10.5. What Is This dB Stuff?
The theoretically plain vanilla, no-padded-seat, economy class antenna is the simple dipole. It is considered to add no increase to the power of the transmitter, hence it is a unity gain antenna. Unfortunately, it is easier to mathematically create a unity gain antenna than it is to construct one, so this antenna is usually theoretical, and it is called an isotropic radiator. This is a useful concept in wireless, because if we build a real antenna and measure its performance, we can express its performance and radiation pattern in terms of the amount of power higher or lower than the theoretical antenna. This information is usually plotted on some kind of graph that shows where the signal strength is greatest and weakest.
One decibel (dB) is a tenth of the power of whatever you start with, either added or subtracted. Because of the way things multiply out, each 3 dB increase in power is a doubling of power with respect to the isotropic antenna (technically 3dBi, where i stands for isotropic). Thus a dish antenna offering 21 dBi in the zone along its focus offers over a hundred times the power of the simple isotropic antenna. This multiplying factor is called antenna gain. A Pringles can Wi-Fi antenna (that's right, an antenna manufactured out of an empty potato chip container), if kept dry, will have a gain of 8-12 dBi.