A Simsmith model of a Ruthroff 1:4 voltage balun – 2843009902 (BN43-7051)

A correspondent asked whether I had a tool similar to An improved simple Simsmith model for exploration of a common EFHW transformer designs (v1.03) to assist in the design of a ferrite cored Ruthroff 1:4 balun for HF.

In fact, the problem is the same as the one discussed in the article above, and the model is suited to application to the ferrite cored HF Ruthroff 1:4 balun case.

This analysis applies to a Fair-rite 2843009902 but may not apply to other manufacturer's BN43-7051.

Above is a screenshot of the model calibrated against measurement. The magenta curve is the prediction and the blue curve is the measurement. Note that very small differences in measured value result in apparently large changes in InsertionVSWR, these two curves reconcile very well, especially considering the tolerances of ferrite material.

Above is a pic of the DUT and test fixture, the floating ground wire is bonded to the external threads of the SMA connector using the plastic clothes peg. All connecting wires are very short, the balun is wound with twisted pair stripped from a CAT5 LAN cable, it has Zo quite close to 100Ω which is ideal for this application.

In fact the uncompensated InsertionVSWR of this balun is well under 1.5 up to 100MHz.

Factors that contribute to very good broadband performance of this balun include:

  • very low leakage inductance (~15nH each side);
  • Zo of the pair close to half the nominal load resistance; and
  • very short connections in the test fixture.

Worst predicted loss is about 0.13dB @ 4MHz, about 3% if input power.

Above is an estimate of the power dissipation for 40° temperature rise in free air (conservatively based on the surface are of the four largest sides). So 2W loss at 3% of input power implies average input power of up to 2/.03=67W.