MVG Presents “An Overview of EMC Chambers”
Click here to download a copy of the presentation.
Overview: The webinar will give a general overview of anechoic chambers in general and will discuss the basics of EMC chambers. We will discuss the common test distances (e.g., 3 m chambers, 10 m chambers) and how this affects the overall performance.
Who should attend: Engineers or technicians involved in EMC precompliance or full compliance testing to commercial & MIL standards and design engineers who need to understand the EMC mperformance of the device they are designing.
Donald J Gray
Donnie Gray received a B.S. in Mathematics and Economics from Allegheny College in 1984 and a MSEE from the University of Colorado in 1990. He has recently joined MVG to initiate the business development, strategies, & marketing activities and to drive sales in the electromagnetic compatibility industry. Donnie has over 25 years’ experience in leading engineering teams and sales & business development with satellite communications, antennas, and electro-magnetic projects. This includes experience in military and commercial applications. He is a Senior Member of the Antenna Measurements and Techniques Association (AMTA, a member of IEEE and of the IEEE EMC Society. He has published numerous technical papers regarding these applications, including antenna measurements, EMC testing, and RF absorber applications.
The following are questions presented to the speaker by the attendees during the webinar, along with answers to each.
How are the chambers tested to make sure they are actually shielded from the outside world?
Answer: Chambers are tested for their shielding attenuation as per IEEE STD- 299, (and/or the older MIL-Std-285A). Typically, they are tested from 10kHz – 18 GHz (4 frequencies), though more frequencies can be added for an additional cost. Some chamber manufacture’s perform self-testing; though it is better, and more impartial, to have the chamber verified by a third-party vendor (e.g., such as Sieberdorf or Shielding Integrity Services).
What is the method to determine the boundaries of the QZ inside a given EMC chamber ?
Answer: The size of the QZ will depend on the overall performance required and the frequency range. Once this is known, then we select the appropriate absorber size and chamber size to accommodate this QZ size. For customized sizes, we need to use the EM properties of the absorber, and do some modeling and analysis via some empirical formulas, ray tracing, or some other type of numerical models. In many instances, this is an iterative process.
You mentioned that the absorber has that sharp tip to gradually increase the impedance seen by the incident wave. Many of your images show a “backwards” white set of absorbers fitted into the blue absorbers on the walls. Do those serve any EMC purpose?
Answer: This “backwards white set of absorbers” is actually a white cap. These white caps reflect visible light and enable a brighter chamber with minimal lighting in the ceiling. The white cap is constructed from a dielectric material (e.g., polystyrene) and is designed to have negligible impact on the propagation of the EM wave as it propagates through.
Thank you for your talk. Why do some absorbers shown have non-sharp/flat top?
Answer: The sole reason for the flat – or truncated absorber tip – is to allow more working space in the chamber. The concept is that although we have a greater impedance mismatch, we maintain the same gradient of resistive material. At the lower frequencies (and thus, longer wavelengths) over which EMC testing is conducted, the absorber can be truncated while having little perceptible effect on the overall performance.
Can we expect new materials coming from stealth technology?
Answer: Good question. If these materials are not classified and become known in the “commercial” sector, then this becomes a possibility. Many companies are continually researching and developing various materials to control EM energy in some form or another. This work involves materials experts, chemists, etc. As more advances are developed, hopefully these materials will be used to enhance the materials being used in anechoic chambers.