By broadband transformer, I mean a transformer intended to have nearly nominal impedance transformation over a wide frequency range. That objective might be stated as a given InsertionVSWR over a given frequency range for a stated impedance. eg InsertionVSWR<2 from 3-30MHz with 3200(+j0)Ω load.

These are used in many things, including medium to high power applications such as EFHW matching transformers.

Leakage inductance is the equivalent series inductance due to flux that cuts one winding and not the other, and vice versa. For most simple transformers, the total primary referred leakage inductance is twice the primary leakage inductance. Since the leakage inductance appears in series with the signal path, it causes degradation of nominal impedance transformation, the very simplest approximation of the frequency response is that of a LR circuit.

Above is a Simsmith model of a 1µH total leakage inductance in series with a 50+j0Ω load, the InsertionVSWR is greater than 1.5 above 3MHz.

Is this a common scenario? Continue reading On ferrite cored RF broadband transformers and leakage inductance

Last update: 4th February, 2022, 8:28 AM