User's Guide

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It measures signal strength relative to some type of reference signal. Decibel measure-
ments are often referenced to the following values:
• The signal at its source
• The center point in the cone radiating from a directional antenna at any dis-
tance from its source.
• One milliwatt. dBm is the unit of measure used when signal strength is mea-
sured in decibels relative to one milliwatt.
• An antenna with no gain. dBi is the unit of measure used when signal strength
is measured in decibels relative to a fictitious isotropic antenna with no gain.
The decibel is a logarithmic measure calculated in base-ten. At 3 dB, a signal has half
the strength of the center lobe signal (not one-third the strength), and a gain of six dBi
is four times stronger than the signal from an antenna with no gain (not six times
stronger).
EIRP. Effective Isotropic Radiated Power (EIRP) is a measure of source signal strength
that combines broadcasting power (measured in watts) with the effect of antenna signal
gain (measured in dBi).
Fading. Temporary reductions in received signal strength are known as fading. Fading
normally results from intermittent interference. Fade margin is a margin of extra signal
strength built into the system to compensate for fading. For terrestrial links, the most
common cause of fading is multipath (deflected signals arriving out of phase) and foliage
within the signal path.
Interference. Interference refers to anything that blocks or degrades an RF signal.
There are three basic forms of interference:
• Atmospheric interference can result from sky noise or lightning in the signal
path.
• RF interference is generally caused by nearby activity in the same RF band (in-
band interference). Also very strong out-of-band activity can interfere with an
RF signal. Any wireless equipment can be a potential source of interference.
• Objects in the signal path can cause interference. The bending of signals as
they pass around obstructions or are deflected by them is known as diffraction.
Diffraction can damage signal quality if the diffracted signals arrive out of phase.
A glass window can attenuate the signal to some extent. Some types of mir-
rored glass can almost totally block the signal. Signals passing through the side
of a wooden building or a forest are also attenuated. Wet leaves can affect a
signal substantially. The success of an RF link depends on a clear line of sight.