hat would the return loss be? We would then have: Return lossda = 20 log(700 + 600)/(700-600) = 20 log(1300/100) = 20 log 13 = 22.28 dB. Good return loss values are in the range of 25 dB to 35 dB. In the case of the telephone network hybrid, the average return loss is in the order of 11 dB. LEARNING DECIBELS AND THEIR APPLICATIONS Subscriber loops 4-W send 2-W switch 2-W side (local exchange) Hybrid L (loop side) Balancing network 4-W receive This diagram is the special situation of the 2-wire/4-wire conversion using the hybrid transformer, a 4-port device. Let us assume that the subscriber loop/local exchange charac- teristic impedance is 600 S2. We usually can manage to maintain good impedance match with the 4-wire trunks, likewise for the balancing network, often called a compromise network. However, the 2-wire side of the hybrid can be switched into very short, short, medium, and long loops, where the impedance can vary greatly. We will set up the equation for return loss assuming that at this moment in time it is through connected to a short loop with an impedance of 450 S2; the impedance of the balancing network is 600 S2, which is Zo. We now calculate the return loss in this situation: Return loss dB = 20 log(600 + 450) / (600 450) = 20 log(1050/150) = 20 log 2.333 = 7.36 dB. This is a fairly typical case. The mean return loss in North America for this situation is again 11 dB. With the advent of an all-digital network to the subscriber, we should see return losses in excess of 30 dB or possibly we will be able to do away with the hybrid all together. C.10 RELATIVE POWER LEVEL: dBm0, pWpO, etc. C.10.1 Definition of Relative Power Level CCITT defines relative power level as the ratio, generally expressed in dB, between the power of a signal at a point in a transmission channel and the same power at another point in the channel chosen as a reference point, generally at the origin of the channel. Unless otherwise specified (CCITT Recs. G.101, 223), the relative power level is the ratio of the power of a sinus