Antennas Notes PDF

ANTENNAS PHIL U. LOMBOY, EcE ANTENNA A structure that is generally a metallic object, often a wire or group of wires, used to convert high frequency signals into electromagnetic waves and vice versa. ANTENNA A device whose function is to radiate elect...

ANTENNAS PHIL U. LOMBOY, EcE ANTENNA A structure that is generally a metallic object, often a wire or group of wires, used to convert high frequency signals into electromagnetic waves and vice versa. ANTENNA A device whose function is to radiate electromagnetic energy and/or intercept electromagnetic radiation  Basic Consideration : Maximum Power Transfer FIELDS OF AN ANTENNA INDUCTION FIELD RADIATION TRANSITION FIELD Considered to extend Considered to out from the antenna ZONE extend out from a to a distance of πD2/8λ distance of 2D2/λ Zone between the two(2) Near Field or Fresnel Far Field or regions Region Fraunhoffer Region  BASIC λ = c/f FORMULAS T = 1/f Where: λ – wavelength Where: T – time F – frequency F - frequency C – velocity in free space CHARACTERISTICS OF ANTENNAS  RADIATION PATTERN A line drawn to join points in space which have equal field intensity due to the source. Major Lobe – the direction of maximum radiation Minor Lobe – the direction of minimum radiation Null – the direction with radiation intensity equal to zero.  FACTORS a. Antenna WHICH height DETERMINE THE b. Power losses RADIATION PATTERN OF AN c. Terminations ANTENNA at its end d. Thickness of the antenna wire  PRINCIPLE The characteristics of antennas, OF such as impedance and RECIPROCIT radiation pattern are identical, Y regardless of use for reception or transmission. ISOTROPIC An antenna that radiates ANTENNA uniformly in all directions in space DIRECTIVE GAIN The ratio of the power density in a particular direction of one antenna to the power density that would be radiated by an isotropic antenna. HERTZIAN DIPOLE: HALF WAVE DIPOLE: 1.5 : 1 (1.76 dB) 1.64 : 1 (2.15 dB)  The longer the antenna, the higher the directive gain Non resonant antennas have NOTES: higher directive gain than resonant antennas The directive gain of all practical antennas is greater than unity  DIRECTIVITY, D Maximum directive gain The gain in the direction of one ofthe major lobes in the antenna’s radiation pattern. POWER GAIN Overall gain considering losses and efficiency Ap = %D Where: % - antenna efficiency D - directivity  ANTENNA 1. RADIATION RESISTANCE RESISTANCE, Rr The ratio of the AC resistance applied voltage The ratio of the to the flowing power radiated by current the antenna to the square of the current at the feedpoint. 2. LOSS RESISTANCE, Rd Antenna and Losses in ground imperfect resistance dielectric very near the Discharge or antenna corona effects Eddy current loss  ANTENNA EFFICIENCY % = Rr / Rr + Rd The ratio of the power radiated by the antenna to the power delivered at the feedpoint The ratio of radiation resistance to the total system resistance An antenna has a radiation resistance of 72 ohms, a loss resistance of 8 ohms, and a power gain of 16. What efficiency and directivity does it have. a. 90% and 17.78 b. 10% and 17.78 c. 90% and 14.4 d. 10% and 14.4 To produce a power density of 1 mw/m2 in a given direction, at a distance of 2 km, an antenna radiates a total of 180 w. An isotropic antenna would have to radiate 2400 w to produce the same power density at that distance. What, in dB, is the directive gain of the practical antenna? a. 11.25 dB b. 13.21 dB c. 10 dB d. 6 dB EFFECTIVE RADIATED EFFECTIVE ISOTROPIC POWER (ERP) RADIATED POWER (EIRP) The product of the power The power radiated by an fed to an antenna and its antenna in its favored power gain. direction, taking the gain of the antenna into account as ERP = Total Radiated referenced to an isotropic Power x Power Gain radiator  FRONT TO BACK RATIO BANDWIDTH Ratio of the power at The operating the optimum frequency direction of the range of an antenna to that of antenna the power 180 degrees from the optimum direction BEAMWIDTH The degree of concentration of the antenna’s radiation Angular separation between two half power points in a major lobe of an antenna radiation pattern  POLARIZATION Space orientation of the waves that the antenna radiates The electric field vector is always parallel to the antenna elements.  