Sontheimer coupler – transformer issues

It is not uncommon that ham designs for Sontheimer coupers (aka Tandem coupler, Grebenkemper coupler) fall short in the design of the magnetic components resulting in one or both of:

  • high InsertionVSWR; and
  • high core loss.

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 a recent published design.

The design is for a coupler embedded in a high efficiency Class-E QRP transmitter (5W), and so one might expect core loss is an important issue, T2 is an Amidon T37-43 core with 7t:1t.

Above is a side by side comparison of the NMG H material datasheet and Amidon 43, the Amidon appears to be a Photoshop treatment of the NMG.

Fair-rite have been a long term manufacturer of a material they designate 43, and it has been resold as such by many sellers. NMG H material is somewhat similar, but to imply it is equivalent to the long established Fair-rite 43 might be a reach.

The above charts are a side by side comparison of estimated magnetising impedance and core loss based on Fair-rite’s published 43 data at left, and Amidon’s published ’43’ data at right. The model does not compensate for transformer self resonance effects, but at low frequencies shown, the error is small… especially below 4MHz where the loss peaks.

There is a significant difference, the model suggests that the Amidon 43 core  has nearly twice the core loss as Fair-rite 43 in this application (12% vs 6.4%). The InsertionVSWR (which is hidden in the embedded coupler) is 1.26 vs 1.13.

Neither transformer is good, one might expect core loss in T2 to be less than 3% in a high efficiency transmitter, certainly not 12% as modelled.

(tr)uSDX / trusdx uses this type of coupler and specifies Amidon cores.


  • Grebenkemper, J. 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.