Antennae — Page 61, Lesson 56

Let’s pick up right where we left off — we’d just built a strong beam along the plane of the aerial, and I told you that directivity comes with a price. That price is the side lobes. Look at the polar diagram: alongside that main beam, we’ve produced many unwanted side lobes, and those lobes will both receive and transmit unwanted signals. When a receiver picks up energy through a side lobe, you get characteristic ghosting on television pictures — that’s usually caused by reflections from buildings and similar structures. These side lobes are a major problem, and they have to be addressed in two specific systems: SSR, which is Secondary Surveillance Radar, and ILS, the Instrument Landing System. They also cause problems in primary radars. Now, ILS actually uses an extension of this same idea to get the narrow beams it needs. The localizer antenna — the part that guides the aircraft along the runway centre line — is an array of 16 or 24 aerials placed in line, with half-wavelength spacing between them. There’s some modification to how the signal is fed to the aerials, but the end result is that two narrow beams of energy are produced, and those beams are symmetrical and close to the runway centre line. That’s what Figure 4.7 shows you — the ILS localizer lobes. Let’s move to a different kind of aerial: the loop aerial, used in the ADF, the Automatic Direction Finder, to detect the direction of an incoming signal. Picture a loop of wire, and think about what happens as a radio wave passes over it. When the loop is aligned with the incoming signal, there’s a phase difference between the signals in each of the two vertical elements of the loop — the two vertical sides of the loop see the wave at slightly different times — and that phase difference produces a net flow of current from the loop. But if you rotate the loop so it’s at right angles to the incoming signal, the induced currents in the two vertical elements become equal, and they cancel each other out, giving you zero output. That’s the null. The resulting polar diagram is the classic figure-of-eight shape — two lobes, and between them two distinct nulls, the points of zero output. Those nulls are what you use to determine the direction from which the radio wave is coming. We’ll go into exactly how that principle is utilized in Chapter 7, so for now just hold onto the idea: the null tells you the direction. Now let’s talk about radar aerials. Radar systems operate in the UHF and SHF bands — that’s Ultra High Frequency and Super High Frequency. At those frequencies, you can’t just use ordinary cables; the transmission of that energy requires waveguides, which are hollow metal pipes that guide the RF energy. The parabolic dish is widely used as a reflector. Here’s the setup: the open end of a waveguide is positioned at the focal point of the parabola — that’s the centre of curvature, marked as point F in Figure 4.10 — and it directs the RF energy towards the dish. The energy from the open waveguide is reflected by the dish as parallel rays. Because of the geometry of the parabola, the path length from the focal point to the dish and out — say, path FXB, or path FYA — those path lengths are all equal. So the transmitted wavefront is made up of parallel rays that are all in phase with each other. In principle, that should produce a very narrow pencil beam, a tight column of energy. But here’s the catch: apart from the region very close to the antenna, the beam actually diverges — it spreads out. In effect, what the parabolic reflector does is convert a point source of energy — the open waveguide at the focal point — into a plane wavefront of uniform phase. That’s the key idea: a point source becomes a flat, in-phase wavefront. So to tie it together: we’ve got the half-wave dipole and its side-lobe problem, the ILS localizer array that uses multiple aerials to shape narrow beams, the ADF loop aerial that finds direction through its nulls, and the parabolic reflector that turns a point source into a plane wavefront for radar. Each one is a different way of shaping or sensing the radio wave.