Another small efficient matching transformer for an EFHW – 2643251002 – #1 – design workup

The article Another small efficient matching transformer for an EFHW – 2643251002 described a correspondent's , Luis, CT2FZI, implementation of the transformer.

I have also had lengthy discussions with Faraaz, VK4JJ, who is experimenting with a similar transformer.

This article describes my own design workup and measurements using a Fair-rite suppression core, 2643251002. The cores are not readily available locally, so I bought a bunch from Digi-key.

I really resist  the tendency in ham radio to design around unobtainium, it is often quite misguided and always inconvenient. In this case, the motivation for these cores that use quite ordinary #43 material is the geometry of the core, they have ΣA/l=0.002995, a quite high and rivalling the better of binocular cores. High  ΣA/l helps to minimise the number of turns which assists broadband performance. See Choosing a toroidal magnetic core – ID and OD for more discussion.

Design criteria

  • EFHW;
  • nominal 49:1 transformation;
  • compensated;
  • autotransformer; and
  • 50W average power handing.

Some key points often overlooked in published designs of EFHW transformers:

  • Insufficient turns drives high core loss; and
  • leakage inductance is the enemy of broadband performance, so the design tries to minimise leakage inductance.

Note that high number of turns drives high leakage inductance, so the design is to a large extent, a compromise between acceptable core loss and bandwidth.

Initial design estimate

From models, I expect that a turns ratio of 2:14 (ie 14t tapped at 2t) is likely to deliver the design criteria (with suitable compensation capacitor).

Above is a perhaps ambitious initial objective using a simple model of the transformer, dotted line is Loss and solid line is InsertionVSWR.

First measurement

The first step is to measure a 2t winding alone on the core.

Above, the 2t winding measurement fixture. The wire is solid 0.5mm wire stripped from CAT5 LAN cable, the one end zip tied to the external threads of the SMA connector and the other end bent and inserted into the female part without damaging the connector.

Above, a plot or impedance. Note the resonance, the self resonant frequency (SRF) is 16MHz.

Above is a screenshot of a Simsmith model that will be used to develop the design.

It is initially configured to simply expose the impedance of the 2t winding using a simple model of the system as a resonator. The model estimates magnetising impedance from manufacturer's complex permeability data and adds equivalent shunt capacitance cse as a first approximation of its first resonance. The model is calibrated by adjusting cse so that the model SRF coincides with the measurement.

In this case there is very good reconciliation between prediction and measurement, especially given the wide tolerance of ‘suppression' ferrite components (see
Using complex permeability to design with Fair-rite suppression products).

The next step…

The next step is to wind the 2:14 autotransformer winding and to make measurements of its SRF and leakage inductance to calibrate a predictive model.

Another small efficient matching transformer for an EFHW – 2643251002 – #2 – prototype bench measurement