Antennas and its Applications

Transcription

1 DRDO Science Spectrum, March 29, pp , DESIDOC DRDO SCIENCE SCECTRUM 29 Antennas and its Applications Pramod Dhande Armament Research & Development Establishment, Dr Homi Bhabha Rd, Pashan, Pune ABSTRACT In the world of modern wireless communication, engineer who wants to specialize in the communication field needs to have a basic understanding of the roles of electromagnetic radiation, antennas, and related propagation phenomena. These papers discuss on the performance, characteristic, testing, measurement and application of antennas in modern wireless communication systems. Antenna is an important part of any wireless communication system as it converts the electronic signals (propagating in the RF Transreceiver) into Electromagnetic Waves (Propagating in the free space) efficiently with minimum loss. We use antennas when nothing else is possible, as in communication with a missile or over rugged mountain terrain where cables are expensive and take a long time to install. The performance characteristics of the parent system are heavily influenced by the selection, position and design of the antenna suite. To understand the concept of antenna one should know the behaviour of Electromagnetic waves in free space. So I am briefly covering the basics of Electromagnetic waves and its propagation modes in free space. Apart from that I am also covering Antenna classifications (based on Frequency, aperture, polarization and radiation pattern), its performance parameters (Gain, Directivity, Beam area and beam efficiency, radiation pattern, VSWR/Return loss, polarization, Efficiency), measurement techniques (Outdoor and Indoor Testing) and its defence applications (Naval antennas, Airborne Antennas and Earth Station Antennas). Finally I discuss about Pyramidal horn antennas, Monopole antennas. Keywords: Antenna, wireless communication, pyramidal horn antennas, monopole antennas 1. INTRODUCTION Antennas are basic components of any electric system and are connecting links between the transmitter and free space or free space and the receiver. Thus antennas play very important role in finding the characteristics of the system in which antennas are employed. Antennas are employed in different systems in different forms. That is, in some systems the operational characteristic of the system are designed around the directional properties of the antennas or in some others systems, the antennas are used simply to radiate electromagnetic energy in an omnidirectinal or finally in some systems for point-to-point communication purpose in which increased gain and reduced wave interference are required. 1.2 Antenna Definitions There are several definitions of antenna, and are as follows: The IEEE Standard Definitions of Terms (IEEE Std ): --A means for radiating or receiving radio waves An antenna is any device that converts electronic signals to electromagnetic waves (and vice versa) effectively with minimum loss of signals as shown in Fig.1. Figure 1. Wireless communication system. An antenna is basically a transforming device that will convert impedance of transmitter output (5/75 Ohm) into free space impedance (12pi or 377 Ohm). Region of transition between guided and free space propagation Concentrates incoming wave onto a sensor (receiving case) Launches waves from a guiding structure into space or air (transmitting case) Often part of a signal transmitting system over some distance. 66

2 DHANDE: ANTENNAS AND ITS APPLICATIONS Antenna placed at nose of the aircraft is a part of guidance RADAR system, which will guide the aircraft. Various jamming antenna are placed on different parts of aircraft for jamming the enemy signals. Antenna placed at the belly of the aircraft for data link application. All these antennas are operated on different frequency bands, so care should be taken that to avoid the interference of radiation pattern of all these antennas. Also when these antennas are placed on the aircraft body, its radiation pattern gets distorted, so one should design an antenna such that it will meet our application. Figure 2. Propagation of EM waves Antenna Definitions The radiation pattern and radiation resistance of an antenna is the same when it transmits and when it receives, if no non-reciprocal devices are used. So, Same antenna can be used for Transmission and Reception of Electromagnetic Waves Does not apply to active antennas. NB: Antenna is a passive device, it does not amplify the signals, it only directs the signal energy in a particular direction in reference with isotropic antenna. 2. IMPORTANCE OF ANTENNA IN AIRBORNE APPLICATION As shown in Fig.3, different frequency band antennas are placed on aircraft/missile body for different communication. 3. ELECTROMAGNETIC WAVES Before understanding the concept of antenna one should know what are Electromagnetic wave and its propagation modes in free space. The full Electromagnetic spectrum is shown in Fig.4. Antennas dimensions are dependent on wavelength of the signal being transmitted. From Fig.4, it Figure 4. Electromagnetic spectrum. (a) is clear that if we move towards high frequency, wavelength of the signal being smaller (from Equation 1); hence the dimensions of the antenna and RF component become smaller. So at higher frequency the size of the wireless system becomes compact. 1 f = (1) λ 3.1 Electromagnetic (em) Wave in Free Space Electromagnetic waves are disturbances to the electrical and magnetic fields. A changing electric disturbance produces a changing magnetic field at right angle to the electric field. (b) Figure 3. Application of airborne antennas. Figure 5. EM wave in free space. 67

