AT-100 ATU Sontheimer coupler using a single binocular ferrite core

A reader of my recent article Sontheimer coupler – transformer issues and the follow up articles giving better transformer designs asked whether the coupler use in the popular AT-100 ATU wouldn't be a good solution.

A quick Google search did not turn up any published design rationale or measurement data for the AT-100 coupler design.

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, and another which is in series with the through line to sense current. To achieve good Directivity, these transformers must be symmetric, nearly ideal, and they must be independent, ie no significant coupling between the transformers by magnetic or electric fields.

The AT-100 uses a Sontheimer coupler, they are very popular with ham users for perceived better performance, notably better Directivity over a wide frequency range.

Above is an extract from the schematic of the AT-100 ATU. Note T1 and T2. The designations might imply they are two separate transformers with negligible coupling (as per Sontheimer), the diagram does not show that they have a magnetic core, nor more importantly, that they share magnetic paths to some extent.

Above, the pic shows T1 and T2 as a binocular core, and the winding configuration can be seen as a single pass of one conductor through an aperture hole, and a bunch of turns on the ‘outer limb' to form T1 and similarly for T2.

The article Thoughts on binocular ferrite core inductors at radio frequencies discusses the behavior of binocular cores from a different but related perspective.

Let's look at the magnetic flux due to current in a wire passing through one aperture hole.

Above is an end view of a binocular core, the blue + is the tail of a current flowing into the page. Oersted's rule tells use that current flowing into the page like that will produce magnetic flux surrounding the current.

At radio frequencies, current distribution is such that most of the current flows in a very thin layer at the surface (skin effect), and as a result there is very little flux interior to the conductor.

Think of the flux as closed loops. In the space between the outer surface of the conductor and the inner surface of the core aperture, there will be flux loops that are approximately circular.

In the space between the inner surface of the core aperture and say half wave to the outside of the core, there will be flux loops that are approximately circular, but much higher density due to the high relative permeability of the ferrite material.

As we approach the outer edge of the core, some of those flux loops are around both core apertures, some of the flux in the left hand limb fringes into the right hand limb.

Outside the core, there is also some flux, out to infinity, but it is much weaker now due to low permeability and greater flux path length.

For a medium permeability core such as #43 material, almost all of the flux is within the core, sufficiently so that we tend to ignore the other flux with little loss in accuracy.

But let's return to the flux loops that go right around the outer thickness of the core, that flux cuts a conductor placed in the right hand aperture.

So, current in at AT-100 T2 1t primary causes some flux to cut the T1 10t primary winding and induce a voltage in it proportional to the through current. This cross coupling of the transformers due to the partially shared magnetic circuit compromises coupler Directivity.


The configuration used in the AT-100 Sontheimer directional coupler creates some magnetic coupling between T1 and T2 due to the geometry of the single binocular core used, compromising coupler directivity.

Since its Directivity is compromised, it begs the question, why is a Sontheimer coupler used over simpler circuits.

In any event, this coupler is principally to guide the auto tuner which appears to have only 256 steps of L and C and one doubts that it has the fineness of control to achieve extremely low VSWR which would require the use of a coupler with high Directivity. A useful metric is the frequency normalised number of steps available in each adjustable component, \(steps_n=\frac{steps}{\frac{f_{max}}{f_{min}}}=\frac{256}{\frac{54}{1.8}}=8.5\), it is a very small number  which suggests quite coarse adjustment steps at the ends of its range.

All in all, a questionable design and to my mind, one that is not a good model to copy.

Using two separate small suppression sleeves such as the Fair-rite 2643250402 with a little spacing might well produce a higher Directivity coupler.

Another example that questions apparent ham preference for a Sontheimer coupler.


  • 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.