Ah, good question and something that you should definitely know if you are working in satellite or communications.
Basically, Bels are too big so we use deciBels (dB) are an easy way to multiply large and small numbers. By using dBs you can add or subtract instead of having to multiply numbers. DBs is a ratio between two levels, an example is the receive signal to noise level (Rx SNR) expressed in dBm (this tells us how strong the signal level is compared to the noise with reference to 1mW).dB = 10 log [ratio of two power levels]
Question: A 30m RG6 coax has a loss of 3dB. What is the output power if the input is 500mW?
Answer: -3dB is 0.5 x power. Therefore the output power is 0.5 x 500mW = 250mW
dBW or dBm?
dBW is reference the power with 1W (Watt)
dBm is reference the power with 1mW (milliWatt)
dBm to dBW Conversion
The conversion between dBm and dBW is quite straight forward. 1 Watt is equal to 1000mW which is an increase of 30dB. So we need to add or subtract 30dB.dBm = dBW + 30
dBW = dBm – 30
Example: Convert 30dBm to dBW
Answer: 35dBm – 30 = 5dBW
Example: Convert -40dBW to dBm
Answer: -20dBW + 30 = 10dBm
Using deciBels (dB) you can quickly calculate the overall gain of a communication system by simply adding or subtracting the different components.