3 DRDO SCIENCE SCECTRUM 29 Electromagnetic Wave originates from a point in free space, spreads out uniformly in all directions and it forms a spherical wave. An observer, however, at a grate distance from the source is able to observe only the small part of the wave in his immediate vicinity and it appears to him as plane wave just as the ocean appears flat to a person who can only see a few miles around him. Thus at a large distance from the source the wave has similar properties to the plane waves in the strip line and so by analogy of strip line the properties of EM waves in free space as follows: 1. At every point in space, the electric vector field E and the magnetic vector field H are perpendicular to each other and to the direction of propagation as shown in Fig Velocity of EM wave in free space is given by c=1/(μå) 1/2 = m/s (2) 3. E and H oscillate in phase and ratio of their amplitude is constant being equal to 12ð or 377 Ohm or (μ/ å) Whatever may be the frequency, the EM waves travels in space with the velocity of light. 5. EM wave propagates in free space as Transverse Electro Magnetic waves (TEM mode). Equation of EM waves in free space is given by: ω f = 2π E x x = 2 2 t με z H λ = E H 2 2 y 1 y = 2 2 t με z 1 με f E = E e ω β x 2π β = λ j( t ± z) H = H e ω β y j( t ± z) Polarization of Electromagnetic Wave The Polarization of Electromagnetic wave is defined as the orientation of electric field vector in space with respect to time. There are three types of EM wave polarization: 1. Vertical Polarization- When E field vector of EM wave is perpendicular to the earth, the EM wave said to be Vertically Polarized.. Z E (3) (4) μ = Z H = ε 2. Horizontal Polarization When E field vector of EM wave is parallel to the earth, the EM wave said to be Horizontally Polarized. Figure 7. Horizontal polarisation. 3. Circular Polarization When E and H field of the EM wave are of same amplitude and having a phase difference of 9o, wave is said to be circularly polarised.. Figure 8. Circular polarisation Properties of Electromagnetic Waves 1. Reflection and Refraction: EM waves gets affected from Reflection and Refraction same as that of light wave. Due to Reflection and Refraction the polarization of the EM wave get changed, so care should be taken that the designed antenna will transmit or receive the EM wave of desired polarization. Figure 6. Vertical polarisation. Figure 9. Reflection and refraction of EM wave. 68

4 DHANDE: ANTENNAS AND ITS APPLICATIONS Reflection, θr = θi Reflection coefficient: E Depends on media, polarisation r ρ of = incident wave and angle E of i incidence. η1 Refraction, sin( θt) = sin( θi) η2 με 1 1 if both media are lossless sin( θt) = με sin( θ 2 2 i) 3.2 Guded Electromagnetic Waves Electromagnetic Wave also exists in guided structure like: Cables : Used at frequencies below 35 GHz Waveguides : Used between.4 GHz to 35 GHz Quasi-Optical Systems : Used above 3 GHz In above structures propagating modes of EM wave gets changed like in waveguide EM wave propagates in Transverse Electric (TE) and Transverse Magnetic (TM) modes. 3.3 Launching of EM Waves EM wave launched into the free space by means of antennas and the selection of antenna is depending on the guided media: Open up the cable and separate wires : Monopole & Dipole antenna Open and flare up wave guide : Aperture (Horn) antenna 4. RADIATION PRINCIPLE OF ANTENNA One of the first questions that may be asked concerning antennas would be How are the electromagnetic fields generated by the source, contained and the guide in the transmission line and antenna, and finally detached from the to form a free-space wave? The best explanation can be given as follows. Let us consider a voltage source connected to a twoconductor transmission line, which is connected to an antenna as shown in Fig. 11. Applying a voltage source across the two-conductor transmission line creates an electric field between the conductors. The electric field associated with it electric line of force, which is tangent to the electric field at each point and the strength, is proportional to the electric field intensity. The electric field forces the charge carriers to be displaced which constitutes the current and hence creates magnetic field intensity. Associated with the magnetic field intensity, the magnetic line of force, which are tangent to the magnetic field. Figure 11. Launching of EM wave from waveguide through aperture antenna. Figure 1. Launching of EM wave from open cable and separated wires through dipole antenna. When a.c. signal is applied to the line from source time varying electric and magnetic fields are created. The creation of time varying electric and magnetic fields between the conductors form electromagnetic waves which travel along the transmission line as shown in Fig. 11. The electromagnetic waves enter the antenna and have associated with them electric charges and corresponding currents. If we remove part of antenna structure as shown in Fig. 11, free space waves can be formed by connecting the open ends of the electric lines. The free space waves are also periodic but a constant phase point moves outwardly with the speed of light and travels a distance of wavelength/ 2 in the time of one half of a period. Before we attempt to explain how guided waves are detached from the antenna to create the free space waves, let us draw a parallel between the guided and free space waves, and water waves created by the dropping of a pebble in a calm body of water or initiated in some other 69

