What Is dBi in Reference to a Wi-Fi Antenna?

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You have many choices in selecting an antenna for a Wi-Fi wireless network adapter for your computer.
 If you use the antenna that came with the adapter and experience weak reception and slow Wi-Fi network performance, 
unscrew the stock antenna and replace it with a higher-gain model. 
Wi-Fi equipment makers rate compatible antennas in units called dBi. The higher the dBi rating,
 the stronger the antenna’s reception.

Isotropic Antenna

Wi-Fi antennas obey the same laws of nature that govern all other radio antennas. 
The isotropic radiator is the simplest theoretical radio antenna.
 It is a point source that produces radio waves equally in all directions. However,
 the isotropic antenna is just an idea; you cannot actually make one because it violates fundamental laws of physics 
-- thus, it is only theoretical. 
However, it can serve as a standard by which you can measure other antennas: 
the decibels --db -- compared to an isotropic radiator.

Antenna Gain

Every antenna has gain, a quality by which it increases the strength of a radio signal. 
Two main factors affect an antenna’s gain: shape and size. 
Shape influences the antenna’s directionality, or how it receives radio  waves from different directions. 
The wavelength of the radio waves determine the radio’s size: an antenna  receives signals best 
if its size matches the size of the radio waves, or is one-half or one-quarter the wavelength. 
For most uses, the better the antenna design, the higher the gain.

Decibels

When radio engineers compare the gain of antennas, they use the decibel system. 
A decibel is ten multiplied by the common logarithm of the ratio of two numbers. 
For example, if you compare 1 milliwatt and 10 milliwatts of radio power,
 you divide the second by the first and obtain 10. Taking the common log of that gives you 1,
 and multiplying it by 10 gives 10. The second amount is larger than the first by 10 decibels, or 10 dB.

dBi Rating

The dBi rating for a Wi-Fi antenna compares its performance to an isotropic antenna. 
If the isotropic antenna produces 1 milliwatt of radio wave power and the Wi-Fi antenna produces 2 milliwatts,
 that is 10 * log(2) or 3 dBi. If another antenna produces 3 milliwatts, that is 10 * log(3) or 4.8 dBi. 
Though the isotropic antenna doesn’t exist, it still serves as a reliable way to compare Wi-Fi antennas. 
You can compare the dBi ratings of the two antennas and choose one with the better dBi rating.