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.