Cisco Cisco Aironet 1260 Access Point Libro blanco

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Think of gain as a description of the distribution of the energy radiated by the antenna. It can’t be stressed enough

that antennas do not “create” power; they merely distribute it over a surface. In general, if the gain is high, the area

over which the power is distributed will be low (at least in one plane). So high gain typically implies narrow

beamwidth.

The gain of an antenna (in any given direction) is defined as the ratio of the power gain in a given direction to the

power gain of a reference antenna in the same direction. It is standard practice to use an isotropic radiator as the

reference antenna in this definition. Note that an isotropic radiator would be lossless and that it would radiate its

energy equally in all directions. That means that the gain of an isotropic radiator is G = 1 (or 0 dB). It is customary

to use the unit dBi (decibels relative to an isotropic radiator) for gain. Gain expressed in dBi is computed using the

following formula:

GdBi = 10*Log (GNumeric/GIsotropic) = 10*Log (GNumeric)

Notice that this definition does not describe a peak value or the gain in any particular direction. The gain describes

the distribution of power over all angles. It is a 3D concept. So a peak gain of 7 dBi indicates that this antenna

directs 7 dB more of the available power in a single direction an isotropic radiator would as shown in Figure 2. This

fictitious isotropic radiator would radiate all its available power equally in all directions and its 3D radiation pattern

would be a sphere. Notice that an isotropic radiator would fit the definition of an omnidirectional antenna, but not

all omnidirectional antennas would fit the definition of isotropic antennas. An isotropic antenna is a “concept”

rather than a description of a real antenna.

Occasionally, a theoretical dipole is used as the reference; in this case, the unit dBd (decibels relative to a dipole)

is used to describe the gain. This unit tends to be used when referring to the gain of omnidirectional antennas of

higher gain, especially in land-mobile-radio and amateur radio circles. In the case of these higher gain

omnidirectional antennas, their gain in dBd would be an expression of their gain above 2.2 dBi. So if an antenna

has a gain of 3 dBd, it also has a gain of 5.2 dBi.

If you look at Figure 2 again, you’ll see that the “gain” of that antenna is about 13 dBi, judging from the peak of the

pattern. Frequently, patterns are shown normalized to the peak gain. This normalization often makes it easier to

determine side lobe levels, front-to-back ratios, and other antenna parameters by inspection from the normalized

patterns. If the antenna patterns from Figure 2 are normalized to the peak gain, the patterns shown in Figure 3 are

the result. The patterns look the same and are shown on a 40-dB scale as before, but the value of the side lobe

levels and other parameters are more obvious.

Figure 3. Normalized Antenna Patterns from Cisco Aironet 1310 Outdoor Access Point/Bridge, a 2 x 2 Array