Transmitting antennae come in various shapes and sizes, each designed to
fulfill a specific electromagnetic mission. This is one end of a type
known as a dipole. The name relates to the opposite poles of a magnet.
Without getting into the boring details of electronic theory, it can be
said that this is an efficient device, producing the maximum signal with
the minimum energy, AND the signal is directional. By that is meant that
it travels primarily in one direction rather than all over the place.
It's the reverse process of shotgun microphone that are seen at
football games. Note the dark area under the rusty portion:
it is explained in another image presentation.