ASTM D4935-18

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Designation: D4935 − 10 D4935 − 18
Standard Test Method for
Measuring the Electromagnetic Shielding Effectiveness of
Planar Materials
This standard is issued under the fixed designation D4935; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method provides a procedure for measuring the electromagnetic (EM) shielding effectiveness (SE) of a planar
material for a plane, far-field EM wave. From the measured data, near-field SE values maycan be calculated for magnetic (H)
2,3
sources for electrically thin specimens. Electric (E) field SE values mayare also able to be calculated from this same far-field
data, but their validity and applicability have not been established.
1.2 The measurement method is valid over a frequency range of 30 MHz to 1.5 GHz. These limits are not exact, but are based
on decreasing displacement current as a result of decreased capacitive coupling at lower frequencies and on overmoding (excitation
of modes other than the transverse electromagnetic mode (TEM)) at higher frequencies for the size of specimen holder described
in this test method. Any Select any number of discrete frequencies may be selected within this range. For electrically thin, isotropic
materials with frequency independent electrical properties of conductivity, permittivity, and permeability, measurements may will
possibly be needed at only a few frequencies as the far-field SE values will be independent of frequency. If the material is not
electrically thin or if any of the parameters vary with frequency, measurements should be made at manymake measurements at
several frequencies within the band of interest.
1.3 This test method is not applicable to cables or connectors.
1.4 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1711 Terminology Relating to Electrical Insulation
3. Terminology
3.1 Definitions—For definitions of terms used in this test method, refer to Terminology D1711.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 dynamic range (DR), n—difference between the maximum and minimum signals measurable by the system.
3.2.1.1 Discussion—
This test method is under the jurisdiction of ASTM Committee D09 on Electrical and Electronic Insulating Materials and is the direct responsibility of Subcommittee
D09.12 on Electrical Tests.
Current edition approved May 1, 2010May 1, 2018. Published June 2010May 2018. Originally approved in 1989. Last previous edition approved in 19992010 as
D 4935–99.D4935 – 10. DOI: 10.1520/D4935–10.10.1520/D4935-18.
Wilson, P. F., and Ma, M. T., “A Study of Techniques for Measuring the Electromagnetic Shielding Effectiveness of Materials,” NBS Technical Note 1095, May 1986.
Adams, J. W., and Vanzura, E. J., “Shielding Effectiveness Measurements of Plastics,” NBSIR 85-3035, January 1986.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4935 − 18
Measurement of materials with good SE require extra care to avoid contamination of extremely low power or voltage values by
unwanted signals from leakage paths.
3.2.2 electrically thin, adj—thickness of the specimen is much smaller (< ⁄100) than the electrical wavelength within the
specimen.
3.2.3 far field, n—that region where vectors E and H are orthogonal to each other and both are normal to the direction of
propagation of energy.
3.2.4 near field, n—that region where E and H are not related by simple rules.
3.2.4.1 Discussion—
The transition region between near field and far field is not abrupt but is located at the distance close to λ/2π from a dipole source,
where λ is the free-space wave length of the frequency of the source. This It is possible this concept of regions is further blurred
by reradiating as a result of scattering by reflecting materials or objects that may be distant from the source. The interior of metallic
structures often contains a mixture of near-field regions.
3.2.5 shielding effectiveness (SE), n—ratio of power received with and without a material present for the same incident power.
3.2.5.1 Discussion—
SE is usually expressed in decibels (dB) by the following equation:
P
SE 5 10log ~dB! (1)
P
where:
P = received power with the material present, and
P = received power without the material present.
If the receiver readout is in units of voltage, use the following equation:
V
SE 5 20log ~dB! (2)
V
where:
V and V = respective voltage levels with and without a material present.
1 2
According to these equations, SE will have a negative value if less power is received with the material present than when it
is absent.
4. Significance and Use
4.1 This test method applies to the measurement of SE of planar materials under normal incidence, far-field, plane-wave
conditions (E and H tangential to the surface of the material).
4.2 The uncertainty of the measured SE values is a function of material, mismatches throughout the transmission line path,
dynamic range of the measurement system, and the accuracy of the ancillary equipment. An uncertainty analysis is given in
Appendix X1 to illustrate the uncertainty that may be probability of uncertainty achieved by an experienced operator using good
equipment. Deviations from the procedure in this test method will increase this uncertainty.
4.3 Approximate near-field values of SE maycan be calculated for both E or H sources by using measured values of far-field
SE. A program maycan be generated from the source code in Appendix X2 that is suitable for use on a personal computer.
4.4 This test method measures the net SE caused by reflection and absorption. Separate measurement of The reflected and
absorbed power may bemeasurement is accomplished by the addition of a calibrated bidirectional coupler to the input of the holder.
