AERIALS AND RADIO AERIALS AND RADIO FREQUENCY FREQUENCY

PROPAGATIONPROPAGATION

By Farhan SaeedBy Farhan Saeed

AERIALSAERIALS

In any radio system information is In any radio system information is superimposed on to a radio frequency carrier superimposed on to a radio frequency carrier which is radiated into the atmosphere in the which is radiated into the atmosphere in the form of electromagnetic (e-m) energy.form of electromagnetic (e-m) energy.

An aerial or antenna, is a device for either An aerial or antenna, is a device for either radiating electromagnetic energy into space radiating electromagnetic energy into space or collecting electromagnetic energy from or collecting electromagnetic energy from space.space.

This electromagnetic energy is in the form of This electromagnetic energy is in the form of electric and magnetic fields, which are in turn electric and magnetic fields, which are in turn related to the alternating currents (ac) which related to the alternating currents (ac) which flow in the aerial. flow in the aerial.

Electro-Magnetic WaveElectro-Magnetic Wave

An EM wave consists of two fields, an oscillating An EM wave consists of two fields, an oscillating Electric field (E) and an oscillating Magnetic field Electric field (E) and an oscillating Magnetic field (H) which are always at right angles to one (H) which are always at right angles to one another.another.

Magnetic Field Electric Field

Electric Field Magnetic Field

Electro-Magnetic WaveElectro-Magnetic Wave

The two fields are always at right angles to The two fields are always at right angles to one another. If the electric field is in the one another. If the electric field is in the vertical position the EM wave is said to be vertical position the EM wave is said to be vertically polarized.vertically polarized.

The electro-magnetic radiation is The electro-magnetic radiation is generated at a transmitter by means of generated at a transmitter by means of alternating current and is transmitted via alternating current and is transmitted via an antenna. It travels in an all-round an antenna. It travels in an all-round direction (omni-directional).direction (omni-directional).

1 Cycle

Wavelength

Time0

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+

Frequency and wavelength

Frequency and wavelengthFrequency and wavelength

Frequency is defined as the number of Frequency is defined as the number of complete series of changes of, for complete series of changes of, for example, an alternating current, which example, an alternating current, which occur in 1 second, i.e. cycles per second. occur in 1 second, i.e. cycles per second. It is measured in Hertz (Hz), i.e. 1 cycle It is measured in Hertz (Hz), i.e. 1 cycle per second = 1 Hertz.per second = 1 Hertz.

Wavelength is defined as the period of Wavelength is defined as the period of time it takes to complete one cycle and is time it takes to complete one cycle and is expressed in metres.expressed in metres.

The velocity of propagation of an The velocity of propagation of an electromagnetic (radio) wave through electromagnetic (radio) wave through space is at the speed of light, i.e. 300 000 space is at the speed of light, i.e. 300 000 000 metres per second.000 metres per second.

Frequency and wavelength

Frequency is defined as the number of Frequency is defined as the number of cycles to pass a point in ONE SECOND OF cycles to pass a point in ONE SECOND OF TIME. It is measured in HERTZ (Hz) where TIME. It is measured in HERTZ (Hz) where 1 cycle per second = 1 Hertz.1 cycle per second = 1 Hertz.

The frequency of Electro-magnetic The frequency of Electro-magnetic radiation is related to the wavelength by radiation is related to the wavelength by the equation:the equation:

Frequency = Velocity (m/s) / Wavelength Frequency = Velocity (m/s) / Wavelength (m) (m)

F = V / F = V / λλ

Frequency and wavelength

The velocity of an EM wave is variable, The velocity of an EM wave is variable, but for navigational aids purposes it is but for navigational aids purposes it is taken as being constant as a speed of taken as being constant as a speed of 300 metres in 1 millionth of a second. 300 metres in 1 millionth of a second. (Known as a MICROSECOND). ( (Known as a MICROSECOND). ( ) )

It can be seen by the formula that if the It can be seen by the formula that if the frequency is increased, the wavelength frequency is increased, the wavelength will decrease or vice versa.will decrease or vice versa.

Frequency and wavelength

The lower the frequency, the greater the The lower the frequency, the greater the range, e. g., frequency of 100 kilohertz range, e. g., frequency of 100 kilohertz (100 thousand Hertz) is approximately (100 thousand Hertz) is approximately 1200 miles, whereas, a frequency of 150 1200 miles, whereas, a frequency of 150 MHz (150 million Hertz) gives a range of MHz (150 million Hertz) gives a range of 25 miles.25 miles.

