F4 Channels of Communication – Student Work Book
1).
Two common communications media for electrical signals are Coaxial Cables
and Twisted Pair Cables.
(a)
(i)
Draw a labelled cross-sectional diagram of a coaxial cable.
(ii)
Describe a common application for this type of cable.
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(iii)
State two limitations of using this type of cable.
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(b)
(i)
Draw a labelled cross-sectional diagram of a twisted pair cable.
(ii)
State two common applications for this type of cable.
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(iii)
Explain why electrical signals carried by this type of cable are less prone to
picking up interference and radiating signals.
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2).
For long distance communication, optical fibres are often used as the transmission medium.
These can be divided into two main types, Multimode and Monomode fibres.
(a)
What are the main differences between these types of fibres?
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(b)
(i)
Draw a labelled cross-sectional diagram of a multimode optical fibre.
State the name of the type of fibre you have drawn and add approximate sizes.
(ii)
Name the other type of multimode fibre and explain how it differs
from the one that you have drawn above.
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(iii)
Explain the limitations of multimode optical fibre and describe how
these are overcome with the use of monomode optical fibre.
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3).
Radio waves are often used for communications when there is no physical connection
(ie cables) between two locations.
(a)
Radio waves are electromagnetic waves.
(i)
Draw a diagram to represent an electromagnetic wave, and label the fields and
direction of propagation.
(ii)
State three properties of all electromagnetic waves
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(b)
(c)
State the approximate frequency ranges used by the following radio transmissions:(i)
FM Radio .......................................................................................................
(ii)
GSM mobile phones ......................................................................................
(iii)
Analogue television .......................................................................................
(iv)
Satellite television ..........................................................................................
(v)
WiFi ................................................................................................................
(i)
What is the purpose of the aerial in a radio communications system?
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(ii)
Explain how it achieves this for both transmission and reception.
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4).
(a)
Explain what is meant by a Geostationary Satellite.
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If the mass of the earth is 6x1024kg, its radius is 6.4x106m and it takes 24 hours to rotate
once, show, by calculation, that for a satellite to be geostationary it should be at an
altitude of approximately 36x106m.
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(b)
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(c)
Two places on the earth, 1000km apart, are in communication by both geostationary
satellite and also optical fibre cable.
If the glass core of the optical fibre has a refractive index of 1.5:(i)
Which route will have the shortest time delay?
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(ii)
What will be the difference in the time delay?
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(d)
The transmission frequency to a satellite is 12GHz. The aerial consists of two pieces of
wire each of length /4. Calculate the overall length of the aerial.
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5).
A company has two offices, separated by a distance of 10km. The company needs to exchange
large quantities of sensitive data between the two offices and is unclear whether to use a radio
or an optical fibre based system. You are required to make a recommendation to the company
and present your findings in the form of a five minute presentation, as to which system they
should use. You should consider such issues as Cost, Bandwidth, Security and Interference
when making your recommendations.
Attach a copy of the slides from your presentation to this booklet.
6).
The aerial used to communicate with satellites is small (see Qu4(d)), but the aerial systems are
often very much larger and consist of a large parabolic dish.
(a)
Draw a labelled diagram of a satellite aerial system, clearly showing the location of the
actual aerial.
(b)
Explain two purposes of the parabolic dish used in satellite aerial systems.
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(c)
A satellite aerial system, for use in the 10GHz band, is to have a beam width of 5°.
Calculate the diameter of the parabolic disk needed to achieve this beam width.
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(d)
(i)
Explain why geostationary satellites are separated by at least 3°.
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(ii)
Estimate the minimum distance between the geostationary satellites in orbit
around the earth.
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7).
As well as placing satellites in geostationary orbit, many satellites are placed in Polar Orbit.
(a)
Explain what is meant by Polar Orbit.
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(b)
Describe two applications which need polar orbiting satellites.
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(c)
A polar orbiting satellite has an altitude of 200km. Estimate its orbit period.
