User's Manual

Figure 10-8: Antenna pattern.
The pattern shown in Figure 10-8 has 20 dB more gain in the forward direction
(270 degrees) than in any other direction. The side lobes occur at various
angles, but only two exceed the 0 dB circle on the graph (at 235 and 315
degrees). This antenna is said to have a “gain” of 20 dBi, that is, 20 dB relative
to an isotropic radiator.
This pattern also shows the Beamwidth of the antenna. This is usually stated as
the distance between the half-power points, or 3 dB points. From the graph, the
antenna beamwidth is 14 degrees.
Antennas also have vertical directionality, called elevation. It looks similar to
Figure 10-8.
10.7 Rain Fade
There are many causes for a radio signal to fade. The only one of concern here
is rain, and we will see that even that mechanism is negligible.
The concept is that if a certain amount of rain happens, then the RF is attenuated
by a few decibels, which reduces the Fade Margin of the link. If this happens a
percentage of the time, then one can predict the Outage of the link, hence
Availability.
ARCXtend manual, August 2003 10-9