Page 332 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
When two wires are close together, each wire picks
up an equal amount of electrical interference. There is no additional potential difference between the two wires and so having the wires close together reduces the interference. Figure 20.14 shows a twisted wire-pair with the wires close together. The connection from your telephone to a socket nearby is likely to use two insulated copper wires placed close together or, more likely, a twisted pair of wires.
Figure 20.14 Twisted wire-pairs in a computer network.
A wire-pair is by far the cheapest transmission medium but it does have some disadvantages.
The changing currents in the wires themselves produce electromagnetic (EM) fields and this makes the wires act as aerials, radiating EM waves. The energy to emit these waves reduces the strength of the signal sent along the wire. As the frequency of the changing current increases, the emission of EM waves increases and so the bandwidth of wire-pairs is low. Energy is also lost in the wires due to electrical heating in the resistance of the wires.
Wire-pairs are often close together in a telephone system and EM waves pass from one wire-pair to another. This is the origin of some forms of crosstalk, where you can hear another telephone conversation.
Wire-pairs are easily ‘tapped’. A connection made to each wire allows an unwanted person to hear a telephone conversation. The security of a wire-pair is low.
Coaxial cable, as shown in Figure 20.15, reduces the amount of crosstalk in wire-pairs when transmission occurs at high speed. The copper core and the finely woven copper wire or braid are the two conductors that transmit the signal. The braid is usually connected to earth,
so, ideally, the potential of this wire does not change. Electromagnetic (EM) waves do not pass easily through metal and so the braid provides a screen or barrier that reduces the interference that reaches the copper core.
An ideal coaxial cable also prevents any emission of EM waves at radio frequencies and has less attenuation than a wire-pair. Although coaxial cable is more expensive than a
wire-pair, it can transmit data faster, over longer distances, and with less electrical interference. Coaxial cable often connects a radio transmitter to an aerial, as coaxial cable has a high bandwidth, which can exceed 100 MHz with a cable 30 m in length. It is also slightly more difficult to ‘tap’ into a coaxial cable than into a wire-pair.
Table 20.3 summarises the advantages and disadvantages of wire-pairs and coaxial cable.
insulation
finely woven copper wire (braid)
is more expensive
is less attenuating
has higher bandwidth
copper
Wire-pairs
Coaxial cable
320
insulation
Figure 20.15 Coaxial cable. are cheap and convenient
strongly attenuate a signal
have low bandwidth
pick up some noise and interference
suffer from crosstalk
have low security
has less electrical interference and noise
has little crosstalk
is more secure
Table 20.3 Comparison of wire-pairs and coaxial cable.
Radio waves and microwave links
Radio waves can travel by a number of different paths from a transmitter to a receiver, as shown in Figure 20.16.
Sky waves greater than 30MHz are known as space waves. They pass through the ionosphere.
ionosphere
Surface waves diffract around the Earth.
Figure 20.16 Radio wave transmissions.
Sky waves up to 30MHz are reflected by the ionosphere.