diferent frequencies ‘Thusa series inductor or capacitor connected with the vertical antenna may alo seve asan antenna tuning circuit . Bee fi fe short notes On Beverage or Wave antenad Wil ss i Deverage or Wave Antenna : This is @ non-resonant anfenna in which ther i 2 standing wave suitable forthe reception of long-wave. signals. This was oigialy ‘developed by HLH. Beverage as Long wave directional antenna. Its based on te principe of iting of wave front of the direct ray due to absorption of energy byte Jrnface over which the wave passes. Beverage or Wave antenna consists of astipht wire ranging from 2/2 to many wavelengths (say upto 10%) long sully cai by insulators on telephone poles at about 9 meters above the ground. It poins towards the desired transmitting station. The end towards transmitter is terminatd by 1 impedance equal to the characteristic or surge impedance of the antensa whe considering as one wire transmission line with ground retum, while the exp abstracted from the radio waves is delivered to the R, atthe other end of ante = shown in igure (2) and). Fig (a) Beverage or wave antenna of single wire type Fig, (b) Directional pats Direction of propagation. \ etnias a E E a E, E, Fig (© Perfect conductor” Fig. (d) Imperfect conductor Fig.) Wave anteastoe oe nN A TE LSA MF ROT F saunas & Wave Propagation 6-6 om \TE ACADEMY PUBLICATIONS : sna To ite pe balanced wave antenna” ts known that electric field of a radio wave travll a uct suc perpendiaar tthe sro min Fig (Howes, teense aes with imperfect conductor like earth’s surface or ground. The electric field lines Ein this case, havea forward tilt near the surface as shown in Fig. (4). : ‘The horizontal component £, is assocated with that part of the wave that enters the surface and appears as heat. However, the vertical component £, continuous to travel along the surface. ‘The ground acts as imperfect conductor. Then emt i induced along the wave antenna by the horizontal component £, as the waves travel towards the A. All the em so induced added up in same phase at the 2, because the current induced in the wire travels withthe same velocity as the radio wave. In fac, theftorizontal component E, ‘exists and is applied in the wave antenna for receiving vertically polarized waves. Further induced emf ariving atthe terminating end due to opposite direction waves, langely absorbed by it. Also waves arziving from ses have litle resultant effect at the 2. Wave antenna possesses a directional pattem in horizontal plane with maximum response in the direction ofthe termination. This directional characteristics partially depend on the length of the antenna, terminating impedance and the velocity of propagation of wave along the wire. If itis not possible to connect R, tthe ead B, then the wave antenna using two wire balanced line with reflection transformer may be used ssshown in figure ()- Advantages: The advantage of Beverage or wave antenna are as follows: () Itis specially suitable for low frequency reception and possesses a good deal of directivity in the horizontal plane. (i) itis brood band antenna and is satisfactory or reception over a wide band of frequencies inthe LF and VLF bands. (Gi) The directivity can be still improved if arranged in simple array of Beverage antenna connected in parallel. : (iv it can receive simultaneously the signals of different wavelengtts if all the ters emiting different wavelengths le in the same direction. Drawback: Like other low frequency antennas it transmission efficiency is low due to oss resistances. (GATE ACADEMY PUBLICATIONS™ 5-7 Practical Antennas GATE ACADEMY PUBLICATIONS™ _5-7_Practical Antonnas_ [5.4 Medium Frequency Antenna and Thelr Characteristics _ ‘Ans. The antenna towers are based on the principle stated by Prof. 5. Ballantine that “For the maximum ground wave radiation in horizontal direction there exists an optimum. height”. The tower radiator consists a vertical mast of ste! lattice structure which is either uniform in cross-section or tapered at the top. The tower radiators with uniform. ‘ross section throughout the height require supports. The current distribution in such radiators is approximately sinusoidal. The