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Antennas

Antennas. Antenna. Antennas. INTRODUCTION Antennas are constructed with conductors in the form of wires or rods. An antenna converts electrical energy into EM waves for transmission and EM waves into electrical energy for reception.

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Antennas

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  1. Antennas • Antenna

  2. Antennas • INTRODUCTION • Antennas are constructed with conductors in the form of wires or rods. • An antenna converts electrical energy into EM waves for transmission and EM waves into electrical energy for reception. • Tx. and Rx. antennas behave identically. The same antenna can be used for transmitting or for receiving.

  3. Antennas • EXCITATION • A Tx. antenna is said to be excited when the electrical o/p of a transmitter is fed to it. • Antenna excitation generates antenna current and causes radio waves to be radiated into the atmosphere. • The radiation will be strongest when the antenna is resonant to the frequency of the antenna current.

  4. Antennas • Half-wave Antenna • A resonant antenna is approximately  /2 long. • A half-wave antenna is called as a dipole, a doublet or a Hertz antenna. • It is  /2 long at its operating frequency.

  5. Antennas • Example of a dipole antenna

  6. Antennas • Standing Waves Pattern of Dipole • Open ends have a current minimum (node) and a voltage maximum (loop). • The center feed point has maximum current and minimum voltage. • Antenna standing wave current and voltage curves are commonly called displacement curves.

  7. Antennas • Standing Waves Pattern of Dipole

  8. Antennas • Antenna Radiation • An excited dipole develops an electric field between its ends because of the high voltage potentials at these points. • A magnetic field develops around the antenna because of the current flow in the antenna. Since the current is maximum at the center, so is the magnetic field. • The combined electric and magnetic field radiate into space as electromagnetic wave (EM).

  9. Antennas • Field Strength • Field strength is a measure of the intensity of an electric (in v/m), magnetic ( in gauss) or electromagnetic field (in watts per square meter). • It depends on the distance from the antenna and on the antenna radiated power. • It varies inversely with distance and it is evaluated by the voltage induced across a wire.

  10. Antennas • Radiation Patterns • An antenna does not perform equally well in all directions. • The directivity of an antenna is expressed by its Radiation Pattern. • The radiation pattern is a graphical plot of the field strength radiated by an antenna in different angular direction at equal distance from the antenna. • The bidirectional radiation pattern of a horizontal dipole shows that maximum radiation occurs broadside and minimum radiation occurs off the ends.

  11. Antennas • Radiation Pattern of a horizontal half-wave dipole • Ground plane plot

  12. Antennas • The omnidirectional radiation pattern of a vertical dipole shows that maximum radiation occurs in all directions in the horizontal plane and minimum radiation occurs directly above or below the antenna. • The polar plot in the horizontal plan is a circle.

  13. Antennas • Radiation Patterns for a vertical half-wave dipole • a. Ground plane plot b. Polar plot

  14. Antennas • Beamwidth • Beamwidth is a measure of the directivity of an antenna. • It is the angle subtended by the points at which the radiated power has fallen to half of its maximum value ( 3dB points), • or the field strength has fallen to 1/ 2 (0.707) of its maximum voltage.

  15. Antennas

  16. Antennas • Gain of Antenna • The gain of an antenna is a measure of its directional properties and indicates the extent to which radiation is concentrated in a particular direction. • The increased power being radiated in a particular direction is obtained at the expense of other directions. • Antenna gain is defined relative to a reference antenna. The reference antenna is either a half-wave dipole or an isotopic antenna. • An isotropic antenna is one which radiate equally well in all directions. (in theory)

  17. Antennas • The gain of a Tx. Antenna is the square of the ratio of the field strength produced at a point in the direction of maximum radiation from the antenna to the field strength produced at the same point by the reference antenna. • It may also be expressed as the ratio of the powers required to be transmitted by the two antennas to produce the same field strength at a particular point in the direction of maximum radiation. • The gain of a half-wave dipole relative to an isotropic radiator is 1.64 times or 2.15dB.

  18. Antennas • Example • An aerial must be fed with 10kW of power to produce the same field strength at a given point as a half-wave dipole fed with 20kW of power. • Calculate the gain of the antenna • a. relative to a half-wave dipole; • b. relative to an isotropic radiator.

  19. Antennas • Solution • Gain of antenna relative to an half-wave dipole • = 10 log10 (20000/10000) • = 3dB • Gain of antenna relative to isotropic radiator • = 3dB + 2.15dB • = 5.15dB

  20. Antennas • Effective radiated power • Effective radiated power, e.r.p., is the power that an isotropic radiator to produce the same field strength at a particular point in the direction of maximum radiation. • e.r.p. = total transmitted power x gain of antenna

  21. Antennas • Example • An antenna with a gain of 20dB relative to an isotopic radiator radiates a power of 10W. Determine the effective power of the antenna. • Solution • 20dB is a power ratio of 100:1 • e.r.p = 100 x 10 = 1kw

  22. Antennas • Radiation resistance • a mathematical quantity that express the relationship between the antenna current and the power radiated in the form of radio waves. • Pr = I2Rr • Pr = power radiated • I = antenna current • Rr = radiation resistance

  23. Antennas • Antenna efficiency • All the antenna losses are represented by a loss resistance RL. • The efficiency  of antenna is the ratio of the power radiated to the power fed to the antenna. •  = I2Rr / (I2RL + I2Rr) = Rr / (RL + Rr) x 100%

  24. Antennas • Example • An antenna has a radiation resistance of 0.3 and a loss resistance of 1.5. If the current fed into the aerial is 50A, calculate the radiated power, the power input and the antenna efficiency. • Solution • Power radiated = I2Rr = 502 x 0.3 = 750W • Input power = I2Rr + I2RL = 502x0.3 + 502x1.5 • = 4.5KW • Efficiency = 750x100/4500 % = 16.67%

  25. Antennas • Impedance • Antenna impedance = voltage / current at the input • Due to energy lost to radiation, input impedance of /2 dipole is not zero, but is approx. 73.

