dB vs dBm

dB 

DB is Power Ratio OR Power Loss OR Variation in Power. 




So, in order to see the loss of your patch cable, you need a transmitter at one end with known power and put the receiver on the other end with known transmitter power. Then the receiver will give you the dB or loss introduced by the patch cable you made. 


dBm

dBm is the actual Power of the signal or value of the quantity. Getting the Signal Strenght of an RF signal is tricky. The Most accurate way to express it is with Milliwatts (mW). But you will end with with huge number if zero in before decimal places because of super-low power. For Example -60dBm is 0.000001 Milli Watts.




So that means 1mW = 0dBm , 10mW = 10dBm, 100mW = 20 dBm, -40db = 0.0001 mWatt




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