equivalent height of the radiator and the {impedance between the lower end of the tower and the ground can be easily obtained {n case of towers with uniform cross-section. In case of tapered cross section radiators, the current distribution departs slightly from the sinusoidal distribution. This gives effective height ofthe tower slightly less than the actual physical height. The tapered. radiators do not require support. Hence structurally the more taper is required but clectrically it affects urrent distribution atthe top. The uniform crase-secton towers are supported with guys which are broken into non-resonant sections by insulators to avoid damage to the structure. The antenna towers of uniform cross-section and ‘tapered cross-section are as shown in the Fig (a) and (b) respectively.Antonnas & Wave Propagation 6-8 __ GATE ACADEMY PUBLICATIONS ™ Tin both the eases, the radial buried wires are used as shown in the Fig (a) and (b)-In practical cases, the length of these wires are equal to the height of tower. In case of antenna towers of guys supporting type, the tower is insulated from the ground, while in case of self- supporting tower, end of the tower is connected to the ground, ‘The tower antennas are excited using two methods namely series excitation and ‘shunt excitation. The uniform cross section towers are excited using series excitation in which voltage is applied between lower end of the tower and ground. This can be treated as the sending end ofthe transmission line with input impedance between the tower base and ground as the sending end impedance. The length of the transmission line ean be considered equal to the height of the tower. But the analysis becomes ‘complicated and it departs from the theory of transmission line due to the Iumped ‘capacity between base and ground. Tn the shunt excitation, the voltage is applied through a coaxial feeder across @ section of tower of height h. This height is selected such that resistance of this section matches the characteristics impedance of the feeder. Tn antenna tower, the current flows from tower to the ground through the capacitance formed between base ofthe tower and ground. Actually this current must yetum to the ground. But due to the skin effec, the current confines to the surface of ‘emerge out from base ofthe tower. The length is idgally equal to the height of the tower, but practically the length used is at least Bn the range 44 to 3/2 The arrangement used for minimising the ground losses is called radial buried wire ground system. The screen of such buried wires provides a very low resistance path © the current, 20 thatthe current flows to the base ofthe tower without confining atthe surface of the earth. In general the antenna towers are fed using a two wire transmission line being inexpensive. Bt the drawback of these lines is that they are greatly troubled due the changing environmental conditions and weather. Two, wire transmission line is 9 balanced line as both wires are symmetrical with respect to ground. The other method for feeding the antenna towers 8 using coma fine feeder. In general coral Tine is ‘unbalanced as one connector is grounded. Besides being very expensive, this feeding system provides superior performance over the two wire transmission line, Uses: (1) For broadcasting purposes, the medium frequency antennas working in between, '300 KHz to 3000 kHz are most widely used. This band is generally utilized by the services which require all around coverage. In such broadcasting services itis necessary that the field strength of the radiation should decrease rapidly beyond certain distance to avoid interference with the similar other broadcasting stations For such broadcasting services, the broadcasting transmitter should transmit maximum energy along the ground surface, while minimum energy towards ‘ionosphere. ti} Practical Ania, For bendcsing purpose, the atennas used are te vet dat rg aoa ele of such towers ranges from 45m to 250 m above te pes 76 to R13), in case of high power tansmters, the towers of hei aie an 2/2 are used which inreases horizontal radiation avoiding athigh wertcal angles. ig [5.5 High Frequency Antenna and Their Characteristics