Predicting Shielding Effectiveness
Authors:
David P. Johns, Flomerics Inc.
Guillaume Girard, Solectron Inc.
Shielding Effectiveness (SE) is one of the major
aspects of any EMC design process. The ability to
accurately estimate the SE of an enclosure can be very valuable in terms of time saving and cost.
The tools available today on the market vary from quick theoretical analysis, experimental
measurement to the new fashion "Numerical Simulation". Like any other technique, Numerical
simulation raises the question of how to excite the structure.
Which is the better technique to find the SE of an enclosure;
Place a source inside and measure the external fields,
Or
Radiate the enclosure from the outside and measure the internal fields?
Are simulated results close to reality?
In this article, TLM computational electromagnetic software is used [1] to explore these questions
and results are compared with experimental measurement. Our Conclusion confirms the
application of the reciprocity theorem and gives direction for future use of SE numerical simulation.
Figure 1: Geometry of SE Problem
Test Case Model
Figure 1 shows a FLO/EMC test case model, consisting of an Aluminum enclosure having
dimensions 1 x 0.5 x 0.25m. The origin is at the center of the enclosure. The rear panel of the
enclosure contains an air vent with 5mm radii holes, 10mm deep and 50% coverage over an area
0.4 x 0.2m. The top panel overlaps the front panel and is screwed to the front panel in the corners
and at the mid-point. This creates two 47cm long seams having a gap width of 1mm and overlap
distance of 10mm. One of the sides has a square 10cm hole and the opposite side is closed. The
test enclosure is deliberately arranged in this way, so that the shielding is dependent on direction.
A 12.5cm long monopole antenna is connected to a 25cm square ground plane centered on the
base of the enclosure. The monopole is terminated in a 50-Ohm load.
External Source Model
In the external source model, the enclosure is illuminated by a 1 V/m vertically polarized plane
wave. The induced current Iis monitored at the monopole load (shielded current). To determine
the minimum shielding, it is necessary to simulate all angles of incidence (f = 0…360°). This
requires many computations, one for each angle of incidence. For the purpose of demonstrating
reciprocity, we will restrict the analysis to one angle of incidence f = 0 (normal to the side
containing the square hole).
In order to calculate the SE, we also calculate the induced current Iref at the monopole with the
enclosure removed (reference current). The results are plotted in Figure 2. We observe that the
shielded current is generally below the reference current. However, at certain frequencies, the
shielded current is higher than the reference current, due to strong resonances in the enclosure.
Monopole Current Against Frequency (External Source)
Figure 2: Induced Current at Monopole
Internal Source Model
In the internal source model, there is no external plane wave; the monopole becomes the
transmitter. A 1V source is connected in series with a 50-Ohm load at the monopole base. The
advantage of the internal source model is that the radiation can be found for all directions in a
single computation. Electric and magnetic fields are monitored at numerous points around the
enclosure on a 3m radius from the origin. The output points are actually defined outside the mesh
and near fields are stored and integrated to calculate the beyond-mesh field values. This minimizes
the mesh size and associated computer requirements. The radiated fields are also found with the
enclosure removed to establish reference results.
Radiated Fields Versus Frequency
Figure 3: Radiated Fields Due to Internal Source
We see that SE depends on both frequency and direction. Plotting the electric field distribution gives us a real insight
into why the peaks are occurring. We see in Figure 4, that the 515.856 MHz peak is due to a TE103 mode inside the
box coupling to the square aperture. At 1.097 GHz, there is leakage of field through the square aperture, seams and
air vent to varying degrees. EMC engineer can use those results for troubleshooting or re-design of the enclosure to
minimize leakage. The E-Field and Surface Current distribution can be used by EMC engineers to determine the
optimal placement of components, cables and access holes in order to avoid "hot spots".
Figure 4: Electric Field Distributions at 515.856 MHz and 1.097 GHz
Shielding Effectiveness
In the external source model, the SE in the f = 0 direction is calculated by:
SE(f) = -20 log10 ( I(f) / Iref(f))
(1)
Where I(f)and Iref(f)are the induced currents due to a plane wave incident in the f = 0 direction.
Similarly, in the internal source model, the SE is calculated by:
SE(f) = -20 log10 ( E(f) / Eref(f))
(2)
Where E(f)and Eref(f)are external fields radiated in the f = 0 direction. The results plotted in Figure
5 confirm that simulated SE is reciprocal. Both curves follow the same trends and show similar
resonance peaks.
Shielding Effectiveness Against Frequency (Internal Source)
Figure 5: Comparison of External and Internal SE (f = 0)
The overall SE is conveniently found by performing a cylindrical scan around the enclosure (for the internal source
case). Near fields are computed and stored over equivalent surfaces encapsulating the enclosure. Field integration
techniques are then used to transform the near field distributions into cylinders having user-defined radius and height.
Figure 6 shows the 3m scans at 304 and 515.856 MHz.
Figure 6: Cylinder Scans at 304 MHz and 515.856 MHz
We see that peak fields occur along the y-axis at 304 MHz (normal to the seams) and along the xaxis at 515.856 MHz (normal to the square hole). These peak fields correspond to dips in the SE
seen in Figure 5.
Experimental Measurement
Experimental tests are often used to estimate the shielding effectiveness of an enclosure.
However, to be able to perform such measurements, a physical representation of the enclosure
must be built. By following the same dimensions as above in the simulated model, an enclosure
was made using 7010 Aluminum.
