What is the effective radiated power (ERP) of a repeater station with 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain?
Why ERP is transmitter power adjusted by the net gain in dB along the path to the antenna. Add the losses: 4 + 3.2 + 0.8 = 8 dB, and subtract from the 10 dBd antenna gain, leaving +2 dB net. A gain of 2 dB is a power ratio of 10^(2/10) = 1.58, so 200 W x 1.58 = about 317 watts. Because the gain is given in dBd (referenced to a dipole), the result is ERP rather than EIRP.
Watch out The choice of 126 watts comes from getting the sign backwards and taking a net loss of 2 dB instead of a net gain; 2,000 watts would be a full 10 dB gain with the losses ignored.
Sum the losses, subtract from gain, then ratio = 10^(dB/10). Remember 2 dB = 1.58x, 3 dB = 2x.