Introduction to Data Communications
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ii. Radio Frequencies

The frequency spectrum operates from 0 Hz (DC) to Gamma Rays (1019 Hz).

Name Frequency (Hertz) Examples
Gamma Rays 10^19 +
X-Rays 10^17
Ultra-Violet Light 7.5 x 10^15
Visible Light 4.3 x 10^14
Infrared Light 3 x 10^11
EHF - Extremely High Frequencies 30 GHz (Giga = 10^9) Radar
SHF - Super High Frequencies 3 GHz Satellite and Microwaves
UHF - Ultra High Frequencies 300 MHz (Mega = 10^6) UHF TV (Ch. 14-83)
VHF - Very High Frequencies 30 MHz FM / TV (Ch2 - 13)
HF - High Frequencies 3 MHz2 Short Wave Radio
MF - Medium Frequencies 300 kHz (kilo = 10^3) AM Radio
LF - Low Frequencies 30 kHz Navigation
VLF - Very Low Frequencies 3 kHz Submarine Communications
VF - Voice Frequencies 300 Hz Audio
ELF - Extremely Low Frequencies 30 Hz Power Transmission

Radio Frequencies are in the range of 300 kHz to 10 GHz. We are seeing an emerging technology called wireless LANs. Some use radio frequencies to connect the workstations together, some use infrared technology.

iii. Microwave

Microwave transmission is line of sight transmission. The Transmit station must be in visible contact with the receive station. This sets a limit on the distance between stations depending on the local geography. Typically the line of sight due to the Earth's curvature is only 50 km to the horizon! Repeater stations must be placed so the data signal can hop, skip and jump across the country.

Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to the large bandwidth.

Advantages:

  1. They require no right of way acquisition between towers.
  2. They can carry high quantities of information due to their high operating frequencies.
  3. Low cost land purchase: each tower occupies small area.
  4. High frequency/short wavelength signals require small antenna.

Disadvantages:

  1. Attenuation by solid objects: birds, rain, snow and fog.
  2. Reflected from flat surfaces like water and metal.
  3. Diffracted (split) around solid objects
  4. Refracted by atmosphere, thus causing beam to be projected away from receiver.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007