PHYSICAL ELECTRICAL LENGTH LENGTH Actual length of the Dependent upon the antenna velocity coefficient or L =λ/2; λ= c/f velocity factor L=kλ λ/2 is the shortest length of a conductor which will where k is the dielectric resonate at a given constant frequency Ex. What is the wavelength of a 500 MHz signal? a. 60 cm b. 6 m c. 0.06 m d. 60 m Ex. What is the electrical length of an antenna operating at a frequency of 500 kHz? a. 500 m b. 570 m c. 600 m d. 630 m GROUNDING SYSTEMS EFFECTS OF GROUND ON ANTENNAS Whereas an ungrounded antenna with its image forms an antenna array, the bottom of the grounded antenna is joined to the top of the image; the system acts as an antenna of double size.  EFFECTS OF GROUND ON ANTENNAS Whereas an ungrounded antenna with its image forms an antenna array, the bottom of the grounded antenna is joined to the top of the image; the system acts as an antenna of double size.  GROUND SCREEN A network of buried wires directly under the antenna, consisting of a large number of radials extending from the base of the tower, like spokes on a wheel, and placed 15 and 30 cm below the ground. COUNTERPOIS E A substitute for ground screen in areas of low conductivity, i.e. rock, mountains and antennas on top of buildings ANTENNA HEIGHT ANTENNA HEIGHT The actual antenna height should at least be λ/4, but where this is not possible, the effective height should correspond to λ/4.  TOP A good method of increasing LOADING radiation resistance by having a horizontal portion at the top of the antenna Effect: to increase the current at the base of the antenna and to make the current distribution more uniform EFFECTIV Antennas behave as though E LENGTH (electrically) they were longer than their physical length END The result of physical antennas EFFECT having finite thickness, instead of being infinitely thin. ANTENNA COUPLING AND IMPEDANCE MATCHING ANTENNA A network composed of reactances and COUPLING transformers, which may be lumped or distributed, to provide impedance matching To tune out the reactive component of the REASONS antenna impedance FOR To provide the transmitter with the correct value COUPLING of load resistance To prevent illegal radiation of spurious frequencies  ANTENNA COUPLERS The antennas are coupled directly to their Direct Coupler transmitters π Coupler Affords a wider reactance range, giving adequate harmonic suppression Symmetrical π Coupler Used for balanced lines IMPEDANCE MATCHING Stub Matching Accomplished by connecting the coax or twin lead to the stub and sliding the connections up or down the stub until the proper SWR is indicated by a meter connected in the system. IMPEDANCE MATCHING Delta Matching Accomplished by spreading the ends of the feedline and adjusting the spacing until optimum performance is reached. IMPEDANCE MATCHING Gamma Matching A sliding clamp is included in the assembly to permit fine tuning for minimum SWR at the time of installation IMPEDANCE MATCHING Quarter A section of transmission line one quarter Wave wavelength long placed Matchin between the load and the g line Used to connect an unbalanced BALUN (coaxial line) to a balanced antenna  CURRENT FED (LOW Z An antenna is FEED) said to be current fed if it is fed at the point of current maximum Includes all feed point impedances below 600 ohms Ex. Center fed half wave dipole or Marconi antenna  VOLTAGE FED (HIGH Z FEED) An antenna is said to be voltage fed if it is fed at the point of voltage maximum Includes all feed point impedances in excess of 600 ohms Ex. Center fed full wave dipole REVIEW QUESTIONS 1. Device that converts high frequency current into electromagnetic waves. a. antenna b. loudspeaker c. microphone d. lightning arrester 2. A polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna. a. Venn Diagram b. Figure 8 pattern c. Lissajous figure d. Radiation Pattern 3. Refers to the orientation of the electric field radiated from an antenna. a. radiation b. polarization c. beamwidth