Antennae — Page 61, Lesson 54

I want to walk you through polar diagrams and directivity — this is where we start turning a simple aerial into something that actually points its energy where we want it. First, the polar diagram. A polar diagram is used to show the radiation or reception pattern of an aerial. It is simply a line joining all points of equal signal strength, and it is generally a plan view perpendicular to the plane of radiation or reception. From here on we talk about radiation only, but the same principle applies to reception. Let me make that concrete. Imagine a dipole aerial — a straight rod. It radiates most energy at right angles to the aerial, with signal strength decreasing towards the ends of the aerial, where there is no radiation at all. If you drew the three-dimensional shape of that radiation, you'd get a torus — a doughnut shape — centred on the centre point of the aerial. That's the 3-D polar diagram. Now, clearly, a 3-D doughnut is cumbersome to draw and read. So we take a plan view of the plane of radiation — we slice through that doughnut and look at it flat. That gives us the horizontal polar diagram and the vertical polar diagram, as two separate flat views. Now here's the key idea: directivity. Many systems require the directional emission or reception of energy — radar, ILS, MLS, and many more. How that directivity is achieved depends on the frequency and the application. The simplest way to achieve directivity is to add parasitic elements to the aerial. Let me explain what that means. If we place a metal rod 5% longer than the aerial, at a distance of a quarter of a wavelength from the aerial, and in the same plane as the aerial, it will act as a reflector. So we have our dipole, and a quarter wavelength away we put a rod that's 5% longer. That reflector re-radiates the energy 180° out of phase. The result: the polar diagram shows no signal behind the reflector, and increased signal in front of the aerial. We've pushed the energy forward. We can take this further. We add other elements in front of the aerial. These elements are known as directors, and they are smaller than the aerial itself. So the reflector is longer than the aerial, the directors are shorter — that's the pattern. You'll recognize this type of aerial array — a reflector behind, directors in front — as the type used for the reception of television signals. The directors have the effect of focussing the signal into, or out of, the aerial, giving a stronger signal than that which would be generated by a simple dipole. So the whole story is: a simple dipole radiates in a doughnut shape, we read it as flat polar diagrams, and we shape that pattern by adding parasitic elements — a longer reflector behind, shorter directors in front — to focus the energy into a beam. That's the foundation of directional aerials.