The decibel is a way of expressing the relative value of something compared to a standard, often electrical power, or sound levels. In this case it is electrical power relative to a milliwatt. The decibel is a logarithmic unit, so 10dBm is ten milliwatts, 20dBm is a hundred milliwatts and 30dBm is a Watt. -30dBm is one microwatt.
A Level Measuring Set (LMS) is a high impedance microvoltmeter calibrated in Decibels relative to one milliwatt. It is very important it is set to the correct circuit impedance otherwise the reading will be incorrect. The calculator above, shows how the impedance affects the emf developed across the load. The impedance of an audio telephone circuit in the U.K. is 600Ω. Wiring carrying radio frequencies usually have impedances of 50Ω or 75Ω but there are other commonly found impedances.
Level Measuring Sets usually have the option to make a through (THRU) or terminated (TERM) reading. Use a through setting if the circuit is unbroken and a terminated reading if the circuit is not connected at the receiving end, the measuring set provides the terminating impedance for the circuit. The wrong setting will give a misleading level measurement.
The diagram illustrates the possible LMS connections. If the LMS is set to TERM when it should be through, it will read 3.5db lower than it should. If set to THRU when it should be in TERM, the reading will be 6db higher than it should. You ought be able to determine how the Level Measuring Set should be set, but if in doubt, switching between through and terminated and noting the difference will show which way is correct. This assumes you have selected the correct circuit impedance and the source and termination impedances are close to their stated value.
Calculator for dB Watts