Insertion Loss is a comparison of the load power available with the transformer in the circuit to the load power with the transformer not in the circuit. Transducer loss is commonly referred to as insertion loss, but this is precisely true only when source and load impedance are equal.

Midcom measures transducer loss, but calls it "insertion loss" for simplicity (except in 'Transformer Analysis Program' analyses where losses are accurately differentiated.) With R_L and R_G known (R_G adjusted to match the ideal reflected PRI impedance) and V_G a constant voltage, V_L²/R _L is the power in R_L. This quantity is inverted and multiplied by ¼(V_G²/R_G ) resulting in the input to output power ratio. The constant ¼ corrects for the fact that V_G² is four times the primary voltage under ideal input match conditions, that is when V_Rg = V_PRI = ½V_G.

Therefore, this expression includes not only the effect of real dissipative losses within the transformer but also the impedance mismatch losses at the input, and is valid for all turns ratios. Typical mid-band transformer insertion loss values are about one dB. Voiceband insertion loss is typically measured at a reference frequency of 1000 Hz, the mid-band frequency. (The use of 1004 Hz is sometimes specified to prevent interference with multiplex sampling frequencies when testing telecom systems.)

* Definitions are actually transducer loss, but are called insertion loss here in keeping with industry parlance. See text for details.