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Transmission Equipment
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Antennae
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  • Transmitting:
    • When an antenna conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current
    • The amplitude of the wave is determined by the voltage
  • Receiving:
    • When electromagnetic radiation passes through an antenna it induces an electric current flow in the conductor
Antennae (cont.)
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  • One Cycle of a Sine Wave:
    • Charge moves: up, down, up down
    • Speed: 300,000 km/sec (ideally)
  • Optimal Antenna Lengths:
    • For 680,000 cycles/sec: One cycle in 1/680000 secs so 1/4 cycle in 1/2720000 secs so charge travels 0.11km in 1/4 cycle (i.e., 360 ft)
    • For 900,000,000 cycles/sec: 8.3cm (i.e., 3 in)
Parabolic Reflective Antennae
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  • Parabola:
    • The set of all points equidistant from a fixed line (called the directrix) and fixed point (called the focus) not on the line
    • images/parabola.gif
  • Reflective Property:
    • images/parabolic-reflection.gif
Towers and Masts
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  • Purpose:
    • Support antennae
  • Differences:
    • A tower is self-supporting or cantilevered
    • A mast is supported by stays or guys
  • Common Media for Communications Towers/Masts:
    • Radio
    • Microwave
    • Light (i.e., laser)
Satellites
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  • Definition:
    • An artificial satellite is an object that has been placed into orbit (intentionally)
  • Common Frequencies for Communications Satellites:
    • C-band, Ka-band, Ku-band, and L-band (i.e., microwave)
    • VHF for GPS
Satellites (cont.)
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  • Orbits:
    • Low Earth Orbit (LEO): 640-1,600km (400-1,000mi; 0.05sec)
    • Middle Earth Orbit (MEO): 10,000-15,000km (6,200-9,400mi; 0.1sec)
    • Geosynchronous Orbit (GEO): 36,000km (22,300mi; 0.25 sec)
  • Terminology:
    • Space Segment - the satellite and its orbit
    • Control Segment - frequency spectrum and signaling techniques
    • Ground Segment - antennae and receivers
There's Always More to Learn
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