What is mismatch loss (ML)?

Impedance mismatch/discontinuity between the transmission line/cable to the connected load/component leads to a small amount of incident signal power reflected back to the source. In transmission line theory, the mismatch loss (ML) is the ratio of incident power (Pi) to the difference between incident and reflected power (Pr). High mismatch loss means high reflected power, so high mismatch loss is not good for a system. The mismatch loss is expressed in dB as follows.

For an ideal system, there is no impedance mismatch between the transmission line & load, hence no reflection, so no reflected power. The mismatch loss is zero. So, all the incident power is delivered to the load. But, for a practical system, there will always be a small impedance mismatch; significant reflection will be there, so the significant reflected power will be there. For a practical system, the mismatch loss is higher than the ideal system.