han the output. By how much? It is smaller by 26 dB. What is the numeric value of 26 dB? Remember, 20 dB is 100; 23 dB is 200, and 26 dB is 400. So the input is 1/400 of the output or 40/400 (mW) = 0.1 mW. C.2 dBm AND dBW These are the first derived decibel units that we will learn. They are probably the most important. The dBm is also a ratio. It is a decibel value related to one milliwatt (1 mW). The dBW is a decibel value related to one watt (1 W). Remember the little m in dBm refers to milliwatt and the big W in dBW refers to watt. The values dBm and dBW are measures of real levels. But first we should write the familiar dB formulas for dBm and dBW: Value (dBm) = 10 log mW), Value (dBW) = 10 log P1/(1W). LEARNING DECIBELS AND THEIR APPLICATIONS Here are a few good relationships to fix in our memories: 1 mW = 0 dBm (by definition), 1 W = 0 dBW (by definition), +30 dBm = 0 dBW = 1 W, -30 dBW = 0 dBm = 1 mW. Who will hazard a guess what +3 dBm is in mW? Of course, it is 3 dB greater than 0 dBm. Therefore it must be 2 mW. Of course, +6 dBm is 4 mW, and -3 dBm is half of 0 dBm or 0.5 mW. A table is often helpful for the powers of 10: 1 mW = 10° mW = 0 dBm, 10 mW = 101 mW = +10 dBm, 100 mW = 102 mW = +20 dBm, 1000 mW = 103 mW = +30 dBm = 0 dBW, 10 W = 104 mW = +40 dBm = +10 dBW(etc.). Likewise, 0.1 mW = 10-1 dBm, 0.01 mW = 10-2 -20 dBm, 0.001 mW = 10-3 -30 dBm, 0.0001 mW = 10-4 -40 dBm. Once we have a grasp of dBm and dBW, we will find it easier to work problems with networks in series. We now will give some examples. ? dBm +23 dB First we convert the input, 8 mW to dBm. Look how simple it is: 2 mW = +3 dBm, 4 mW = +6 dBm, and 8 mW = +9 dBm. Now watch this! To get the answer, the power level at the output is +9 dBm +23 dB = +32 dBm. Another problem will be helpful. In this case the unknown will be the input to a network. ? dBm 10 mW -17 dB In each case like this we ask ourselves, is the output greater than the input? Because the network is lossy, the input is 17 dB greater than the output. Convert the output to dBm