d. bandwidth 4. Pertains to a wire structure placed below the antenna and erected above the ground which is a form of capacitive grounding system. a. image b. counterpoise c. antenna orientation d. polarization 5. What is the technique used to electrically increase the antenna length? a. loading b. using image antenna c. using antenna arrays d. increasing antenna height 6. Antenna supported by insulators seems electrically longer than its physical length due to a. image b. reflection c. end effect d. broadside effect 7. The ratio of the power radiated by the antenna to the total input power. a. power gain b. directive gain c. antenna efficiency d. radiation efficiency 8. The ratio of the front lobe power to the back lobe power a. front to side ratio b. front to back ratio c. back to front ratio d. minor to major ratio 9. The standard reference antenna for directive gain a. infinitesimal dipole b. isotropic antenna c. elementary doublet d. half wave dipole 10. The gain of a hertzian dipole with respect to an isotropic antenna a. 1.76 dB b. 2.15 dB c. 1.5 dB d. 1.64 dB 11. A half wave dipole antenna is capable of radiating 2000 watts and has a 2.15 dB gain over an isotropic antenna. How much power must be delivered to the isotropic antenna to match the field strength of the directional antenna? a. 1640 watts b. 1219 watts c. 3280 watts d. 3520 watts 12. An ungrounded antenna near the ground a. acts as a single antenna of twice the height b. is unlikely to need a ground screen c. acts as an antenna array d. must be horizontally polarized 13. Top loading is sometimes used with an antenna in order to increase its a. effective height b. bandwidth c. beamwidth d. input capacitance 14. Very low signal strength in an antenna a. minor lobes b. nulls c. antenna patterns d. major lobes 15. A horizontal antenna is ______ polarized. a. vertically b. horizontally c. centrally d. circularly 16. An antenna with unity gain. a. rhombic b. half wave dipole c. isotropic d. whip 17. What is the front to back ratio of an antenna which radiates 500 watts in a northernly direction and 50 watts in a southernly direction? a. 25000 dB b. 10 dB c. 100 dB d. 20 dB 18. Good grounding is important for a. Horizontal antennas b. Broadside array c. Vertical antennas d. Yagi Uda Antennas 19. If the radiated power increases 10.89 times, the antenna current increases by a. 3.3 times b. 6.6 times c. 1.82 times d. 10.89 times 20. Shortening effect of an antenna that makes it appear as if it were 5% longer a. end effect b. flywheel effect c. skin effect d. capture effect 21. If an antenna is too short for the wavelength being used, the effective length can be increased by adding a. capacitance in series b. inductance in series c. resistance in parallel d. resistance in series 22. Actual height of an antenna should be at least a. 1λ b. λ/2 c. λ/4 d. ¾ λ 23. The directivity pattern of an isotropic radiator a. figure 8 b. a sphere c. unidirectional cardioid d. parabola 24. A Hertz antenna is operating on a frequency of 2182 kHz and consists of a horizontal wire that is hanged between two towers. What is the frequency of its third harmonic? a. 727 kHz b. 6546 kHz c. 436 kHz d. 6.546 kHz 25. What is the gain of an antenna over a half wavelength dipole when it has 6 dB gain over an isotropic radiator? a. 6 dB b. 8.1 dB c. 3.9 dB d. 10 d BASIC TYPES OF ANTENNAS  ISOTROPIC ANTENNA ELEMENTARY DOUBLET a standard reference antenna , A theoretical antenna shorter radiating equally in all than a wavelength used as a directions, so that the radiation standard to which all other pattern is spherical. antenna characteristics can be compared   = 60 le I sin θ / λr Where θ – angle of axis and point of maximum radiation  – field strength Le – antenna length I – antenna current r – distance Ex. An elementary doublet is 10 cm long. If the 10 MHz current flowing through it is 2 A, what is the field strength 20 km away from the doublet in a direction of maximum radiation? a. 6.28 uV/m b. 62.83 uV/m c. 15.92 uV/m d. 1.59 uV/m DIPOLE An antenna made up of two wires bent at 90 degrees to each other so as to be in the same line and signal is fed at the center  HALF WAVE DIPOLE Length is λ/2 and radiation pattern is a toroid (bidirectional) VOLTAGE AND CURRENT CHARACTERISTICS NON-RESONANT ANTENNA One in which there are no standing waves Radiation pattern is directional Standing waves are suppressed by the use of a correct termination to ensure that no power is reflected, so that only a forward traveling wave will exist.  