User's Guide

Appendix A. Radio Signal Propagation
User's Guide A-2 BreezeNET BU-DS.11/RB-DS.11
P
dBm
= 10 x Log P
mw
For example: 1 Watt = 1000 mW; P
dBm
= 10 x Log 1000 = 30 dBm
100 mW; P
dBm
= 10 x Log 100 = 20 dBm
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:
Figure A-2. Attenuation of an RF signal
P
in
is the incident power level at the attenuated input
P
out
is the output power level at the attenuated output
Attenuation is expressed in dB as follows: P
dB
= -10 x Log (P
out
/P
in
)
For example: If, due to attenuation, half the power is lost (P
out
/P
in
= 1/2),
attenuation in dB is -10 x Log (1/2) = 3
dB
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
Free Space Loss
Attenuation of the electromagnetic wave while propagating through space. This attenuation is
calculated using the following formula:
Free space loss = 32.4 + 20xLog(F
Mhz
) + 20xLog(R
Km
)
F is the RF frequency expressed in Mhz.
R is the distance between the transmitting and receiving antennas.