Shielding Effectiveness Simulation of Small Perforated ... - MDPI

energies Article

Shielding Effectiveness Simulation of Small Perforated Shielding Enclosures Using FEM Zdenˇek Kubík * and Jiˇrí Skála Department of Applied Electronics and Telecommunications, University of West Bohemia, Pilsen 30614, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-377-634-268 Academic Editor: Rodolfo Araneo Received: 6 January 2016; Accepted: 18 February 2016; Published: 25 February 2016

Abstract: Numerical simulation of shielding effectiveness (SE) of a perforated shielding enclosure is carried out, using the finite element method (FEM). Possibilities of model definitions and differences between 2D and 3D models are discussed. An important part of any simulation is verification of the model results—here the simulation result are verified in terms of convergence of the model in dependence on the degrees of freedom (DOF) and by measurements. The experimental method is based on measurement of electric field inside the enclosure using an electric field probe with small dimensions is described in the paper. Solution of an illustrative example of SE by FEM is shown and simulation results are verified by experiments. Keywords: electromagnetic compatibility (EMC); measurement; shielding effectiveness (SE); shielding enclosure; finite element method (FEM); simulation

1. Introduction Nowadays, electronic devices contain a lot of highly energized parts, including processors, field-programmable gate arrays (FPGAs), power supplies and high-speed communications buses. But all these parts work on relatively low voltage levels and, therefore, they may often become victims of electromagnetic interference. One of its sources is high-speed data transmission. For these reasons, there exist many standards and recommendations concerning electromagnetically compatible environments. One part of electronic devices supporting a good level of EMC is electromagnetic shielding. Electromagnetic shielding mostly consists of shielding enclosures, gaskets, and ventilation structures. The shielding effectiveness (SE) is the term describing the quality of the shielding. There exist standards for SE measurement of shielding enclosures, but these standards do not respect the current trend—reduction of the device dimensions. For example, the IEEE standard for measuring the effectiveness of electromagnetic shielding enclosures [1] describes the measurement procedure for any enclosure having a smallest linear dimension greater or equal to 2.0 m. The question is, how to measure SE of such enclosures; the dimension of the measuring antenna depends on the wavelength and this antenna must be placed inside the shielding enclosures. If this is not possible, numerical or analytical methods can be used for determining SE. Analytical methods for SE calculation of the shielding enclosures with aperture are described in [2,3]. The numerical methods are well usable here, because it is easy to change geometry of the model. There are many suitable methods, including finite element method (FEM), method of moments (MoM), finite difference time domain method (FDTD) and others derived from them. Many papers can be found that deal with SE modelling—very interesting ones are [4–7]. This paper starts from previous articles [8–10] that are related to the problem of SE simulation.

Energies 2016, 9, 129; doi:10.3390/en9030129

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1.1. Shielding effectiveness The shielding effectiveness for electric and magnetic field is defined as the ratio (expressed in decibels) between the absolute value of electric field E1 (or magnetic field H1 ) at a point in space without the shielding and the absolute value of electric field E2 (or magnetic field H2 ) at the same point in space with the shielding [1]: SEE “ 20 ¨ log

|E1 | rdBs , |E2 |

(1)

|H1 | rdBs |H2 |

(2)

SEH “ 20 ¨ log

These definitions are applicable for theoretical calculations and for situation where dimensions of the measuring antenna are negligible compared with the dimensions of the measured shielding enclosure. In real measurement cases, however, the measuring antenna does not have negligible dimensions and the measured values depend on the location of the antenna inside the enclosure. This problem is caused by the cavity resonances of the enclosure that decrease the SE. Similar problems exist for simulations—there it is necessary to use the definition where the field is integrated through the whole cavity of the enclosure. The global shielding effectiveness (GSEs) for an electric/magnetic field is defined by [11]: r E1 dV r GSEE “ 20 ¨ log V rdBs , V E2 dV

