Antennae — Page 61, Lesson 52

We're starting a brand-new topic now: antennae, or aerials. These are the physical means by which radio energy is radiated out into space and received back. The type of antenna you use is dictated entirely by the job the radio system has to do, and this chapter is about the principles common to all of them, plus the special requirements for specific radio navigation systems. Let's begin with the two basic types used for transmitting and receiving basic communications: the half-wave dipole and the Marconi, or quarter-wave, aerial. First, the half-wave dipole. Power is fed to the centre of this aerial, and it radiates in all directions perpendicular to the aerial itself. So if the aerial is vertical, the energy goes out horizontally in every direction. Now the Marconi aerial. It's set on, but insulated from, a metal surface. That metal surface acts as the second half of a dipole. The radio energy radiates perpendicular to the aerial, just like the dipole. And here's the key point for us: because of its better aerodynamic qualities, the Marconi aerial is the one used on aircraft. For any aerial to operate at maximum efficiency, it must be the correct length for the wavelength of the frequency in use. As the names imply, the ideal length is half or quarter of the wavelength being transmitted. The dipole is half a wavelength, the Marconi is a quarter. Now, there's a subtlety here. We tend to regard the speed of propagation of electromagnetic energy as constant, but that's only true within a specified medium. If the energy passes from one medium to another, the speed changes. The denser the medium, the slower the speed. This has to be taken into account when you're determining aerial length. Let me give you a worked example. What's the optimum length for a Marconi aerial transmitting on 125 MHz? First, recall the wavelength formula from Chapter 1: wavelength equals 300 divided by the frequency in megahertz. So 300 divided by 125 gives us a wavelength of 2.4 metres. Since the Marconi is a quarter-wave aerial, we divide that by 4, giving 0.6 metres, or 60 centimetres. Now, how do we get the energy from the transmitter to the aerial, and from the aerial to the receiver? That's the job of aerial feeders, and the type used depends on the frequency and the power levels. At low and medium frequencies, a simple wire is adequate. It carries the signal over reasonable distances with little energy loss. As frequency increases, power losses increase. So into HF and VHF, a twin wire feeder is more efficient. But at UHF frequencies, the losses in those simple feeders become unacceptably high, and you need a coaxial cable. Now, in the upper part of the UHF band, and in the SHF and EHF bands, dipole and Marconi aerials are precluded entirely. That's because of the high energy losses and the way the energy is produced at those frequencies. Instead, a waveguide is used to carry the energy to or from the aerial. The waveguide is a hollow, rectangular metal tube. Its internal dimensions are determined by the frequency in use, being half the wavelength. So to summarise the progression: simple wire for low and medium frequencies, twin wire for HF and VHF, coaxial cable for UHF, and a waveguide for the upper UHF, SHF, and EHF bands. And remember, the Marconi quarter-wave aerial is the one you'll find on aircraft because of its aerodynamic advantages.