Sontheimer coupler – transformer issues – an alternative design – FT37-43

Sontheimer coupler – transformer issues discussed problems with the Sontheimer coupler in a recently published QRP transceiver ((tr)uSDX / trusdx), suggesting that the core loss in transformer T2 was excessive.

This article presents an alternative design for the transformer for a coupler for a 5W transmitter.

The above circuit is from (Grebenkemper 1987) and is an embodiment of (Sontheimer 1966). In their various forms, this family of couplers have one or sometimes two transformers with their primary in shunt with the through line. Let's focus on transformer T2. It samples the though line RF voltage, and its magnetising impedance and transformed load appear in shunt with the through line. T2's load is usually insignificant, but its magnetising impedance is significant and is often a cause of:

  • high InsertionVSWR;
  • high core loss;

In the case of couplers embedded in a transmitter, the InsertionVSWR is hidden and frustrates obtaining expected power and PA efficiency.

Let's model the effect of the magnetising impedance of T2 on both of these parameters using an alternative design.

The alternative design is guided by the concept that small is beautiful when it comes to design of broadband ferrite transformers with nearly ideal response. Of course practical transformers are limited by power handling considerations, but low power transmitters permit quite small transformers.

The alternative design uses a Fair-rite 5943000201 (FT37-43), note that an Amidon core is NOT a substitute (see Sontheimer coupler – transformer issues). T2 has a 14t primary, and 1t or 2t secondary. Whilst a 1t secondary is preferred, if the transformer was to be used as a replacement of the 7t:1t transformer discussed in the previous article, 14t:2t is more directly compatible (eg with firmware).

It should go without saying that best directivity is obtained with symmetric transformers, ie, T1 and T2 are identical, they have almost equal leakage inductances etc, and almost equal amplitude and phase response.

A first step is to confirm that the core is unlikely to approach saturation.

Above, expected flux density is 21mT @ 1.8MHz, well below the onset on non-linear B-H response at about 200mT, and less at higher frequencies.

Above, the test inductor with 14t of 0.3mm ECW winding on the 9.3mm OD core for measurement.

A Simsmith model was constructed to estimate the InsertionVSWR and core loss due to T2 based on an estimate of the magnetising impedance of the primary winding. The impedance model is based on (Duffy 2015).

Above, the model for calibration of cse.

The prototype transformer was measured and its measurement s1p file merged with the Simsmith model to compare estimated with measured, and adjust cse for reconciliation of the self resonant frequency (SRF) of the model with measurement.

The model calibrates with cse=0.415pF, SRF=41MHz, the two sets of curves reconcile quite well validating the model and measurement.

Above is the calibrated model showing estimated InsertionVSWR and core loss.

InsertionVSWR due to T2's magnetising impedance alone is 1.03 @ 3.5MHz, and core loss is 0.08W @ 5W (0.07dB loss), both should be quite acceptable.

The real world

This is a desk study of an alternative design, it produces a candidate for testing of the important coupler parameters (like InsertionVSWR and Loss).

Thermals are best confirmed from the working prototype, expected temperature rise is towards 20° @ 0.08W average dissipation (5W transmitter @ 3.5MHz), much lower for SSB telephony.

Above is a thermograph of the inductor in free air showing 25.9-12.5=13.4° rise over ambient, stabilised after 3m of continuous 3.5MHz 5W carrier through signal.

References / links

  • Duffy, O. 2015. A method for estimating the impedance of a ferrite cored toroidal inductor at RF.
  • Grebenkemper, L. Jan 1987. The tandem match – an accurate directional wattmeter In QST.
  • Sontheimer,C & Frederick,RE. Apr 1966. Broadband directional coupler. US Patent 3,426,298.