Page 335 - Physics Coursebook 2015 (A level)

Chapter 20: Communications systems
from the other person also takes 0.24 s and so there always seems to be a delay of at least 0.48 s in the conversation.
Geostationary satellites used for communication can transmit to each other around the world but they cannot always receive from regions close to the poles, as the curvature of the Earth blocks the signal. The delay when communicating with a polar satellite is much smaller but you may have to wait until the satellite is overhead to transmit or receive.
The features of a geostationary satellite are:
■■ the satellite rotates with the same period as the Earth
■■ the satellite is in orbit above the equator with a period of
1 day
■■ the satellite appears to remain fixed in position above a
point on the equator and so satellite dishes do not need to be moved.
Compared to a geostationary satellite, a satellite in polar orbit:
■■ travels from pole to pole, with an shorter period of orbit
■■ is at a smaller height above the Earth and can detect objects
of smaller detail
■■ is not always in the same position relative to the Earth and
so dishes must be moved
■■ has smaller delay times.
An alternative for long-distance communications is the optic fibre, a very thin glass or plastic fibre that carries light or infrared. Optic fibres use glass and infrared for long distances, rather than plastic or light, as the glass can be very pure and does not absorb or scatter infrared. As optic fibres have very low signal attenuation, the distance between repeater amplifiers can be high.
Figure 20.19 shows a photograph of a number of optic fibres and the internal structure of one fibre. The three rays of light are totally internally reflected from one end of the fibre to the other.
When used for communication, an electrical signal causes a laser or a light-emitting diode (LED) to emit pulses of light or infrared, with a frequency of the order of 2 × 1014 Hz (2 × 108 MHz). Because the frequency is so high, the potential bandwidth available is also very high. The pulses of light or infrared provide the digital signal that passes along the fibre. With a cable containing more than a hundred fibres and each fibre carrying a large number of pulses per second, the whole cable can carry ten million telephone conversations at the same time.
Fibre optic cables have replaced the use of satellites for long-distance transmission. Just a few fibre optic cables running across the oceans and from city to city link almost the whole world and make the internet possible
– the ‘information superhighway’. The delay between
optic fibre
Figure 20.19 An optic fibre passing through the eye of a needle, and its internal structure.
cladding
transmission and reception is less than with a satellite as the distances travelled round the world by the signal are less than up to a satellite and back down again. The disadvantages of fibre optic cables are that an electrical signal must first be converted to pulses of light and the optic fibres are difficult to connect to one another as two fibres cannot just be glued together.
Compared to a metal cable, a fibre optic cable:
■■ has much greater bandwidth and can carry more information per second
■■ has less signal attenuation, so repeater and regeneration amplifiers can be further apart
■■ is more difficult to tap, making the data it carries more secure ■■ does not suffer from electrical interference and crosstalk
■■ weighs less and so large lengths can be handled more easily ■■ is immune to lightning and the effects of nearby power lines ■■ can be used in flammable situations as no sparks are
produced
■■ is cheaper than the same length of copper wire.
QUESTION
17 A television signal can be transmitted using a coaxial cable to pass an analogue signal, using a space wave in the UHF band, using a sky wave linked to a satellite or using fibre optic cable and the internet.
a Explain what is meant by the terms in bold type.
b State approximate values for the frequencies and wavelengths used by the carrier wave in each case.
c Give one advantage and one disadvantage of each of these four methods.
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