There are various forms of electromagnetic radiation that can be used to
communicate between two places.
All forms of electromagnetic radiation suffer from
attenuation – this is the loss of energy with distance.
Attenuation reduces the signal as the travels from the transmitter and receiver. The signal
needs to be amplified every so often by 'repeater' stations.
(a) Radio
We are all
familiar with radio communication – long wave (1500m) to FM (wavelength about 3m). You
will also now probably use DAB (Digital Audio
Broadcasting).
(b)
microwaves – used for satellite links and mobile phones. Using microwaves rather than
cables for sending signals is that you don't have to get permission to transmit a signal across
someone else's land. However some people may object to having a mobile phone mast near
their house.
A disadvantage is that the path between stations must be clear and free
from obstacles – 'line of sight' and microwaves are also affected by weather – particularly
rain (microwaves are absorbed by water – see Microwave ovens). Microwave dishes are
often placed in groups on a tower in cities and these towers have become a familiar feature
on city skylines. The photograph is of the Post Office tower in London.
(c) infra red –
used for relatively short distance communication in remote controllers for TVs, CD players
and computer mice. Infra red lasers are used to send beams down optical fibres.
Communication by infrared is interference free and it is therefore used in high security local
communication networks. Most of the new mobile phones and Pocket PCs now have an
infrared (IRDA) port and so this can now be used to transmit information between the two
devices.
One advantage of infrared over microwaves is that the transmitters are smaller
because the wavelength of infrared radiation is smaller than that of microwaves. The main
disadvantage is that the attenuation is greater because
of the higher frequency. It is for this reason that infrared
is only used over short distances.