Radio transmitter RF output power, through feed line (cable) to the antenna by theelectromagnetic waves radiated by the antenna. Electromagnetic waves reach the receiving site, the antenna then (receives only a very small part of power), and through the feeder to the radio receiver. Visible antenna is transmitting and receiving electromagnetic waves of an important radio equipment, no antenna, no radio communications.
Antenna a variety for different frequencies for different purposes, different situations, different requirements in different situations.
For many kinds of antennas, for proper classification is necessary:
- Classified by use: including communications antennas, TV antennas, radar antenna and so on;
- Classification by frequency band: including HF antennas, VHF antennas, microwaveantennas, such as;
- Classification by directional: can be divided into Omni-directional antenna, a directional antenna and so on;
- Classified by shape: can be divided into linear antennas, antenna and so on.
Dipole is a classic, by far the most widely used type of antenna, a single half-wave dipole can be simply a separate site or use as a feed parabolic antenna, also can usemultiple half-wave dipole antenna array.
Arms of equal length dipole is called a dipole.
Antenna is a basic function of energy obtained from the feeder to the surrounding space radiating second basic feature is most of the energy in the required direction of radiation. Vertical half-wave dipole is flat on the "doughnut"