Handy ISM Band Propagation Aids

The Decibel

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

 Power(mW) Power(dBm) 10 10 100 20 1000 30

Table 1:
Easy to Memorize Conversions
of Power from mW to dBm

 Power(dBm) Power(mW) 10 10 13 20 16 40 19 80 22 160 25 320

Table 2:
Working up from 10 mW
by 3 dB

 Power(dBm) Power(mW) 30 1000 27 500 24 250 21 125

Table 3:
Working down from 1000
mW by 3 dB

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
= -20 + 67.60 + 32.44
= 80 dB

 Distance(meters) Free Space Loss(dB) 1 40 dB 10 60 dB 100 80 dB 1000 100 dB

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)

 Band Frequency (MHz) Max Xmitter Power Max Antenna Gain EIRP Notes 6.7 MHz 6.765 to 6.795 13 MHz 13.553 to 13.567 26 MHz 26957-27283 40 MHz 40.66 to 40.70 430 MHz 433.05 to 434.79 This band is usable in Region 1 (Europe) only. 900 MHz 902 to 928 This band is usable in Region 2 (Americas) only. 2.4 GHz 2400 to 2483.5 30 dB 6 dB 36 dB Non directional antenna, non point-to-point, non fixed 2.4 GHz 2400 to 2483.5 30 dB 6 dB 36 dB Directional antenna, point-to-point, fixed. Reduce xmitter power 1 db for every 3 dB increase in antenna gain over 6 dB. 29 dB 9 dB 38 dB  28 dB 12 dB 40 dB  27 dB 15 dB 42 dB  26 dB 18 dB 44 dB  5.2 GHz 5250 to 5350 5.4 GHz 5470 to 5725 5.8 GHz 5725 to 5825 5.8 GHz 5725 to 5825 24 GHz 24000 to 24250 60 GHz 57000 to 64000 122 GHz 12200 to 12300 244 GHz 24400 to 24600

References:
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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. Dont 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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