An Omni-directional antenna is usually described as a low-gain, dipole-like antenna, while Directional antenna is one with a predominant beam in one direction and significantly more gain. Using another antenna, whether indoor or outdoor, is one of the options you have for extending your wireless (WiFi) network.
An antenna gives the wireless system three fundamental properties: gain, direction and polarization. Gain is a measure of increase in power. Gain is the amount of increase in energy that an antenna adds to a radio frequency (RF) signal. Direction is the shape of the transmission pattern. As the gain of a directional antenna increases, the angle of radiation usually decreases. This provides a greater coverage distance, but with a reduced coverage angle. The coverage area or radiation pattern is measured in degrees. These angles are measured in degrees and are called beamwidths. [Cisco]
In deciding which of these two antennas you should be using, you must consider the range that you want to achieve as well as the obstructions/interferences that are present. Like for example, would you want to focus the signal to one direction only, that is towards the other building, or to two buildings on opposite sides of the central one? If to only one building, then maybe you could consider a directional antenna, otherwise, an omni might be more practical. Also, consider the walls that might be obstructing the area you wish to reach with your wireless signal. If there are too many obstructions, you might also consider putting up wired connections in addition to the wireless part.
Do look and compare the gain, the direction, and polarization of various antennas if you do wish to purchase one and evaluate if it is what you wish to achieve for extending the range of your wireless network.
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