(3)

r H1 dV GSEH “ 20 ¨ log rV rdBs V H2 dV

(4)

Numerous factors decrease the SE of enclosures. There exist two most important factors: the first one is the cavity resonance of the box and the second one are the presence of apertures in the enclosure. The cavity resonance frequencies f r are given by the formula [12]: 1 fr “ ? 2 µε

c´ ¯ m 2 a

`

´ n ¯2 b

`

´ p ¯2 c

rHzs

(5)

where µ and ε represent the material parameters—permeability and permittivity of the cavity, a, b and c are the dimensions of the cavity (inside dimensions of the enclosure) and indexes m, n and p correspond to the resonance mode. The influence of the cavity resonances on SE is not easy to determine, because the influence of the field strength inside the cavity on the resonance depends on the quality factor Q of the resonator, which is often unknown. Apertures in the enclosures have also a significant effect on the SE. The theoretical SE of a circular aperture with diameter r placed in a perfect shielding plate is [13]: SEap “ 20 ¨ log

λ rdBs 2π ¨ r

(6)

and the SE for structure with n apertures (where the distance between apertures is less than a half-wavelength) is given by [13]: SEn´ap “ 20 ¨ log

λ ? rdBs 2π ¨ r ¨ n

(7)

One of the ways for obtaining a high SE of the shielding enclosure is using cable glands, shielded air vent panels (honeycomb structures), and shielding gaskets.

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1.2. Definitions of Numerical Models 1.2. Definitions of Numerical Models  The most important numerical methods for SE simulation are the finite element method, finite The most important numerical methods for SE simulation are the finite element method, finite  difference time domain method, method of moments, and their modifications. The general geometries difference time domain method, method of moments, and their modifications. The general geometries  of the models are similar for all these methods, but the definitions—initial and boundary conditions, of the models are similar for all these methods, but the definitions—initial and boundary conditions,  distribution of be be  different. The The  model consists of a free surrounding the excitation a  free  space  surrounding  the  distribution  of mesh—could mesh—could  different.  model  consists  of space source and geometry of the shielding enclosure (see Figure 1). Of course, from the definition of the SE, excitation source and geometry of the shielding enclosure (see Figure 1). Of course, from the definition  there necessary calculate to  two models—one with the enclosure andenclosure  one without (only with the of  the isSE,  there  is tonecessary  calculate  two  models—one  with  the  and itone  without  it  observation area of the same dimensions as the internal dimension of the enclosure). (only with the observation area of the same dimensions as the internal dimension of the enclosure). 

  Figure 1. General description of SE model.  Figure 1. General description of SE model.

The  setting  of  correct  boundary  conditions  of  the  model  is  necessary  for  obtaining  correct  The setting of correct boundary conditions of the model is necessary for obtaining correct results. results. For example, the model boundary is usually modeled as a perfect magnetic ( n  H  0 ) or  For example, the model boundary is usually modeled as a perfect magnetic (n ˆ H “ 0) or electric n  E  0 ) conductor, but a better choice is to use perfectly matched layer (PML), where the  electric ( (n ˆ E “ 0) conductor, but a better choice is to use perfectly matched layer (PML), where the transition transition of the source signal is improved. The boundary of the box should be modeled as a perfect  of the source signal is improved. The boundary of the box should be modeled as a perfect electric electric conductor (PEC), when the material of the box is made from a highly conducting material.  conductor (PEC), when the material of the box is made from a highly conducting material. The The definition of the source depends on the method used, and for FEM it is possible to use a source  definition of the source depends on the method used, and for FEM it is possible to use a source of of electric or magnetic field.  electric or magnetic field. The  SE simulations simulations  can  solved  in  two  or  three  dimensions.  For  a  2D  simulation,  it  is  The SE can be be  solved in two or three dimensions. For a 2D simulation, it is necessary necessary  to two compute  two  models  represented  by  two  slices  of  the  enclosure.  sets  of  results  to compute models represented by two slices of the enclosure. Two sets Two  of results from 2D from 2D simulations exhibit results comparable with a 3D simulation. The main difference is in the  simulations exhibit results comparable with a 3D simulation. The main difference is in the solution solution time, the 3D simulation requires much longer time. On the other side, 2D FEM models can  time, the 3D simulation requires much longer time. On the other side, 2D FEM models can be calculated be calculated by a solver with  by a solver with hp-adaptivity hp of‐adaptivity of the mesh, which provides very accurate results [8].  the mesh, which provides very accurate results [8]. 2. 3D Finite Element Method (FEM) Numerical Model  2. 3D Finite Element Method (FEM) Numerical Model For finite element analysis in the frequency domain, the governing wave equation can be written as:  For finite element analysis in the frequency domain, the governing wave equation can be written as: ˆ ˙ 1 1    E   k 22  ε  jσjσ E     E p∇ ˆ Eq “ k00  εrr ´ ∇ˆ ωε µr μ r ωε 0 0 

