In telecommunications, **insertion loss** is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber and is usually expressed in decibels (dB).

If the power transmitted to the load before insertion is *P*_{T} and the power received by the load after insertion is *P*_{R}, then the insertion loss in dB is given by,

- $IL(\mathrm {dB} )=10\log _{10}{P_{\mathrm {T} } \over P_{\mathrm {R} }}$

## Electronic filters

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 (|*V*_{1}|) to the signal level with the filter installed (|*V*_{2}|). This ratio is described in dB by the following equation:

- ${\mbox{Insertion loss (dB)}}=10\log _{10}{\left|V_{1}\right|^{2} \over \left|V_{2}\right|^{2}}=20\log _{10}{\left|V_{1}\right| \over \left|V_{2}\right|}$

For passive filters, |*V*_{2}| will be smaller than |*V*_{1}|. In this case, the insertion loss is positive and measures how much smaller the signal is after adding the filter.

## Link with scattering parameters

In case the two measurement ports use the same reference impedance, the insertion loss ($IL$) is defined as:^{[1]}^{[2]}

- $IL=-20\log _{10}\left|S_{21}\right|\,{\text{dB}}$.

Here $S_{21}$ 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.

## See also

## References

**^** Pozar, David M.; *Microwave Engineering, Third Edition*
**^** Collin, Robert E.; *Foundations For Microwave Engineering, Second Edition*