.5.1 Calculating Watt and Milliwatt Values When Given dBW and dBm Values We will find that the process of calculating numeric values in watts and milliwatts is very similar to calculating the numeric value of a ratio when given the equivalent value in decibels. Likewise, the greater portion of these conversions can be carried out without a calculator to a first-order estimation. In the case where the dB value is 10 or a multiple thereof, the value will be exact. Remember: 0 dBm = 1 mW; 0 dBW = 1 W by definition. Furthermore, lest we forget: +3 dBm is twice as large as the equivalent 0 dBm value, thus where 0 dBm = 1 mW, +3 dBm = 2 mW. Also, +10 dBm numeric value is 10 times the equivalent 0 dBm value (i.e., it is 10 dB larger). So +10 dBm = 10 mW; -10 dBm = 0.1 mW; -20 dBm = 0.01 mW. In addition, -17 dBm is twice the numeric magnitude of -20 dBm. So -17 dBm = 0.02 mW, and SO forth. Try calculating the numeric equivalents of these dBm and dBW values without using a calculator. C.7 dB APPLIED TO THE VOICE CHANNEL 1. +13 dBm 2. -13 dBm 3. +44 dBm 4. -21 dBm 5. +27 dBW 6. -14 dBW 7. -11 dBm 8. +47 dBW Answers: 1: 20 mW. 2: 0.05 mW. 3: +14 dBW, 25 W. 4: 0.008 mW. 5: 500 W. 6: 40 mW. 7: 0.08 mW. 8: 50 kW. C.6 ADDITION OF dBs AND DERIVED UNITS Suppose we have a combiner, a device that combines signals from two or more sources. This combiner has two signal inputs: +3 dBm and +6 dBm. Our combiner is an ideal combiner in that it displays no insertion loss. In other words, there is no deleterious effect on the combining action, it is "lossless." What we want to find out is the output of the combiner in dBm. It is not +9 dBm. The problem is shown diagrammatically as +6 dBm Output ? dBm +3 dBm Combiner Some texts provide a nomogram to solve such a problem. We believe the following method is more accurate and, with the advent of affordable scientific calculators, easier. It is simple: Convert the input values to their respective numeric values in mW; add and convert the sum to its equivalent value in dBm. The +3 and +6 dBm val