An electromagnetic compatibility (EMC) or Electromagnetic Interference (EMI) chamber is a highly specialized piece of test equipment used in the testing of electronic devices against regulatory standards concerning both radiated and conducted radio frequency (RF) emissions and immunity. There are many different types EMC test facilities, test cells, open area test sites (OATS) and EMC test chambers used for EMC testing. Out of all of the different types of EMC test facilities used to perform EMC testing, the most common one is undoubtedly the semi-anechoic chamber.
Semi-anechoic chambers along with OATS are the preferred test facilities described in nearly all radiated and conducted emissions testing standards including ANSI C63.4 and CISPR 16. These standards describe in great detail the methods of measurement of radio-noise emissions from low-voltage electrical and electronic equipment.
The term “anechoic” means non-reflective, non-echoing, and echo-free. The term “semi” denotes that a portion of the chamber is reflective, usually the sheet metal ground reference plane located on the floor of the chamber. The reason the floor is kept reflective and not loaded with absorber is because it replicates the real world more closely to that of a free space environment and because radiated emissions measurements are more repeatable and accurate if the RF energy emanating from the equipment under test (EUT) is fully reflected. In contrast, the reflections that pass-through foam absorber material attenuate differently (sometimes none at all) depending of the angle of incidence of the RF field impinging upon it.
Although the floor is reflective, the remaining five sides of the chamber are treated with ferrite tile, RF foam absorber, or a hybrid combination of the two to make the chamber appear more like free-space to any RF energy produced within it. See references 2 and 3 for further details on how RF absorber materials work and for help in choosing the RF absorber material that best suits your needs.
RF shielded room
The RF shielded room forms the basis for a semi-anechoic chamber. It’s a near RF-impenetrable metal box, that offers electric and magnetic field shielding effectiveness up to 100 dB over a wide range of frequencies. It’s good for emissions testing because no signals or ambient noise from the outside world can penetrate it, making it much easier to measure the emissions generated only by the EUT. It’s also good for immunity testing because no signals generated inside the enclosure can escape to the outside world, possibly causing harmful interference to equipment or personnel located nearby.
Keep in mind there are different types of shielding materials and configurations for semi-anechoic chambers and each of them have different shielding performance levels. If your application requires maximum shielding effectiveness, then you may want to consider shielding material that is double electrically isolated. This shielding has two layers of metal (one on the inner surface and one on the outer surface), typically connected at only the main grounding point of the chamber. If anything needs to be mounted to either the inside or outside of the chamber then the mounting screws selected should be keep short enough to penetrate just the first shielding layer and not go through the second.
Designing a semi-anechoic chamber
Specifying an anechoic chamber and associated equipment that precisely fulfills all of your EMC testing wants and needs is complicated. Some of the things that need to be considered when setting up an EMC test facility include a semi-anechoic chamber are:
- Size of the EUT and number of cables
- Desired size of the quiet zone
- The quiet zone is a volume of space where the least amount of disturbance from unwanted signals is incurred.
- Antenna separation requirements
- Frequency range
- Cable and auxiliary equipment (AE) filtering requirements
- AE needed to properly exercise the EUT during test (power supplies, bit error rate testers, peripheral equipment, etc.)
- Measurement accuracy
- Typically, +/- 4 dB according to ANSI C63.4
- Full or pre-compliance
- Emissions only, immunity only, or both
- Door type, size, location and maintenance
- Ground plane flatness
- Absorber type(s)
- Installation location (where will it go within your facility)
- Proper grounding
- Types of test equipment to go with it (spectrum analyzers or receivers, cables, pre-amplifiers, RF amplifiers, attenuators, antennas, antenna masts, turntables, etc.)
- Location of test equipment (when in use and when not in use)
- Anti-room size and location
- EUT staging area
- EUT troubleshooting (assortment of passive components: capacitors, ferrite beads, cable clamps, inductors, resistors; EMI filters, soldering equipment, ESD workbench, etc.)
- Cost for calibration of equipment
- Cost for accreditation
3, 5, 10-meter chambers
The most common chambers are the 3m, 5m and 10m types. 3m refers to an EUT to antenna separation distance of 3 meters, 5m is an EUT/antenna separation distance of 5-meters, and 10m is an EUT/antenna separation distance of 10 meters.
The smaller the chamber the less costly it is to purchase mainly because it doesn’t need as much expensive absorber material. The downside is that it is tougher to achieve the measurement accuracy needed to perform full-compliance testing, especially at frequencies below 300 MHz, and the quiet zone is limited to about 1.5m, so it’s only good for testing smaller EUTs. It’s also not usually possible to achieve full antenna scan height of 1-4 m in the smaller chambers however there are full-compliance 3m chambers available on the market which do not possess many of these limitations. The dimensions of a full-compliance 3-meter chamber is ~ 9.0m x 6.0m x 6.0m.
A 5-meter chamber is really just oversized 3-meter chamber where the 5m chamber dimensions are ~ 11.5m x 7.5m x 6.0 m. 5-meter chambers are longer, wider, have a bigger door, and larger quiet zone than a 3-meter chamber (the quiet zone is ~ 2 to 4 m in a 5m chamber). 5-meter chambers can accommodate even larger EUTs than 3-meter chambers and they possess excellent performance capabilities.
10-meter chambers are considered the benchmark with which all other anechoic chambers are measured. They are twice as long as 5-meter chambers and also wider and higher. 10-meter chambers can accommodate larger EUTs due to their large quiet zone area (> 4m) and very large entry door (~3m by 3m). If budget and available floor space are not a problem then a 10-meter chamber is the way to go.
Other points to consider when specifying a semi-anechoic chamber:
- Provide chamber installers with a level pad in which to install the ground plane. To maintain measurement accuracy the ground plane should not have deformations exceeding 5cm at a 3m antenna separation distance.
- If testing to Class B emissions limits, try to find AE that also meets Class B limits. Trying to test a Class B device using AE that only meets Class A limits can be problematic.
- Ensure no cables enter or exit the shielded enclosure unless adequately filtered. One unfiltered cable penetration can totally defeat shielding effectiveness of the entire semi-anechoic chamber.
- Keep RF immunity testing equipment (signal generators, RF amplifiers, transmitting antennas and cables) far away from any cables or equipment used to perform emissions testing. The emissions test equipment can easily pickup signals produced by the RF immunity equipment and this can greatly hamper the emissions testing capability of the semi-anechoic chamber. If the chamber is used for both emissions and immunity this shouldn’t be much of a problem since only one type of test (emissions or immunity) can be performed at one time.
- Eadie, A. (2016, June 29). The Anechoic Chamber Guide For EMC and RF (Wireless) Testing.
- In Compliance. (2018, September 28). What Every Electronics Engineer Needs to Know About: Absorbing Materials.
- Glen, D. (2018, October). In Compliance, 10 (10), pp. 60-63.
- Gray, D.J. (2015, November 12). An Overview of EMC Chambers.