Handy ISM Band Propagation Aids
The decibel is a handy way to calculate loss and gain in RF systems. Instead of messing with times 10 type calculations, one simply converts gains to dB and adds them up. This is why almost all RF specs are given in dB. Antenna gain is given in dBi but can be thought of as dB. Cable loss is given in dB/100 feet or dB/100 meters and can also be added to a system gain equation.
dB = 10log10(Ratio to a standard)
dBm is a handy power measurement for ISM radios that generally have less than one watt output. dBm is power referenced to one milliwatt.
dBm = 10log10(power/1mW)
Decibels have some handy properties. Adding 3 dB increases the result by 2 times. Subtracting 3 dB decreases the result by 1/2.
Example: 10 dBm + 3 dB is 13 dBm or double the power
If you have ever looked at antenna specifications you may have noticed that have a gain given in decibels. This makes it easy to add a calculation for the antenna (or any gain or loss) to the power of your radio. Look ahead to Link Budget for examples.
Free Space Loss
Free space loss is the loss purely due to distance from the transmitter. It can be approximated by the following handy equation when the units are MHz and kilometers.
Free Space Loss(FSL) in dB = 20log(distance from transmitter in km) + 20log(freq in MHz) + 32.44dB
Example with a distance of 100 meters and frequency of 2.4 GHz:
FSL = 20log(.100) + 20log(2400) + 32.44
Table 4: Free Space Loss for 2.4 GHz at sample distances in Meters
Link budget calculations can be simple or complex. An adequate calculation takes into account transmitter gain, line loss, transmitting antenna gain, free space loss, receiving antenna gain, line loss, and receiver sensitivity.
ISM/UNII Band Power Limits (under construction)
Choice of equipment
Please note that other users in your area may not appreciate use of an omni-directional antenna even if you think that your application calls for it. Dont assume that there are no other user of an ISM band in your area. Even in rural areas (such as where we are located in Bozeman MT), the ISM bands are used extensively by the local ISPs and utilities companies. Most areas have a volunteer frequency coordinator that helps find open sections of spectrum and has a list of used spectrum. In any radio link design using unlicensed frequency, interference reduction by antenna directionality and polarization is prudent and probably required.
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