Antennae — Page 61, Lesson 58

I want to walk you through the modern radar antennae now. We've been looking at parabolic reflectors, and I want to make sure you understand one important behaviour of them before we move on. When a parabolic reflector reflects the radar energy, the reflection isn't perfectly even. Because of that uneven reflection, some of the energy literally spills out of the reflector, and that spillage forms what we call side lobes. You can see this in Figure 4.11, which is the polar diagram of a parabolic reflector. Now, side lobes matter because they contain enough energy to produce valid returns outside the main lobe or beam. That's a real operational concern — a return that appears to come from one direction might actually be coming from a side lobe, which can confuse your target picture. Now, modern radar development has introduced a completely different type of aerial, and it goes by a few names: the Flat Plate Array, the Phased Array, or the Slotted Antenna. Let me describe its construction. The antenna is a flat plate, and it has numerous waveguide-size slots cut into it. Each of those individual slots is fed with RF energy from behind the plate. RF, of course, is radio frequency energy. So the transmitted radar beam is not produced by a single reflector — it's the result of the interaction of all those numerous individual beams coming from the slots. Here's the key point about performance. This type of antenna is more efficient than the parabolic reflector. Why? Because it wastes much less energy in the side lobes. And for a given frequency, the RF energy is concentrated into a narrower beam. That's a direct consequence of the design. And because the flat plate array is a more efficient means of transmission, radars that use this technology require less power to do the same job. Let me give you the full list of advantages of the phased or flat plate array over the parabolic reflector, because you'll want these memorised. First, a narrow beam. Second, reduced side lobes. Third, less power required for a given range. Fourth, a narrower pulse. And fifth, improved resolution. Each of those follows from what we just discussed — the efficiency of the flat plate means the energy stays concentrated in the main beam, so you get a tighter beam, less wasted energy in the side lobes, you need less power to reach a given range, the pulse can be narrower, and your overall resolution improves. So when you compare the two, the parabolic reflector gives you that classic dish shape with the feed at the focus, but it suffers from side-lobe spillage. The flat plate array replaces that whole concept with a plate full of slots, each fed individually, and the combined beams give you a cleaner, more efficient, higher-resolution radar picture.