Numerous SE measurement methods have been published for small enclosures in the past [2] [3]
[5]. Most of them require special tools and handling, and are only applicable to specific
experiments. The method used in this study is based on the measured transmitted field (radiated
field from the box). Reciprocity is assumed.
The test method is basically a delta measurement of the E-field strength due to a noise source in
free space and the E-field strength of the same noise source place inside the enclosure. The noise
source is a discrete clock powered by a 7.5V DC camera battery. The source is mounted with a
12.5 cm monopole antenna. The use of a clock limits the SE value to discrete frequencies
corresponding to the fundamental and harmonics of the 150.00MHz waveform. Position of the
noise source inside the enclosure was chosen arbitrarily in the center, as in the simulation.
The experimental setup required a Horn antenna, turntable and spectrum analyzer. Luxury of an
ambient-free Chamber was possible for our measurement, but is not mandatory. Figure 7 presents
the experimental set-up. The turntable is place 1m from the ground and 3m from the antenna,
which can be considered to be in the far field of our enclosure under test.
Figure 7: SE Measurement Setup
Measurements were taken using a spectrum analyzer at the fundamental and even harmonics of
the TTL clock for different permutations of the noise source and enclosure as in Table 1. The noise
source emissions are elliptically polarized so the permutations are use to help sample the strongest
field strength. For each discrete frequency the noise source and/or enclosure is rotated 360
degrees and the maximum field strength is noted. Antenna factors and measurement uncertainties
are not taken into account, since this is a delta measurement. For the experiment to be valid no
displacement or change to the setup and EM environment is made, except for the enclosure
presence.
Figure 6 shows the field profile monitored from the front of the enclosure (where the bezel is
located). These results showed that the prominent resonance was still at 7.4 GHz that was the one
of the resonances investigated with the single transceiver. Due to this all the additional analysis in
this article will be centered on this frequency. Discussions with a user of these types of
transceivers confirmed that the resonance's that were being seen in the model corresponded with
those seen in the measurement chamber.
Configuration
Antenna Orientation
Noise Source/ Enclosure
1
Vertical
Horizontal
2
Horizontal
Vertical
3
Vertical
Vertical
4
Horizontal
Horizontal
Table 1: Antenna and Noise Source/Enclosure Permutations
The following graph presents the field strength for the noise source with and without the
enclosure. As we can see, the enclosure provides a varying amount of field attenuation, which is
possible to quantify in terms of SE by subtracting the curves as in equation 3.
SE(f) = (E(free space) - E(with enclosure))
(3)
Where E(free space)and E(with enclosure)are in dBµV/m.
Figure 8: Field Strength Measurement
The SE results are presented in Figure 9 with the corresponding simulation results. The two curves
show a similar trend. Variations between experiment and simulation are likely to be due to noise
source impedance changes and cavity loading in the test. The most important frequency here is at
1244 MHz where we find the lowest SE. By observation from Figure 9, we can reinforce our
conclusion that the reciprocity theorem is applicable for both simulation and experiment.
Shielding Effectiveness Against Frequency (Internal Source)
Figure 9: SE Measurement and Simulation Comparison
Conclusions
As expected, we have confirmed reciprocity of SE. That is, the SE found with the external source is
equivalent to the SE found with the internal source, provided that the internal antenna is present
in both analyses.
It is essential for the SE analysis to consider all angles of propagation around the enclosure since
the result will depend on direction. It is computationally more efficient to place the source inside
the enclosure and determine the radiated fields around the enclosure, rather than radiate the
enclosure with plane waves incident from different angles.
In the internal source approach, the source-type can affect the measurement. It is known that a
monopole source gives different SE results to a wire loop source. The effect can be particularly
pronounced at lower frequencies. At lower frequencies, the apertures of the enclosure will be in
the near field of the source. Hence the wave impedance will affect the coupling and the resulting
radiation from the enclosure.
Experimental results follow similar trends as the simulation results. Generally, variations of less
than 10dB are observed. It is difficult to measure SE. Physical representation of the enclosure
must be provided at some cost. Lab equipment and time is another factor. Electromagnetic
modeling reduces the burden by simulating different designs rather than iterating around the
build->test->rebuild->retest cycle. However, experimental measurement is ultimately desired to
verify the integrity of the final enclosure design.
Computational TLM electromagnetic software enables virtual investigations of EMC problems,
helping Engineers to find and view EM phenomena. Including numerical simulation in the EMC
design process can provide a major improvement in design optimization.
References
1. FLO/EMC Electromagnetic Field Simulation Software available from Flomerics INC., 257
Turnpike Road, Southborough, MA 01772.
2. Cromarty, "Effect of Apertures and Enclosure Dimension on the Shielding Effectiveness of
Electronic Product Enclosures", Concordia University, Montreal, January 2001.
3. M. Li, S. Radu, J. Nuebel, W. Cui, J. L. Drewniak, T. H. Hubing, T. P. Van Doren, "EMI from
Apertures at Enclosure Cavity Mode Resonances", 1997 IEEE Electromagnetic Compatibility
Symposium, Austin, TX, pp.183-187.
4. IEEE Standard for Measuring the Effectiveness of Electromagnetic Shielding Enclosure,
IEEE Std 299-1991, IEEE, July 1991.
5. Bruce Archambault, Omar Ramahi, "Evaluating Tools which Predict the Shielding
Effectiveness of Metal Enclosure Using a Set of Proposed Standard EMI Modeling
Problems", 1998 International Symposium on Electromagnetic Compatibility Record,
pp.517-521, August 1998.
The authors may be contacted by email:
david.johns@flomerics.com
GuillaumeGirard@solectron.com