Introduction of Antenna
A transmitter generates a radio frequency (RF) signal, and then sends the signal through a transmission line to an antenna. The antenna broadcasts the signal in the form of electromagnetic (EM) wave into the space. When the EM wave is received by another antenna at the designated location, a transmission line passes the signal from the antenna to the receiver, and the communication path is completed.
In a word, antenna acts as a main role in wireless communication network. No antenna, no wireless communication.
There are many types of antennas that can be classified into different categories under different circumstances.
For example:
* Classify by application: communication antenna, television   antenna, radio antenna,radar antenna, etc…
* Classify by frequency: VHF antenna, microwave antenna, UHF   antenna, etc…
* Classify by style: decorative antenna, dish antenna,Yagi antenna,panel antenna, parabolic antenna, Omni-directional antenna, etc…

Gain of Antennas:
Antenna gain relates the intensity of an antenna in a given direction to the intensity that would be produced by a hypothetical ideal antenna that radiates equally in all directions and has no losses. Since the radiation intensity from a lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4π steradians, we can write the following equation:

Although the gain of an antenna is directly related to its directivity, the antenna gain is a measure that takes into account the efficiency of the antenna as well as its directional capabilities. In contrast, directivity is defined as a measure that takes into account only the directional properties of the antenna and therefore it is only influenced by the antenna pattern. However, if we assumed an ideal antenna without losses then Antenna Gain will equal directivity as the antenna efficiency factor equals 1 (100% efficiency). In practice, the gain of an antenna is always less than its directivity.

Antenna Radiation Direction--Gain
Under the same input power, gain is the ratio between the highest antenna's radiated power intensity and the standard unit antenna. Antenna gain represents the convergence of the radiation energy at the pointing direction.
If Gain G=13dB=20 , to radiate 100W with the antenna, it only needs 100W / 20 = 5W input power.
A half wave length dipole = 2.15dBi.