GATE ACADEMY PUBLICATIONS ™ antenna form a standing wave pattem. ‘The features of these antennas are: 0) teingtctarcountmniconisacimatiplects (@) These antennas are open at both ends. @) These are not terminated in any resistance. (8) They are used ata fixed frequency. (6) Inthese antennas, forward/ incident and backward] reflected waves est (6) A.standing wave exists in these antennas. (7) The radiation patterns of these antennas are multi-directional. (8) The current distribution in resonant antennas are shown in igure (a) 3h 2 Rig. (2) Current distribution in resonant antenna iig. (b) Radiation patter of different resonant dipole (10) The halt-wave antenna hasdistbuted inductance apd capacitance and ita ike ‘a resonant circuit. (11) The voltage and current onitarenot in phase. (12) The length of a resonant antenna is found from. where, Fe velocity factor (The velocity factor of wire compared to air, F= 0.95) gr velocity of propagation. Lara Fig, (0 Voltage distribution (13)Resonant antennas are known as periodic antennas. ‘Along wie or tavelling wave antenaa which doesnot have natural frequencies of ‘uclation and responds equally well fo radiation over abroad range of frequencies. “The features ofthese antennas are: (0) Thelenghof anon resonant antensioher han in mulpes of (2) Atone end ofthe antenna its excited and the other end itis terminated, (@) Ttoperates overa range of frequencies. (@) Iehasa wide bandwidth. (©) There are no reflected waves on this antenna. (6 There are no tanding waves. (7) Itisa teaveling wave antenna. (©) tis terminated by characteristic impedance. (6) lt produces uni-directional radiation pattems. (10) Example of nonresonant antenna s Rhombic antenna, (11) Non-resonant antennas can alsobe cal direonal antennas (02)The typical nonresonant antenna, current, voliage distribution and its radiation patter are shown in gure. ((13)Non resonant antennas are known as aperiodic antennas. “tes & (snr + + Fig (a) Current and voltage distribution Fig (b) Non resonant antenna radiation pattern (14)The patter expression ofa travelling wave antenna is given by, 30l, sind 2 Been? LM-on0) ® ‘maximum current in the element, r= distance from the source, length ofthe element. (15)The patter is not symmetic about 0 = 90°Antennas & Wave Propagation 5.6 Long Wire Antenna or Harmonic Antenna 5-12 GATE ACADEMY PUBLICATIONS ™ Hartt is integral multiple of half wave length. When an antenna is more than a half wave long i is callded as a long wire antenna or a harmonic antenna.” “Thus the long wire antenna isa single long wire, generally two or more wavelength (Le. 4/4 or more 2/2) long at the operating frequency. The higher the number of 42 the better its directivity. It radiates a horizontally polarised wave at low angles from about 17 to 2° relative to the earth surface. In long wire antenna, the currents in adjacent half wave section must be out of phase and hence any feeder system can be used that disturb this condition. This condition can be satisfied if long wire antenna is fed at ether end or at ‘any current loop. A long wire antenna is generally made a half wavelength at the lowest frequency of operation and fed at the end. a_i Trazsmission line Fig (a) Long wire resonant antenna ‘The long wire may assume two forms : (Resonant (unterminated) : In resonant long wire antenna, standing wave exists along its length and the pattem is bidirectional corresponding to incident waves and reflected waves. (di) Non-resonant (terminated at characteristics impedance): In case of non-resonant tong wire antenna all the incident waves are absorbed ini terminating impedance and there is no reflected waves. Ths is why the patter is only given to incident ‘waves ie. unidirectional only and uniform cirrent and voltage exist along the axis of the wire. ‘The angle of radiation with reference to wire axis depends on number of wavelength ie. even or odd, For =3 and =4 directional patter is shown in fig. (9, (4) and (© for example maximum radiation from a long wire antenna of ®.long wat. wire axisis at 175*with many small minor lobes. ‘The physical length of along wire antenna can be extended from the physical length of M2antenna as follows from equation (i) a, 468 2° 7M) Fig.