Antennae — Page 61, Lesson 52

Let’s start with the basics. An antenna — also called an aerial — is the means by which radio energy is radiated and received. The type of antenna used is determined by the function the radio system is required to perform. So different radio navigation systems will need different aerials, but there are principles common to all of them. There are two basic types of aerial used for receiving and transmitting basic communications. The first is the half-wave dipole. The second is the Marconi, or quarter-wave, aerial. Let’s look at the half-wave dipole first. With the dipole aerial, the power is fed to the centre of the aerial, and it radiates in all directions perpendicular to the aerial. So the energy goes out sideways, at right angles to the wire itself. Now the Marconi aerial. It is set on, but insulated from, a metal surface. That metal surface acts as the second part of a dipole. So the Marconi is effectively half a dipole, with the metal surface completing the other half. The radio energy radiates perpendicular to the aerial, just like the dipole. Because of the better aerodynamic qualities, Marconi aerials are used on aircraft. That’s a key point — on aircraft, we use the Marconi aerial for its aerodynamic advantage. Now, 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. So a half-wave dipole is half a wavelength long, and a Marconi is a quarter wavelength long. But here’s an important subtlety. We regard the speed of propagation of electromagnetic energy as constant, but that 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. So the speed isn’t truly constant — it depends on the medium, and that affects the physical length you need. Let me walk you through the example. What is the optimum length for a Marconi aerial transmitting on a frequency of 125 MHz? First, we recall the wavelength formula from Chapter 1: wavelength, lambda, equals 300 metres divided by the frequency in MHz. So for 125 MHz, we have 300 divided by 125, which gives us a wavelength of 2.4 metres. Now, for a Marconi aerial, the optimum length is a quarter of the wavelength. So we take 2.4 metres and divide by 4, which gives us 0.6 metres, or 60 centimetres. That’s the optimum length for that Marconi aerial. Now let’s talk about aerial feeders. The means by which energy is carried between the aerial and the 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. So into HF and VHF, a twin wire feeder is more efficient. At UHF frequencies, the power losses in these simple feeders become unacceptably high, and a coaxial cable is required. Now, in the upper part of the UHF band, and in the SHF and EHF bands, the use of dipole or Marconi aerials is precluded — that means it’s not possible — 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. So the waveguide’s internal size is set to half the wavelength of the frequency it carries. Let me just recap the key points. Two basic aerials: the half-wave dipole, fed at the centre, radiating perpendicular to the aerial. The Marconi, quarter-wave, set on an insulated metal surface that acts as the second half of the dipole, and used on aircraft for aerodynamic reasons. Optimum length is half or quarter wavelength, but the speed of propagation depends on the medium — denser medium, slower speed. For feeders: simple wire at low and medium frequencies, twin wire into HF and VHF, coaxial at UHF, and waveguide in the upper UHF, SHF, and EHF bands — a hollow rectangular metal tube with internal dimensions of half the wavelength.