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What Is Antenna Gain ?

What Is Antenna Gain ?

What are “dBi” and “dB” ?

Imagine an antenna that radiates energy equally in all directions, much like our sun does. In scientific lingo, this is said to be an “isotropic radiator”, because it has no preference for radiation in any direction …  in other words it has no “directivity”.  This type of isotropic antenna is said to have “no gain”. “No gain” can be expressed in linear terms like x1 (times 1), meaning that all directions have the same energy radiation, and are equal to the average energy radiation.

Antenna engineers like logarithmic terms, and we say this no-gain situation is 0 dBi (pronounced “zero dee bee eye”). Imagine a giant stellar sized mirror beside our sun, and how it would change this energy distribution and give it directivity. With such an imaginary mirror, one half of our solar system would be dark (behind and shadowed by the mirror). The other half would be twice as bright (seeing the sun plus it’s reflection). Mirrors or lenses have the appearance of intensifying energy in some preferred directions by stealing and redirecting it from disadvantaged directions. Antennas do the same thing. This is called gain. Please remember, no new energy is created, it is simply redirected or given directionality (directivity). The amount of intensification in a preferred direction is quantified as gain. Thus a mirror can redirect half of the energy from the sun (or a candle), and make it look twice as bright (i.e. two candles). It is said to have a gain of 2x (times two) or doubling. We also answer the age often asked question: “Is More Antenna Gain Better?

The Logarithmic dB Scale

Antenna engineers use a logarithmic scale to express this apparent 2x (times two) mirror power doubling as +3 dBi. It still means “doubling”. Here are some other examples of ratios or multipliers on the engineering dB log scale.

Gain in dBi

What It Really Means

-10 dBi
One tenth , 1/10, or "10 % of" (loss, not gain)
-6 dBi
One quarter, 1/4, or "25 % of" (loss, not gain)
-3 dBi
One half, 1/2, or "50% of" (loss, not gain)
0 dBi
No gain, "same", 100% (no gain, no loss)
+1 dBi
12% higher, times 1.12, or 112%
+2 dBi
58% higher, times 1.58, or 158%
+3 dBi
100% higher, times 2, "double", or 200%
+6 dBi
300% higher, times 4
+9 dBi
Times 8 (% scale in not useful for large multiples)
+10 dBi
Times 10 (% scale in not useful for large multiples)
+13 dBi
Times 20 (% scale in not useful for large multiples)
+20 dBi
Times 100 (% scale in not useful for large multiples)

 

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