Enclosing circuitry in a shielded enclosure is a good way to control radiated emissions and to improve immunity to radio-frequency interference. For example, an enclosure fabricated from highly permeable materials can be used to protect sensitive circuitry or devices against magnetic interference. The shield acts as a barrier to unwanted electromagnetic radiation. The material and construction of the enclosure determines its properties as a shield. In the majority of cases, enclosures are used to contain fields generated in a system as a by-product of the system's operation, and thus avoiding interference to nearby systems and complying with emission-control requirements of relevant international standards such as CISPR 25, EN 55022, etc. Enclosures also protect system components against internally or externally generated interference.

Shields function on the basis of two major electromagnetic phenomena: reflection from a conducting surface and absorption in a conductive volume. An electromagnetic plane wave (far field) striking a metallic surface encounters both types of losses. Part of the wave is reflected while the remainder is transmitted and attenuated as it passes through the media. The combined effect of these losses (reflection and absorption) determines the effectiveness of the shield (see Figure 1).

Figure 1. Shield attenuation of electromagnetic fields. SE = sheilding effectiveness, R = reflection loss, and A = absorption loss.

Reflection from a shield results when the impedance of the wave in free space is different from the impedance of the electromagnetic wave in the barrier. This phenomenon is independent of the barrier thickness and is a function of the material's conductivity, magnetic permeability, and frequency. Because the wave impedance is different for magnetic fields (low impedance), electric fields (high impedance), and plane wave fields, the barrier reflection follows a different characteristic for each wave type. In addition to the reflection phenomenon described, the incident wave is re-reflected inside the shield.

Absorption is the transformation of the wave energy in the shield to heat. This loss is frequently defined by the term skin depth, which, in turn, is a function of the shield's conductivity and permeability. The absorption loss does not depend on the wave impedance of the impinging field, and thus it is not directly related to near- or far-field conditions of the system. The attenuation of the wave in the shield is described by an exponential decaying expression given by:

The shield's effectiveness varies with frequency, shield geometry, positioning within the shield, type of field being attenuated, directions of incidence, and polarization.

Various techniques are employed in industry to determine the performance of enclosures. These techniques provide a measure of the enclosure's effectiveness in shielding against unwanted electromagnetic interference (EMI). The basic measurement principle is the determination of the reflection or absorption properties of the material from which the enclosure is fabricated. The two primary methods can be referred to as the insertion-loss method and the twin-antenna method. The main factor differentiating the two methods is the use of an antenna to perform the measurement in the twin-antenna method.

Base Material. In general, the shielding effectiveness of the base material is determined using the insertion-loss method. This technique (described in ASTM D4935) involves irradiating a flat, thin sample of the base material with an electromagnetic wave over the frequency range of interest.¹ The test method utilizes a coaxial transmission line with an interrupted inner conductor and a flanged outer conductor. A reference measurement for the empty cell is required for the shielding-effectiveness assessment (see Figure 2). The reference sample is placed between the flanges in the middle of the cell, covering only the flanges and the inner conductors. A load measurement is performed on a solid disk whose diameter is the same as that of the flange (see Figure 3). The reference and the load measurement are performed on the same material. The shielding effectiveness is determined from Equation 3, which is the ratio of the incident field to that which passes through the material.

Figure 3. Load sample in the jig for capacitive method.

Other methods involve the use of a transverse electromagnetic (TEM) cell with an interrupted inner conductor and a rectangular cross section for E-field measurements or a modified TEM cell with two loop antennas for magnetic field measurement. Although such methods are used, they are not referenced in any test standard.

For the evaluation of large flat samples, test methods based on MIL-STD 285 are used. It is worth mentioning that MIL-STD 285 has since been withdrawn and replaced with IEEE-STD 299.^2,3 The test method requires a shielded enclosure with an open window (see Figure 4). An initial measurement is performed with the enclosure, and another is performed with the window covered with the conductive material. At lower frequencies, coplanar loop antennas are used for magnetic-shielding measurement and rod antennas are used for electric field–shielding measurement. Tuned dipoles or broadband antennas are used at higher frequencies.

Figure 4. Typical measuring setup for larger samples.

Prototype Enclosure. The most common technique for evaluating the shielding effectiveness of an enclosure is the twin-antenna method. This method requires either the use of an emitter and a receptor (both antennas) or the use of a combination of a noise source and receive antenna or a combination of a transmit antenna and a receiver (such as an isotropic field probe). The test usually involves fabricating a prototype enclosure from the base material and comparing its shielding performance against the product's functional requirements or against a metallic or conductive-coated enclosure.

