Tutorial of RF Propagation for 2.4 GHz

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

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:
1
2
3
4

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.