Perhaps the best way to describe the receivers is this analogy:
Let us say that the different types of music and talk radio were regulated to broadcast at certain frequencies. For example, say country and western at frequencies 1 to 100, rock from 101 to 200, talk radio from 201 to 300, and so on.
In order to listen to your favorite type of radio broadcast, you have to buy a module that you plug into your radio. This module will pick up only certain broadcasts: the country and western module would pick up frequencies 1 to 100 - and no other frequencies. Same for the rock module, the talk module, and so on.
The nice thing is that the module will not pick up any other frequencies. If you only wanted to listen to rock, you'd pick up the rock module and no others.
The receivers in the antenna are like the modules above: they have a certain frequency range and can be tuned within this range. Anything outside that range and they don't function.
The receivers are in the box at the bottom of the feed horn. This box is actually a Dewar flask (a thermos that keeps your coffee warm is a simple type of Dewar flask) that keeps the receiver chilled to about 15 K (-432 F). When this cold, the radio noise of the electronics from the receiver are minimized so as not to interfere with the radio signal.
The power of the radio signal is very small. The unit of measurement is called a Jansky, and it is 3x10-27 watts/m^2 or:
and many of the brightest radio sources are milliJanskys, so tack on three more zeros.
When the signal comes out of the receiver it is split into four channels called A, B, C, and D. Channels A and B carry right circular polarized data, while channels C and D carry left circular polarized data. (Right circular and left circular polarization is an indication of which way the incoming radio signal is spinning. If the signal is spinning counter-clockwise, it is left circular polarized; if spinning clockwise, it is right circular polarized.)
These four channels then make up the signal path that travel throughout the systems.