95/TX Operation & Maintenance Manual, Revision 1.0
Page 33
13
WIRELESS RANGE CALCULATIONS
In order for reliable wireless communication to occur, transmitter power output, antenna ‘gain’ and
receiver sensitivity must exceed the ‘loss’, or attenuation experienced by radio signals as they travel
between transmitter and receiver. RF signals experience two kinds of ‘loss’ as they travel through space.
“Free space loss” is due strictly to the fact that RF signals ‘spread out’ as they propagate from a single
point source. Additionally, RF signals experience loss as a result of interference and interaction with
buildings, trees, structures and other physical barriers.
This gain or loss is measured in decibel-milliwatts (dBm)
and is a function of both distance and frequency. An
estimate of free space loss for 900 MHz and 2.4 GHz is
shown at right.
EXAMPLE POINT TO POINT GAIN CALCULATION
In a typical 2.4 GHz system shown below, for example, the transmitter power output is +21 dBm, each
antenna provides an additional gain of +3 dBm and the receiver sensitivity allows reliable reception of a
signal at -95 dBm power. Therefore, the total system gain is 21+3+3-(-95) =
122 dBm.
Figure 13-1: RF Transmission Margin
Since a reliable system requires a gain margin of +20 dBm, this system will work acceptably if the total
free space loss is less than
102 dBm
(122 dBm – 20 dBm). However, this theoretical calculation does not
take into account additional loses due to intervening structures or foliage.
WAYS TO MINIMIZE RF SIGNAL LOSS
One way to minimize free-space loss is to ensure that the combined height of the transmitter antenna and
receiver antenna exceeds the diameter of the “RF Transmission Zone” (see Fig. 13-2) and that at least 60%
RF Power
+21 dBm
Antenna
Gain = +3 dBm
Antenna
Gain = +3 dBm
Receiver
Sensitivity
-95 dBm
Free Space Loss @ 900 MHz:
-71 dBm at 300 ft., -91 dBm at 3000 ft.
Free Space Loss @ 2.4 GHz:
-80 dBm at 300 ft., -100 dBm at 3000 ft.
