Alternatively, if an antenna is placed into an electromagnetic field, it will produce an alternating voltage in response to the field: see radio frequency induction.
There are two basically different types of antenna. One type couples to the electric field of an electromagnetic wave. It's usually a length of wire in which an electric charge moves back and forth. The other type couples to the magnetic field of an electromagnetic wave. It is usually a coil or loop of wire forming an electromagnet.
By adding additional conducting rods or coils, called "elements", and varying their length, spacing and layout, one can make an antenna with different properties as required. Typically, antennae are designed to operate at a specific frequency and to either radiate or receive.
The vast majority of antennae are simple vertical rods, which are inexpensive, and both radiate and receive from all points of the compass with equal efficiency. One crucial limitation of this type is that it does not radiate or receive very well in the direction in which the wire points. This is called the antenna blind cone.
Antennae have practical use for the transmission and reception of radio signals, which can pass through (nonconducting) walls at the speed of light over great distances.
Theoretically, arrays of microscopic antennae could be designed, capable of converting light, which is of course an electromagnetic wave, and other ambient radio waves, into usable electricity. Where standard photovoltaic cells reach no more than 25% efficiency, this technique could convert virtually all frequencies, including infrared and ultraviolet.
Types of antenna
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