Calculation !Calculation !

Propagation of Radio WavePropagation of Radio Wave

Propagation is concerned with the way that radio Propagation is concerned with the way that radio waves travel between a transmitter (Tx) and a waves travel between a transmitter (Tx) and a receiver (Rx) at some distant point.receiver (Rx) at some distant point.

The radio frequency spectrum is divided into The radio frequency spectrum is divided into major bands, i.e.major bands, i.e. VLFVLF Very Low FrequencyVery Low Frequency LFLF Low FrequencyLow Frequency MFMF Medium FrequencyMedium Frequency HFHF High FrequencyHigh Frequency VHFVHF Very High FrequencyVery High Frequency

MF

LF

VLF

Transmitter Aerial

Very Low FrequencyVery Low Frequency

3 – 30 kHz3 – 30 kHz In this band the radio wave follows the In this band the radio wave follows the

curvature of the earth’s surface and is curvature of the earth’s surface and is known as a known as a groundground or or surfacesurface wave. wave.

Given sufficient transmitter output power Given sufficient transmitter output power and high aerial arrays, world-wide and high aerial arrays, world-wide communication is possible.communication is possible.

Very Low FrequencyVery Low Frequency

Since there is not much bandwidth in this band of Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals the radio spectrum, only the very simplest signals are used, such as for radionavigation. are used, such as for radionavigation.

VLF waves can penetrate water to a depth of VLF waves can penetrate water to a depth of roughly 10 to 40 m , depending on the frequency roughly 10 to 40 m , depending on the frequency employed and the salinity of the water. employed and the salinity of the water.

VLF is used to communicate with submarines VLF is used to communicate with submarines near the surface. near the surface.

VLF is also used for radio navigation beacons VLF is also used for radio navigation beacons (alpha) and time signals (beta).(alpha) and time signals (beta).

VLF is also used in electromagnetic geophysical VLF is also used in electromagnetic geophysical surveys.surveys.

Low FrequencyLow Frequency

30 kHz – 300 kHz 30 kHz – 300 kHz In this band the radio wave again follows the In this band the radio wave again follows the

curvature of the earth’s surface, i.e. curvature of the earth’s surface, i.e. groundground or or surfacesurface wave. wave.

However, because the frequency is now higher, the However, because the frequency is now higher, the radio wave is attenuated by the earth more quickly radio wave is attenuated by the earth more quickly and so the range is reduced to approximately 1 to and so the range is reduced to approximately 1 to 2 thousand miles dependant upon transmitter 2 thousand miles dependant upon transmitter output power.output power.

Loran C transmissions at 100 kHz, give reliable Loran C transmissions at 100 kHz, give reliable accurate ground wave coverage up to 1200 miles.accurate ground wave coverage up to 1200 miles.

Low FrequencyLow Frequency

Used forUsed forAM Broadcast serviceAM Broadcast serviceLORANLORANWeather systemWeather systemTime signals Time signals

Medium FrequencyMedium Frequency

300 kHz – 3000 kHz300 kHz – 3000 kHz Uses Uses groundground or or surfacesurface wave, but because the wave, but because the

frequency is now even higher, the range is reduced. frequency is now even higher, the range is reduced. The actual range of communication now depends The actual range of communication now depends

upon both the transmitter output power and on the upon both the transmitter output power and on the type of information being transmitted. type of information being transmitted. MF RT 2182 kHz 150 to 200 miles, MF RT 2182 kHz 150 to 200 miles, MF DSC 2187.5 kHz approximately 400 milesMF DSC 2187.5 kHz approximately 400 miles Navtex 518 kHz Navtex 518 kHz

The range on MF RT is less because the bandwidth is The range on MF RT is less because the bandwidth is higher and therefore susceptible to attenuation.higher and therefore susceptible to attenuation.

High FrequencyHigh Frequency

High FrequencyHigh Frequency3 MHz – 30 MHz3 MHz – 30 MHzThe HF band is so big that we tend to The HF band is so big that we tend to

sub-divide it into those which are used sub-divide it into those which are used for maritime communications, i.e. 4, 6, for maritime communications, i.e. 4, 6, 8, 12, 16 and 22 MHz. 8, 12, 16 and 22 MHz.

