对于无线工程师来说更常用分贝dBm这个单位，dBm单位表示相对于1毫瓦的分贝数，
dBm和W之间的关系是：dBm=10*lg(mW)1w的功率，
换算成dBm就是10×lg1000＝30dBm。2w是33dBm，4W是36dBm……大家发现了吗？
瓦数增加一倍，dBm就增加3。为什么要用dBm做单位？
原因大致有几个：
1、对于无线信号的衰减来说，不是线性的，而是成对数关系衰减的。用分贝更能体现这种关系。
2、用分贝做单位比用瓦做单位更容易描述，往往在发射机出来的功率几十上百瓦，到了接收端已经是以微微瓦来计算了。
3、计算方便，衰减的计算公式用分贝来计算只用做加减法就可以了。  

以1mW 为基准的dB算法，即0dBm=1mW，dBm=10*log(Power/1mW)。
发射功率dBm－路径损失dB＝接收信号强度dBm
最小通信功率dBm－路径损失dB≥接收灵敏度下限dBm
最小通信功率dBm≥路径损失dB＋接收灵敏度下限dBm

                        
                               功率与dbm的对照表
                              
                                 mWdBm 25Watts
                                 0 1.0 mW
                                 1 1.3 mW
                                 2 1.6 mW
                                 3 2.0 mW
                                 4 2.5 mW
                                 5 3.2 mW
                                 6 4.0 mW
                                 7 5.0 mW
                                 8 6.0 mW
                                 9 8.0 mW
                                 10 10 mW
                                 11 13 mW
                                 12 16 mW
                                 13 20 mW
                                 14 25 mW
                                 15 32 mW
                                 16 40 mW
                                 17 50 mW
                                 18 64 mW
                                 19 80 mW
                                 20 100 mW
                                 21 128 mW
                                 22 160 mW
                                 23 200 mW
                                 24 250 mW
                                 25 320 mW

                               
                              
                                 dBm Watts
                                 26 400mW
                                 27 500mW
                                 28 640mW
                                 29 800mW
                                 30 1.0W
                                 31 1.3W
                                 32 1.6W
                                 33 2.0W
                                 34 2.5W
                                 35 3.0W
                                 36 4.0W
                                 37 5.0W
                                 38 6.0W
                                 39 8.0W
                                 40 10W
                                 41 13W
                                 42 16W
                                 43 20W
                                 44 25W
                                 45 32W
                                 46 40W
                                 47 50W
                                 48 64W
                                 49 80W
                                 50 100W
                                 60 1000W

                         　
                         附：各种馈线的衰减表
                         这些数值均为近似值。准确的数值请和电缆厂家核实
                          Table B - Typical Cable Attenuation Values
                               电缆类型 衰减 / 100 米@ 2.4 GHz(dB)
                               Belden 9913 8.0
                               LMR 200 16.8
                               LMR 240 12.9
                               LMR 400 6.9
                               LMR 600 4.4
                               1/2”LDF 3.9
                               1/2”Superflex 6.1
                               3/8” LDF 5.9
                               3/8” Superflex 6.8
                               1/4” Superflex 9.8