LONG WIRE ANTENNA Lengths in the order of several wavelengths When an antenna is 2 or more wavelengths long, it provides gain and a multilobe radiation pattern. When terminated at one end, it becomes unidirectional. RHOMBIC ANTENNA Consists of non-resonant antenna elements arranged differently, i.e. planar rhombus Length of equal radiators = 2 to 8 ‘s Angle of tilt: 40 to 75˚ Rt = 800 ohms Rin = 650 to 700 ohms RHOMBIC ANTENNA Non-resonant antenna used for long distance sky wave transmission or reception of horizontally polarized waves over distances from 200 to over 3000 miles at frequencies from 4 to 22 MHz. RESONANT ANTENNA Standing waves exist, caused by the presence of both a reflected traveling wave and the forward wave. Antenna whose length is a multiple of λ/4’s HERTZ ANTENNA An antenna system in which the ground is not an essential part Half wave antenna used for frequencies above 2 MHz Half Wave Dipole  MARCONI ANTENNA Grounded Quarter Wavelength antenna Quarter Wavelength antenna used for frequencies below 2 MHz; omnidirectional Vertical Monopole ANTENNA ARRAYS  ANTENNA ARRAY A radiating system consisting of individual radiators or elements placed close together so as to be within each other’s induction field  DRIVEN PARASITIC ELEMENT ELEMENT Element of an array Radiation not directly connected to the connected to the output output of the of the transmitter transmitter Receives energy through the induction field of a driven element  REFLECTOR DIRECTOR A parasitic element A parasitic element longer than the shorter than the driven element and driven one from which close to it reduces it receives energy; signal strength in its tends to increase own direction and radiation in its own increases it in the direction opposite direction. BROADSIDE ARRAY Simplest array which consists of a number of dipoles of equal size, equally spaced along a straight line with all dipoles fed in the same phase from the same source. Typical antenna length: 2 to 10 ‘s Typical spacing: λ/2 or λ/4 Number of elements: dozens  ENDFIRE ARRAY Physical arrangement is the same as that of the broadside array The magnitude of the current in each element is still the same as in every other element, there is now a phase difference between these currents. TURNSTILE ARRAY Consists of two horizontal, half wave antennas mounted at right angles to each other  YAGI UDA ANTENNA An array consisting of a driven element and one or more parasitic elements arranged collinearly and close together. FOLDED DIPOLE Single antenna which consists of 2 elements, one is fed directly and the other coupled conductively at the ends. LOG PERIODIC ANTENNA Main feature is frequency independence for both radiation resistance and pattern Bandwidths of 10:1 are achievable with ease Radiation patterns: uni and bidirectional LOG PERIODIC ANTENNA Main feature is frequency independence for both radiation resistance and pattern PYRAMIDAL ANTENNA It looks and works in much the same way as a standard log periodic antenna, with one big difference: the two halves of the transmission line are separated and positioned as a V, so each half of the transmission line is in effect a single wire transmission a type of log periodic antenna line. UHF AND MICROWAVE ANTENNAS  PARABOLIC ANTENNA Works on the principle of a parabola Parabola – a plane curve defined as the locus of a point which moves so that its distance from another point (called the focus) plus its distance from a straight line (directrix) is constant. PARABOLIC ANTENNA All waves coming from the source and reflected by the parabola will travel in the same distance by the time they reach the directrix, no matter from what point on the parabola they are reflected.  