  26. Antennas • Marconi Antenna • Physical size is /4 (quarter-wave) • Erected vertically and connected to ground at one end • A perfect ground will produce a mirror image of the quarter-wave • The earth acts as additional /4 of the antenna • Marconi behaves electrically as a half-wave antenna.

  27. Antennas • Marconi antenna Direct wave Reflected wave /4 antenna Ground /4 mirror antenna

  28. Antennas • Parasitic Element • Parasitic elements are secondary antennas which are placed in close proximity to the main or driven antenna. • They are not directly fed, but have currents induced in them from the main element (or from the received wave in the case of a receiving antenna). • The secondary antennas are tuned so as to cause energy reradiation from them, and this changes the radiation pattern of the main antenna

  29. Antennas • Parasitic Reflectors • The reflector element is placed behind the driven dipole and is made about 5% longer than the driven l/2 dipole. • Maximum radiation occurs in the front direction along array axis when the reflector is placed about 0.2l behind the driven element.

  30. Antennas • Parasitic Reflectors

  31. Antennas • Parasitic Directors • The director element, which is placed in front of the driven dipole, is made about 5% shorter than the dipole. • It is spaced to provide maximum radiation in the forward direction, and optimum spacing is again found experimentally to be about 0.1l.

  32. Antennas • Parasitic Directors

  33. Antennas • Yagi Antenna • Yagi antennas are the most commonly used parasitic arrays, used primary for TV reception. • A Yagi antenna consists one driven element, one reflectors and one or more directors. • One or more directors are placed in front of the driven element to direct the wave in a forward direction. • A reflector is placed behind the driven element to reflect the wave back to the driven element.

  34. Antennas • Yagi Antenna

  35. Antennas • Yagi Antenna

  36. Antennas • Folded Dipole • Input impedance of a /2 dipole is 73 resistive. • Addition of parasitic elements reduces the input impedance to much lower value. • Impedance mismatch with a normal 50  or 75  coaxial feeder. • It can be overcome by using a higher input impedance driven element -folded dipole. • The input impedance of a folded dipole is is four times of a simple dipole (292  ).

  37. Antennas • Folded Dipole /2 292

  38. Antennas • Log Periodic Antenna • The log periodic antenna is basically an array of dipoles, fed with alternating phase, lined up along the axis of radiation. • The element lengths and their spacing all conform to a ratio, given as • t=L(n+1)/Ln=X(n+1)/Xn • Also, the angle of divergence is given as • a=tan-1(Ln/Xn) • The open-end length L must be larger than 1/2l if high efficiency (90%) is to be obtained.

  39. Antennas • Log Periodic Antenna • This antenna has the unique feature that its impedance is a periodic function of the logarithm of the frequency-hence its name. • The antenna characteristics are broadband, and it has the directional characteristics of a dipole array. • This type of antenna is often used for mobile-base-station operations

  40. Antennas

  41. Antennas • Dish Antenna • A dish antenna is a high-gain antenna used for reception and transmission of UHF and microwave signals. • E.g. microwave relay system, satellite communication system and TV receiving systems. • It consists of a driven element and a large spherical or parabolic reflector. The driven element is placed at the focal point of the reflector. • Signals arriving from distance and in parallel waveforms are reflected off the disc and brought together at the focal point.

  42. Antennas • Dish Antenna • Energy radiated by the driven element is reflected by the disc and sent out as parallel wave. • D = Diameter of the dish • Gain = 6(D/ )2 • Beamwidth = 70 /D

  43. Antennas • Dish Antenna Reference: http://www.sat-ant-sys.com/frames.htm

  44. Antennas • Longwire Antenna • The long-wire antenna is a wire with several wavelengths in length that is suspended at some height above the earth. • The wire is driven at one end and has a resistive termination at the remote end which is matched to the characteristic impedance of the line at that end. • When an alternating current wave is transmitted down this line toward the terminated end, about half of the energy is radiated into space.

  45. Antennas • Longwire Antenna • Longwire antennas are sometimes used in MF/HF band transmission and reception. • Gain is proportional to length of wire. • Inexpensive and easy to install. • Disadvantages: • Inconvenient to change direction. • Large space required. • The radiation pattern of a longwire antenna consists of 2 main lobes.

  46. Antennas • Longwire Antenna Longwire antenna Ground

  47. Antennas • Longwire Antenna • Radiation patterns

  48. Antennas • Rhombic Antenna • A wideband antenna used for h.f. links. • The rhombic antenna consists of 4 long wires (4 - 8  in length) connected horizontally together in a geometric shape of a rhombus. • All the 4 wires will radiate energy in the directions indicated by its radiation pattern. • The transmission line feeds one end and transmits an unreflected current wave down each side toward the resistive termination at the far end.

  49. Antennas • Rhombic Antenna  :Title angle

  50. Rhombic antenna Reference: http://users.neca.com/cummings/rhombic.html

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