5. Apparatus
5.1 A basic equipment setup is shown in Fig. 1.
FIG. 1 General Test Setup
D4935 − 18
5.2 Specimen Holder—Physical dimensions of a specimen holder are given in Annex A1. The specimen holder is an enlarged,
coaxial transmission line with special taper sections and notched matching grooves to maintain a characteristic impedance of 50
Ω throughout the entire length of the holder. This impedance is checked in accordance with 7.1, and any variations greater than
60.5 Ω are corrected. There are three important aspects to this design. First, a pair of flanges in the middle of the structure hold
the specimen. This allows capacitive coupling of energy into insulating materials through displacement current. Second, a
reference specimen of the same thickness and electrical properties as the load specimen causes the same discontinuity in the
transmission line as is caused by the load specimen. Third, nonconductive (nylon) screws are used to connect the two sections of
the holder together during tests. This prevents conduction currents from dominating the desired displacement currents necessary
for the correct operation of this specimen holder.
5.3 Signal Generator, a source capable of generating a sinusoidal signal over the desired portion of the frequency range specified
in 1.2. A 50-Ω output impedance is needed to minimize reflections caused by mismatches. Precision step attenuators are useful in
increasing the effective dynamic range for SE measurements.
5.4 Receiver, a device with a 50-Ω input impedance capable of measuring signals over the same frequency range as the signal
generator in 5.3. A wide dynamic range is desirable to achieve a wide dynamic range of measured SE values. Typically, either a
spectrum analyzer or a field intensity meter is used.
5.5 Coaxial Cables and Connectors—These are devices for connecting power between specific components without causing
interference with other components. These shouldshall all have a 50-Ω characteristic impedance. Double-shielded cables provide
lower leakage than single-shielded cables. Type N connectors provide more reliability and less leakage than BNC connectors.
Precision 14-mm connectors give lower mismatch errors and are more reliable under heavy usage than other connectors but are
more expensive and are not used on most generators or receivers.
5.6 Attenuators—These are devices are used to isolate the specimen holder from the signal generator and the receiver. Their
main purpose in this system is for impedance matching. A10-dB, 50-Ω attenuator shouldshall be used on each end of the specimen
holder. The material under test usually causes a large reflection of energy back into the signal generator. This may generator, which
also cause variations of the incident power by changing the generator impedance loading. Use of a bidirectional coupler allows
monitoring and correcting any changes in incident power as a result of this loading. Attenuators greater than 10 dB will excessively
decrease the dynamic range of the measurement system.
6. Test Specimens
6.1 The reference and load specimens shall be of the same material and thickness. Both are shown in Fig. 2. Dimensions are
shown in Fig. 3. The load specimen can be larger than the outer diameter of the flange on the holder but keeping them to the
dimensions shown in Fig. 3 will expedite handling.
6.2 Specimen thickness is a critical dimension. For the best repeatability of SE measurements, reference specimen and load
specimen shall be identical in thickness. For this test method, two specimens are considered to have identical thickness if the
difference in the average thicknesses is less than 25 μm and the thickness variation within and between specimens is less than 5 %
of the average.
6.3 Materials of the specimens may It is permissible for the specimen materials to be either homogeneous or inhomogeneous,
single or multiple layered, and conducting or insulating. Measured SE values of inhomogeneous materials are dependent on
geometry and orientation, and results are less repeatable than for homogeneous materials.
6.4 Before tests, condition test specimens for 48 h at 23 6 2°C and 50 6 5 % relative humidity. Tests shall be performed
immediately upon removal from the conditioning environment.
FIG. 2 Illustration of Reference and Load Specimens
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FIG. 3 Drawing That Gives Dimensions of Reference and Load Specimens
7. Preparation of Apparatus
7.1 An Performed an initial check of the specimen holder should be performed with a time-domain reflectometer or other
suitable instrument to ensure that a characteristic impedance of 50 6 0.5 Ω has been achieved during construction and that this
impedance has not been degraded during shipment or handling. A time-domain system can give location of a mismatch in addition
to its magnitude.
7.2 Each time the ancillary equipment is connected to the specimen holder, good practice requires measurement of a reference
specimen to ensure the measurement system is in proper working order.
7.2.1 The dynamic range (DR) of the system can be checked by comparing the maximum signal level obtained with a reference
specimen to the minimum signal level obtained when using a metallic load specimen. The lower limit of the measurement system
sensitivity is a function of the sensitivity and bandwidth of the receiver. Narrowing the bandwidth of the receiver lowers the
detectable level but increases the measurement time. Leakage It is possible that leakage caused by connectors or cables maywill
reduce the DR of the system by providing a parallel signal path that does not pass through the specimen. If a step attenuator placed
in series with the specimen holder causes a change in the minimum signal detected that corresponds to a change in attenuator
setting, and if the step attenuator itself does not cause a leakage path, leakage is negligible and the DR measured above is correct.
If the levels do not correspond, the attenuation shouldshall be increased until a one-to-one correspondence is achieved to determine
the DR. Since Recheck connections since leakage from a coaxial connector is determined not only by the quality of the connector,
but also by the amount of torque used in tightening the connector, connections should be rechecked.connector.
7.2.2 If a standard reference specimen such as gold film deposited on mylarMylar® is available, measurement of its SE value
can provide assurance that the entire system is working properly. A specimen with the surface resistivity of 5 ΩΩ commonly
possess SE = –32 6 3 dB. Any Use any other known specimen may be used to check setup-to-setup repeatability.
7.2.3 Careful handling of the specimen holder and specimens is important.
7.3 Preparation of 7.2 shouldshall
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