Most simply, a piece of
wire connected to a
radio or
television to pick up
transmissions. A
tuned electrical conductor used to
transmit or
receive broadcasts.
There are many types of antennas, from whips, dipoles, beams or Yagis, to the directional antenna arrays used by radio stations that you see on the highway at night. Cell-phone antennas on towers have proliferated in recent years.
How worketh an antenna, ask you?
Consider a guitar. Pluck the low E string and a pitched tone is created. If another guitar is lying nearby, the incoming sound energy will cause its strings to vibrate sympathetically. The low E string of the second guitar, in particular, will vibrate strongly. This is because its length is resonant with the wavelength of the incoming sound energy. The other, non-resonant strings will vibrate less, although the high-E string will vibrate quite a bit.
Much like the second low-E string, an antenna will most efficiently "pick up" signals whose wavelength is closest to its physical length. So an antenna 2 meters long will most strongly resonate with radio signals with a frequency around 150 megahertz. The signal induces an electrical voltage in the antenna, causing a current to flow in the circuit attached to the antenna.
Also like a guitar string, the antenna will still "pick up" signals of other nearby frequencies (and those which are harmonic or subharmonic multiples of its resonant frequency. For example a half- or quarter- wavelength antenna is still quite efficient.
But as the signal gets farther and farther from the antenna's natural or resonant frequency, the induced current is less and less. The antenna is acting as a bandpass filter.