Well, it’s this. Although they’re not 100% the same, a passive radiator in the antenna world can be thought of as another word for a director. They are parts of an antenna that essentially help focus the signal on a particular point.
This brings up the fact that on a lot of antennas you see, the actual “driven element,” in other words the part that actually receives the signal, is quite small. It is usually found in the middle of the antenna. There’s a lot of metal surrounding it that isn’t connected to the driven element at all, and this may lead you to think that a lot of the metal on an antenna is just for show. That couldn’t be further from the truth.
The posts on the front of the antenna, before the driven element, help send more signal toward that driven element by “passively radiating.” In other words, the signal gets intercepted and essentially pointed in one direction rather than bouncing all around. Another way of saying this is “the beam’s directionality” is improved. This has a negative side effect as well– the antenna becomes able to see only a narrower portion of the sky. However, the part it does see has stronger signals.
This is all part of the design of a Yagi antenna, where elements are arranged in such a way to maximize reception. Elements that connect directly to your antenna cable are said to be “active” while the others are said to be “passive.”
Yes, it does sort of seem like I’m talking in circles here, because it’s hard to define one part of an antenna without defining the others. That’s why it’s better off, assuming you’re not shooting for an engineering degree, to talk about what a passive radiator does for the overall design of the antenna, rather than talking about the complex physics that make it work.
So, here’s the bottom line — a passive radiator focuses signal on one point. That point is where the active part of the antenna is, and the more signal it gets, the better your reception is. Simple as that.