The twin-antenna method involves positioning a receptor (usually a dipole antenna) inside the enclosure and placing a transmitter at a far-field distance. The basic setup simulates the enclosure's performance in shielding the enclosed components against interference. Placing the emitter within the enclosure and placing the receptor at some distance away simulates the shield's ability to inhibit the transmission of electromagnetic waves from the enclosed circuitry to neighboring devices or equipment. Appropriate positioning with respect to field polarization and the direction of induced current enables the acquisition of relevant shielding-effectiveness data. Small loop antennas or rod antennas can be used for this measurement; loop antennas have the advantage of creating a low-impedance-level measuring setup, which is not easily influenced by ambient conditions. Small rod antennas require adequate grounding connections to the conductive layer of the enclosure to ensure accurate results. In addition, these antennas make a high-impedance measuring system and are sensitive to ambient conditions.

The procedure requires an initial measurement without the enclosure. The reference value is the coupling measured between the emitter and the receptor antennas. The measurement is then repeated with only the enclosure in place. IEEE-STD 299 requires measurement at various positions within the enclosure and specifies that the transmitting antenna be positioned outside the enclosure. The standard, which also applies to enclosures whose smallest linear dimension is no less than 2 m, specifies the use of standard log-periodic and biconical antennas.

A number of test standards and methods are available for determining the shielding effectiveness of conductive plastic materials. Too many methods lead to inconsistent results and overspecification by materials specialists and enclosure designers. Shielding-effectiveness data obtained from ASTM D4935 often do not provide equipment enclosure designers with relevant and sufficient data to facilitate an enclosure design that provides adequate shielding for a given application.

The shielding performance of conductive plastic materials is greatly influenced by the molding process parameters. Furthermore, the shielding performance of the base material is not necessarily the same as that of the enclosure fabricated from the same material. In the design and development of the base material, manufacturers evaluate the suitability of the material through performance testing and production process–variation optimization. The production process does not lend itself to one-off sampling; it is usual for a batch run of 50 samples or more to be produced before production variations are minimized to obtain an optimum shielding level. For manufacturers of conductive materials, evaluation of a flat, thin sample of the base material in accordance with ASTM D4935 not only is cost-effective but also provides a means of material comparison during product development.

Equipment enclosures, however, are not infinite planar structures. Typically, they would have a few discontinuities, the geometry of which are predetermined by functional and ergonomic requirements. Because shielding effectiveness is the accepted criterion for assessing the performance of a material or enclosure to attenuate electromagnetic fields, it is important to have standardized test methods to measure it. Furthermore, the industrial global market dictates that harmonized test standards or methods must be in place to ensure that measurement results obtained in one laboratory can be confirmed in another, thereby facilitating the free movement of goods.

For the shielding industry, a harmonized approach for evaluating the shielding performance of materials and enclosures is crucial. This harmonized approach should specify the procedures, measurement conditions, test equipment, and test limits (if applicable) that the test should meet. Manufacturers of shielding materials, enclosure designers, and end-users require a common reference document. Although such a document could be voluntary and not legally binding, it would remove the current confusion and proliferation of company-specific test methods and the mismatching of test techniques. By demonstrating compliance with the requirements of such standards, manufacturers of enclosures and materials would be able to self-certify that products conform to the requirements of the standard.

Many test methods and standards address the shielding-effectiveness measurement of enclosures. Although nearly all are based on reflection and absorption measurement, some relate to the measurement of the material's conductivity and permeability. The fundamental differences between each method seems to involve the frequency range, the size of the material or enclosure, field condition, and measurement instrumentation. These differences are significant enough to deem published data sheets meaningless because it is impossible to know the exact test condition in which a measurement was performed. Following is a brief review of standards used or referenced in the shielding enclosure industry.

MIL-STD 285 (Withdrawn). This was a military standard introduced in 1956 for the shielding-effectiveness measurement of metal enclosures. The standard has an upper frequency limit of 400 MHz. It was specifically developed for large-enclosure and shelter assessment. The standard has since been withdrawn and superseded by IEEE-STD 299. Testing to MIL-STD 285 is inappropriate; however, uncontrolled versions of this standard are in abundance and in use by manufacturers and end-users of enclosures. The derivatives of this standard are used primarily to assess the shielding effectiveness of large, flat materials and small enclosures. The accuracy and repeatability of results is questionable across laboratories.

