Campbell Scientific RF401-series and RF430-series Spread Spectrum Data Radios/Modems - Manual (Page 85)

Appendix H. Distance vs. Antenna Gain, Terrain, and Other Factors
Where:
Pt => transmitter output power, in dBm (20 dBm in the case of the RF401
or RF411)
Lt => cable loss between transmitter and antenna in dB (see Cable Loss section)
Gt => transmit antenna gain in dBi (dBi = dBd + 2.15)
Lp => path loss between isotropic antennas in dB (see TABLE H-1, TABLE
H-2)
Gr => receive antenna gain in dBi
Lr => cable loss between antenna and receiver in dB
Pr => signal power at the radio receiver in dBm
The signal power at the receiver (Pr) must exceed the receiver sensitivity
(−110 or –104 dBm) by a minimum of 6 dB for an effective link. The amount
that Pr exceeds –110 dBm or –104 dBm (2.4 GHz) is the link margin.
All of these elements are known, or are easily determined, with the exception
of Lp. Unfortunately, signal path loss can make the difference between a
marginal link ½ mile apart, and a reliable link 10 miles apart!
Transmitter Power
Transmitter output power is often expressed in dBm, which is a decibel power
rating relative to 1 milliWatt. The formula is: dBm = 10 log (Pt) with Pt
expressed in milliWatts.
Transmitter Power (Pt)
(milliWatts)
dBm
1 0
10 10
50 (RF416, RF432) 17
100 (RF401, RF411, RF430, RF431) 20
1000 30
5000 37
Cable Loss
Cable loss is a function of cable type, length, and frequency and is usually
specified as attenuation (dB) per 100’ of cable. Using a low loss cable
becomes very important as the cable run distances increase. Here are some
typical cable types and their properties:
Cable Type Outside Diameter Loss (dB/100’) @ 900 MHz Loss (dB/100’) @ 2.4 GHz
RG-58A/U .195” 21.1
COAX RPSMA-L .195” 11.1 18.8
RG-8 .405” 6.9
COAX NTN-L .405” 4.5 8.1
LMR-400 .405” 3.9 6.7
*CSI stocked antenna cables are shaded.
H-3