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8).
“Satellite communication provides quick, secure, cheap and reliable communication.”
Write report of about 1000 words in which you examine the validity of this statement.
Attach your report to this booklet.
F4 Channels of Communication – Student Work Book – Responses
1).
(a)
(i)
Outer insulation
Copper braid (outer conductor)
Copper wire (inner conductor)
Insulation
(ii)
(iii)
(i)
(b)
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Linking aerials to transmitters and receivers.
Bulky, susceptible to interference, radiates signals, limited bandwidth etc
wires
conductors
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(ii)
(iii)
2).
(a)
insulation
Network cables (100m), computer peripheral cables (eg USB)
The signals passing along a twisted pair cable are transmitted so that when the
one wire is positive, the other is negative and vice versa. These are known as
differential signals. On transmission, the magnetic and electric fields produced
by one wire therefore cancels out the magnetic and electric fields produced by
the other wire, so minimising any radiated signal. On reception, the receiver is
only sensitive to differences in the signals carried by each wire, so therefore
noise or other interference induced in both wires is cancelled out.
A multimode fibre allows the electromagnetic waves to travel by multiple paths, a
monomode fibre only allows one path
multimode fibres are 50 - 65µm in diameter, monomode fibres are 5 – 8 µm in diameter
(i)
Step Index fibre
Cladding (n1)
(b)
a
b
125m
50m
c
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(ii)
(iii)
Core (n2)
Graded index fibre - the refractive index of the fibre is reduced gradually from
core to cladding rather than having a step change.
Multiple paths causes dispersion of the signals, so limiting bandwidth. Monomode fibre only allows one path so eliminating this problem.
3).
(a)
(i)
Displacement
Electric field
Magnetic field
time
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(ii)
(b)
(c)
4).
(a)
(b)
(c)
(d)
5).
(i)
(ii)
(iii)
(iv)
(v)
(i)
(ii)
All travel at the speed of light, 3  108m s1, in a vacuum.
All can undergo reflection, refraction, diffraction, interference and polarisation
under the appropriate conditions.
All will obey the inverse square law in the absence of absorption.
FM Radio 88 – 108MHz
GSM mobile phones 900 and 1800MHz
Analogue television 400 – 800MHz
Satellite television 10GHz
WiFi 2.5 and 5GHz
convert electrical signals to em waves and vice versa
For transmission, an alternating current is sent to the aerial, which makes the
electrons in the aerial accelerate to and fro. When an electron accelerates it
produces an electromagnetic wave - the radio wave, which then travels away
from the aerial. For reception, when the aerial is placed in an alternating
electromagnetic field (radio wave), electrons in the aerial are forced to move to
and fro in time with the varying electromagnetic field. This movement of
electrons forms an alternating current which can then be processed by the radio
receiver to recover the transmitted information.
Their period of orbit is 24 hours so they maintain their positions above a particular point
on the Earth's surface.
Equate angular centripetal force to Newton’s law of gravitation. Replace  with 2/T,
where T is the period in seconds (24 x 3600) and solve for R (radius of orbit).
Remember to subtract the radius of the earth to get the altitude.
(i)
Fibre cable
(ii)
approx 0.36s for the satellite and 0.005s for the fibre cable
12.5mm
See the Topic booklet, F4.2, for ideas and details.
6).
(a)
parabolic
reflector
dipole
feed cable
(b)
(c)
(d)
Collects as much em waves/energy as possible to give a larger signal output from the
aerial,
produce a narrow field of transmission/reception.
0.84m
(i)
Diffraction effects limit the number of satellites that can be placed in
geostationary orbits - if they are too closely spaced then their signals will
interfere with each other.
(ii)
1.1x106m
7).
(a)
(b)
(c)
Satellites which rotate around the earth passing over each pole.
Surveying and weather plus descriptions
1.5hr
8).
See section 4.6 for issues to consider.