Antennae — Page 61, Lesson 54

Let's start with polar diagrams, because they are the language we use to describe what an aerial actually does. A polar diagram is a graph that shows the radiation or reception pattern of an aerial. Think of it as a map of where the signal is strong and where it is weak. It is drawn as a line that joins all points of equal signal strength. So if you measure the signal all around the aerial and connect every point where the strength is the same, that line is the polar diagram. It is generally drawn as a plan view, meaning you are looking down on the aerial from above, perpendicular to the plane of radiation or reception. From here on, we will talk about radiation only, but remember the exact same principle applies to reception. Now, let's look at the simplest aerial, the dipole. A dipole aerial radiates most of its energy at right angles to the aerial itself. So if the aerial is lying horizontally, the strongest signal shoots out sideways, perpendicular to it. As you move towards the ends of the aerial, the signal strength decreases, and right at the very ends there is no radiation at all. If we drew this in three dimensions, the shape of the radiation would be a torus — that is, a doughnut shape — centred on the centre point of the aerial. That three-dimensional picture is what we call the 3-D polar diagram, and you can see it in Figure 4.3. Obviously, drawing a doughnut every time is cumbersome, so we use a plan view of the plane of radiation instead. That gives us the horizontal polar diagram and the vertical polar diagram, as shown in Figure 4.4. So the plan view is our practical, two-dimensional way of representing that three-dimensional doughnut. Now, let's move on to directivity. Many systems require the directional emission or reception of energy — for example, radar, ILS, and MLS. ILS is the Instrument Landing System, and MLS is the Microwave Landing System. The way we achieve this directivity depends on the frequency and the application. The simplest way to achieve directivity is to add parasitic elements to the aerial. A parasitic element is a metal rod that is not connected to anything electrically — it just sits near the aerial and interacts with its field. Here is the key rule: if we place a metal rod that is 5% longer than the aerial itself, at a distance of a quarter of a wavelength from the aerial, and in the same plane as the aerial, that rod will act as a reflector. So the reflector is longer than the aerial, positioned a quarter wavelength away, and lying in the same plane. What does this reflector do? It re-radiates the energy 180° out of phase. That means the reflected wave is exactly inverted relative to the original. The result is shown in the polar diagram: there is no signal behind the reflector, and the signal in front of the aerial is increased. So we have taken the omnidirectional dipole and made it directional — energy is pushed forward and cancelled behind. We can take this process further by adding other elements in front of the aerial. These elements are called directors, and they are smaller than the aerial itself. So now we have a reflector behind, which is longer than the aerial, and directors in front, which are shorter. This combination is shown in Figure 4.6, and you will all recognize this as the type of aerial array 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 a simple dipole would generate on its own. So to summarise the whole picture: the polar diagram is our map of signal strength; the dipole radiates a doughnut shape; and to get directivity we add a reflector behind — longer, a quarter wavelength away — and directors in front — shorter — to focus the energy. That is the fundamental principle behind directional aerials.