Very High FrequencyVery High Frequency

30 MHz – 300 MHz30 MHz – 300 MHzOn VHF, UHF and SHF bands, the radio On VHF, UHF and SHF bands, the radio

waves travel in straight lines and are waves travel in straight lines and are known as known as directdirect or or spacespace waves, i.e. waves, i.e. line of sightline of sight communication. communication.

The main consideration which The main consideration which determines the range obtainable is the determines the range obtainable is the height of both the receiving and height of both the receiving and transmitting aerials are above sea level, transmitting aerials are above sea level, i.e. an increase in height gives an i.e. an increase in height gives an increase in range.increase in range.

Frequency and wavelengthFrequency and wavelength

Any Questions ?Any Questions ?

Hyperbolic Line Hyperbolic Line

A hyperbolic line may be defined as a line A hyperbolic line may be defined as a line joining all points where the DIFFERENCE IN joining all points where the DIFFERENCE IN DISTANCE from two places IS THE SAME.DISTANCE from two places IS THE SAME.

The distance mentioned can be a The distance mentioned can be a measurement of any unit, e.g., metres or measurement of any unit, e.g., metres or miles OR, in the case of navigational miles OR, in the case of navigational systems, radio waves where the difference systems, radio waves where the difference can be measured either by phase can be measured either by phase difference or time difference.difference or time difference.

Hyperbolic Line Hyperbolic Line

Hyperbolic LineHyperbolic Line

Long base lines have the advantage Long base lines have the advantage over short baselines because the over short baselines because the hyperbolic lines are nearly parallel hyperbolic lines are nearly parallel and therefore do not diversify as and therefore do not diversify as greatly. greatly.

A ship at position P and position Q ?A ship at position P and position Q ?Ambiguity !Ambiguity !

Layout of a Hyperbolic Layout of a Hyperbolic SystemSystem

Time DifferenceTime Difference

Hyperbolic systems depend upon the fact that Hyperbolic systems depend upon the fact that if signals are transmitted from separate shore if signals are transmitted from separate shore stations, the difference in the times of their stations, the difference in the times of their arrival at a ship is a measure of the difference arrival at a ship is a measure of the difference in distance of the ship from the two stations.in distance of the ship from the two stations.

The signals may be sent in pulses so that the The signals may be sent in pulses so that the time between receiving the pulses may be time between receiving the pulses may be measured. measured.

Loran C uses this system, and once again the Loran C uses this system, and once again the system is operated in such a way that the system is operated in such a way that the Master station pulse is always received first.Master station pulse is always received first.

Time DifferenceTime Difference

Given a time difference and knowing the Given a time difference and knowing the velocity of radio waves, the distance velocity of radio waves, the distance difference can be found.difference can be found.

In a system measuring time difference In a system measuring time difference (Loran C), the hyperbolic lines are drawn (Loran C), the hyperbolic lines are drawn on the chart representing the time on the chart representing the time difference in micro-seconds between difference in micro-seconds between receiving the two transmissions.receiving the two transmissions.

Time DifferenceTime Difference

Given a time difference and knowing the Given a time difference and knowing the velocity of radio waves, the distance velocity of radio waves, the distance difference can be found.difference can be found.

In a system measuring time difference In a system measuring time difference (Loran C), the hyperbolic lines are drawn (Loran C), the hyperbolic lines are drawn on the chart representing the time on the chart representing the time difference in micro-seconds between difference in micro-seconds between receiving the two transmissions.receiving the two transmissions.

Lattice chartLattice chart

A lattice chart is a chart which has a family A lattice chart is a chart which has a family of hyperbolic lines drawn on it. Since the of hyperbolic lines drawn on it. Since the lines cross and cut one another, it gives lines cross and cut one another, it gives the appearance of ‘lattice’ work, hence the the appearance of ‘lattice’ work, hence the word, ‘lattice’ word, ‘lattice’

In the Loran C system, the lattice lines are In the Loran C system, the lattice lines are drawn on a Loran chart. When the ship’s drawn on a Loran chart. When the ship’s position is fixed on one of these charts, position is fixed on one of these charts, the position may be transferred to the the position may be transferred to the Admiralty chart.Admiralty chart.

Hyperbolic SystemHyperbolic System

Any Questions !Any Questions !