It’s the round part of a satellite dish. But that’s not all. Reflectors are found on many different antennas, and while they look diffeent, they all have much the same function.
The reflector on a dish is called a parabolic reflector. It acts like a lens, focusing as much of the satellite’s transmission as possible on a single spot. Here’s a simple drawing of how it works:
The reflector takes up a lot of space. In the case of satellite dishes, that space tends to be less than one meter (39″) in diameter to comply with OTARD rules. The bigger the reflector, the more of that transmission can be used — that’s why those big dishes at the Los Angeles Broadcast Center are so big. The transmission is focused on the LNB, which actually receives and processes the signal so that it is usable. Without a reflector, the LNB itself doesn’t get enough signal to actually do anything. That’s why you can’t just put the LNB on top of the TV.
Parabolic reflectors are thousands of years old. They were used to start fires or just to focus light. The first time anyone used one for radio waves was back when Heinrich Hertz invented the first dish-shaped antenna in 1888. There’s another type of reflector, though, and it looks really quite different from a dish’s parabolic reflector:
This antenna has a reflector too. The X-shaped elements at front are actually the antenna’s working part, while the bars at the back are the reflector. (The black plastic at the edges is to hold the bars in place.
Antenna signals are much more powerful than satellite ones, so it’s not necessary to use a parabolic reflector. As everyone who looks in a mirror knows, a reflector can be perfectly straight and the reflector on the back of antenna does a great job of sending signals to the back of the antenna. (Because radio waves bounce a little differently than visible light, the reflector doesn’t have to be solid.) That way, both sides of the antenna element can be used, making it twice as powerful.