Pt IIB 0730206
MSc 0780150
UNIVERSITY OF YORK
Department of Electronics
Part IIB Examinations 2010/
MSc in Communications Engineering Examinations 2010
Antennas and Propagation
Time allowed: ONE HOUR AND THIRTY MINUTES
You should attempt 2 of the 4 questions.
All questions carry equal marks.
Use only black or blue pens for your answers. The use of red pens is
forbidden and may result in questions not being marked.
Clearly write your examination number on every answer booklet and piece of
paper used. Begin each question on a new page and clearly write the
question number in the space provided on each page.
Please mark clearly on your booklet all the questions attempted. Failure to do
this may result in questions not being marked.
Materials supplied:
Standard University Calculator
Special requirements for Q1 —
The speed of light is 3.0 x 10 8 m/s.
The radius of the Earth is 6380 km.
The radius of the nth Fresnel ellipse is given by:
hf = nAdid2
(cli+d2)
where all symbols have their usual meanings.
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Special requirements for Q2
The speed of light is 3.0 x 10 8 m/s.
The impedance of free space is 377 ohms.
At a distance of d from an antenna with a figure of merit of V, the electric field
strength is given by:
V.VPt (kW)
E(mV/m)= d(km)
The figure of merit of a short vertical monopole is 300.
The directivity of a short vertical monopole is 3.
120
100
80
5 60
ELI 40
20
10 20 50 100 200 500 10002000
d (km)
Figure 1 - Normalised Signal Strength for Land Paths
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Pt IIB 0730206
MSc 0780150
120
100
80
E
60
03
0.7 MHz
40 1.0 MHz
1.5 MHz
20 = 4000 mS/m
Er = 80
10 20 50 100 200 500 1000 2000
d (km)
Figure 2 - Normalised Signal Strength for Sea Paths
Special requirements for Q4
Boltzmann's Constant = 1.38x10 -23 J/K
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A microwave link between a mobile phone base station and its mobile
switching centre (MSC) operates at 3.5 GHz. The link is 15 km long, and
both antennas are 10 meters above ground level. In between the base
station and the mobile switching centre is completely flat ground.
For successful operation most of the time, it is required that the clearance
between the direct line-of-sight path between the two antennas and the
ground should be at least 0.7 of the radius of the first Fresnel ellipsoid at
every point for true Earth radius propagation conditions (k = 1).
(a) [10 Marks]
Is this condition satisfied for this link?
Show your working, and state any assumptions you make.
(b) [4 Marks]
If not, by how much must both the antennas be raised for
successful operation to be achieved?
(c) [2 Marks]
After the link has been successfully installed and tested, it was
found that at certain times, it failed to work. Suggest one reason
why this might happen.
(d) [4 Marks]
Another technique to make this link work would be to increase the
frequency, and leave the antennas where they are. What is the
minimum frequency that could be used to satisfy the
requirement?
What would be the disadvantages of moving to this frequency?
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Pt IIB 0730206
MSc 0780150
(a) [4 Marks]
Ground waves (sometimes known as surface waves) propagate
around the surface of the Earth.
(I )
Why are ground waves used for long-distance propagation
always vertically polarised?
(ii) Ground wave propagation equations cannot be used for
frequencies above 20 MHz. Why not?
(iii) As the distance of a receiver from a land-to-sea transmitter
is increased the signal strength can also increase. What
causes this effect?
(b) [8 Marks]
A 300 kW transmitter using a short vertical monopole antenna is
sending a signal using a 300 kHz carrier. 500 km away from the
transmitter, over a path that covers 300 km of land followed by
200 km of sea, a small boat is listening to the signals.
What is the received signal strength at the receiver?
(c) [3 Marks]
If the receiver has an effective aperture of 12 m 2 , what is the
maximum received power?
(d) [5 Marks]
During the night, the signal strength received by the boat
increases by a large amount. Why? What's happening?
Estimate the maximum signal strength at the receiver at night,
stating any assumptions you make.
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(a) [4 Marks]
Define the terms ISOTROPIC and OMNIDIRECTIONAL as
applied to an antenna.
(b) [4 Marks]
By using an appropriate set of approximations, derive an
approximate relationship between the gain and the beamwidth of
a high directivity antenna.
(c) [6 Marks]
A base-station antenna with horizontal beamwidth 90° and
vertical beamwidth 6° is required to produce a field strength of
10mV/m at a distance of 10km. The antenna has an input
voltage reflection coefficient of 0.70 when connected to its feed
cable. What power should be fed to the antenna along the cable?
(d) [6 Marks]
The antenna of part (c) is used to receive a signal from a device
which radiates a power of 10pW with a dipole-like radiation
pattern. What power is delivered to the antenna feed cable? The
system operates at a frequency of 1.8 GHz.
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Pt IIB 0730206
MSc 0780150
(a) [5 Marks]
What is meant by the term "Noise Temperature" applied to an
antenna? List three physical phenomena that can contribute to
the noise temperature of an antenna.
(b) [15 Marks]
A spacecraft tracking system uses an 80m diameter circular
parabolic reflector antenna to receive signals from a spacecraft at
a frequency of 3GHz. The sky noise temperature is 10°K. The
associated receiver has an input noise temperature of 15°K and
operates with a bandwidth of 10Hz. This receiving system is
used to track a beacon on the spacecraft that has a carrier power
of 10W and radiates from a low-gain antenna that has an
omnidirectional pattern in one plane and a cosine pattern in the
orthogonal plane.
The spacecraft's orientation drifts. What angle can the
orientation drift from the optimum such that the received carrier to
noise ratio is 0dB? The spacecraft is a distance of 800 million km
from Earth. State any assumptions.
End of Paper
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