A modular TDR probe assembly is presented. The probe assembly includes a first section with a central conductor and an outer conductor having a first impedance, and a second section with a central conductor and an outer conductor having a second impedance. An impedance matching elbow joins the first section and the second section. The elbow contains a solid dielectric insert with a bore passing through it housing a terminal pin that conveys electrical signals between the center conductor of the first section and the center conductor of the second section.