Chapter
4
Antennae
Introduction |
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Basic Principles . . . . . . . . . . . . |
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Aerial Feeders |
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Polar Diagrams . . . . . . . . . . . . |
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Directivity . . . . . . . . . . . . . . |
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Radar Aerials . . . . . . . . . . . . |
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Modern Radar Antennae |
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Questions . . . . . . . . . . . . . . |
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Answers . . . . . . . . . . . . . . |
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4 Antennae
Antennae 4
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Antennae 4
Introduction
Antennae or aerials are the means by which radio energy is radiated and received. The type of antenna used will be determined by the function the radio system is required to perform. This chapter will look at the principles which are common to all antennae and at the specialities required for particular radio navigation systems.
Basic Principles
There are two basic types of aerial used for receiving and transmitting basic communications, the half-wave dipole and the Marconi or quarter-wave aerial.
Antennae 4
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Figure 4.1: Half-wave dipole
With the dipole aerial the power is fed to the centre of the aerial and radiates in all directions perpendicular to the aerial. The Marconi aerial is set on, but insulated from, a metal surface which acts as the second part of a dipole, with the radio energy radiating perpendicular to the aerial. Because of the better aerodynamic qualities, Marconi aerials are used on aircraft.
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Figure 4.2: Marconi aerial
For an aerial to operate with maximum efficiency it must be the correct length for the wavelength of the frequency in use. As the names imply the ideal length for an aerial is half or quarter of the wavelength of the frequency being transmitted. However, whilst we regard the speed of propagation of electromagnetic energy as being constant, this is only true in a specified medium. If the energy passes from one medium to another the speed will change. In the case of electromagnetic energy, the denser the medium the slower the speed. This needs to be taken into account in the length of aerials.
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Example:
What is the optimum length for a Marconi aerial transmitting on a frequency of 125 MHz?
Recall from Chapter 1:
Wavelength (λ) = |
300 m |
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f (MHz) |
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300 m |
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2.4 m |
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125 |
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With a wavelength of 2.4 m, the optimum length will be:
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0.6 m or 60 cm |
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Aerial Feeders
The means by which energy is carried between the aerial and transmitter or receiver is dependent on the frequency in use and the power levels. At low and medium frequencies a simple wire is adequate to carry the signal over reasonable distances with little energy loss. As frequency increases the power losses increase and into HF and VHF a twin wire feeder is more efficient. At UHF frequencies, the power losses in these simple feeders becomes unacceptably high and a coaxial cable is required.
In the upper part of the UHF band and in the SHF and EHF bands the use of dipole or Marconi aerials is precluded because of the high energy losses and the way the energy is produced. At these frequencies a waveguide is used to carry the energy to or from the aerial. The waveguide is a hollow, rectangular metal tube. The internal dimensions of the tube are determined by the frequency in use, being half the wavelength.
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