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RF Power Level
RF power level at either the transmitter output or the receiver input is expressed in
Watts. It can also be expressed in dBm. The relation between dBm and Watts can be
expressed as follows:
P
= 10 x Log P
dBm
For example:
1 Watt = 1000 mW; P
For link budget calculations, the dBm convention is more convenient than the Watts
convention.
Attenuation
Attenuation (fading) of an RF signal is defined as follows:
P
is the incident power level before attenuation
in
P
is the output power level after attenuation
out
Attenuation is expressed in dB as follows:
For example: If, due to attenuation, half the power is lost (P
attenuation in dB is -10 x Log (1/2) = 3
Path Loss
Loss of power of an RF signal traveling (propagating) through space. It is expressed
in dB. Path loss depends on:
The distance between transmitting and receiving antennas
Line of sight clearance between the receiving and transmitting antennas
Antenna height
mw
100 mW; P
Figure 28: Attenuation of an RF signal
= 10 x Log 1000 = 30 dBm
dBm
= 10 x Log 100 = 20 dBm
dBm
P
= -10 x Log (P
dB
dB
58
/P
)
out
in
/P
= 1/2),
out
in
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