Some Definitions

To get a handle on our main question, we need to establish some background. Wireless communications, such as Wi-Fi, rely on transmitting radio waves at a specific frequency in specific patterns. These patterns are then picked up by a receiving antenna, and translated to usable data. In order for the receiver to understand the incoming signal, it must be strong and clear enough to be distinguished from background noise. The power of a transmission decreases with distance from the transmitter, so to increase the range of a given transmission, you can increase the power in the signal.

The power for wireless communications transmissions is specified in terms of watts, and dBm (decibel-milliwatts). The dBm unit is a logarithmic scale, where 0dBm is one milliwatt, and every additional 10dBm is 10 times more power.

Antennas can be used to boost the effective power of a wireless transmission. They do this by focusing the power that would otherwise be dispersed in all directions into a smaller area. The amount the antenna increases signal strength in its focal area is called the gain. The gain of an antenna is specified in in dBi, which stands for decibels over isotropic, and indicates the amount the input signal is boosted in the focal area relative to a hypothetical antenna that emits power equally in all directions.

Connecting an antenna to a transmitter, the output power of the system is rated in terms of Equivalent Isotropic Radiated Power or EIRP. This describes the intensity of the signal in the focal area of the antenna, relative to an antenna that emits power equally in all directions. To calculate the EIRP, you multiply the power of the transmitter by the gain of the antenna. Since both are rated in terms of decibels, the dBm of the transmitter and the dBi of the antenna can be added together to get the EIRP of the sytstem.

How much can I boost my Wi-Fi signal?

Now that we have all the techno-babble established, we can answer this question.

For a Wi-Fi transmitter to be allowed to operate in Canada and the United States, the maximum transmitter output power for a 2.4GHz Wi-Fi transmitter is 1W (30dBm), with a maximum EIRP of 4W (36dBi), before you need to licence your application. [Canadian Regulations] [FCC Regulations]

These regulations are less strict if you are using directional antennas in a fixed point-to-point configuration with no moving parts, but our aim is to control a rover over Wi-Fi, so while this may help in some situations, it does not apply to our test.

For transmitters with less power, you can subtract the power of the transmitter in dBm from 36dBi, and the remaining dBi is the maximum gain you can use for your antenna.

In practical terms, what this means is that you can take a standard Wi-Fi transmitter with 0.1W (20dBm) of output power, and attach it to an antenna with up to 16dBi of gain, and be just within the limits for licence-exempt RF operation in the United States and Canada.

What about 5GHz Wi-Fi?

As a general rule, as the carrier frequency of a transmission is increased, it suffers greater losses as it travels and will have a shorter range. Because of this, 2.4GHz communications will have a longer range than an equivalent 5GHz signal, so we will focus on the former.