User's Manual

302395 User’s Guide and Repeater Network Design Guide
16 February 2004
3.3.2 Relative Gain
As shown in Figure 4, the strength of an antenna’s signal varies as you move around the antenna’s signal
pattern. You might measure a signal a certain distance from an antenna where the signal is at its peak
power, and then measure the signal at another location the same distance from the antenna where the
signal might be only half as powerful as at the first location.
This difference in signal strength, or gain, is measured in decibels.
3.3.2.1 dB (Decibels)
When comparing antenna gains where one of the signals is not a laboratory standard, “decibel” is
abbreviated as “dB.” (For use of the “dBi” abbreviation for an antenna’s gain specification, see section
3.1.2.1.) Typically, when measuring signal strength in an antenna’s pattern, you compare your readings to
the antenna’s peak signal rather than to a laboratory standard.
A graph of an antenna’s pattern uses the decibel scale to indicate how an antenna’s gain varies across the
pattern.
Figure 5 Antenna Pattern dB Scale
Figure 6 shows a horizontal antenna pattern. The outermost ring signifies 100 percent signal power, or 0
(zero) percent signal loss. The next outermost circle (labeled —5) shows the region where the signal
strength has dropped 5 dB from the original 100 percent (0 dB) level. The next circles show where the
signal has dropped off 10 dB and 15 dB from the 0 dB level.
Note: A negative (—) dB value indicates a reduction in signal power.
Full radio signal power is obtained at 0 degrees, where there is 0 dB signal drop off. As you move to
either side of 0 degrees the signal power starts to drop off, and crosses the —5 dB point, the —10 dB
point, the —15 dB point, and finally the —20 dB point.