IEEE-STD 299. Developed by the Institute of Electrical and Electronics Engineers (IEEE), this standard specifies an upper frequency limit of up to 100 GHz and suggests a frequency-sweep measurement rather than the fixed-frequency measurement specified in MIL-STD 285. This is a voluntary standard with no legally binding requirement. The test method described in this standard is only applicable to an enclosure whose smallest linear dimension is > 2 m. In its current form, this standard does not apply to small- and medium-sized enclosures (e.g., IC packaging, enclosures for automotive and avionics applications, handheld and portable devices enclosures, etc.). IEEE-STD 299 is currently being revised, and an extension that covers enclosure sizes with much shorter linear dimensions is expected.

ASTM E1851. This standard was developed by the American Society for Testing and Materials (ASTM) for the evaluation of shelters.⁴ It requires magnetic shielding-effectiveness measurements between 140 and 160 kHz and between 14 and 16 MHz; far-field shielding measurements between 300 and 500 MHz, 900 and 1000 MHz, and 8.5 and 10.5 GHz are required. This standard, like the first two, was developed for measuring the shielding effectiveness of large enclosures and shelters.

VG 95373 Part 15. This German military standard describes the shielding effectiveness of enclosures.⁵ The standard covers the frequency range above 30 MHz; between the frequency range of 30 and 200 MHz, a minimum antenna-to-enclosure separation distance of 2.5 m is specified. This distance can be reduced to 1 m for frequencies greater than 200 MHz. The test method specifies that a receiving antenna—which must be small compared with the enclosure under test—be built into the enclosure for the test. Although there are technical problems with this standard, it is currently the only standard applicable to small- and medium-sized enclosures. It is not surprising that the test method described in VG 95373 Part 15 (or variations of it) is used for shielding-effectiveness evaluation of small- and medium-sized enclosures and is favored by manufacturers of conductive plastic enclosures.

ASTM D4935. ASTM developed this standard for evaluation of flat, thin samples. Plane-wave shielding-effectiveness measurements are made from 30 MHz to 1 GHz; higher-frequency measurements are possible by modifying the coaxial transmission jig. As previously discussed, this standard is used extensively by manufacturers of conductive plastic materials. The test procedure is simple to set up and does not require the use of an anechoic chamber. The standard only covers the use of a coaxial transmission line, but similar measurements can be made in a structure similar to a TEM cell.

Current standards covering the shielding-effectiveness evaluation of enclosures are designed for evaluating the effectiveness of large enclosures and shelters to attenuate electromagnetic waves. The standards are applicable to enclosures whose smallest linear dimension is 2 m; this limitation excludes a wide range of enclosures used for a variety of applications whose smallest linear dimension ranges from about 10 cm to just under 2 m. Within this range, test methods that require the use of standard EMC measurement antennas (biconical, log-periodic, and horn antennas) cannot be used as receiving antennas because they are physically too large to fit inside an enclosure. Equally important, the functional requirement of an enclosure within this range differs from that of large enclosures and shelters. It is more likely that small enclosures would be used to contain noise generated inside the system than to protect the system from external interference. A significant number of small enclosures within this range are fabricated from conductive plastics.

Although numerous techniques can be used to assess the shielding performance of small enclosures, the shielding industry currently lacks voluntary or mandatory standards to refer to. The industry requires a standard that accounts for different shielding-level requirements as well as different enclosure sizes. Such a standard may need to incorporate a number of measurement techniques to address the variety in shape, size, material composition, and other electrical parameters of an enclosure that could influence its ability to attenuate electromagnetic waves.

The structure of such a standard could take the form shown in Table I. This structure, which is much like ISO documents, is similar to many international standards in which multiple parts are the norm. One of the benefits of such a structure is that the individual parts are self-contained, with details of equipment setup and test procedures. Each part can be updated as and when required. This feature would become important in any standard designed to cover enclosures whose smallest linear dimension ranges from 10 cm to 2 m.

The proposed program ensures the standardization of measurement setup and instrumentation wherever possible. Being able to reference common test specifications would assist manufacturers of shielding materials and enclosures in the certification of products. Each part of the standard should describe the measurement setup with appropriate test limits.

The industry needs a product standard for assessing the shielding effectiveness of enclosures and materials. A standard with a structure similar to that shown in Table I would result in the reduction of unnecessary and uncontrolled test standards and methods. The standard need not be mandatory, but the inclusion of test procedures and methods that adequately cover the measurement of the shielding effectiveness of small enclosures would be of great use to manufacturers and enclosure designers of small enclosures.