An antenna has three main properties: direction, gain and polarization.
Wireless Antenna Gain is a term used for measuring of increase in signal power, as decibels (dB). Indoor dipole antenna come in different gain values, 2dbi, 5dbi, and 9dbi.
Antenna Direction is the shape of the wireless signal after leaving the antenna.
Contrary to common knowledge, an antenna does not add any additional power to your WLAN device. In fact, the wireless signal is only shaped in a specific pattern by the antenna. The pattern in which the wave spreads itself is defined by properties of the antenna.
Polarization of the wireless antenna is the orientation of the wireless signal.
It can be vertical/E-Plane, horizontal/H-Plane, circular or combinations of these.
Following diagram showing the relationship between the E and H planes for a 9dB omni-directional dipole antenna (ARS-N19MBP).
You also need to make sure that the receiving end is in the wave-path of the antenna as shown in following.
You might want to use 9dB omni-directional antenna if your wish to shoot your
signal in great distance in horizontal elevation. Or 5dB dipole antenna if you wish
to shoot your wifi signal over a couple of levels of a building.