This article documents a review of the magnetics of the ‘voltage’ transformer in the VK3AMP Sontheimer directional coupler. It is typically the most important component in determining InsertionVSWR and ReturnLoss at the lowest frequencies.
The transformer of interest is the one to the left, and if you follow the tracks, the multiturn winding is connected between ground and a track that routes across to the through line. The transformer primary appears in shunt with the through line.
The transformer appears to comprise a 24t primary on a 2646665802 #46 toroid. Note that #46 is a MgZn ferrite, where other designs tend to use either powdered iron or NiZn ferrite.
For the purpose of this article, flux leakage is ignored as it will have very little effect on the calculated ReturnLoss due to this component alone.
Above is the calculated ReturnLoss due to the magnetising admittance of the voltage transformer that is in shunt with the through line based on the published permeability characteristics and assuming 3.5pF of equivalent shunt capacitance to model self resonance. ReturnLoss is presented here as InsertionVSWR is very very small, VSWR=1.02 equates to ReturnLoss=40dB.
ReturnLoss at the lowest specified frequency, 1.8MHz, is 47dB.
Now this is not the only source of mismatch that would drive low ReturnLoss, but in most ham designs, this single component is responsible for ReturnLoss less than 20dB at their specified lower frequency limits.
There is little point having an instrument that indicates VSWR down to say 1.1 when it invisibly causes VSWR=1.3 looking into it (eg
Grebenkember’s original Tandem match).