Different Kinds of Antennae
Antennae have a variety of properties that define their operation and performance characteristics.
Frequency: Each antenna is tuned to a specific frequency by virtue of its size. You must match the frequency of the radio with the size of the antenna.
Power: Antennae are rated with a maximum power capacity (in terms of watts). Note, however, that given the relatively small power output of a typical WLAN radio, this is hardly ever a consideration. Most antennae are rated to withstand several watts of power, whereas most WLAN applications operate in the 30-100 milliwatt (mW) range.
Direction: Each antenna shapes the energy field in a particular direction. This pattern is often plotted in multiple dimensions to help visualize the coverage area that the antenna provides. Antennae are categorized according to this pattern, such as omnidirectional or directional.
Gain: Antennae use a rating system to describe the amount of signal that the antenna transmits or receives. Gain is measured in dB, short for decibels. The bel (B) is named after Alexander Graham Bell. A decibel is 10 times the logarithm (base 10) of one power level divided by another power level, 10(log10[Power X/Power Y]), where X represents the output power and Y represents the input power. Therefore, decibels represent a ratio between two power values. In order for this information to be meaningful, most manufacturers rate antennae by how well they perform compared to an isotropic antenna (dBi) or relative to 1 milliwatt (dBm).