A TEM cell is a stripline device for radiated emissions and immunity testing of electronic devices. It is not a replacement, but due to its size and cost it is a convenient alternative to measurements in an anechoic chamber.
A TEM cell consists of a septum, the conductive strip in the centre section and walls which are connected to ground. The geometry is designed to present a 50Ω stripline. The device under test (DUT) is placed in between the bottom wall and the septum.