Antenna wrt frequency


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Antenna wrt frequency

  2. 2. VLF/ LF/ MF Antennas & Antenna Systems
  3. 3. Instrument Configuration IDPU: Instrument Data Processor Unit SPB : Spin Plane Booms (4x) AXB : Axial Booms (2x) SST : Solid State Telescope (2x) ESA : Electrostatic Analyzer FGM : Fluxgate Magnetometer SCM : Search Coil Magnetometer 4
  4. 4. Probe Configuration EFI Axial Booms (2, Stowed) Antenna ESA Miniature Sun Sensor Fuel Tank IDPU EFI SPB Thruster A2 Repress Tank Fuel Tank Transponder Thruster T1 EFI SPB Thruster A1 BAU Battery Gyros Thruster T2 AEB EFI SPB 5
  5. 5. EM Spectrum ISM band 902 – 928 Mhz 2.4 – 2.4835 Ghz 5.725 – 5.785 Ghz LF 30kHz 10km MF 300kHz 1km VHF HF 3MHz 30MHz 100m 10m UHF 300MHz 1m SHF 3GHz EHF  30GHz 300GHz 1cm 100mm 10cm  X rays  infrared visible UV 1 kHz 1 MHz 1 GHz 1 THz 1 PHz Gamma rays 1 EHz Propagation characteristics are different in each frequency band
  6. 6. Frequency Band Allocations IR RADIO VISIBLE UV X-RAYS GAMMA RAYS RADIO VLF 3k LF 30k MF 300k HF 3M VLF: Very Low Frequency MF: Medium Frequency VHF: Very High Frequency SHF: Super High Frequency VHF 30M UHF SHF 300M 3G EHF 30G 300GHz LF: Low Frequency HF: High Frequency UHF: Ultra High Frequency EHF: Extremely High Frequency 7
  7. 7. Wavelengths of Frequency Bands      c meters sec meters   f cycles sec cycle  VLF, LF  long waves Propagate well beyond line of sight MF  medium waves The distance the signal travels HF, VHF  short waves UHF, SHF  microwaves  Decreases as the frequency increases  EHF  millimeter waves  Above microwave region, only certain windows of frequencies propagate freely through air, rain, etc. Infrared and visible light will not penetrate walls X-rays and gamma rays interact with matter   8
  8. 8. TX R=100 m RX
  9. 9. VLF Antennas • They operates on VLF Band. • They are electrically small. This simplifies analysis. • They are physically large structures. – – Generally have a number of towers 200-300 m high. Generally cover areas of up to a square kilometer or more. • Support worldwide communicatipn. • The principal objective is to radiate specified amount of power over a sufficient bandwidth of frequency.
  10. 10. Problems with VLF Antennas 1. Bandwidth is less than 200 Hz. 2. Small radiation resistance. 3. They are expensive structures. 4. Antenna system covers a large area. 5. Designing an efficient transmitting antenna is difficult. 6. High power levels are needed for transmission.
  11. 11. Vertical Electric Monopole Antenna -Antenna Model-
  12. 12. Vertical Electric Monopole Antenna -E and H FieldsAssume a uniform vertical electric current I along a monopole of effective height he rms vertical electric field rms tangentical magnetic field
  13. 13. Vertical Electric Monopole Antenna -Radiated Power- The vertical electric field in terms of radiated power is:
  14. 14. Vertical Electric Monopole Antenna -Equivalent Antenna Circuit-
  15. 15. Vertical Electric Monopole Antenna -Radiation Efficiencywhere and antenna loss resistance Effective power = (power capacity of the transmitter) x (antenna system efficiency)
  16. 16. Vertical Electric Monopole Antenna -Antenna BandwidthThe 3 dB bandwidth b in (c/s) for a single resonant circuit is: f : resonant frequency Q: the circuit reactance resistance ratio X/R0 R0: Total series resistance
  17. 17. Multiple Tuned VLF Antennas To have sufficient bandwidths:  Huge antenna systems can be built. or  Several small multiple-tuned elements can be used.
