In general, insertion loss is defined as the loss in signal power, when a device is inserted in the transmission path of a signal on a transmission line or circuit.Insertion loss is usually stated in decibels (dB).

So if the transmitted power is PT and the received power at the load, after the insertion of the device is PR , then the insertion loss is defined as

IL ( dB) = 10Log(PT/PR)

Where the Log is to the base of 10.

Insertion loss is also defined for filters as the ratio of the ouput signal level in a test configuration *without* the filter installed, to the signal level *with* the filter installed. So if the output signal level *without* the filter is V1 and the signal level *with* the filter installed is V2 then the insertion loss in dB is:

20Log(|V1|/|V2|) dB

If using scattering parameters use the following expression:

10Log(|s21|^{2 }/1 – |s11|^{2 })

where the symbols have their usual meaning.

Transfomers also have an insertion loss specification. This is a figure of merit for a RF transformer.

The low end ( or low frequency) loss is determned by the primary inductance. The high frequency insertion loss is dependent on the losses in the inter – winding capacitance and the series inductance.

In addition, for transformers with metallic cores, the permeability is directly proportional to the temperature of operation. As the temperature decreases, the permeability *decreases* which causes an *increase* in the insertion loss.