When deploying wireless gear, it’s very tempting to grab the highest gain antenna you can to get the ‘best’ coverage. In reality, a deployment is often best served by a lower gain antenna. Understanding what gain really means and how to plan for it is a key component to wireless design and deployment. Antennas are often misunderstood, particularly when discussing gain.
The vast majority of antennas are passive devices (i.e. they do not have their own power source), which means that they do not generate any power themselves. The gain that an antenna provides is not an increase in overall power, but rather an effective gain in signal in some directions at the cost of gain in one or more other directions. Thus a higher gain antenna likely has its energy focused into a smaller area than a lower gain antenna. This may be perfectly acceptable for some deployments, but inappropriate for others.
When an antenna is specified, it is most commonly the ‘peak’ gain that is listed, not the average. Thus a “3.5dBi antenna” may have a radiated gain in one area that shows 3.5dBi of gain, but its average gain in the same plane could be at 1dBi. It’s always important to look at the antenna’s radiation pattern to understand how the peak and average values are related and see if the antenna is right for your application.
When setting the power on an access point, regulatory authorities expect the installer to compensate for antenna gain to ensure that regulatory power maximums are respected. Putting a higher gain antenna onto an access point will require backing the power the AP is configured for further down to ensure the overall regulatory requirements are met. This can have unexpected consequences if too high an antenna gain is used, many AP radios lose their linearity at low power settings, and may perform worse at these levels than they would with a lower gain antenna but a higher power setting.