There are many types, sizes and forms of antenna. Basic antenna usually are in the form of a pole, a rod or an array of rods. Advanced antennas are usually covered by things, such as a box, a case, a protecting shield or a radome. Despite of the different shapes and sizes, the function of an antenna is always to receive and/or transmit radio signals. The basic principles behind these antenna are the same.
Let's briefly discuss this subject without going into too much technical details.
We are surrounded by antennas. Your TV aerial is a receive antenna. Your satellite dish is also a receive antenna. Your mobile phone has a transmit and receive antenna (inside, in the form of a printed circuit board). Your Wi-Fi router has a transmit and receive antenna. Taxis, police vehicles, fire trucks, ambulances are all equipped with antennas. Even your kid's walk-talkie has an antenna.
Antennas "push out" or "accept" (ie. transmit or receive) signals from a range of directions. The "range" is known as the "beam" (ie. radiation patten). Imagine it as a beam of light. A candle beams light into many directions; but a hand held torch only beams into one particular direction. This is also the case in radio transmissions. A television broadcast antenna push out signals into a wide range of directions; and every aerials at the roof top in an neighborhood always faces the same direction because they can only accept signals from one direction. It makes little sense to broadcast into all possible directions as it is a waste of energy and might also lead to co-ordination issues. For non-broadcasting purpose, the beam of antenna is even narrower (concentrated). This reduces the energy loss. Imagine it as the lightsaber used by Jedi.
The radiation pattern ("beam") of advanced antennas such as TV broadcast and cellphone base station antennas at radio sites are usually well calculated and well defined. The beam can be modified by varying the electrical signals controlled by computer programs. The exact beam of a FM radio or car aerial receive antenna are often not well defined. The radiation patten of a cellphone antenna is difficult to define as human tissues will vary its pattern when in use.
Are antennas dangerous or harmful? Antennas are passive devices. They are not dangerous (apart from being heavy and sometimes sharp). It's the transmitter connected to these antennas that we should be cautioned of. We will talk more about that if we have the chance later.
(Read about other technologies that makes life simple under the Lifestyle tab.)