() Long wire non-resonant antenna 0) (GATE ACADEMY PUBLICATIONS™ 5-13 Practical Antennas (length) 2/2 ana fone wavelength in waveengy (length) 172 a ‘fone half wavelength in wavelngh (length) 1.02 a it two half wavelength in wavelengh Hence form half wavelength long wire antenna (eet) =O if half wavelength in length ‘A long wire antenna can be used for harmonically related frequencies and along wie (ot harmonic antenna isthe basis of multiband operation with one antenna, “The field strength of a long wire antenna of odd and even number of wavelengths givenby, . - cos cos soap tials") sath 5 od (i) £(r.04)= =i) where, /,, = ms curren at current maximum point Amp) -Blecrc field strength in Vin) 1=Distance in metres. ors ‘The above figure (c), (4) and (¢) shows non-resonant long wire pattem.Fig(f) Current and voltage distribution when operated at harmonics of fundamental frequency. In case, along wire antenna is made non-resonant or travelling wave antenna by termination, then ls sin ‘The directional patter for odd (n = 3) and even number (7 = 4) of wavelengths are shown in the fig (c) (A) and (es derived from the above equations. Ifthe number of half wavelengths is made larger, the number of lobes increases proportionately with ‘major lobe radiation closer to antenna wire axis. ‘The radiation resistance of a resonant long wire antenna, 1 half wavelengths long, in fre space is given by an approximate formula R= 73+ 69}050" °) where, = number of half wavelength. Further the angle of maximum radiation ie. angle between acisis given by, nl 2080, = 7 7 On putting the value of cos from tis equation to equation (i) and i) (eroding © nis odd or even) the value B,., canbe calculated. és ‘maximum lobe and wire fi) {@ Resonant and nonresonant long wire antennas are used for transmision and reception i. from 500 kHz to 30 MHz. GATE ACADEMY PUBLICATIONS ™ 5-45. (il) They provide a simple and effective method of obtaining directional pattem and power gain. (it A tong wire antenna has practical value because of its structural simplicity and relatively low cost, irrespective of theoretical complications. 5.7 The V- Antenna and Inverted V - Antenna, ‘Ans. (i) Vantenna in V shape. Itis balanced ~ fed at the apex and the included angle, length and elevation are chosen. to obtain the desired directional properties. ‘Thestructure ofa V antenna is shown in Fig. (2) Apex 4 Diresjon of maximum rats Feed line Fig (@) V antenna Salient features of Vaantenna ae a follows: (2) Ttconsists of two long wire antennas arranged in the form of V and itis fed at the apex. (2) The excitation to each wires out of phase. (@) Tokfers greater gain and directivity when length of each leg or wire is increased. (@) Itsradiation patter i bi-directional (6) Vantennas are of two types : The frst oneis resonant and the second one is non - resonant. (6) The pattem of resonant V antenna is shown in Fig (b). —— SED ieee ced ine Fig (0) Radiation pattem of resonant V antenna (2) Thepatter of non-resonant V antenna is shown in Fig (0). (®) These antennas are useful in HF band, (©) Themain disadvantages that high side lobes exist. (20) The apex angle ranges between 36 to TPfor V antennas of 8. to 22, length.GATE ACADEMY PUBLICATIONS ‘Antannas & Wave Propagat 6-46 _ GATE ACADEMY PUBLICATIONS’ a Aa shape oan inverted -V. structure is thown in Fig. (9- Feo ling a —) stoce (aia RZ, Fig(c) Non-resonant V antenna with radiation pattern (qaytis easy to construct and they are cheap. ((2)Using V antennas, end-fire and broadside antennas can be easily constructed. ‘He tnverted V antenna ‘These are shown in Fig) and Fig) respecte Salient features of inverted ~ V antenna are as follows: (1) Risa traveling antenna. He (2) The direction of maximum radiation s towards the terminated end. Direction of (@) Inputs given at point B. (see Fig.(0) oo {4) Feading is done through ransssson Lines with espet to adil earth wires. eed (@) Antenna wire at Cis terminated in a resistance. line (©) Theangle ¢ is the tilt angle. Fig (a Endre array of V antenna, (7) The terminating resistances about 40002. (®) Angle of main lobe corresponds to 4. ' (0) Gainis function of angle fle engl and terminating resistance (10) These antennas are useful in HF band. P (11) They have considerable band width. Fig.