A Quadrax to Twinax conversion apparatus includes stacked trace layers of transmission line with a ground plane between the trace layers. Embodiments include trace layers of stripline or microstrip. Orthogonal plated through holes include a diagonal pair of through holes in electrical contact with traces on one of the trace layers and another diagonal pair of through holes in electrical contact with another trace layer. Contact pins extend through these orthogonal plated through holes with one pair of pins making electrical contact with one trace layer and the other pair of pins making electrical contact with another trace layer. The conversion apparatus electrically connects Twinax cables to respectively different trace layers without crossing over or disturbing the relative positions of the Quadrax diagonal pairs for very efficient high-speed data transfer from four wire Quadrax to two wire Twinax cables.