5 DRDO SCIENCE SCECTRUM 29 manner. Once the disturbance in the water has been initiated, water waves are created which begin to travel outwardly. If the disturbance has been removed the waves do not stop or extinguish themselves but continue their course of travel. If the disturbance persists, new waves are continuously created which lag in their travel behind the others. The same is true with the electromagnetic waves created by an electric disturbance. If the initial electric disturbance by the source is of short duration, the created electromagnetic waves will travel inside the transmission line, then into the a antenna, and finally will be radiated as free space waves, even if the electric source ceased to exist. If the electric disturbance is of continuous nature, electromagnetic waves will exist continuously and follow in their travel behind the others. When the electromagnetic waves are within the transmission line and antenna, their existence is associated with the presence of the charges inside the conductors. However, when the waves are radiated, they form closed loops and there are no charges to sustain their existence. This leads us to conclude that electric charges are required to excite the fields but are not needed to sustain them and may exist in their absence. This is in direct analogy with water waves. 5. ELECTROMAGNETIC WAVE PROPAG-ATION MODES: Electromagnetic wave can propagate into the free space by three modes: 1. Ground-wave propagation 2. Sky-wave propagation 3. Line-of-sight propagation 5.1 Ground-wave propagation The ground wave is a wave that is guided along the surface of the earth just as an electromagnetic wave is guided by a waveguide or transmission line. Surface wave permits the propagation around the curvature of the earth. This mode of propagation exists when the transmitting and receiving antennas are closed to the surface of the earth and is supported at its lower edge by the presence of the ground. Follows contour of the earth. Can propagate considerable distances. Frequencies up to 2 MHz. Example AM radio 5.2 Sky-wave Propagation The sky waves are of practical importance at medium and high frequencies for very long distance radio communications. In this mode of propagation electromagnetic waves reach the receiving point after reflection from the ionized region in the upper atmosphere called ionospheresituated between 5Km to 4 Km above earth surfaceunder favorable conditions. Figure 13. Sky wave propagation. Signal reflected from ionized layer of atmosphere back down to earth. Signal can travel a number of hops, back and forth between ionosphere and earth s surface. Reflection effect caused by refraction. Frequency: 2-3MHz. Examples Military Comm. Amateur radio 5.3 Line-of-sight propagation In this mode of propagation, electromagnetic waves from the transmitting antenna reach the receiving antenna either directly or after reflections from the ground in the earth s troposphere region. Troposphere is that portion of the atmosphere which extends upto 16Km from the earth surface. Frequency: More then 3MHz Figure 12. Ground wave propagation. Figure 14. Line of sight propagationa 7

