Exploring the Use of 5-m Semianechoic Chambers for Compliance Testing

Most engineers with experience in electromagnetic compatibility (EMC) engineering know that many EMC tests on electronic products can be performed in shielded semianechoic chambers (SACs). SACs are usually either of two sizes: a 3-m chamber, in which the antenna is placed 3 m from the product under test, or a 10-m chamber, in which the antenna is placed 10 m from the product under test. SACs can also be made into fully anechoic chambers (FACs). In addition to the absorbent material placed on the sidewalls and ceiling of an SAC, the floor of an FAC is lined with absorbent material. The absorbent material on the floor can be permanent; it can also be temporary for applications such as the radiated radio-frequency field immunity test required in IEC 61000-4-3.¹

Internal view of a 5-m semianechoic chamber.

Another chamber size has recently been gaining popularity: the 5-m chamber, which enables placement of the antenna 5 m from the turntable holding the product under test.

A typical 3-m chamber measures approximately 9 m long x 6 m wide x 6 m high. A standard 5-m chamber measures approximately 11 x 7 x 9 m. And a 10-m chamber typically measures 19 x 12 x 9 m. Thus, a 3-m chamber requires roughly 54 m² of floor space, a 5-m chamber requires 77 m², and a 10-m chamber requires 228 m². In other words, a 5-m chamber calls for about 50% more floor space than a 3-m chamber but only one-third of the floor space necessary for a 10-m chamber. Similarly, the volume required by a 5-m chamber is more than twice that of a 3-m chamber but only about one-third that of a 10-m chamber.

Most 3-m chambers use a 1-m-diam turntable, which limits the size of the product that can be tested. Most recently built 5-m chambers use a 2-m-diam turntable, which obviously allows for testing of a larger product or system. And for very large products, a 10-m chamber has the capacity for still larger turntables: 3- and 4-m-diam turntables are currently popular.

The use of 2-m-diam turntables instead of 1-m-diam turntables could prove important in the future. The 2-m-diam turntables allow easy correlation with several studies being conducted in Europe on FACs. These studies relate to the proposed European Standard prEN 50147-3, which has the following criteria: a maximum turntable diameter of 1.2 m for a 3-m FAC, a maximum turntable diameter of 2 m for a 5-m FAC, and a maximum turntable diameter of 4 m for a 10-m FAC.² Therefore, a 5-m SAC with a 2-m-diam turntable provides direct correlation with this European approach to measurement.

Near-Field Technical Concerns

This comparison of test-chamber sizes assumes that the chambers meet the normalized site attenuation (NSA) criteria required by the U.S. Federal Communications Commission (FCC) to be listed as an acceptable test site for generating data that complies with FCC regulations and for testing products under FCC's declaration of conformity rules. Both developers and users of 5-m chambers have demonstrated that it is easier to meet the NSA criteria with a 5-m chamber than with a 3-m chamber because of the greater electrical distances between the antenna and the chamber walls.

External view of a 5-m chamber.

In a 5-m chamber, the antenna is 2 m farther away from the product under test than the antenna in a 3-m chamber. The 5-m chamber's antenna is therefore immersed in the far field at many important frequencies. At 60 MHz, one wavelength is 5 m. Using the definition of far field as greater than one wavelength from the source, all measurements above 60 MHz in a 5-m chamber are in the far field. For a 3-m chamber, measurements must be above 100 MHz to be one wavelength from the product under test.

When testing at an open-area test site (OATS), many licensed signals (television and FM radio) broadcast in the electromagnetic ambient between 60 and 100 MHz, which gives the 5-m chamber a decided advantage over a 3-m OATS: in a 5-m chamber, there are no ambient signals to interfere with emission measurements.

Rear view of a 5-m chamber.

Correlation to 10-m Regulatory Limit

When measurement of a product is made at 3 m, the correlation to a regulatory limit at 10 m assumes an inverse distance relationship for the generated fields (the magnitude of the field decreases inversely with the distance from the product source). A signal measured from a product under test at 3 m is therefore assumed to fall off about 10 dB in magnitude as it propagates out to 10 m. This correlation is not always accurate, due to near-field effects such as significant capacitive coupling to the receiving antenna. However, when the antenna is moved back to the 5-m location, the near-field effects diminish and the correlation becomes more accurate. Assuming the inverse-field correlation, the drop-off in signal strength from 5 to 10 m should only be 6 dB.

The 5-m chamber is approximately only 15% more expensive than a 3-m chamber and requires just 50% more building space. Including building costs, a 5-m chamber is only moderately more expensive than a 3-m chamber. And a 5-m chamber is only one-third the cost of a typically priced $1.2 million 10-m chamber. Obviously, a 5-m chamber also requires a smaller building than a 10-m chamber, which further reduces building costs. Unfortunately, like the 10-m alternative, a 5-m chamber requires a high ceiling.

Chamber control station.

Conclusion

Five-meter chambers are technically superior to 3-m chambers for many products over a wide frequency range. Because they are larger, 5-m chambers have less opportunity for reflections from the chamber walls. These chambers are correlatable to current research projects in Europe on fully anechoic chambers, and they easily meet the NSA requirements specified in ANSI C63.4.³ Their correlation to 10-m regulatory requirements is, in general, superior to the correlation of 3-m chambers to the same requirements. The cost of 5-m chambers is lower than that of 10-m chambers by a wide margin, but only moderately higher than that of 3-m chambers. And by utilizing a 2-m turntable, testing of larger products is possible without precluding smaller products.

This is not to say that the 5-m chamber is without technical challenges, including:

References