Campbell Scientific RF500M Radio Modem - Manual (Page 31)

Appendix B. Power Calculations
There must be enough transmission power in any RF link to complete
communication. The sources of power are the radio and the antennas.
Conversely, power is lost both through the cables (coax loss) and over the
distance of communication (path loss). The power of the signal received
(Signal Power) can be calculated as stated below.
The signal power must be greater than -95 dBm (-80 dBm @ 2.4K baud) to
have a good radiotelemetry link. Decibel milliwatts (dBm) is a scale of power,
0 dBm represents one milliwatt of power. The lower limit of power for good
data transmission is approximately 0.0000000000003 Watts (3X10-13), which
represents -95 dBm.
Signal Power
SP = TP + AG - PL - CL
where, SP = Signal Power (dBm) Power of the signal received,
TP = Transmit Power (dBm) Rated output power of
transmitting radio,
PL = Path Loss (dB) Power lost over the distance of
communication (calculated below),
AG = Antenna Gain (dB) Total power gained by both the
transmit and receive antennas,
and, CL = Coax Loss (dB) Total power lost through both lengths
of cable connecting the transmit and receive radios to
the antennas.
Path Loss
PL = 36.6 + 20*Log(F) + 20*Log(D)
where, PL = Path Loss,
F = Frequency (MHz),
and, D = Distance (miles).
Coaxial Cable Loss
Typical coaxial cable losses are listed below.
200 MHz 400 MHz
Cable Type Loss/100 ft.
RG-8A/U 3.1 dB 5.0 dB
RG-58A/U 6.2 dB 9.5 dB
Transmit Power
5 Watt Radio 36.99 dBm
4 Watt Radio 36.02 dBm
B-1