dB / dBm Terminology

The dB terminology is used for ratio between two power levels in communication equipment for example to compare power coming out of a component to that going into it. The dB number system can also be used to express absolute value of power as dBm. Power measured in dBm is just the power value that is being measured referenced to 1 mW (respect to mili). That's what the m stands for. To see how dB / dBm is used look at Figure above.

The transmitter power of 10 000 mW is 40 dBm; the loss of 0.4 in cable is -4 dB; the gain of the transmitting antenna is 300 which is 25 dB; the path loss is 4 x 10 power of -19 or -184 dB. The gain of the receiving antenna is 3000 which is 35 dB; the loss of the cable in receiving systems is -4 dB; the low noice receiver gain of 3 billion is 95 dB. The beauty of dB is we can easily add / subtract the losses of gain which gives a received power of 3 dBm which is 2 mW. We can try the calculation on the amateur radio transmission system. See yaa again. Bye2.

Reference : Allan W. Scott, 1993, Understanding Microwave, John Wiley & Sons.