BN-43-2402 balun example

An online poster recently sought to design a broadband 9:1 transformer for HF.

Choosing a BN-43-2402 balun core, he planned to use a 2t primary and 6t secondary for a nominal 50Ω input. He subsequently posted measurements of the prototype.

What might we expect… is it a good starting point.

A first approximation at the low frequency end with a medium µ core is that it is like an ideal transformer withe the magnetising impedance in shunt with the primary.

Lets calculate the magnetising impedance (or admittance)

The datasheet for a BN-43-2402 balun core gives Al=1440nH/1t.

Finding the magnetising impedance of the primary at 3.5MHz with Calculate ferrite cored inductor (from Al) we get the following.

screenshot-03_10_16-13_00_46

For this calculations, it is more convenient to work with the admittance Y. I have ignored Cs as it will make little difference to this calculation.

Lets add to it the transformed test load of 470Ω (G=1/470) for a combined Y=0.02473-0.011S.

Lets calculate input impedance, VSWR, RL etc

Entering that admittance into Calculate VSWR and Return Loss from Zload (or Yload) and Zo, we get:

screenshot-03_10_16-13_02_48

Reconciling

The expected input impedance and VSWR of the simple model is 33.8+j15.0Ω and 1.7, not too far off his measured 36.6+j16.1 and 1.6.

For a low Insertion VSWR 9:1 transformer, the low end VSWR might be higher than desired and adding 1t to the primary and 3t to secondary would be worth measuring.