(8)

where  μrr  represents  electric  where µ represents the  the relative  relative permeability,  permeability, εrr  stands  stands for  for the  the relative  relative permittivity,  permittivity, σ  σ is  is electric conductivity, ε conductivity, ε00 represents the vacuum permittivity, and  represents the vacuum permittivity, and k00 is the wave number of free space defined  is the wave number of free space defined by the equation:  by the equation: ω ? k 0 “ ω ε 0 µ0 “ ω (9) k0  ω ε0μ 0  c   (9) The source is modeled as an electric point dipole, thecvector of electric current dipole moment beingThe source is modeled as an electric point dipole, the vector of electric current dipole moment  P “ r0; 0; 1s m ¨ A. The P shielding 0;0;1   mbox being   A is .  approximated by a PEC, where: The shielding box is approximated by a PEC, where:  nˆE “ 0 (10) nE0  (10) For the analysis of fringing fields, a sphere of air was added around the model that is overlaid with For the analysis of fringing fields, a sphere of air was added around the model that is overlaid  a perfectly matched layer (PML). PML absorbs all radiated waves with small reflections by stretching with  a  perfectly  matched  layer  (PML).  PML  absorbs  all  radiated  waves  with  small  reflections  by  stretching the virtual domains into the complex plane using the following coordinate transform for  the general space variable t: 

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the virtual domains into the complex plane using the following coordinate transform for the general space variable t: ˙ ˆ Energies 2016, 9, 129  4 of 12  t n p1 ´ iq λF (11) t1 “ ∆w n Energies 2016, 9, 129  4 of 12   t 

λF   order, λ is the frequency, and(11) t´    i PML where ∆w is the width of the PML region, n represents F denotes  1the   w n 2 the scaling factor. The imaginary unit i satisfies the t  equation i = ´ 1. λF  PML  order,  λ  is  the  frequency,  and  t´ n  represents  (11)F   1  i the  where  Δw  is  the  width  of  the  PML  region,   w 

i satisfies the equation i2 = − 1.  denotes the scaling factor. The imaginary unit  3. Illustrative Example

where  Δw  is  the  width  of  the  PML  region,  n  represents  the  PML  order,  λ  is  the  frequency,  and  F 

