One of the common misconceptions that people tend to have is that higher gain = better. This results in people blindly buying the highest dBi rating omnidirectional antennas they can get their hands on, and often not achieving the desired results. With respect to wireless security and auditing, this can mean having your wireless network spilling out into the streets as a potential target for an attacker, or in the case of wireless security auditing, not being able to pinpoint the location of a rogue AP or client.
So what is gain?
Imagine an ideal omnidirectional antenna (technically known as an isotropic antenna) as a reference. This antenna should theoretically emit radio waves evenly in all directions in the shape of a sphere. A real world antenna doesn’t actually do that. Commercial whip type “omni” antennas tend to emit in a doughnut shape, with farther horizontal reach and limited vertical reach. Other types of antennas like the Yagi-Udas are more directional and typically have a forward facing lobular reach with some smaller side lobes. Gain is simply the measure of directivity with respect to the reference of a perfect sphere multiplied by the efficiency of the antenna design. Of course, all the extra reach and directivity isn’t free due to the laws of physics where you can’t get energy out of nowhere, and a gain in one direction will always come at a sacrifice of another.