  18. 18. Multiple Tuned VLF Antennas  Ground losses are reduced.  Radiation resistance and efficiency are increased.  Instead of one and vulnerable antenna, several and smaller elements can achieve the same bandwidth-efficiency product.  If one element is shunt off servicing, the others still can be operated.  The effective ground loss with multiple-tuning will be less than for a single element.  Tuning and retuning after the system is disturbed is difficult.  Each antenna has to be matched to a transmitter.
  19. 19. Multiple Tuned VLF Antennas Goliath Antenna
  20. 20. Some Applications of VLF Antenna 1.Submarine:  Requires EM Wave at VLF because of skin effect.  Propagation in sea water is almost vertical so only electric and magnetic type of dipoles can be used.  Transmitted wave will be attenuated in the sea-water so output power must be high enough to reach receiver.
  21. 21. Some Applications of VLF Antenna (II) 2.Underground Mine Communication:  Especially it is designed for the event of mine disaster.  Provide wireless communication between earth’s surface and miner.  Normal radio frequency get attenuated rapidly so VLF Band is used.  VLF Loop antenna can be used for this purpose.
  22. 22. Some Other Applications Water resource exploration.  Geological mapping.  Human body SAR detection. 
  23. 23. Simplified VLF Transmitting Antenna
  24. 24. Pictures Triatic Type Antenna
  25. 25. Maine Antenna Installation
  26. 26. Goliath Antenna
  27. 27. Goliath Antenna (2)
  28. 28. Goliath Antenna
  29. 29. Conclusion @ VLF band
  30. 30. VLF Band EM waves penetrate well into the sea water. (Communications with submerged submarines) Low atmospheric attenuation. Appropriate for long range communication.
  31. 31. VLF Antennas • Ground and Sky waves • Frequeny range: 3-30 KHz • Antennas : very large • Power: kW levels and even more
  32. 32. Some Problems Associated with VLF Antenna Systems • Small Bandwidth (usually less than 200 Hz) • Small radiation resistance. • High cost. • Antenna system covers a large area. • Need for very high power levels for transmission.
  33. 33. LF Band
  34. 34. LF Antennas  Ground and Sky waves  Frequeny range: 30-300 KHz  Antennas: large  Power: kW levels and even more
  35. 35. Some Disadvantages  High cost  Large Dimensions  Trouble with efficiency, power capacity, bandwidth
  36. 36. VLF and LF antennas are “electrically small” antennas :  problem: high capacitive reactance and small antenna radiation resistance  remedy: top loading
  37. 37. Top-loading  Top-loading increases gain bandwidth (by decreasing reactance)  In VLF large top-loading supported by towers
  38. 38. 3/5/2014 39
  39. 39. INTRODUCTION • Usually: Vertical radiators operating in the MF band (300-3000 kHz). • The towers may be guyed or selfsupporting.
  40. 40. APPLICATION AREAS • AM Broadcasting • Maritime Radio • Coast Guard Communication • Direction Finding
  41. 41. CHARACTERISTICS OF RADIATORS • • Maximum radiation in the horizontal plane Antennas taller than one-half wavelength have a minor lobe
  42. 42. Characteristics of the Radiators  Requirement for metallic ground plane to minimize losses  Vertical polarization is preferred due to superior propagation characteristics
  43. 43. • • • Other features of the radiators Shunt fed radiators Top loaded radiators Sectionalized radiators
  44. 44. Circuits for MF antenna systems • • • Antenna tuning units for matching purposes Phase shifter networks for directional antenna systems Power dividing networks FROM TRASMITTER PHASE CONTROL NETWORK T-LINE ANTENNA TUNING UNIT T-LINE ANTENNA TUNING UNIT POWER DIVIDER NETWORK PHASE CONTROL NETWORK
  45. 45. Ground Systems • • • • • 120 buried (1/4 length) copper wires Extending radially outward 120-180 cm depth is sufficient Individual ground systems are required for each tower of the array. Copper-mesh ground system may also be used.
