Transmission Media

A signal can travel as an electrical current down a copper wire, as pulses of light along a glass fibre, or as an invisible wave through empty space — and once it's in free space, it can reach a receiver by several genuinely different routes. This lesson surveys the physical media available for the "channel" stage of a communication system, and looks closely at how radio waves actually get from a transmitter to a receiver that isn't always in a straight line of sight.

Wired media: metal wire and optic fibre

The simplest way to connect a transmitter to a receiver is a physical cable running directly between them. Two very different technologies dominate here.

• Metal wire. An electrical signal is carried as a varying voltage or current along a conducting wire, typically copper. Wire is cheap, simple to install, and easy to connect to standard electronic circuits at each end. Its main drawbacks are resistive power loss along the length of the cable (so the signal weakens with distance and needs periodic amplification for long runs), and vulnerability to electromagnetic interference — nearby electrical equipment, or even other cables running close by, can induce unwanted noise onto the wire, since the signal itself is an electrical/electromagnetic quantity.
• Optic fibre. The signal is carried as pulses of light, guided along a thin glass or plastic fibre by repeated total internal reflection at the boundary between the fibre's core and its surrounding cladding. Optic fibre offers a very high data-carrying capacity (bandwidth) compared with metal wire, because light has an extremely high frequency, allowing far more information to be encoded per second. It's also strongly immune to electromagnetic interference, since the signal travelling down the fibre is optical, not electrical — external electrical noise simply has nothing to couple into. The main practical cost is that fibre and its associated transmitting/receiving optoelectronics are typically more expensive to install than simple wire, especially over short runs.

Radio and microwave: three ways to get past the horizon

Radio and microwave signals travel through free space as electromagnetic waves, without needing any physical cable at all. This makes them essential wherever a fixed cable link is impossible or impractical — communicating with a moving vehicle, aircraft or ship, or reaching a receiver anywhere a cable simply can't be run. But free-space propagation isn't a single simple mechanism: depending on frequency, radio waves reach a distant receiver by one of three genuinely different routes.

• Ground wave propagation. Lower-frequency radio waves travel close to the Earth's surface and are able to follow its curvature to some extent, diffracting slightly over the ground rather than travelling in a perfectly straight line into space. This lets ground waves reach receivers beyond the direct line of sight from the transmitter, though the signal's strength steadily decreases with distance as energy is absorbed by the ground itself.
• Sky wave propagation. Radio waves transmitted upward at a suitable frequency can be refracted, and effectively reflected, back down toward the Earth by the ionosphere — a region high in the atmosphere containing a significant density of charged particles (ions and free electrons), produced mainly by solar radiation. By bouncing off this layer, a sky wave can reach a receiver far beyond the horizon, well out of any possible line of sight or ground-wave range. This mechanism is sensitive to the exact frequency used and to the state of the ionosphere, which itself changes with time of day and solar activity.
• Long-wavelength diffraction. Diffraction — the spreading of a wave as it passes an obstacle or edge — becomes significant when a wave's wavelength is comparable to the size of the obstacle it encounters. Very long-wavelength radio waves can therefore diffract noticeably around large obstacles such as hills, buildings, or even around the curve of the Earth itself, reaching receivers that a geometrically straight ray would never reach. Shorter-wavelength waves, by contrast, are far less able to bend around large obstacles and behave much more like a straight-line beam.

Satellite systems

For very long-distance communication, or communication with remote and inaccessible areas where laying cable or building ground transmitters is impractical, signals can instead be relayed via an orbiting satellite. A ground station transmits a signal up to the satellite — the uplink — and the satellite retransmits it back down to a receiver, potentially on the opposite side of the Earth — the downlink.

Satellite links typically use microwave frequencies. Microwaves have short wavelengths, meaning they travel essentially in straight lines (they are not significantly diffracted around obstacles, nor bent back down by the ionosphere the way lower-frequency sky waves are), and they pass through the atmosphere with comparatively little absorption. Their high frequency also allows a high data-carrying capacity, letting a satellite link carry a large volume of information. Satellites are particularly valuable exactly where ground-based infrastructure would be impractical or prohibitively expensive — connecting ships at sea, aircraft, or communities separated from the rest of the world by terrain that makes cabling difficult.

Why uplink and downlink use different frequencies: receiver de-sensing

A communications satellite has to do two things simultaneously: receive the relatively weak uplink signal arriving from a ground transmitter possibly thousands of kilometres away, and transmit its own downlink signal back down to Earth — usually at a much higher power, since it must be detectable after travelling the same enormous distance back down. Both of these have to happen using antennas mounted close together on the same small spacecraft.

If the uplink and downlink used the same frequency, the satellite's own outgoing downlink transmission — necessarily powerful, since it has to reach the ground clearly — would swamp the satellite's own receiver circuitry, which is simultaneously trying to detect an incoming uplink signal that is comparatively extremely weak after its own long journey up from the ground. The receiver would effectively be deafened by its own transmitter, a problem called de-sensing (loss of receiver sensitivity due to interference from a nearby powerful transmission). Using two different frequencies for uplink and downlink keeps the two functions electrically separated, so the receiver's sensitive circuitry isn't overwhelmed by the satellite's own much stronger outgoing signal.

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