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Thorn · 07-30-2002

Q: What is all this talk about dB and deciBels. I thought radio power is expressed in Watts?
Q: Why are deciBels expressed in negative numbers in NetStumbler?
Q: Why doesn't SNR equal Signal/Noise?

A: Decibels (or dB) is a REFERENCE value only. You have to reference it to a known value such as power in watts, milliwatts or voltage in volts, millivolts and microvolts. For power, dB
is expressed as:

dB = 10 * log10(p)

Now with that in mind:

Transmitters, such as those used in commercial microwave transmitters are referenced to 1 milliwatt. So

0 dBm = 10 log10(1 mW)

therefore a transmitter of 1 watt would be a +30 dbm transmitter, a 1/2 watt transmitter would be +27 dbm or a 0 dbm transmitter would be 1 milliwatt.

0 dbm= 1 milliwatt
10 dbm= 10 milliwatts
20 dbm= 100 milliwatts
30 dbm= 1000 milliwats or 1 watt

Received signal strength reads negative numbers because of the free space and path loss of the signal; a negative number simply means the power is less than 1mW. Thus, a negative dB value represents a loss, and a positive one represents a gain.

The noise floor is the signal strength of the background noise, which is caused by man made devices and nature, such as the sun.

Your signal to noise ratio (SNR) is how high your signal is above that noise floor. So if your noise floor is -92dB and your signal is -82dB, then your SNR is 10. Note that the SNR is obtained by subtracting the noise value from the signal instead of dividing because the signal and noise are expressed in dB. Thus:

SNR = S(mW)/N(mW) = S(dBm) - N(dBm)

Questions posted by many people. Answered by mshimek
Edited for the FAQ by Thorn

07-30-2002	#2 (permalink)
Thorn Did you do the math? Join Date: Apr 2002 Location: Villa Straylight Posts: 10,361	deciBels (dB) Q: What is all this talk about dB and deciBels. I thought radio power is expressed in Watts? Q: Why are deciBels expressed in negative numbers in NetStumbler? Q: Why doesn't SNR equal Signal/Noise? A: Decibels (or dB) is a REFERENCE value only. You have to reference it to a known value such as power in watts, milliwatts or voltage in volts, millivolts and microvolts. For power, dB is expressed as: dB = 10 * log10(p) Now with that in mind: Transmitters, such as those used in commercial microwave transmitters are referenced to 1 milliwatt. So 0 dBm = 10 log10(1 mW) therefore a transmitter of 1 watt would be a +30 dbm transmitter, a 1/2 watt transmitter would be +27 dbm or a 0 dbm transmitter would be 1 milliwatt. 0 dbm= 1 milliwatt 10 dbm= 10 milliwatts 20 dbm= 100 milliwatts 30 dbm= 1000 milliwats or 1 watt Received signal strength reads negative numbers because of the free space and path loss of the signal; a negative number simply means the power is less than 1mW. Thus, a negative dB value represents a loss, and a positive one represents a gain. The noise floor is the signal strength of the background noise, which is caused by man made devices and nature, such as the sun. Your signal to noise ratio (SNR) is how high your signal is above that noise floor. So if your noise floor is -92dB and your signal is -82dB, then your SNR is 10. Note that the SNR is obtained by subtracting the noise value from the signal instead of dividing because the signal and noise are expressed in dB. Thus: SNR = S(mW)/N(mW) = S(dBm) - N(dBm) Questions posted by many people. Answered by mshimek Edited for the FAQ by Thorn __________________ Thorn "Read Altas Shrugged. Compare it to today. Repeat as necessary" Last edited by lincomatic : 07-30-2002 at 05:47 PM.