NSA and resonance in a semi-anechoic chamber in low frequency

As a result of the continuing growth of high speed electronic systems and equipment in the world, EMI (electromagnetic interference) is becoming a big problem. Specifications for the purpose of imposing maximum acceptable levels of noise that can be radiated from electronic systems and equipment have been published. Almost all the electronic products in the world should be tested to ensure compliance to these specifications. The tests should be performed over a wide frequency range of 30 to 1000 Mhz, either in open field test sites or inside anechoic chambers. For the open field test sites, the tests can be time consuming and expensive at the mercy of the environment, both natural and manmade. Many absorber lined chambers have been established for EMC tests in the world. To simulate an open test site, designers of these chambers must consider controlling reflection within the chamber and shielding the chamber from transmitting or receiving radio wave interference. To ensure whether a chamber is suitable for EMC tests, many regulations have been published such as ANSI C63.4, CISPR 16 and prEN50147-2. The criterion is the deviation from an ideal free-field transmission factor between a source and a receiving antenna, the so-called normalized site attenuation (NSA). The NSA is often measured to confirm whether the chamber satisfies the characteristics required. Problems often come out in the low frequency range. In this paper one reason which may cause the irregularity is discussed.