Transmission Equipment

JMU

Transmission Equipment
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Antennae

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.)

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

Parabola:
- The set of all points equidistant from a fixed line (called the directrix) and fixed point (called the focus) not on the line
Reflective Property:

Towers and Masts

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

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.)

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