(e) Broadside array of V antennas ((12) The length of the leg equal to Ais used. (13) The optimum included angle, «is (13) The main disadvantages is that it has high side lobes in its radiation pattern. . (Qajlnverted V antenna with its image looks lke a rhombus. n-1093(£) eona( ty asf )eas {ipisusd upto Mi for rsving pups. : (16)Ground and surface waves ae best received in upper HF band. For aSsosI5 (17) Arrays can be used for high gain. x : (18) tis not preferred for transmitting purposes. and a=1339(2) -nar(j) 169.27 (19)Its radiation pattem contains high side lobes. : 1 5.8 Rhombic Antenna or Rhombic Arrays = For issis3 7 (04) The maximum directivity A paasi(!)st 1 For 055553. ear ‘Ans. This is an antenna which is in the shape of a rhombus. It is usually terminated it» resistance. The side of the rhombus, the angle between the sides, the eewaion termination and height above the earth are chosen to obtain the desied characteristic. A typical Rhombic antenna and radiation pattem are shown in BESS il) PaseiiceeittnET | Amv 5-19 Practical Antonnas (GATE ACADEMY PUBLICATIONS (13) The terminating resistance is about 6000 « (14) The input impedance of Rhombic antenna lies between 650 0700.0 - (15) The directivity of Rhombic antenna varies between 20 and 90. (16)The power gain lies between 15 and 60 after taking power loss in terminating resistance into account. (i7tis a very useful antenna for transmission and reception in HE band. (18)Tts easy and cheap to erect. (19)Its main disadvantages are: () Terequires more space for installation. (Gi) Is efficiency is les, as some power is lost i termination. (2yIts radiation pattem in a vertical plane is shown in below figure. Fig. Radiation patter of Rhombic antenna in vertical plane In the design of Rhombie antennas the maximum point of the main lobe of the radiation patter is aligned with the desired angle of elevation. The angle of elevation || "Ans. Salient features of Rhombic antenna are a8 follows: (Q) tis long wire antenna and consists four non-resonant wires. @ Itprovides greater directivity than V antenna. @ Its bandwidth is high. (@ Itis a HF non-resonant antenna. is also called angle of radiation. {6) Itis very useful for point-to-point communications. f ‘The design parameters of Rhombic antenna are: (6 Itisa travelling wave antenna and there are no reflections. (1) Rhombic height, « @) Angle of elevation, { ications where the angle of elevation of the main lobe @) Wielngth/. (7 It also finds wide app! (measured from the plane (@ Atclevation angle above 30°, (0) The directivity of each wire is ‘of the antenna to the radiation axis) is less than 30° - ‘the gain is very low for practical applications. D(0)= sind ‘Ans. The levation angle are wed tn define he apparent positon ofan obectin the sky, relative ta speic observation point. The elevation angles the angle between the horizontal plane and line of sight measured in vertical plane. The reference direction is ‘a horizontal line in the direction to horizon, starting from antenna, | va where, = The magnitude ofthe current in element é 6- The polar angle de Wave length (11) The tilt angle ¢ varies between (12) ¢is determined from length.5-20 _GATE ACADEMY PUBLICATIONS™ ‘Ans. Loop Antenna : Its an antenna whichis in the form of a loop. ‘An antenna which consists of one or more turns of wire forming a DC short circuit is called a loop antenna. The loop antenna can be of circular, square of rectangular. ‘Typical oop antennas are shown in figure : (© Square loop (Rectangular folded dipole Fig. Different types of loop antenna ‘Small vertical loops are used for finding the direction. The oop is oriented until a null ‘or zero fields is obtained. This gives the direction ofthe received signal. * Loop antennas have advantage over the other antennas in direetion-finding as they are ‘small in ize. These are more suitable for mobile communication applications. “The polarization of the loop antennas isthe same as that of a short dipole. ‘5% Horizontal loop antenna produces horizontal polarization and vertical loop produces vertical