dBmV (decibels relative to one millivolt) is a measure of the signal strength in wires and cables at RF and AF frequencies. A millivolt is 1/1000 of a volt (0.001 V or 10 -3 V). This unit is defined in terms of root-mean-square ( rms ) alternating current ( AC ) signal voltages in circuits in which the impedance is a pure resistance of some specified value.
S dBmV = 20 log 10 E
A 1-mV signal across a pure resistance has a level of 0 dBmV. Signals weaker than 1 mV have negative dBmV values; signals stronger than 1 mV have positive dBmV values, assuming the impedance remains a pure resistance and remains constant.