6 DHANDE: ANTENNAS AND ITS APPLICATIONS Transmitting and receiving antennas must be within line of sight Satellite communication signal above 3 MHz not reflected by ionosphere Ground communication antennas within effective line of site due to refraction Refraction bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums Examples: TV, satellite, optical comm. 6. ANTENNA CLASSSIFICATION Antenna can be classified on the basis of: 1 Frequency - VLF, LF, HF, VHF, UHF, Microwave, Millimeter wave antenna 2 Aperture - Wire, Parabolic Dish, Microstrip Patch antenna 3. Polarization - Linear (Vertical/Horizontal), Circular polarization antenna 4. Radiation - Isotropic, Omnidirectional, Directional, Hemispherical antenna 6.1 Frequency Basis Frequency Band 3-3 KHz 3-3 KHz 3-3 KHz 3-3 MHz 3-3 MHz 3-3 MHz 3-3 GHz 3-3 GHz Designation Very Low frequency (VLF) Low Frequency (LF) Medium Frequency (MF) High Frequency (HF) Very High Frequency (VHF) Ultra High Frequency (UHF) Super High Frequency (SHF) Extremely High Frequency (EHF) Typical service Navigation, SONAR. Radio beacons, Navigational Aids. AM broadcasting, maritime radio, coast guard communication, direction finding. Telephone, Telegraph and Facsimile, amateur radio, ship-to-coast and shipto-aircraft communication. Television, FM broadcast, air traffic control, police, navigational aids. Television, satellite communication, radiosonde, surveillance RADAR, navigational aids. Airborne RADAR, Microwave Links, Satellite Communication. RADAR, Experimental Examples of Antenna on Frequency basis 1. Very Low Frequency (VLF) & Low frequency (LF) antenna: Vertical Radiators, Top-loaded Monopoles, T and Inverted L antennas, Triatic antenna, Trideco antenna, Valleyspan antenna. 2. Medium Frequency (MF) antennas: Radiators (monopoles and dipoles), directional antennas. 3. High Frequency (HF) antennas: Log periodic antenna, conical monopole and Inverted Cone antennas, Vertical whip antenna, Rhombic antenna, Fan dipole antenna. 4. Very High Frequency (VHF) & Ultra High Frequency (UHF) antennas: Yagi-Uda antennas, log periodic antennas, Helical antennas, Panel antennas, Corner reflector antennas, parabolic antennas, discone antennas, 5. Super High Frequency (SHF) & Extremely High Frequency (EHF) antennas: Parabolic antenna, pyramidal horn antennas, discone antennas, monopoles and dipoles antennas, Microstrip patch antennas, fractal antenns. 6.2 Aperture Antennas Aperture antennas transmit and receive energy from its aperture. Wire antennas Horn Antenna Parabolic reflective antenna Cassegrain antenna Wire Antenna A wire antenna is simply a straight wire of length ë/ 2 (dipole antenna) and ë/4 (monopole antenna), where ë is the transmitted signal wavelength. A wire antenna can be a loop antenna such as circular loop, rectangular loop, etc. Basically all vertical radiators are come in to wire antenna categories. A whip antenna is the best example of wire antenna Vertical Monopole antenna Length <.64l Self impedance: ZS = Z ANT +R GND + R REF Efficiency: η = Z ANT / Z S η ranges from < 1% to > 8% depending on antenna length and ground system Efficiency improves as monopole gets longer and ground losses are reduced Figure 15. ë /4 Vertical Monopole: (Fig.16) Figure 16. ë /4 Vertical monopole 71

7 DRDO SCIENCE SCECTRUM 29 Length ~.25l Self impedance: ZS ~ 36-7 W The l /4 vertical requires a ground system, which acts as a return for ground currents. The image of the monopole in the ground provides the other half of the antenna The length of the radials depends on how many there are Take off angle ~ 25 deg ë /4 Vertical Monopole: (Fig.17) one end and open at the other end. If flaring is done in one direction, then sectorial horn is produced. Flaring in the direction of Electric vector and Magnetic vector, the sectorial E-plane horn and sectorial H-plane Horn are obtained respectively. If flaring is done along both walls (E and H) of the rectangular waveguide, then pyramidal horn is obtained. By flaring the walls of a circular waveguide, a conical horn is formed. Figure 17. ë /4 Vertical monopole. Figure 19. Corrugated conical horn antenna Figure 2. Pyramidal and conical horn antennas. Length is approximately.48l Self impedance ~ 2 W Antenna can be matched to 5 ohm coax with a tapped tank circuit Take off angle ~ 15 deg Ground currents at base of antenna are small; radials are less critical for l/2 vertical The Rectangular Loop: (Fig.18) Figure 18. Rectangular loop Parabolic Reflective Antenna A parabola is a two dimensional plane curve. A practical reflector is a three dimensional curved surface. Therefore a practical reflector is formed by rotating a parabola about its axis. The surface so generated is known as paraboloid which is often called as microwave dish or parabolic reflector. The paraboloid reflector antenna consists of a primary antenna such as a dipole or horn situated at the focal point of a paraboloid reflector. The important practical implication of this property is that reflector can focus parallel rays on to the focal point or conversely it can produce a parallel beam from radiations originating from the focal point Prime Focus Paraboloid Reflector antenna Shaped reflector: parabolic dish, cylindrical antenna. Reflector acts as a large collecting area and concentrates power onto a focal region where the feed is located The total length is approximately 1.2 l. The self impedance is 1-13 W depending on height. The Aspect Ratio (A/B) should be between.5 and 2 in order to have Zs ~ 12 W. SWR bandwidth is ~ 4.5% of design frequency. Directivity is ~2.7 dbi. Note that the radiation pattern has no nulls. Max radiation is broadside to loop Antenna can be matched to 5 Wcoax with 75 W l / 4 matching section Horn Antennas A horn antenna maybe regarded as a flared out or opened out waveguide. A waveguide is capable of radiating radiation into open space provided the same is excited at Figure 21. Prime focus paraboloid reflector antenna. 72

