FT37-43 for a 49:1 EFHW transformer enquiry

A correspondent asked whether Sontheimer coupler – transformer issues – an alternative design – FT37-43 could be used to inform design of a 49:1 EFHW transformer based on the same core, but with a 2 or 3t primary.

In the case of the Sontheimer coupler the winding with the higher number of turns appears in shunt with the nominal 50Ω load, and its effect on InsertionVSWR and the core loss can be predicted reasonably well and confirmed by measurements as in the referenced article.

In that instance, a 7t winding in shunt with the nominal 50Ω load causes excessive core heating, a 3t winding will be worse, and 2t worse again.

The case of an EFHW transformer is somewhat similar, the difference is now that the winding with less turns in approximately in shunt with the nominal 50Ω primary referred load. The same Simsmith model can be used to predict likely InsertionVSWR due to primary magnetising admittance, and the core loss.

Let’s try the 3t case first, with the experience of the referenced article we can expect it will have insufficient turns for good performance.

Above is the Simsmith model of a Fair-rite 5943000201 core (equivalent dimensions to FT37-43) with a 3t winding. Note this does not apply to Amidon #43 as their material is significantly different in characteristic.

So, the model scenario is of a 50Ω source that would deliver 5W into a matched load. With the magnetising admittance appearing in shunt, at 3.5MHz core loss is around 1.2W and power delivered to the transformed load is 3.4W. InsertionVSWR due to the magnetising admittance is 1.8.

So, will it work?

Well, anything ‘works’, it depends on your meaning of ‘works’.

There are two main questions:

  1. is the loss of power to the antenna a concern; and
  2. will the associated heat be an issue?

Is the loss of power to the antenna a concern?

It seems that QRP aficionados are less concerned about loss of power to the antenna, QRP^2 if you like.

It is a personal matter.

Will the associated heat be an issue?

Above, the predicted temperature rise for the core in free air of just 0.1W of dissipation is 22°, so on a hot day at 40° ambient, the core would reach 62° which is just enough to burn skin.

The good news is that the average power of uncompressed SSB telephony is around 5% of PEP, so this core scenario should withstand that mode at up to 2W PEP, less for compressed speech.

If your choice was FT8, the core would get dangerously hot, and even reach the Curie temperature (where the core loses its magnetic properties) in time.

If the transformer is enclosed, its power rating is reduced.

Similar sized cores

I have had enquiries from hams about similar transformers on small #43 cores, FT82-43 matching transformer for an EFHW discusses one of the popular designs. It seems everyone is a designer, and adapting an often poor design by changing the core size is seen and an obvious path to success.

The promotion of these ‘designs’ speaks to the credibility of the ‘designer’.

What does work well?

There are several designs on this web site, start with Small efficient matching transformer for an EFHW.

Search the net for published designs that include exposition of the magnetic design, prediction of core loss, measurement of a prototype, and thermographs to validate the design performance.


This 3t primary on a Fair-rite 5943000201 (FT37-43) is not a good choice for 80m, it has high InsertionVSWR and high core loss.

References / links