Transcript Frequency and wavelength

AERIALS AND RADIO
FREQUENCY
PROPAGATION
By Farhan Saeed
AERIALS
In any radio system information is superimposed
on to a radio frequency carrier which is radiated
into the atmosphere in the form of
electromagnetic (e-m) energy.
 An aerial or antenna, is a device for either
radiating electromagnetic energy into space or
collecting electromagnetic energy from space.
 This electromagnetic energy is in the form of
electric and magnetic fields, which are in turn
related to the alternating currents (ac) which
flow in the aerial.
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Electro-Magnetic Wave
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An EM wave consists of two fields, an oscillating Electric
field (E) and an oscillating Magnetic field (H) which are
always at right angles to one another.
Electric Field
Magnetic Field
Magnetic Field
Electric Field
Electro-Magnetic Wave
The two fields are always at right angles to one
another. If the electric field is in the vertical
position the EM wave is said to be vertically
polarized.
 The electro-magnetic radiation is generated at a
transmitter by means of alternating current and
is transmitted via an antenna. It travels in an
all-round direction (omni-directional).
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Frequency and wavelength
+
Time
0
1 Cycle
Wavelength
Frequency and wavelength
Frequency is defined as the number of complete
series of changes of, for example, an alternating
current, which occur in 1 second, i.e. cycles per
second. It is measured in Hertz (Hz), i.e. 1 cycle
per second = 1 Hertz.
 Wavelength is defined as the period of time it
takes to complete one cycle and is expressed in
metres.
 The velocity of propagation of an
electromagnetic (radio) wave through space is at
the speed of light, i.e. 300 000 000 metres per
second.
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Frequency and wavelength
Frequency is defined as the number of cycles to
pass a point in ONE SECOND OF TIME. It is
measured in HERTZ (Hz) where 1 cycle per
second = 1 Hertz.
 The frequency of Electro-magnetic radiation is
related to the wavelength by the equation:
Frequency = Velocity (m/s) / Wavelength (m)
F = V/λ
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Frequency and wavelength
The velocity of an EM wave is variable, but
for navigational aids purposes it is taken
as being constant as a speed of 300
metres in 1 millionth of a second. (Known
as a MICROSECOND). (  )
 It can be seen by the formula that if the
frequency is increased, the wavelength
will decrease or vice versa.
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Frequency and wavelength
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The lower the frequency, the greater the range,
e. g., frequency of 100 kilohertz (100 thousand
Hertz) is approximately 1200 miles, whereas, a
frequency of 150 MHz (150 million Hertz) gives a
range of 25 miles.
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Calculation !
Propagation of Radio Wave
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Propagation is concerned with the way that radio waves
travel between a transmitter (Tx) and a receiver (Rx) at
some distant point.
The radio frequency spectrum is divided into major
bands, i.e.
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VLF
LF
MF
HF
VHF
Very Low Frequency
Low Frequency
Medium Frequency
High Frequency
Very High Frequency
Transmitter Aerial
MF
LF
VLF
Very Low Frequency
3 – 30 kHz
 In this band the radio wave follows the
curvature of the earth’s surface and is known as
a ground or surface wave.
 Given sufficient transmitter output power and
high aerial arrays, world-wide communication is
possible.
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Very Low Frequency
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Since there is not much bandwidth in this band of the
radio spectrum, only the very simplest signals are used,
such as for radionavigation.
VLF waves can penetrate water to a depth of roughly 10
to 40 m , depending on the frequency employed and the
salinity of the water.
VLF is used to communicate with submarines near the
surface.
VLF is also used for radio navigation beacons (alpha)
and time signals (beta).
VLF is also used in electromagnetic geophysical surveys.
Low Frequency
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30 kHz – 300 kHz
In this band the radio wave again follows the curvature
of the earth’s surface, i.e. ground or surface wave.
However, because the frequency is now higher, the radio
wave is attenuated by the earth more quickly and so the
range is reduced to approximately 1 to 2 thousand miles
dependant upon transmitter output power.
Loran C transmissions at 100 kHz, give reliable accurate
ground wave coverage up to 1200 miles.
Low Frequency
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Used for
 AM
Broadcast service
 LORAN
 Weather system
 Time signals
Medium Frequency
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300 kHz – 3000 kHz
Uses ground or surface wave, but because the
frequency is now even higher, the range is reduced.
The actual range of communication now depends upon
both the transmitter output power and on the type of
information being transmitted.
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MF RT 2182 kHz 150 to 200 miles,
MF DSC 2187.5 kHz approximately 400 miles
Navtex 518 kHz
The range on MF RT is less because the bandwidth is
higher and therefore susceptible to attenuation.
High Frequency
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High Frequency
3
MHz – 30 MHz
 The HF band is so big that we tend to subdivide it into those which are used for
maritime communications, i.e. 4, 6, 8, 12, 16
and 22 MHz.
Very High Frequency
 30
MHz – 300 MHz
 On VHF, UHF and SHF bands, the radio waves
travel in straight lines and are known as
direct or space waves, i.e. line of sight
communication.
 The main consideration which determines the
range obtainable is the height of both the
receiving and transmitting aerials are above
sea level, i.e. an increase in height gives an
increase in range.
Frequency and wavelength
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Any Questions ?
Hyperbolic Line
A hyperbolic line may be defined as a line joining
all points where the DIFFERENCE IN DISTANCE
from two places IS THE SAME.
 The distance mentioned can be a measurement
of any unit, e.g., metres or miles OR, in the case
of navigational systems, radio waves where the
difference can be measured either by phase
difference or time difference.
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Hyperbolic Line
Hyperbolic Line
Long base lines have the advantage over
short baselines because the hyperbolic
lines are nearly parallel and therefore do
not diversify as greatly.
 A ship at position P and position Q ?
 Ambiguity !
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Layout of a Hyperbolic System
Time Difference
Hyperbolic systems depend upon the fact that if
signals are transmitted from separate shore
stations, the difference in the times of their
arrival at a ship is a measure of the difference in
distance of the ship from the two stations.
 The signals may be sent in pulses so that the
time between receiving the pulses may be
measured.
 Loran C uses this system, and once again the
system is operated in such a way that the
Master station pulse is always received first.
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Time Difference
Given a time difference and knowing the velocity
of radio waves, the distance difference can be
found.
 In a system measuring time difference (Loran
C), the hyperbolic lines are drawn on the chart
representing the time difference in microseconds between receiving the two
transmissions.
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Time Difference
Given a time difference and knowing the velocity
of radio waves, the distance difference can be
found.
 In a system measuring time difference (Loran
C), the hyperbolic lines are drawn on the chart
representing the time difference in microseconds between receiving the two
transmissions.
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Lattice chart
A lattice chart is a chart which has a family of
hyperbolic lines drawn on it. Since the lines
cross and cut one another, it gives the
appearance of ‘lattice’ work, hence the word,
‘lattice’
 In the Loran C system, the lattice lines are
drawn on a Loran chart. When the ship’s
position is fixed on one of these charts, the
position may be transferred to the Admiralty
chart.
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Hyperbolic System
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Any Questions !