Figure 1: Cutaway view of a dummy-load
A DUMMY LOAD is a device which absorbs RF energy in a transmission line. It is a resistive load that is matched to the characteristic impedance of the waveguide to prevent the origin of standing waves.
Radar sets use resistive dummy loads for high power as illustrated in the figure. The dummy load shown is a short-cuited section of waveguide filled over a length of several wavelength with mixture of sand and graphite. When the fields enter this mixture, they induce a current flow. All the energy traveling down the waveguide is dissipated in the form of heat by the sand and grafite, thus no energy is reflected (or radiated). The outside of the waveguide section used to as waveguide terminator has cooling fins which help dissipate the heat.
Figure 2: Different models of dummies