Understanding RF Power Values
RF signals are subject to various losses and gains as they pass
from a transmitter through the cable to its antenna, then through the air (and
other obstructions such as walls and doors), to the receiving antenna, through
that cable, and finally to the receiving radio. With the exception of the walls
and other obstructions, most of these signal-loss factors are known and can be
used in the design process to determine whether an RF system such as a WLAN will
work. To understand how to evaluate systems, a good understanding of how RF
parameters are measured is important. The following sections discuss measurement
values such as decibels and RF power, as well as antennas, cables, and RF
propagation in a WLAN environment.
Decibels
The decibel (dB) scale
is a logarithmic scale used to denote the ratio of one power value to
another:
An increase of 3 dB indicates a doubling (2x) of power. An
increase of 6 dB indicates a quadrupling (4x) of power. Conversely, a decrease
of 3 dB is a halving (1/2) of power, and a decrease of 6 dB is a quarter (1/4)
the power. Table 2-3 shows some
examples.
Table 2-3. Decibel Values and Corresponding
Factors
|
Increase |
Factor |
Decrease |
Factor |
|
0 dB |
1x (same) |
0 dB |
1x (same) |
|
1 dB |
1.25x |
1 dB |
0.8x |
|
3 dB |
2x |
3 dB |
0.5x |
|
6 dB |
4x |
6 dB |
0.25x |
|
10 dB |
10x |
10 dB |
0.10x |
|
12 dB |
16x |
12 dB |
0.06x |
|
20 dB |
100x |
20 dB |
0.01x |
|
30 dB |
1000x |
30 dB |
0.001x |
|
40 dB |
10,000x |
40 dB |
0.0001x |
Power Ratings
The transmitter power rating of most WLAN equipment is usually
specified in decibels compared to known values such as mW or watts.
Transmit power and receive sensitivity are specified in dBm,
where m means 1 milliWatt (mW). A value of 0 dBm is equal to 1 mW. From
there you can use the previously mentioned 3-dB rule and calculate that 3 dBm is
equal to 2 mW, 6 dBm is equal to 4 mW, and so on. For example, a radio with a
rating of 100-mW transmit power is equal to a radio specified at 20-dBm transmit
power.
Common mW values to
dBm values are shown in Table
2-4.
Table 2-4. Common mW to dBm Values
(Approximate)
|
dBm |
mW |
dBm |
mW |
|
0 dBm |
1 mW |
0 dBm |
1 mW |
|
1 dBm |
1.25 mW |
1 dBm |
0.8 mW |
|
3 dBm |
2 mW |
3 dBm |
0.5 mW |
|
6 dBm |
4 mW |
6 dBm |
0.25 mW |
|
7 dBm |
5 mW |
7 dBm |
0.20 mW |
|
10 dBm |
10 mW |
10 dBm |
0.10 mW |
|
12 dBm |
16 mW |
12 dBm |
0.06 mW |
|
13 dBm |
20 mW |
13 dBm |
0.05 mW |
|
15 dBm |
32 mW |
15 dBm |
0.03 mW |
|
17 dBm |
50 mW |
17 dBm |
0.02 mw |
|
20 dBm |
100 mW |
20 dBm |
0.01 mW |
|
30 dBm |
1000 mW (1 W) |
30 dBm |
0.001 mW |
|
40 dBm |
10,000 mW (10 W) |
40 dBm |
0.0001 mW |
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