8 DHANDE: ANTENNAS AND ITS APPLICATIONS Cassegrain Antenna In cassegrain antenna primary feed radiator is positioned around an opening near the vertex of the paraboloid instead of at focus. Cassegrain feed system employs a hyperboloid secondary reflector whose one of the foci coincides with the focus of paraboloid. The feed radiator is aimed at the secondary hyperboloid reflector or sub-reflector. As such, the radiations emitted from feed radiator are reflected from cassegrain secondary reflector which illuminates the main paraboloid reflector as if they had originated from the focus. Then the paraboloid reflector colliminates the rays as usual. plane. The major disadvantages of patch or microstrip antennas are their inefficiency and very narrow bandwidth which is typically only a fraction of a percent or at the most a few percent. 6.3 Antenna Classification on Polarization Basis Antenna polarization is governed by the polarization of Electromagnetic waves. Based on that: 1. Linearly (Vertically/Horizontally) Polarized antenna. 2. Circularly Polarized antenna Linearly (Vertically/Horizontally) polarized antenna If antenna is transmitting/receiving Vertical E field vector, then antenna is said to be vertically polarized antenna. If antenna is transmitting/receiving horizontal E field vector, then antenna is said to be horizontally polarized antenna. Figure 22. Cassegrain antenna Advantages of cassegrain antenna Less prone to back scatter than simple parabolic antenna Greater beam steering possibility: secondary mirror motion amplified by optical system Much more compact for a given f/d ratio. Reduction in spill over and minor lobe radiation. Ability to get an equivalent focal length much greater than the physical length. Ability to place the feed in a convenient location. Capability for scanning or broadening of the beam by moving one of the reflecting surfaces Microstrip Patch Antenna In spacecraft or aircraft applications, where size, weight, cost, performance, ease of installation, and aerodynamic profile are constraints, low profile antennas are required. In order to meet these specifications Microstrip Patch antennas are used. These antennas can be flush mounted to metal or other existing surfaces and they only require space for the feed line which is normally placed behind the ground Figure 25. Examples of linearly polarised antennas Circularly Polarized antenna If the antenna is able to transmit or receive E field vectors of any orientation, then antenna is said to be circularly polarized antenna. Figure 23. Microstrip patch antenna. Figure 24. Various shapes of patch antenna. Figure 26. Examples of circularly polarised antennas. 73

9 DRDO SCIENCE SCECTRUM Antenna classification on Radiation Pattern Basis On the basis of radiation pattern antenna can be classified as: 1. Isotropic antenna. 2. Omnidirectional antenna. 3. Directional antenna. 4. Hemispherical antenna Isotropic Antenna An isotropic antenna is a fictitious antenna and is defined as a antenna which radiates uniformly in all directions. It is also called as isotropic source or omnidirectional antenna or simply unipole. An isotropic antenna is a hypothetical lossless antenna, with which the practical antennas are compared. Thus an isotropic antenna is used as reference antenna. Although sometimes, a half-wave dipole antenna is also used as reference antenna but these days use of isotropic antenna as reference antenna is preferred. Let us assume that practical antenna is having a gain of 3 dbi means that gain of practical antenna is three times more than that of isotropic antenna when both the antenna are connected with same source Omnidirectional Antenna Omnidirectional antennas are those antennas which will cover equally well in azimuth direction and having some angle in elevation direction. Basically most of the wire antennas are having omnidirectional radiation pattern. Examples are Whip antenna, Dipoles antennas, etc. The radiation patterns of omnidirectional antennas are shown below Hemispherical Antenna Antenna whose radiation pattern will cover the one half of the hemisphere either upper hemisphere or lower hemisphere is said to be antenna with Hemispherical Radiation pattern. Such types are antennas are implemented on aircraft body to cover the lower hemisphere for data link purpose. Examples are all Monopoles antennas with large ground plane. The radiation pattern of these antennas are shown below. Figure 28. Directional radiation pattern. 7. ANTENNA CHARACTERISTICS Before designing an antenna one should know its performance parameters or characteristics of antenna for particular applications. The beam pattern of any antenna is shown below in Fig.29 and 3. Figure 29. Upper hemispherical radiation pattern. Figure 27. Omnidirectional antenna Directional Antennas Antennas which directs its energy in one particular direction is said to be directional antennas. These antennas are having very high gain and directivity to cover large wireless distance. Examples are paraboloid reflector antenna, Yagi-Uda antenna, Log periodic antenna, etc. Radiation pattern of these antennas are shown below. Figure 3. Antenna pattern showing main beam and side lobes. 74

10 The performance parameters of the antennas are discussed below: 7.1 Radiation Pattern The radiation pattern of any antenna determines its coverage area in free space. The radiation pattern of any antenna looks like as shown in Fig.31. DHANDE: ANTENNAS AND ITS APPLICATIONS Figure 32. Antenna radiating regions. Figure 31. Antenna Parameters definitions are based on the geometry of the antenna gain pattern Properties of Radiation Pattern of antenna Always measured in Far field. 2 D Far field: r > 2 D: largest dimension of the antenna λ Field intensity decreases with increasing distance, as 1/r. Radiated power density decreases as 1/r2. Pattern (shape) independent on distance. Usually shown only in principal planes Antenna Regions 2 D Far-Field (Fraunhoffer) Region r > 2 λ Where D is the largest linear dimension of the antenna This is the region where the wavefront becomes approximately planar The apparent gain of the antenna is a function only of the angle (i.e., the antenna pattern is fully formed) 2 λ D Radiating Near-Field (Transition region) < r <2 2π λ The region between near and far field E and H are equal, but inverse square law does not apply The antenna pattern is not fully formed λ Reactive Near-Field r< 2 π Gain is not a meaningful parameter here E and H are not equal Reactive components 1% or more of radiating components may cause error in field measurements 7.2 Gain (G) Gain of an antenna without involving the efficiency is defined as the ratio of maximum radiation intensity in given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same power input. Gain is also defined as the increase in signal strength as the signal is processed by the antenna for a given incident angle Usually expressed in db Can be negative An isotropic antenna has unity gain db A general Gain equation is given by- G ç (4ð/ë2) Ap where ç efficiency of the antenna ð wavelength in meters Ap the physical area of the aperture in m2 7.3 Directivity (D) Directivity of an antenna is defined as the ratio of Maximum radiation intensity to its average radiation intensity. Relation between Directivity and Gain of antenna- G ç D where ç efficiency of the antenna 7.4 Antenna Efficiency (ç ) The efficiency of antenna is defined as the ration of power radiated to the total input power supplied to the antenna and is denoted by ç. Thus, Antenna Efficiency, ç =Power Radiated/Total Input Power In terms of resistances, ç= [R r /(R r +R l )] 1 where, R r = Radiation resistance; R l = Ohmic loss resistance of antenna conductor 7.5 Beam Area and Beam Efficiency 2π π Beam area : Ω = P( θφ, ) sin( θ) dθdφ= P ( θφ, ) dω A n n 4π 75

