User guide

RLX-IH ♦ 802.11b Reference
Industrial Hotspot
Page 78 of 99 ProSoft Technology, Inc.
April 10, 2007
6.4.1 Antenna pattern
Information between radios is transferred via electromagnetic energy radiated by
one antenna and received by the second. More power is radiated in certain
directions from the antenna than others, a phenomenon known as the antenna
pattern. Each antenna should be mounted so the direction of strong radiation
intensity points toward the other antenna(s) to which it is linking.
Although complete antenna patterns are three-dimensional (3D), a two-
dimensional (2D) slice of the pattern is often shown because the antennas of
interest are often located horizontally from one another, along the ground rather
than above or below each other.
A slice taken in a horizontal plane through the center (or looking down on the
pattern) is called the azimuth pattern. A vertical plane slice, which is seen from
the side, is the elevation pattern.
An antenna pattern that has equal or nearly equal intensity in all directions is
omnidirectional. In two dimensions, an omnidirectional pattern is a circle. An
antenna is considered omnidirectional if one of its 2D patterns is omnidirectional.
(No antenna has an omnidirectional pattern in 3D.)
Beam width is an angular measurement of how strongly the power is
concentrated in a particular direction. Beam width is a 3D quantity, but it can be
broken into 2D slices just like the antenna pattern. The beam width of an
omnidirectional pattern is 360°, because the power is equal in all directions.
6.4.2 Antenna gain
Antenna gain is a measure of how strongly an antenna radiates in its direction of
maximum radiation intensity compared to how strong the radiation would be if the
same power were applied to an antenna that radiated all of its power equally in
all directions. Using the antenna pattern, the gain is the distance to the furthest
point on the pattern from the origin. For an omnidirectional pattern, the gain is 1,
or equivalently 0 dB. The higher the antenna gain is, the narrower the beam
width, and vice versa.
The amount of power received by the receiving antenna is proportional to the
transmitter power multiplied by the transmit antenna gain, multiplied by the
receiving antenna gain. Therefore, the antenna gains and transmitting power can
be traded off. For example, doubling one antenna gain has the same effect as
doubling the transmitting power. Doubling both antenna gains has the same
effect as quadrupling the transmitting power.
6.4.3 Antenna polarity
Antenna polarization refers to the direction in which the electromagnetic field
lines point as energy radiates away from the antenna. In general, the polarization
is elliptical. The simplest and most common form of this elliptical polarization is a
straight line, or linear polarization. Of the transmitted power that reaches the
receiving antenna, only the portion that has the same polarization as the
receiving antenna polarization is actually received. For example, if the