A rectangular prism shielding enclosure was used for this illustrative 3. Illustrative Example  i satisfies the equation  i2 = − 1.  example of SE simulation. denotes the scaling factor. The imaginary unit  Its dimensions are 291 mm ˆ 277 mm ˆ 243 mm and the thickness of its wall is 1 mm. The circular A rectangular prism shielding enclosure was used for this illustrative example of SE simulation.  aperture3. Illustrative Example  of 10 mm diameter is placed in the center of the 277 mm 243is mm (see Figure 2a). The Its  dimensions  are  291  mm  ×  277  mm  ×  243  mm  and  the area thickness  of  its ˆ wall  1  mm.  The  circular  cavity dimensions are 289 mm ˆ 275 mm ˆ 241 mm. A rectangular prism shielding enclosure was used for this illustrative example of SE simulation.  aperture of 10 mm diameter is placed in the center of the area 277 mm × 243 mm (see Figure 2a). The  cavity dimensions are 289 mm × 275 mm × 241 mm.  Its  dimensions  are  291  mm  ×  277  mm  mm  and model. the  thickness  its  visible wall  is  1 four mm. regions, The  circular  Figure 2b represents a spherical area× of243  the FEM Thereof are the first of Figure 2b represents a spherical area of the FEM model. There are visible four regions, the first  aperture of 10 mm diameter is placed in the center of the area 277 mm × 243 mm (see Figure 2a). The  them representing the PMLs (on the model boundary) followed by a free space (surrounding enclosure). cavity dimensions are 289 mm × 275 mm × 241 mm.  of  them  representing  the  PMLs  (on  the  model  boundary)  followed  by  a  free  space  (surrounding  Inside the free space region there are placed the excitation source and shielding enclosure. The model Figure 2b represents a spherical area of the FEM model. There are visible four regions, the first  enclosure).  Inside  the  free  space  region  there  are  placed  the  excitation  source  and  shielding  represents the worst case; radiation themodel  source impactsfollowed  the wallby  with thespace  aperture. All parts were a  free  (surrounding  of  them  representing  the  PMLs from (on  the  boundary)  enclosure. The model represents the worst case; radiation from the source impacts the wall with the  computed using Equations (8)–(11) by the software COMSOL Multiphysics. enclosure).  Inside  the  free  space  region  there  are  placed  the  excitation  source  and  shielding  aperture. All parts were computed using Equations (8)–(11) by the software COMSOL Multiphysics.  enclosure. The model represents the worst case; radiation from the source impacts the wall with the  aperture. All parts were computed using Equations (8)–(11) by the software COMSOL Multiphysics.   

 

(a) 

(b) 

 

  The  Figure  2.  (a)  Dimensions  of  shielding  enclosure  in  millimeter;  (b)  area  of  the  FEM  model. 

Figure 2. (a) Dimensions of shielding the FEM model. The excitation (b)  (a)  enclosure in millimeter; (b) area of excitation source is placed at the red point, the enclosure is placed on the opposite side of the model.  source isHere the dimensions are in m.  placed at the red point, the enclosure is placed on the opposite side of the model. Here the Figure  2.  (a)  Dimensions  of  shielding  enclosure  in  millimeter;  (b)  area  of  the  FEM  model.  The  dimensions are in m. excitation source is placed at the red point, the enclosure is placed on the opposite side of the model.  The FE mesh is shown in Figure 3. The size of the mesh was chosen with respect to the accuracy  Here the dimensions are in m.  Theof the model—its important parts (shielding enclosure, free space area) were meshed more densely,  FE mesh is shown in Figure 3. The size of the mesh was chosen with respect to the accuracy while for PMLs we used sparser mesh. Five layers were used for modeling PMLs.  The FE mesh is shown in Figure 3. The size of the mesh was chosen with respect to the accuracy  of the model—its important parts (shielding enclosure, free space area) were meshed more densely, of the model—its important parts (shielding enclosure, free space area) were meshed more densely,  while for PMLs we used sparser mesh. Five layers were used for modeling PMLs. while for PMLs we used sparser mesh. Five layers were used for modeling PMLs. 

  Figure 3. Distribution of the FE mesh. (a) FE mesh in the whole model. There are visible five layers of    the PMLs, the black box inside the model represents a very fine FE mesh of the shielding enclosure;  (b) detail of the FE mesh around the circular aperture of the enclosure.  Figure 3. Distribution of the FE mesh. (a) FE mesh in the whole model. There are visible five layers of 

Figure 3. Distribution of the FE mesh. (a) FE mesh in the whole model. There are visible five layers of the PMLs, the black box inside the model represents a very fine FE mesh of the shielding enclosure;  the PMLs, the black box inside the model represents a very fine FE mesh of the shielding enclosure; (b) detail of the FE mesh around the circular aperture of the enclosure.  (b) detail of the FE mesh around the circular aperture of the enclosure.

