Antennas
Gain

The gain of any antenna is essentially a measurement of how well that antenna focuses radiated RF energy, in a particular direction. There are different methods for measuring this, depending on the reference point chosen. To ensure a common understanding, Cisco is standardizing on dBi to specify gain measurements. This method of measuring gain uses a theoretical isotropic antenna as a reference point. Some antennas are rated in dBd, which uses a dipole type antenna, instead of an isotropic antenna as the reference point. Remember, to convert any number from dBd to dBi, simply add 2.14 to the dBd number. Figure summarizes the basics of antenna gain.

High-gain antennas direct energy more narrowly and precisely. Low gain antennas direct energy in a broader, wider pattern. With dish-type antennas, for example, operation is similar to the operation of the reflector on a flashlight. In this example, the reflector concentrates the output of the flashlight bulb into one predominant direction to maximize the brightness of the light output in that direction. Very little light goes in other directions. This principle also applies to any gain antenna, as there is always a tradeoff between gain, which is comparable to brightness in a particular direction, and beamwidth, which is comparable to the narrowness of the beam. Therefore, an antenna gain and its radiation pattern are fundamentally related. Higher gain antennas always have narrower beamwidths or patterns. Lower gain antennas always have wider beamwidths. Figure illustrates this relationship. Figure shows the relationships between antenna gain and size or frequency.

Many access points come with omnidirectional antennas that provide basic coverage. To extend the transmission range, a higher gain antenna should be used. The choice will depend on the range and the coverage desired.

Due to U.S. Federal Communication Commission (FCC) laws shown in Figure , each WLAN vendor must use different connectors for their antennas. This helps to ensure that antennas are designed to work with the WLAN equipment. Cisco Systems, Inc. uses the RP-TNC connector, as described in Figure .

When using 802.11a 5 GHz equipment, external antenna options are limited. The U.S. FCC restricts the usage of this unlicensed U-NII band, which includes a total of 300 MHz of spectrum, from 5.15 to 5.825 GHz. U-NII-1 includes the frequencies between 5.15 to 5.25 GHz. It is for indoor access only, using a fixed antenna. U-NII-2 goes from 5.25 to 5.35 GHz and is for indoor or outdoor use, with a flexible antenna. U-NII-3 goes from 5.725 to 5.825 and is only for outdoor bridging applications. Different restrictions apply in Europe for HiperLAN.


Web Links