Campbell Scientific RF300-Series DRL VHF/UHF Radio Transceivers - Manual (Page 43)

C-1
APPENDIX C. 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
Power Conversion
Conversion of Watts to dBm can be done with the following formula.
dBm = 10 * Log((Watts)/0.001)