What is the difference between db, dbi and dbm?

Endorsed Question
Aug 19th, 2002

Hi,

I always see db, dbi and dbm in documentation about wireless but I've never seen, what those are doing exactly.

I would understand, What does mean db, dbi and dbm?

Thanks

I have this problem too.
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Endorsed by Scott Fella
manjeets about 11 months 2 weeks ago

•        

Desible:-

         Decibels – If a receiver is very sensitive to RF signals,  it may be able to pick up signals as small as 0.000000001 Watts.

•         A logarithm  is the exponent to which the number 10 must be raised to reach some  given value. If we are given the number 1000 and asked to find the  logarithm (log), we find that log 1000 = 3 because 10^3 = 1000. Notice  that our logarithm, 3, is the exponent. An important thing to note about  logarithms is that the logarithm of a negative number or of zero does  not exist.

•         Log (-100) =  undefined!

•         Log (0) =  undefined!

•         Decibels  are a relative measurement unit unlike the absolute measurement of  milliwatts.

dBM:-

•         1 mW = 0  dBm

•         The m in  dBm refers simply to the fact that the reference is 1 milliwatt (1 mW)  and therefore a dBm measurement is a measurement of absolute power.

•         The  relationship between the decibels scale and the watt scale can be  estimated using the following rules of thumb:

•         +3 dB will  double the watt value:

•         (10 mW +  3dB ≈ 20 mW)

•         Likewise,  -3 dB will halve the watt value:

•         (100 mW -  3dB ≈ 50 mW)

•         +10 dB will  increase the watt value by ten-fold:

•         (10 mW +  10dB ≈ 100 mW)

•         Conversely,  -10 dB will decrease the watt value to one tenth of that value:

•         (300 mW -  10dB ≈ 30 mW)

dBI:-

•         The unit of  measurement dBi refers only to the gain of an antenna. The “i” stands  for “isotropic”, which means that the change in power is referenced  against an isotropic radiator.

•         An  isotropic radiator is a theoretical ideal transmitter that produces  useful electromagnetic field output in all directions with equal  intensity, and at 100-percent efficiency, in three-dimensional space.  One example of an isotropic radiator is the sun.

Thank you,

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blue.modal Wed, 08/21/2002 - 10:22

A concise explanation:

dBm = decibels referenced to milliwatts

dB = gain or loss (no inherent power, just an adjustment to somthing with power)

dBi = decimels referenced to an isotropic radiator

Most antenna manufacturers market their product with the dBi rating because it is 2.15 higher than the dB rating. But if you want to calulate expected results, you need to use dB. (Just subtract 2.15).

Matthew Wheeler

Chief Wireless Architect

www.BlueModal.com

manjeets Thu, 05/09/2013 - 07:35

•        

Desible:-

         Decibels – If a receiver is very sensitive to RF signals,  it may be able to pick up signals as small as 0.000000001 Watts.

•         A logarithm  is the exponent to which the number 10 must be raised to reach some  given value. If we are given the number 1000 and asked to find the  logarithm (log), we find that log 1000 = 3 because 10^3 = 1000. Notice  that our logarithm, 3, is the exponent. An important thing to note about  logarithms is that the logarithm of a negative number or of zero does  not exist.

•         Log (-100) =  undefined!

•         Log (0) =  undefined!

•         Decibels  are a relative measurement unit unlike the absolute measurement of  milliwatts.

dBM:-

•         1 mW = 0  dBm

•         The m in  dBm refers simply to the fact that the reference is 1 milliwatt (1 mW)  and therefore a dBm measurement is a measurement of absolute power.

•         The  relationship between the decibels scale and the watt scale can be  estimated using the following rules of thumb:

•         +3 dB will  double the watt value:

•         (10 mW +  3dB ≈ 20 mW)

•         Likewise,  -3 dB will halve the watt value:

•         (100 mW -  3dB ≈ 50 mW)

•         +10 dB will  increase the watt value by ten-fold:

•         (10 mW +  10dB ≈ 100 mW)

•         Conversely,  -10 dB will decrease the watt value to one tenth of that value:

•         (300 mW -  10dB ≈ 30 mW)

dBI:-

•         The unit of  measurement dBi refers only to the gain of an antenna. The “i” stands  for “isotropic”, which means that the change in power is referenced  against an isotropic radiator.

•         An  isotropic radiator is a theoretical ideal transmitter that produces  useful electromagnetic field output in all directions with equal  intensity, and at 100-percent efficiency, in three-dimensional space.  One example of an isotropic radiator is the sun.

Thank you,

Scott Fella Thu, 05/09/2013 - 07:45

Good information!!!!

Thanks,

Scott

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