A test system includes a test socket assembly for capturing low energy electromagnetic emissions from radio frequency (RF) integrated circuits (ICs). The test socket assembly is structured to direct electromagnetic radiation from the device under test (DUT) to a socket port coupled to one end of a waveguide for transmission to a tester. The combination of the materials comprising the socket assembly is selected to more efficiently couple electromagnetic emissions from the DUT into the waveguide. For example, a reflective plane with an adjustable position may be located below the DUT in order to increase coupling of electromagnetic radiation from the DUT into the waveguide.