1 / 52

VLF LF MF and HF ANTENNAS

VLF LF MF and HF ANTENNAS. CLASSIFICATION ACCORDING TO FREQUENCY BANDS. VLF/LF Antennas and Antenna Systems. VLF Band . EM waves penetrate well in to the sea water . (Communications with submerged submarines) Low atmospheric attenuation .

edan-mccoy
Download Presentation

VLF LF MF and HF ANTENNAS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. VLF LF MF and HF ANTENNAS CLASSIFICATION ACCORDING TO FREQUENCY BANDS

  2. VLF/LF Antennas and Antenna Systems

  3. VLF Band • EM waves penetrate well into the sea water. (Communications with submerged submarines) • Low atmospheric attenuation. • Appropriate for long range communication.

  4. VLF Antennas • Ground and Sky waves • Frequeny range: 3-30 KHz • Antennas : very large • Power: kW levels and even more

  5. 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.

  6. LF Antennas • Ground and Sky waves • Frequeny range: 30-300 KHz • Antennas: large • Power: kW levels and even more

  7. Some Disadvantages • High cost • Large Dimensions • Trouble with efficiency, power capacity, bandwidth

  8. VLF and LF antennas are “electrically small” antennas : • problem: high capacitive reactance and small antenna radiation resistance • remedy: top loading

  9. Top-loading • Top-loading increases gain bandwidth (by decreasing reactance) • In VLF large top-loading supported by towers

  10. A simple VLF/LF Transmitting Antenna

  11. VLF / LF Ground Systems • Radial-wire: radial wires buried in the ground • Multiple-star: small radial-wire systems forming a star topology

  12. Basic Theory: The Vertical Electric Monopole Antenna

  13. Vertical Electric Monopole Antenna Assume uniform electric current I along a vertical monopole of effective height he : electric field magnetic field

  14. Vertical Electric Monopole Antenna-Radiated Power- The vertical electric field in terms of radiated power is:

  15. Vertical Electric Monopole Antenna-Equivalent Antenna Circuit-

  16. Vertical Electric Monopole Antenna(Radiation Efficiency) and where antenna total loss resistance Effective power = (power capacity of the transmitter) x (antenna system efficiency)

  17. Vertical Electric Monopole Antenna-Antenna Bandwidth- The 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

  18. Multiple Tuned VLF Antennas To have sufficiently large bandwidths: • Huge antenna systems must be built. or • Several small multiple-tuned elements must be be used.

  19. Multiple Tuned VLF Antennas

  20. 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-efficiencyproduct. • If one element is out of service, the others can still operate. • The effective ground loss with multiple-tuning will be less than for a single element. • Tuning and retuning the system is difficult. • each antenna has to be matched to the transmitter.

  21. Triatic Type Antenna

  22. Cutler, Maine Antenna Installation

  23. Goliath Antenna

  24. Goliath Antenna

  25. References (1) “VLF Radio Engineering”, A. D. Watt, Perg. Press, 1967 (2) “High Power Very Low Frequency/Low Frequency Transmitting Antennas”, P Hansen, Military Communications Conf., 1990. MILCOM '90, Conference Record, 'A New Era'. 1990 IEEE, 30Sept.-3Oct.1990 Pages:1091 - 1096 vol.3 (3) Technology Conference, 1991.IMTC-91.Conference Record. ,8th IEEE , 14-16 May 1991 Pages:330 - 334 (4) “Multiple Tuned VLF Antennas”, Manfred Schopp, IEEE Transactions on Broadcasting, Vol. 39, No.4, Dec. 1993. References for the photos & figures: [1] http://hawkins.pair.com/nss.shtml [2] http://www.tpub.com/neets/book17/77.htm

  26. MF ANTENNAS & ANTENNA SYSTEMS

  27. INTRODUCTION • Usually: Vertical radiators operating in the MF band (300-3000 kHz). • The towers may be guyed or self-supporting.

  28. APPLICATION AREAS • AM Broadcasting • Maritime Radio • Coast Guard Communication • Direction Finding

  29. CHARACTERISTICS OF RADIATORS • Maximum radiation in the horizontal plane • Antennas taller than one-half wavelength have a minor lobe

  30. Characteristics of the Radiators • Requirement for metallic ground plane to minimize losses • Vertical polarization is preferred due to superior propagation characteristics

  31. Other features of the radiators • Shunt fed radiators • Top loaded radiators • Sectionalized radiators

  32. Circuits for MF antenna systems • Antenna tuning units for matching purposes • Phase shifter networks for directional antenna systems • Power dividing networks

  33. Ground Systems • 120 buried (/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.

  34. Ground Systems A typical ground system for a two-element directional antenna

  35. HF Antennas & Antenna Systems

  36. 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

  37. 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.

  38. Types of HF Antennas:

  39. 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

  40. Long-wire Antenna A long terminated wire radiator

  41. 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.

  42. 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.

  43. Resonant HF Antennas: • Monopole Antenna • Elevated-feed Monopole • Double-cone Monopole • Inverted-L and –T Antenna • Dipoles and Slot Antennas • Loop Antennas

  44. Monopole Antennas Outside half-wave resonance, elevation pattern breaks up into main lobes as input impedance becomes very high. Efficiency decreases

  45. Dipole Antennas

  46. Loop Antennas Usully used for reception and direction finding.

  47. 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

  48. 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

  49. 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

  50. Broadside Arrays Beam steering by phase variation is possible.

More Related