The performance of RF transformers can be understood with the help of the equivalent circuit in Figure 8. L 1 and L 2 are the primary and secondary leakage inductances, caused by incomplete magnetic coupling between the two windings. Because their reactance is proportional to frequency, these inductances increase insertion loss and reduce return loss at high frequency. R 1 and R 2 are the resistance, or copper loss, of the primary and secondary windings. Skin effect increases the resistance at high frequencies, contributing to the increase in insertion loss.
L p is the magnetizing inductance, which limits the low frequency performance of the transformer.
It is determined by the permeability and crossectional area of the magnetic core, and by the number of turns. Insertion loss increases and return loss decreases at low frequency. Further, permeability of many core materials decreases with a decrease in temperature, and increases above room temperature. This accounts for the spread of the lower frequency portion of the insertion loss curves in Figure 3 as explained above. Temperature variation of the capacitances and the leakage inductances is relatively small.