CASSEGRAIN ANTENNA an antenna in which the radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex secondary reflector slightly inside the focus of the main reflector. BEAMWIDTH Where: D= mouth diameter, meters Ф = 70λ / Ф0 = beamwidth between nulls, D degrees Ф = beamwidth between half power Ф0 = 2 points, degrees Ф λ = wavelength, meters GAIN OF A PARABOLIC ANTENNA Ap = 6 ( D/λ)2 Where: D= mouth diameter, Ap =meters Power Gain λ = wavelength, meters GAIN OF A PARABOLIC ANTENNA Ap =  2 D2/ 2 Where: Ap = Power Gain D= mouth diameter, meters  = antenna efficiency λ = wavelength, meters  Ex. Calculate the beamwidth between nulls of a 1m paraboloid reflector used at 6GHz. a. 3.5˚ b. 7˚ c. 1.75˚ d. 14˚ Ex. Calculate the gain of the paraboloid reflector in the previous problem. a. 9600 b. 2400 c. 1600 d. 2800 HORN ANTENNA Ideal as primary feed antenna for parabolic reflectors and lenses  LENS ANTENNA Used as a collimator of frequencies in excess of 3 GHz  HELICAL ANTENNA Broadband VHF and UHF antenna which is used when it is desired to provide circular polarization characteristics Consists of a loosely wound helix, backed up by a ground plane, which is simply a screen made of chicken wire DISCONE ANTENNA A combination of a disk and a cone in close proximity Characterized by an enormous bandwidth for both input impedance and radiation pattern A constant angle, low gain antenna; omnidirectional LOOP ANTENNA Used for direction finding, because they do not radiate in a direction at right angles to the plane of the loop. For portable domestic Circular or square shaped receivers PHASED ARRAY Group of antennas, connected to one transmitter or receiver, whose radiation beam can be adjusted electronically without physically moving parts; used in radars. WHIP ANTENNA the most common example of a monopole antenna, an antenna with a single driven element and a ground plane. The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a ground plane. SLOT ANTENNA consists of a metal surface, usually a flat plate, with a hole or slot cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna.  NOTCH ANTENNA An open ended slot antenna Since the currents are zero at the middle of the slot, we may cut the ground plane here to make a notch antenna. ANTENNA SUPPORT STRUCTURES ANTENNA SUPPORTING STRUCTURES POLE Used to signify one piece of structure similar to the common telephone pole MAST TOWER Used to designate a Applied to a structure made of very large, high metal or wood which structure, may be either in a which in most form of a one piece or cases, is sectionalized structure constructed of metal REVIEW QUESTIONS 1. A non-resonant antenna that is capable of operating satisfactorily over a relatively wide bandwidth, making it ideally suited for HF transmission. a. end-fire array b. rhombic c. broadside array d. log periodic 2. Antenna that is independent of their radiation resistance and radiation pattern to frequency. It has bandwidth ratios of 10:1 or greater. a. loop antenna b. helical c. Yagi Uda antenna d. Log periodic antenna 3. A half wave antenna a. Hertz b. Marconi c. Parabolic d. Vertical Monopole 4. Antenna which is used very frequently but almost entirely as a reception antenna and is usually found at the back of table radios. a. loop antenna b. folded antenna c. rhombic d. log periodic 5. One of the special purpose antennas which has broadband VHF and UHF that is ideally suited for applications for which radiating circular rather than horizontal or vertical polarized electromagnetic waves are required. a. loop antenna b. phased array c. folded dipole d. helical 6. What are the two types of antenna elements? a. driven and reflector b. director and reflector c. parasitic and director d. driven and parasitic 7. Which one of the following terms does not apply to the Yagi Uda array? a. good bandwidth b. parasitic elements c. folded dipole d. high gain 8. Indicate the antenna that is not wideband. a. discone b. folded dipole c. helical d. Marconi 9. One of the following is not an omnidirectional antenna a. halfwave dipole b. log periodic antenna c. discone d. Marconi 10. One of the following consists