The purpose of an antenna is to radiate efficiently the power supplied to it by the transmitter. A simple antenna, consisting of a single vertical element over a ground plane can do this job quite well at low-to-medium frequencies. Antenna systems may also be required to concentrate the radiated power in a given direction and to minimize radiation in the direction of other stations sharing the same or adjacent frequencies. To achieve such directionality may require a complicated antenna system that incorporates a number of individual elements and matching networks.

As the operating frequency increases into VHF and above, the short wavelengths permit the design of specialized antennas that offer high directivity and gain.

Wavelength ? is the distance traveled by one cycle of a radiated electric signal. The frequency f of the signal is the number of cycles per second (Hz). It follows that the frequency is inversely proportional to the wavelength. Both wavelength and frequency are related to the speed of light c per the formula c = f ?. The velocity of electric signals in air is essentially the same as that of light in free space (2.9983 10 ¹⁰ cm/s).

Bandwidth is a general classification of the frequencies over which an antenna is...