GASMAX/TX Operation & Maintenance Manual, Revision 1.0
Page 15
TRANSMISSION DISTANCE
In order for reliable wireless communication to occur, transmitter power output, antenna ‘gain’ and
receiver sensitivity must exceed the ‘free space loss’ attenuation experienced by radio signals as they
travel between transmitter and receiver. This gain or loss is measured in decibel-milliwatts (dBm) and is a
function of both distance and frequency. Typical free space loss at 900 MHz is -71 dBm at 300 ft / 100
meters and -91 dBm at 3000 ft / 300 meters, and for 2.4 GHz the free space loss is -80 dBm at 300 ft/ 100
meters and -100 dBm at 3000 ft / 1000 meters.
Figure 5-2: RF Transmission Margin
In a typical 2.4 GHz system shown above, for example, the transmitter power output is +21 dBm, each
antenna provides an additional gain of +3 dBm and the receiver sensitivity is -95 dBm. Therefore, the total
system gain is 21+3+3-(-95) = 122 dBm. Since a reliable system requires a margin of +20 dBm, this system
will work if the free space loss between the transmitter and receiver is less than 102 dBm, suggesting a
distance of approximately 1000 meters. However, this calculation does not take into account additional
loses due to intervening structures or foliage.
To minimize free-space loss, the combined height of the transmitter antenna and receiver antenna should
exceed the diameter of the “RF Transmission Zone” (see Fig. 5-3) and at least 60% of the area inside the
zone should be free of obstacles. Although both 900 MHz and 2.4 GHz signals can travel around or
through most towers or buildings, structures such as large metal tanks or solid metal buildings may
attenuate the signal to the point where reception is marginal.
Raising the antenna at either end of the path will improve signal strength and reduce transmission errors.
GDS Corp recommends placing the GASMAX/TX at least 5 feet above the surrounding terrain, and even
more if possible. However, note that the standard antenna transmits its maximum signal strength in a
relatively flat ‘donut-shaped’ pattern which may affect the performance of GASMAX/TX monitors located
in close proximity to an elevated C2/TX Site Manager or receiver/controller.
RF Power
+21 dBm
Antenna
Gain = +3 dBm
Antenna
Gain = +3 dBm
2.4 GHz,
1000m
Free Space
Loss =
-102 dBm
Receiver
Sensitivity
-95 dBm