  46. 46. Ground Systems A typical ground system for a two-element directional antenna
  47. 47. HF Antennas & Antenna Systems
  48. 48. HF Antennas and Antenna Systems  Frequency Range: 3 to 30 MHz ( 10 to 100 meters; in wavelength)  For medium- and long- distance communications and broadcoasting
  49. 49. Characteristics of HF Antennas:  Signals are distorted as the ionosphere is neither regular nor smooth.  High powers and high antenna gains may be needed for communication.
  50. 50. Types of HF Antennas: Non-Resonant HF Antennas Long-wire Antenna Vee Antenna Rhombic Antenna Resonant HF Antennas Monopole Antenna Dipoles and Slot Antennas Loop Antennas Log Periodic HF Antennas Early Log-Periodic Antenna Logarithmic Dipole Antenna Directional HF Antennas End-fire Arrays Broadside Arrays Circular Arrays
  51. 51. Non-Resonant HF Antennas: • wave propagates along the radiator in one direction only • remaining power is absorbed in a matched load TYPES    Long-wire Antenna Vee Antenna Rhombic Antenna
  52. 52. Long-wire Antenna A long terminated wire radiator 3/5/2014 53
  53. 53. Vee Antenna    Single mast (one wire radiator terminated in a resistive load at the far end). Radiation pattern exhibits large side lobes near the main beam. The efficiency is low (almost half of the total input power may be exhausted in the matched load. 3/5/2014 54
  54. 54. Rhombic Antenna • • • • 4 radiating wires of equal length mounted on four masts one of the wires are load-matched. high directivity the large rhombics are used for long-range communications. 3/5/2014 55
  55. 55. Resonant HF Antennas:  Monopole Antenna • Elevated-feed Monopole • Double-cone Monopole • Inverted-L and –T Antenna  Dipoles and Slot Antennas  Loop Antennas 3/5/2014 56
  56. 56. Monopole Antennas Outside half-wave resonance, elevation pattern breaks up into main lobes as input impedance becomes very high. Efficiency decreases 3/5/2014 57
  57. 57. Dipole Antennas
  58. 58. Loop Antennas Usully used for reception and direction finding.
  59. 59. The Log-Periodic Antenna    Fed from the vertex. Signal travells along the structure until reaches its resonant region. The signal radiates from the resonant region
  60. 60. Directional HF Antennas:  End-fire Arrays • Horizontal Array of Dipoles • RCA Fishborne Antenna • Series Phase Array  Broadside Arrays • Broadside Dipole Array • Wide-Band Curtain Array  Circular Arrays
  61. 61. End-fire Arrays      Higher directivity. Provide increased directivity in elevation and azimuth planes. Generally used for reception. Impedance match difficulty in high power transmissions. Variants are:  Horizontal Array of Dipoles  RCA Fishborne Antenna  Series Phase Array
  62. 62. Broadside Arrays Beam steering by phase variation is possible.
  63. 63. Circular Arrays    Used for direction finding. Consists of 30 – 100 elements, with equi-spaced and fed from a central source – goniometer. Band-width seperation is possible:
  64. 64. Ship’s Antenna Arrangement
  65. 65. Ship’s Antenna Arrangement Watch keeping RX Radar VHF DSC RX Scanner E.B. Saturn 35 Radome DSC RX VHF T/RX no. 2 MF/HF Radio Telephone RDF loop VHF DSC TX Loran RX Navtex RX VHF TX/RX
  66. 66. E-layer F1-layer D-layer Night F2-layer Day
  67. 67. VHF Direct Wave TX RX Reflected Wave
  68. 68. MF Reflected Wave Ground Wave TX RX
  69. 69. HF 8 Mhz 16 Mhz
  70. 70. E- Layer’s Effect F - Layer E - Layer DAY
  71. 71. E- Layer’s Effect F - Layer E - Layer NIGHT
  72. 72. E- Layer’s Effect F - Layer E - Layer