If the power transmitted to the load before insertion is PT and the power received by the load after insertion is PR, then the insertion loss in dB is given by,
Insertion loss is a figure of merit for an electronic filter and this data is generally specified with a filter. Insertion loss is defined as a ratio of the signal level in a test configuration without the filter installed (|V1|) to the signal level with the filter installed (|V2|). This ratio is described in dB by the following equation:
For passive filters, |V2| will be smaller than |V1|. In this case, the insertion loss is positive and measures how much smaller the signal is after adding the filter.
Link with scattering parameters
Here is one of the scattering parameters. Insertion loss is the extra loss produced by the introduction of the DUT between the 2 reference planes of the measurement. The extra loss can be introduced by intrinsic loss in the DUT and/or mismatch. In case of extra loss the insertion loss is defined to be positive.