Evanescent modes in a unique mode-stirred chamber the Advanced Technology Chamber (ATC)-design, construction, operation and data

Abstract

Mode-stirred (tuned reverberating) chambers are used for performing EMC measurements. These chambers have several advantages over traditional EMC test facilities. These advantages include the ability to simulate high intensity radiated field (HIRF) levels by using low power amplifiers to produce a statistically isotropic electromagnetic environment. The major disadvantage of the mode-stirred chambers has been the inability to produce an electromagnetic environment near the cutoff frequency of the chamber. Typically, a mode-stirred chamber begins to operate at six times the cutoff frequency or approximately 200 MHz for an average-sized chamber. This limitation has a detrimental impact on electronic equipment assessment within the high frequency (HF) spectrum (2-30 MHz) where a vast majority of electromagnetic interference (EMI) events occur. This paper describes a unique chamber which combines the mode-stirred technique with a method which enables test personnel to perform HIRF testing into the HF spectrum. The Advanced Technology Chamber (ATC) can be used as a standard mode-stirred chamber above 200 MHz and HIRF testing can be continued into the HF spectrum without reinstalling the equipment under test into a different test cell. HF susceptibility testing is usually performed on subsystems utilizing the test techniques described in MIL-SID-462D, Test Method RS103. The wave impedance associated with a parallel plate antenna is 377 ohms. This impedance has little relevance to the wave impedance of an environment containing HF transmitting antennas located within 100 meters of electronic equipment. This environment is of particular interest to the US Navy with its extremely harsh HF topside EM environments. The major advantage of the ATC over parallel plate antennas and hybrid chambers is the ability to produce a true HF wave impedance. The majority of HF disruptions occur close (with respect to the electrical wavelength) to the HF source. Since the wave mode is not TEM in the near field of the antenna, the present plane-wave test techniques can lead to erroneous results.