3 dB T junction Power Divider

the Bel is a "unit" that can express a ratio.
dB is 1/10 of Bel.

dB = 10 log ( Power/Reference Power )
dB = 20 log ( U / U reference )
The reference is essential!
( 1mW can be 30 dB in reference to μW , 0 dB in reference to 1mW or -30 dB in reference to 1 W)
Power i related with U to the second power , hence:
P = U^2/R
dB = 10 log (P/Pr) =
10 log (U^2 / Ur^2) =
10 log (U/Ur) ^2 =
2 x 10 log ( U/Ur) =
20 log (U/Ur)

3dB means that the output power in question is two times greater than the reference power.
10 dB means that the output power is ten times greater than the reference power.
20 dB means that the output power is hundred times grteater than the reference power.

I put unit in quotation mark because dB has no real unit ( W/W) , (V/V) .

Hi,
you are talking about a coupler, therefore a 3dB coupler (or power divider) means that it just divides your input power. If you put 0dBm power in, you get on each of the two output ports -3dBm power out. In that case -3dB means 1/2, -6dB would mean 1/4, -9dB means 1/8 and so on. Just continue that row and you will find out that -10dB equals to 1/10 and -20dB means 1/100.
You put 1W (equals to 30dBm) into the divider and you will get only 0.01W (equals to 10dBm) out. So 20dB less.