The use of phase reflection gratings in a screened room

Screened rooms are used for both radiated emission tests and immunity tests. The data obtained from such tests are influenced by the size and shape of the room as well as by the position in the room of the equipment being tested. The test results will be reliable only at frequencies where the mode density inside the room is greater than a defined minimum. This mode density depends on both the frequency and the size of the room. Methods of increasing the mode density are therefore sought after so that the screened room can be used at lower frequencies without increasing the size of the room. The method of increasing the mode density described uses phase reflection gratings on the walls of the screened room to produce diffuse reflections. These gratings are constructed from sequences of integers which represent the depths of the gratings. The authors describe the use of binary sequences whose auto-correlation is zero. They assume that sequences with this property, which work well as sound diffusers, will also work well for the present purposes. In order to decide which sequences work best it would be useful to have some measure which would predict the increase in modes obtainable for each particular sequence. The authors describe such a measure and also provide verification that this measure does in fact give a valid indication of the increase in mode density inside the screened room.