polarization, WH vs ‘Salient features of loop antenna are as follows: {@) Smal loops, whose circumferences are less than 0.12 at the highest frequencies, © ae suitable for receiving signals upto about 30 MHz. eee eee GATE ACADEMY PUBLICATIONS ™ (6) A vertical loop an {receives bidiectiona (©) A vertical lop antenca, if shielded, receives uni-directional signal © Ithas excellent directivity. : : (7) Vertical loop antennas are very seal for direction finding application, {@) These are suitable for L, MF, HF, VHF and UHF ranges {©) The radiation pattem is in the shape ofa doublet. (G0) The direction patters of loop antennas are independent ofthe xc stop loop. : (ann direction finding applications, a small vertical lop is rotated about te vey ass, The plane ofthe oop is perpendicular tothe direction of radiation. (azyLoop antennas have ferrite cores to increase the effective diameter fp, “These are used as broadcast receivers "Fig. Radiation pattem (Horizontal plane) ~ ()Cloverleaf and Adcock antennas are examples of loop antennas. (44) The radiation pattem of the vertical loop antenna in horizontal planes hon figure. ((5)induced rms voltage, ¥,., in the loop is given by 2E AN cost * where, £,=Maximum electric field on the wave, (in Vim) . 220 etectve eight ofthe loop, Ae Wavelength (0), 1 area cl the lop. [N= numberof tums, angle between plane ofthe Joop and direction ot wave. (16)The radiation efficiency of a small oop antenna is poor. : (7) The dimensions of the antenna should be of the order of Mor wes! ‘wanemitters Vu = (18)The radiation resistance of small oop antennas R, =3117] ‘where, N= Number of tums, A = Area of the loop, A Wavelength (19)Radiation resistance ofa loop antennas is x.=3.10(2)}c0m Where, a radius ofthe loop (20) The radiated power of loop antennas is F, = 10K‘43/2 watts. (@1)Loop antennas are used extensively in radio receiver, scat >H, this effect is small on the patch. For the microstrip Hine where w/tis small the fringif effect is accounted for by proximately adding two coll. This gves more accurate formula forte impedance ofthe microstrip ine. Thats given by, pom soi) ve [+2 Radiation pattem ofthe patch antenna is broad. Its typical cof half space of or « steradian. “Therefore, the directivity D of the patch antenna. eters a, D= 1010p 4 = 6.021 4B “The effective height A, of the antenna is given by, PRA Re A R, = radiation resistance in 2 14, eliectve aperturein 1? or in mt ny =intrinsic impedance of free space in -R,= 50Mfor a typical patch value of beam area 91,i8 1/2 where, GATE ACADEMY PUBLICATIONS 5-33 ve For matching purposes, feed point can be moved from the edge. Arrays of patches an bbe used to have more directivity. ‘The conductance of patch antenna is given by, Methods of bandwidth control : It is a narrow bandwidth antenna. Honeys, bandwidth can be increased by, 1. Increasing the thickness () of parallel plate transmission line. 2. Cutting holes or slotin the patch. . 3. Using high dielectric constant (¢,) substrate. 4. Adding reactive components to the patch. (These reduces VSWR). Application of patch antenna: 1, They are used in spacecrafts and aircrafts. 2. They are used in applications where aerodynamic drag due to antennas shouldbe nil. 3, They are used in telemetry, satellite communication and defense radar gtensto Senden sadvantag ‘Ans. Advantages of patch antenna are as follows: é 1. Small size. 2. Less weight, 3. Low cost. 4. Ease of installation. 5._ It does not give rise to aerodynamic drag when used in airerafts. 6. ‘These are low profile antenna. 7. These antennas can be flush mounted to a metallic conductor orto other Su ‘They do not require space for feed line. The feed line can be placed bet ground plane, Z Disadvantages of patch antenna are as follows : = 1. Their efficiency is les. 2, Their bandwidth is small and is typically a few percent.eee RTT _ Tema (a) Angle of elevation, 4 =10" Tiltangle, 4-90 -4.