of nonresonant antennas a. rhombic b. folded dipole c. end fire array d. broadside array 11. Which of the following is best excited from a waveguide? a. biconical b. horn c. helical d. discone 12. An antenna that is circularly polarized a. parabolic reflector b. Yagi Uda c. Helical d. Circular loop 13. What is the polarization of a discone antenna? a. vertical b. horizontal c. circular d. spiral 14. When speaking of antennas, ____ is a section which would be a complete antenna by itself. a. image b. top loading c. bay d. quarterwave 15. ______ is an antenna with a number of half wave antennas in it. a. antenna array b. tower c. omnidirectional d. rhombic 16. Which antenna radiates an omnidirectional pattern in the horizontal plane with vertical polarization? a. Marconi b. Discone c. Horn d. Helical 17. An antenna with very high gain and very narrow beamwidth. a. helical b. discone c. horn d. parabolic dish 18. An open ended slot antenna a. helical b. rhombic c. notch d. cassegrain 19. Which antenna is properly terminated? a. Marconi b. Rhombic c. Dipole d. Yagi Uda 20. What is the radiation characteristic of a dipole antenna? a. omnidirectional b. bidirectional c. unidirectional d. hemispherical 21. An antenna which is one tenth wavelength long. a. hertz antenna b. loop antenna c. Marconi antenna d. Elementary doublet 22. What is the minimum number of turns a helical antenna must have? a. 3 b. 4 c. 5 d. 6 23. An antenna made up of a number of full wavelengths a. elementary doublet b. log periodic c. long wire d. whip 24. Which of the following improves antenna directivity? a. driven element b. reflector c. director d. parasitic element 25. The frequency of operation of a dipole antenna cut to a length of 3.4 m a. 88 MHz b. 44 MHz c. 22 MHz d. 11 MHz 26. Where are the voltage nodes in a half wave antenna? a. at the ends b. three quarters of the way from the feedpoint towards the end c. one half of the way from the feedpoint towards the end d. at the feedpoint 27. A simple half wavelength antenna radiates the strongest signal a. at 45 degrees to its axis b. parallel to its axis c. at right angles to its axis d. at 60 degrees to its axis 28. An antenna array which is highly directional at right angles to the plane of the array a. Broadside b. End fire c. Turnstile d. Log periodic 29. What is the usual electrical length of a driven element in an HF beam antenna? a. λ/4 b. λ/2 c. 3/4 λ d. 1 λ 30. The input terminal impedance at the center of a folded dipole antenna a. 72 Ω b. 300 Ω c. 50 Ω d. 73 Ω 31. The length of a Marconi antenna to operate with 985 kHz is a. 200 ft b. 500 ft c. 250 ft d. 999 ft 32. The purpose of stacking elements on an antenna a. sharper directional pattern b. increased gain c. improved bandpass d. all of these 33. Which of the following antennas is used for testing and adjusting a transmitter for proper modulation, amplifier operation and frequency accuracy? a. elementary doublet b. real c. isotropic d. dummy 34. A resonant half wave dipole in free space has a feed line current of 1 amp. How much power appears at the input to the feedpoint? a. 73 w b. 146 w c. 219 w d. 292 w. 35. What would be the physical size of a Hertz antenna operating at 74.784 MHz? (k=0.95) a. 71 in b. 75 in c. 79 in. d. 150 in 36. The efficiency of an antenna is 82%. Its radiation resistance is 30 ohms. What is the value of its loss resistance? a. 75 ohms b. 15 ohms c. 12 ohms d. 6.58 ohms 37. System of buried conductors that improves ground conductivity a. Counterpoise b. Antenna Coupler c. Earth Mat d. Top Load 38. The antenna property of interchangeability for transmitting and receiving electromagnetic energy under the same radio frequency is referred to as a. Back to back b. maximum energy c. retransmission d. reciprocity 39. A region in front of a paraboloid antenna a. Fresnel b. transition zone c. fraunhoffer d. any of these 40. Calculate the gain (relative to an isotropic) of a parabolic antenna that has a diameter of 3 m, an efficiency of 60% and operates at a frequency of 4 GHz. a. 39.8 dBi b. 41.94 dBi c. 37.66dBi d. 79.53 dBi  Phil Lomboy po, CERTIFIED KAPUSO!