Antennae — Page 61, Lesson 56

I want to walk you through what happens when we push directivity too far, because we've just seen how concentrating energy into a beam is useful, but it comes with a price. Look at the polar diagram we've built: we've produced a strong beam along the plane of the aerial, but we've also produced many unwanted side lobes. Those side lobes will receive, and transmit, unwanted signals. On a television picture, signals received in these side lobes produce characteristic ghosting, usually caused by reflections from buildings and so on. And these side lobes give major problems that have to be addressed in SSR and ILS, and they also produce problems in primary radars. Now, the Instrument Landing System, the ILS, actually uses an extension of this idea to produce the narrow beams, or lobes, of energy required to guide an aircraft along the runway centre line. The ILS 'localizer' antenna that produces this is an array of 16 or 24 aerials placed in line with half wavelength spacing. There is some modification to the way the signal is fed to the aerials, but the end result is that two narrow beams of energy are produced which are symmetrical, close to the centre line of the runway, as shown in Figure 4.7. Now let's move to the Automatic Direction Finder, the ADF. Here a loop aerial is used to detect the direction of an incoming signal. When the loop is aligned with the incoming signal, there is a phase difference between the signals in each of the vertical elements of the loop, and there will be a net flow of current from the loop. But if the loop is placed at right angles to the incoming signal, then the induced currents will be equal and will cancel each other out, giving a zero output. The resulting polar diagram will have two distinct nulls, which can be used to determine the direction from which the radio wave is coming. How this principle is utilized will be discussed in detail in Chapter 7. Now let's look at radar aerials. Radar systems operate in the UHF and SHF bands, and the transmission of such frequency energy requires the use of 'waveguides' rather than cables. The parabolic dish is widely used as a 'reflector': the open end of a waveguide is positioned at the focal point of the parabola, the centre of curvature, designated by point F, and it directs the RF energy towards the dish. The energy from the open waveguide is reflected by the dish as parallel rays; the path length FXB, FYA, and so on will therefore be equal, and the transmitted wavefront will be made up of parallel rays that are all in phase. In principle, a very narrow pencil beam should be produced, but apart from the region very close to the antenna, the beam, in fact, diverges. In effect, the parabolic reflector converts a point source of energy, the open waveguide, at the focal point into a plane wavefront of uniform phase.