When comparing add-on antennas, what does dBi mean? How do I choose?
dBi is “decibels relative to isotropic radiator” which means that the radiator (an antenna, usually) sends the signal equally strong in all directions, including above and below. Such a radiator would have 0 dBi. (I have yet to see an antenna like that — it may exist only in math theory.) A higher dBi is obtained by designing the antenna so that the strength of the signal is more focused. The little vertical ‘rubber duck’ antenna that you see on most wireless routers or Access Points (AP’s) focusses the signal such that it is stronger from the sides of the antenna and weaker above and below. It is omni-directional. If the signals were visible, it would resemble a fat donut shape with the radiator in the middle. This focusing gives a relative strength of 2.15 dBi. After-market vertical antennas get stronger dBi by narrowing the beam. The signals can be “heard” more from farther away, but less from above and below.
