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Unguided Tr. Me

| | Friday, April 17, 2009

Unguided Transmission Media

Unguided transmission media is data signals that flow through the air. They are not guided or bound to a channel to follow. They are classified by the type of wave propagation.

RF Propagation

There are three types of RF (radio frequency) propagation:

  • Ground Wave

  • Ionospheric

  • Line of Sight (LOS)

Ground wave propagation follows the curvature of the Earth. Ground waves have carrier frequencies up to 2 MHz. AM radio is an example of ground wave propagation.

Ionospheric propagation bounces off of the Earth's ionospheric layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30 - 85 MHz. Because it depends on the Earth's ionosphere, it changes with the weather and time of day. The signal bounces off of the ionosphere and back to earth. Ham radios operate in this range.

Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called space waves or tropospheric propagation. It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

Radio Frequencies

The frequency spectrum operates from 0 Hz (DC) to gamma rays (1019 Hz).

NameFrequency (Hertz)Examples
Gamma Rays1019+
Ultra-Violet Light7.5 x 1015
Visible Light4.3 x 1014
Infrared Light3 x 1011
EHF - Extremely High Frequencies 30 GHz (Giga = 109)Radar
SHF - Super High Frequencies3 GHzSatellite & Microwaves
UHF - Ultra High Frequencies300 MHz (Mega = 106)UHF TV (Ch. 14-83)
VHF - Very High Frequencies30 MHzFM & TV (Ch2 - 13)
HF - High Frequencies3 MHz2Short Wave Radio
MF - Medium Frequencies300 kHz (kilo = 103)AM Radio
LF - Low Frequencies30 kHzNavigation
VLF - Very Low Frequencies3 kHzSubmarine Communications
VF - Voice Frequencies300 HzAudio
ELF - Extremely Low Frequencies30 HzPower 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.


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 their large bandwidth.


  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 only a small area.

  4. High frequency/short wavelength signals require small antennae.


  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.


Satellites are transponders (units that receive on one frequency and retransmit on another) that are set in geostationary orbits directly over the equator. These geostationary orbits are 36,000 km from the Earth's surface. At this point, the gravitational pull of the Earth and the centrifugal force of Earth's rotation are balanced and cancel each other out. Centrifugal force is the rotational force placed on the satellite that wants to fling it out into space.

The uplink is the transmitter of data to the satellite. The downlink is the receiver of data. Uplinks and downlinks are also called Earth stations because they are located on the Earth. The footprint is the "shadow" that the satellite can transmit to, the shadow being the area that can receive the satellite's transmitted signal.

Iridium Telecom System

The Iridium Telecom System is a new satellite system that will be the largest private aerospace project. It is a mobile telecom system intended to compete with cellular phones. It relies on satellites in lower Earth orbit (LEO). The satellites will orbit at an altitude of 900 - 10,000 km in a polar, non-stationary orbit. Sixty-six satellites are planned. The user's handset will require less power and will be cheaper than cellular phones. There will be 100% coverage of the Earth.

Unfortunately, although the Iridium project was planned for 1996-1998, with 1.5 million subscribers by end of the decade, at the time of this writing, it looked very financially unstable.

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