=50" 108-00" ; a Wire length, pg “6smn ‘ $80", 1214396 mf 1658 m (0) Angle of elevation, A=15° Tikangle, $50" -1S=T9° Rhombicheight, H=z2-=0966% =0960, Wirelength, 3 arash, 3 4273 H=9660, 1-746 (0 Angleofelevation A=20° Titangle, $290-20= a ‘i = a07 =73m Rhombicheight Wazoo 073 x tata Wirelngty Iara fe10 He 1301=0.70 (@ Angleot elevation 4-25" Titangl, G-M-28 65" a ; = 90 5910 Rhombicheight H+ Tink A 22792-2190 a 4-68, H=$91m,[=279 {@) Angleof elevation, A=30" Titaxgle =s0" <3 =" Wirelengthy (8 Angleofelevation, 4=35" Titangle, $5 35 Rhombicheight = =0405, =435m Wire length, 4259, H=435m, [=152m ‘Ans, (@ Angleofelevation, A=40" Tiltangle, * $=90°-40"=50" * Rinne, eA ota «390 120, =m 2078 $-50',H=39m/=121m. Ans. Height of Rhombic, # =10x1/2=5m Tangle, $= 50 -A= 60° Titangle, — $=90-4=90-10=80"GATE ACADEMY PUBLICATIONS ™ 5-87 Practical Antennas Sol. Given: E_,=001V/m, h=8 metres, ¢ =1MHz, N=12 Tums; 0=0", 6-86 __ GATE ACADEMY PUBLICATIONS™ Am1 square metre (a) Voltage induced in vertical antenna of 8 metres high antenna i given by, Vg = Engh S08 Pul=Leni =0.01x8 =0.080 volts = 60 milli- volts. 7m (b) Voltage induced in frame antenna (ie. loop antenna) is given by, IRE AN 56 yottge EXO aoe pp x 300 ¥, = ot “-man] 2wxO.01 _ 23.1401 _ 628 194 sot 35 3S 25.12x10* volts =2512 mV. ‘Sol. Given : Diameter of loop antenna = 1.5 Radius of the loop antenna, a=! oo79, 2 ‘The expression of radiation resistance, 3720%0.15 =2790.0. p antenna 3x10" 50010 Sol. Given : ¥,,=2 mV =2xl0°V, N= 10, A= ‘The directivity ofthe loop antenna is, metre, E,, =? 2) = 4.25x0.75 =3.1875 Ans, Feld strengths evaluate from euation AN cst vale feat ef AN rs Yack _2o10? 600 Bei” Bevivd 95 025m 2 f=tMite Vim Gag Eq = 2x10" Vim =03185%60 min Eq =19.11 mVin x05 =025square metre N25, 0=0°, V =150RV,E, =?Gare Acasa PUBLICATIONS GATE ACADEMY PUBLICATIONS™ 6-89 Practical Anonaan Yak 2a A 28 AN t -cos® volts: 150%10* x600_ 72 2nx100x10* x1x12xe080" _ 23.14%1200x10* 5, =1Staltxea0 7, 1 BML aden? 202 Masonio+ aes gO Vin [ a x0 30 Eu, = 031857210" Vim =22.932«10¢ Vim 1 =628x40x104 =Q51.20x104V =25120.V ‘Ans. Eq = 2293%10" Vin = 2293 nVin Ans, Sol, Given: E,, = 0.02 Vim, 4=200m, A=1m* ‘Voltage induced in frame antenna, YEA yy Poe ee ~ "Fa TaxDe200x05 ~ 628xI0 B_ = OD4716x10° Vin Eq =4 716310 Vin =x 10* = 5x10" Vin ‘Sol. Given: 4=Im*, N= 400 tums, ‘Voltage induced in frame antenna, 1, = BRAIO1O* x 40060560" pons fm 300 7,-3*40,408 vote = 2520 volts = 4186x104 vols ae 3 _ Input voltage to the receiver V, = 7,2 volts =41.89x10*x100 volts V, = 4186x104 volts = 4.186 mV Sol Given: W =10 Tums, A=Im*, f=1ME, V, iy =4010" Hin + Induced voltage air cooled loop antenna, ¥,=@R4¥=0u HAN vhs fe Buy(GATE ACADEMY PUBLICATIONS ™ Practica An Antennas & Wave Propagation 5-40 (GATE ACADEMY PUBLICATIONS ™ Glamae c erect reese fae ae eee cee () Singe Su2 2 oe Snax [Chouno D=2h Aden Foxtel ded H=1,793x10% Alm Raton rene K-32 3002 samc i | a Diep Dotan (0 vinviyn-0en(2)-06n(2) wota( 5) cosassat-4ans D =4283 (dimensionless) ‘Alternative method 1 0 Vy x2exfxi0"xbe10x = a) «8-0 aad ce | 7amayrree" 14x03x10* _628x03x10* _1884 igs ee ae ale V,=1332«10* volts “L332 Hal poer beam widths givenby, : vette : Besochansy aes. qr Gees ‘The power gain of pyramidal hom antenna is given by, ASWd _ 1995 2 GL, =10log,(12.96) =11.12 eB Directivty of pyramidal hom antenna is given by, TSwd _75x12x6 ei Sol. Given: Y_=13aW, N=10 Tur, 4-1", i . 3x0", 300000 = i 2M iponteF aso 2 Bika Pa ti Sol. Given: d=8,, w=8h ‘The power gain of square hom antenna is given by, : AS 45x, op ona ;(exind) a 3S tee ee = =24.59 ‘The pateris obtained from above Bessel function. Mota) = 24.59Antennas & Wave Propagation 5-42 _ GATE ACADEMY PUBLICATIONS ™ | = 6 Gz, d=10 em, w= Sem 3x0 ra SIO 0.05 m=5 em P= 005 mes Power gain of hom antennais given by, Gn fSud _ASuI0%5. Se Directivity of hom antennas given by, tied ne poled TI ney , : ce Operating frequency, f= 200M => A= 32 015m delsm f ‘Now, we calculate length of six elements as Length of driven element, 1, =04161= 069m Length of reflector, 1,=0475h=0.7125m Length of director elements, 1, =0442.= 0.66 m 1, =0402.=060 m ‘Spacing between reflector and driven element, §,=025)=0375 m ‘Spacing between director and driving element, $,=031)=0465m Diameter of elements, d=001A=0.015m ‘The length of array=1.5h=225 m. o00e Practical Antonnas (GATE ACADEMY PUBLICATIONS ™ 6-43 Last Minute Revision (LMR) Rhombie Antena? AL Ble engl in degre) Til angle m4 =~ (in degree) Rhonbichght =H =