Unidirectional antennas concentrate the beam in one direction, with a specific beam angle. By concentrating all of the signal in one direction, the signal and receive sensitivity is stronger. This is ideal for short-range intra-building links using WDS. Note that the unidirectional aerials actually have a considerably narrower vertical beam, hence you actually don’t want your PC/client to be too close and high or low to the aerial as otherwise you can actually ‘miss’ the signal area.