Evaluating semi-anechoic rooms using an accurate and ultra-repeatable source

Semi-anechoic rooms are very popular for making commercial EMI emissions measurements. They allow measurements in a RF quiet environment, so that low level fields may be measured without masking by ambient signals. The absorber material on the walls prevent reflections etc. which would change the value of the received signal level. Standard test procedures exist to test the anechoic room RF environment to insure that measurements can be made accurately enough for the test standard, but these tests are not well suited to analyzing where the cause of problems within a room might be located. This paper shows how a small transmitting ultra-repeatable electric field source was used to carefully examine an anechoic room, and to diagnose where unwanted reflections were occurring. A series of measurements were made within an anechoic test room, and the data examined to show the uncertainty as the test setup was moved to various locations within the room. From this data, the primary reflection from side and end walls could be considered, and areas of absorbing materials could be shown to be faulty. This process shows promise for future evaluation of both new and older anechoic rooms.