User Manual

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2009 Industrial Wireless Guidebook
Understanding Industrial WLAN – IEEE 802.11
2
Transmit Power and Received Sensitivity
When a radio signal is being transmitted through the air, it will experience a great loss in signal strength
caused by attenuation introduced by free space. Therefore, when evaluating a wireless system, one needs
to be aware of the signal power level at the transmitter end and at the receiver end. The signal power
received cannot be so weak as to break the communication link, or too strong as to saturate the receiver’s
amplifiers.
These concerns call for estimating the “power budget” of a wireless system. By making a power budget
estimation, you will have an idea of how far you can extend your wireless link without losing communication.
Please note that the following calculations are pure theoretical estimations that are not meant to guarantee
communication distance. There are many other factors involved that will affect transmission distance.
dBm Watt dBm Watt
+40dBm 10W +12dBm 16mW
+30dBm 1W +9dBm 8mW
+20dBm 100mW +6dBm 4mW
+10dBm 10mW +3dBm 2mW
0dBm 1mW 0dBm 1mW
-10dBm 100uW -3dBm 500uW
-20dBm 10uW -6dBm 250uW
-30dBm 1uW -9dBm 125uW
-40dBm 100nW -12dBm 62.5uW
The dB is a unit of relative quantity, which means it is merely a multiplication factor used to represent the gain
or loss of signal power. A useful rule of thumb is an addition or subtraction of 3 dB is equivalent to a multiple of
2 or 0.5. An addition or subtraction of 10 dB is equivalent to a multiple of 10 or 0.1.
In dealing with antenna gain specifications, the gain factor is often represented by “dBi”. The “i” stands for
“isotropic”, which means the gain is relative to an isotropic radiator (i.e., a radiating sphere in space). This
ideal radiation is impossible to realize but
its pattern is the reference for all realizable
antennas. The gain of a passive antenna is
measured by how effectively the antennas
can focus the energy (how narrow is the
antenna angle), rather than the actual boost
in transition power. Therefore, the narrower
the antenna angle, the higher the antenna
gain. The diagram below shows the antenna
angles of a high and low gain antenna.
f is the frequency in mHz, pt and pr in dBm, and gt and gr in dBi, which are easier to obtain from product
specifications. To get the effective range d in km, all we have to do is plug in the values for pt, pr, gt, gr, and
f.
Pt : Transmit Power
Gt, Gr: Antenna Gain
Pr: Sensitivity
The following table shows some common conversion values between
dBm and mW: