antennas

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antennas

  1. 1. All About Antennas Class Presentation
  2. 2. All About Antennas <ul><li>History </li></ul><ul><li>Antennas </li></ul><ul><li>Currents and Voltages </li></ul><ul><li>Receiving antenna </li></ul><ul><li>Short Dipole Antennas </li></ul><ul><li>Quarter-Wave Antenna </li></ul><ul><li>Half-Wave Antenna </li></ul><ul><li>Dipole Characteristic </li></ul><ul><li>Set-Top TV Antenna </li></ul><ul><li>Frequencies </li></ul><ul><li>Field Strength </li></ul><ul><li>Conclusion </li></ul>
  3. 3. History <ul><li>A dipole antenna was developed by Heinrich Hertz around 1886. The antenna had a center-fed driven element for transmitting or receiving radio frequency energy. These antennas are the simplest antennas from theoretical point of view. </li></ul>
  4. 4. Antennas <ul><li>Antenna is an efficient radiator of electromagnetic energy. (radio waves) into space. </li></ul><ul><li>The same principles apply to both transmitting and receiving antennas to radiate as much RF power as possible either in all directions (omnidirectional antenna) or in specified direction (directional antenna). </li></ul>
  5. 5. Currents and Voltages <ul><li>Because of the current and voltage distribution on the dipole, there must be electric and magnetic fields in the vicinity of the antenna. Since the instantaneous voltage and current on the antenna are 90 degrees out of phase, the same phase relationship applies to the E and H fields. These fields are continously expanding out from the antenna and collapsing back with the velocity of light. However, because the action is not instantaneous, the collapse will only be partial, so closed electric and magnetic loops will be left in space. These loops represent the radiated electromagnetic energy that is propagated into free space and travels with the velocity of light. Those flux lines that collapse back into the antenna represent 5the induction field, which is only strong in the immediate vicinity of the antenna. </li></ul>
  6. 6. Receiving Antenna <ul><li>Receiving antenna is used to intercept a RF signal voltage which is efficiently large, compared to the noise existing within the receiver’s bandwidth. </li></ul>
  7. 7. Short dipole antennas <ul><li>Short dipole antennas are physically feasible dipole formed by two conductors with a total length L very small compared with the wavelength λ . The two conducting wires are fed at the center at the dipole. </li></ul>
  8. 8. Quarter-wave antenna <ul><li>The quarter wave or unipole antenna is a single element antenna feed at one end, that behaves as a dipole antenna. It is formed by a conductor λ /4 in length. It is fed in the lower end, which in near a conductive surface which works as a reflector. </li></ul>
  9. 9. Half-wave antenna <ul><li>Half-wave antenna is formed by two quarter wavelength conductors or elements placed back to back for a total length of λ /2. A standing wave on an element of a length ~ λ /4 yield the greatest voltage differential, as one end of the element is at a node while the other is at an antinode of the wave. The larger the differential voltage, the greater the current flow between the elements. </li></ul>
  10. 10. Dipole characteristic <ul><li>Hertzian dipole antenna is much smaller than the wavelength of the signal. These have a very low radiation resistance and a high reactance, making them inefficient, but they are often the only available antennas at very long wavelengths. The theoretical maximum gain of a Hertzian dipole is 10 log 1.5 or 1.76 dB. The maximum theoretical gain of a λ /2-dipole is 10 log 1.64 or 2.15 dB. </li></ul>
  11. 11. Set-top TV antenna <ul><li>The most common dipole antenna is t5he “ rabbit ears” type use with televisions. While theoretically the dipole element should be along the same line, “rabbit ears” are adjustible in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio equipment on the ground or the ends on the ground with the center supported. </li></ul>
  12. 12. Frequencies <ul><li>The bandwidth of an antenna may be regarded as the frequency range over which the antenna is expected to perform well. As practical examples, Amplitude Modulation (AM) broadcast stations have antenna systems that radiate vertically polarized waves, whereas Television broadcast stations use horizontal polarization. </li></ul>
  13. 13. Frequencies <ul><li>For example, at a frequency of 100MHz (which lies within the FM commercial broadcast band of 88 -108 MHz), the length required for the Hertz antenna is: 468/100 = 4.68 feet. Such an antenna is made from two thin conducting rods, each 2.34 ft. long, and positioned remote from ground. At 100 MHz, the antenna behaves as a series resonant circuit with a Q of approximately 10. Therefore the thin dipole is capable of operating effectively within a narrow range that is centered on the resonant frequency. However, if the operating frequency is below the resonant frequency, the antenna will appear to be too short and will behave capacitively. In orde4r to bring such an antenna into resonance, it is necessary to add an inductor in series. </li></ul>
  14. 14. Field Strength <ul><li>The field strength at a position one mile (= 1609.3 m) from the antenna is: </li></ul><ul><li>E = 60IA/d </li></ul><ul><li>= 60 x 3.696/1609.3 </li></ul><ul><li>=0.1378 volts per meter </li></ul><ul><li>= 137.8 millivolts per meter </li></ul><ul><li>The value is used as the FCC standard to determine the gain of more sophisticated antennas. </li></ul><ul><li>For example, when parasitic elements as reflectors and directors are added to a dipole, the radiated RF power is concentrated in particular directions, so the antenna gain is increased. In addition : Antenna power gain in dB: </li></ul><ul><li>=10log10 antenna power gain ratio. </li></ul><ul><li>= 20 antenna field gain ratio </li></ul><ul><li>= power gain of antenna system in dB + transmission line loss in dB. </li></ul>
  15. 15. Field Strength <ul><li>When the Hertz dipole is resonant and the R F power is applied at the center of the antenna, the input impedance at the feed point is a low resistance, which mathematically can be shown to have a value of 73.2 ohm (for this reason, the antenna is often spoken of as a “70 ohm dipole”). This is referred to as the radiation resistance of the dipole and is the ohmic load that the ^/2 antenna represent at resonance. For a practical dipole, the value of the radiation resistance is the determined by the length: diameter ratio of each rod and the location of the antenna with regard to nearby metallic objects and structures. The field strength is therefore directly proportional to the antenna current, and inversely proportional to the distance from the transmitting antenna. This relationship is only true for the radiated field; for the induction field, the field strength is inversely proportional to the square of the distance. Therefore, the inductance field is only important in the immediate vicinity of the antenna. If the power gain of an antenna system is increased, the principal radiation lobe will be narrowed and the beam width is reduced. </li></ul>
  16. 16. Conclusion <ul><ul><li>Least but not last, antennas are very good electrical equipment and they are effectively used to transmit and receive signals at various ranges depending on the antennas’ mechanism. If it were not for antennas, proper electrical communications would be useless in today’s communication world. </li></ul></ul>
  17. 17. <ul><li>Delroy Christie </li></ul><ul><li>Electronic Engineering </li></ul><ul><li>Prof. Michael Thomas </li></ul><ul><li>LATTC, Summer 08 </li></ul>

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