11 DRDO SCIENCE SCECTRUM 29 Main Beam area : Minor lobes area : Main Beam Efficiency : 7.6 Effective Aperture and Aperture Efficiency Effective aperture of the antenna is that aperture that will actively take part in transmission and reception of electromagnetic waves. The relation between physical and effective aperture of the antenna is given by- Effective Aperture=K Physical Aperture, < K <1 Receiving antenna extracts power from incident wave: P rec = S in A e Aperture and beam area are linked: 2 λ Ae = Ω Ae Aperture efficiency can be defined: ε ap = A Radiation Resistance The radiation resistance is a hypothetical resistance and does not correspond to a real resistor present in the antenna but to the resistance of space coupled via the beam to the antenna terminals. Antenna presents impedance at its terminals, Z A = R A + jx A Resistive part is radiation resistance plus loss resistance, R A = R R + R L Ω (, ) M = Pn θφdω min or lobes Main beam Ω (, ) m = Pn θφdω ε M Ω = Ω Frequency Coverage The frequency coverage of an antenna is the range of frequencies over which an antenna maintains its parametric performance Antennas are generally rated based upon their stated centre frequency Example: GHz, fc = 1. GHz 7.7 Bandwidth (B) The bandwidth (B) of an antenna is the frequency M A A p range in units of frequency over which the antenna operates Often stated in percentage bandwidth 7.8 Beamwidth (èb, ÖB) The n -db beamwidth (èb, ÖB) of an antenna is the angle defined by the points either side of boresight at which the power is reduced by n-db, for a given plane. For example if èb, represents the beamwidth in the horizontal plane, ÖB represents the beamwidth in the orthogonal (vertical) plane. The 3-dB beamwidth defines the half-power beam. 7.9 Polarization The polarization of an antenna defines the orientation of the E and H waves transmitted or received by the antenna Linear polarization includes vertical, horizontal or slant (any angle) Typical non-linear includes right- and left-hand circular (also elliptical) 7.1 VSWR/Return loss VSWR or Return Loss determines the matching properties of antenna. It indicates that how much efficiently antenna is transmitting/receiving electromagnetic wave over particular band of frequencies Impedance Antenna must be terminating with 5 Ohm impedance in order to transfer maximum power from transmitter into free space. 8. ANTENNA MEASUREMENT Antenna must be undergoing various measurements before installing on the system. Basically there are two types of measurement conducted on antennas: 1. Passive Measurement/Laboratory Measurement VSWR/Return Loss Impedance Bandwidth 2. Active Measurement Radiation Pattern (Elevation And Azimuth) Gain Directivity Half Power Beamwidth Cross Polarization 8.1 Passive Measurement/Laboratory Measurement VSWR/Return Loss and Impedance Bandwidth measurement can be done on Vector Network Analyzer. Antenna port is connected to one port of the network Analyzer and can see its VSWR/Return Loss and Impedance Bandwidth directly on the screen of the Network Analyzer. 8.2 Active Measurement In active measurement, the following properties of antenna can be tested: Radiation Pattern (Elevation And Azimuth) Gain 76

12 DHANDE: ANTENNAS AND ITS APPLICATIONS Figure 34. Set up for measuring VSWR/Return loss and impedance of antenna using vector network analyser. Directivity Half Power Beamwidth Cross Polarization 8.3 Radiation Pattern Measurement Open field Outdoor Elevated Range Ground Reflection Range Anechoic chamber Rectangular Anechoic Chamber Compact Antenna Test Range Open Field Radiation Pattern of Mobile antennas 9. ANTENNA APPLICATIONS 9.1 Astronomical Antenna Anechoic Chamber Helical Antenna 1. Highly Directional Antenna 2. Circularly Polarized Antenna 3. Use in Radio Astronomy 9.2 Defence Antennas Radiation Pattern of Some Antennas A close-up view of the conical high-frequency Dipole antenna mounted on the bow of the Ship Paraboloid Grid Reflector Antenna 77

13 DRDO SCIENCE SCECTRUM 29 A view of the antenna array on the island structure of the nuclear-powered aircraft USS Theodore Roosevelt (CVN-71). A close-up view of the antenna masts and bridge structure aboard the guided missile cruiser as seen from off the ship`s starboard bow. A view of the AN/SPN-46(V) radar antenna for the automatic carrier landing system (ACLS) aboard the nuclear-powered aircraft carrier USS Abraham Lincoln (CVN-72). A view of the antenna rig aboard the guided missile frigate USS DOYLE (FFG-39). 78