Transmission media comparison

A transmission medium carries a signal from a transmitter to a receiver. The choice of medium affects how the signal travels, the data rate that can be achieved, the cost of the link and how easily the signal can be intercepted.

Metal wire carries an electrical signal. It can be inexpensive to install over a short distance, but the signal is susceptible to electrical interference. The cable can also be tapped to obtain the signal.

Optical fibre carries a light signal that is guided along the fibre. It can support very high data transmission rates and is difficult to tap without access to the cable. However, installing and connecting fibre can be costly.

Wireless links use radio waves or microwaves to carry electromagnetic signals through space. They do not need a continuous cable between transmitter and receiver, but they do require transmitting and receiving equipment. Wireless signals can be intercepted, and the available data rate depends on the bandwidth and conditions of the link.

No medium has a fixed advantage in every situation: its relative cost, data rate and security depend on the particular system and how it is installed.

Ground-wave propagation

A ground wave is a radio wave that travels near the Earth's surface. Radio waves with long wavelengths can diffract, or spread, around the curved surface. This allows a signal to reach a receiver beyond the direct line of sight, where the Earth's curvature would block a straight path from the transmitter.

As a ground wave travels, its signal becomes weaker. Some of its energy is lost through interaction with the ground, so the received signal eventually becomes too weak to be useful. Ground-wave propagation therefore has a limited range, even though it can reach beyond the direct line of sight.

Sky-wave propagation

A sky wave is a radio wave directed upwards that can return to Earth after interacting with the ionosphere. The ionosphere is a region of the upper atmosphere containing charged particles.

The properties of the ionosphere change with height. As a suitable radio wave travels through it, these changes can progressively bend, or refract, the wave until it travels back towards Earth. This return is often described as reflection by the ionosphere, although the path may bend gradually rather than turn sharply at a single surface. Reflection can also occur when a wave meets a sufficiently abrupt change in the medium.

The returning wave can reach a receiver beyond the direct line of sight from the transmitter. Whether a wave returns to Earth depends on its frequency and the state of the ionosphere; some waves travel through the ionosphere instead.

Satellite communication links

A communication satellite receives a signal sent from Earth and retransmits it towards Earth. The path from an Earth station to the satellite is the uplink; the path from the satellite back to Earth is the downlink.

Satellite links commonly use microwaves at frequencies in the gigahertz range. For example, a C-band link may use an uplink near $$6,\mathrm{GHz}$$ and a downlink near $$4,\mathrm{GHz}$$. The satellite receives the uplink signal and retransmits it at the downlink frequency.

Why uplink and downlink frequencies differ

A transmitter produces a strong signal, while a receiver at the same location may need to detect a much weaker incoming signal. If both signals used the same frequency, the strong transmitted signal could overwhelm the receiver.

Using different uplink and downlink frequencies allows a receiver to filter out much of the unwanted transmitted signal while accepting the wanted incoming signal. This reduces receiver desensitisation: a loss of the receiver’s ability to detect a weak wanted signal when a strong unwanted signal is present.