Antennae — Page 61, Lesson 58

Let’s pick this up right where the parabolic reflector leaves off. I want to walk you through the modern radar antennae, because this is where the technology really changes the game. First, the key point: the parabolic reflector has a problem. Because the reflection off the dish is uneven, some energy “spills out” around the edges of the reflector. That spillage forms what we call side lobes — you can see them in Figure 4.11, the polar diagram of the parabolic reflector. Those side lobes sit outside the main lobe, or main beam, and here’s the critical part: they contain enough energy to produce valid returns — real, usable radar echoes — from targets that are outside the main beam. That sounds like a bonus, but it’s actually a hazard. A return from a side lobe can show up on your display as if the target were straight ahead, when it’s actually off to the side. So side lobes are a source of false or misleading information. Now, modern radar development introduced a completely different type of aerial. It goes by three names, and you need to know all of them: the Flat Plate Array, the Phased Array, or the Slotted Antenna — see Figure 4.12. The antenna itself is a flat plate with numerous slots cut into it, and those slots are sized like waveguides — that is, they’re cut to the dimensions appropriate for the radar’s frequency. Each individual slot is fed with RF energy — radio frequency energy — from behind the plate. So the transmitted radar beam is not a single beam; it’s the result of the interaction of all those numerous individual beams from the slots combining together. Why does that matter? Because this type of antenna is more efficient than the parabolic reflector. Let me give you the three concrete ways. First, it wastes much less energy in the side lobes — so the side-lobe problem is greatly reduced. Second, for a given frequency, the RF energy is concentrated into a narrower beam. And third — and this is a big operational point — 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 lay out the full list of advantages of the phased/flat plate array over the parabolic reflector, because this is exactly the kind of comparison you’ll be tested on. There are five. Narrow beam — the beam is tighter. Reduced side lobes — less wasted energy and fewer false returns. Less power required for a given range — you get the same range performance with less transmitted power. Narrower pulse — the transmitted pulse is shorter in duration. And improved resolution — because of the narrower beam and narrower pulse, you can distinguish between two targets that are close together, both in bearing and in range. So the takeaway is this: the flat plate array trades the simplicity of the parabolic dish for a more precise, more efficient, and more discriminating radar. The side lobes that plagued the reflector are suppressed, the beam is tighter, and you need less power to achieve the same range — all of which adds up to better resolution on your display.