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The model was solved in the frequency domain. The simulation started at 500 MHz and ended at 2500 The model was solved in the frequency domain. The simulation started at 500 MHz and ended  MHz. The frequency step—with respect to the computation time—was chosen 2.5 MHz. Overall, at  2500  MHz.  The  step—with  respect  to  the  computation  time—was  chosen  2.5  MHz.  800 simulation runsfrequency  were carried out. Overall, 800 simulation runs were carried out.  Important part of the simulation is verification of the result accuracy. Computing of the total Important part of the simulation is verification of the result accuracy. Computing of the total  electric energy inside the model is one possibility to check the solution accuracy: electric energy inside the model is one possibility to check the solution accuracy:  We we “ We rJ{m 33s (12) w e V [J/m ]   (12) V where V represents the whole volume of the model and energy We is calculated from the expression: where V represents the whole volume of the model and energy We is calculated from the expression:  ż ż 1 1 11 2 We “ ED dV “ (13) rJs  We  2 ED dV  2 εε00E dV [J] E2 dV (13) 2 2 V V



V



V

The accuracy of the solution is good if the total electric energy does not change with the number The accuracy of the solution is good if the total electric energy does not change with the number  of the (DOFs). For thethe  same number of DOFs, the accuracy will be better for lower of  the degrees degrees ofof freedom freedom  (DOFs).  For  same  number  of  DOFs,  the  accuracy  will  be  better  for  frequencies; much more DOF must be used forbe  high frequencies. 4 shows the dependences of lower  frequencies;  much  more  DOF  must  used  for  high  Figure frequencies.  Figure  4  shows  the  total electric energies onelectric  the number of DOFs thenumber  model. There are three dependences corresponding dependences  of  total  energies  on  of the  of  DOFs  of  the  model.  There  are  three  to frequencies 500 MHz, 1500 MHz and 2500 MHz. The accuracy of the result at 500 MHz is obviously dependences corresponding to frequencies 500 MHz, 1500 MHz and 2500 MHz. The accuracy of the  good. Different situation occurs with frequency 1500 MHz; the good accuracy of result begins result at 500 MHz is obviously good. Different situation occurs with frequency 1500 MHz; the good  at 1.15 million of DOFs. The worst situation is at frequency 2500 MHz—the total electric energy accuracy of result begins at 1.15 million of DOFs. The worst situation is at frequency 2500 MHz—the  of the model is changing and the accuracy of the result cannot be guaranteed. A model with more   total electric energy of the model is changing and the accuracy of the result cannot be guaranteed.  DOF must be solved for high frequencies. The solution of such a model with a high number of DOF, A model with more DOF must be solved for high frequencies. The solution of such a model with a  however, requires longer calculation time and higher demands on hardware. high number of DOF, however, requires longer calculation time and higher demands on hardware.  -3

x 10 1.2

0.8 0.6

e

w [Jm-3]

1

500 MHz 1500 MHz 2500 MHz

0.4 0.2 0 0

0.5

1 DOF [-]

1.5

2

6

x 10

 

Figure  Figure 4.  4. Total  Total energy  energy dependence  dependence on  on degree  degree of  of freedom  freedom in  in the  the model  model with  with shielding  shielding enclosure.  enclosure. Example of the model convergence for frequencies 500 MHz, 1.5 GHz and 2.5 GHz.  Example of the model convergence for frequencies 500 MHz, 1.5 GHz and 2.5 GHz.

Examples  of  simulation  results  are  shown  in  Figures  5  and  6.  These  figures  represent  Examples of simulation results are shown in Figures 5 and 6. These figures represent distributions distributions of electric fields in the model at frequencies 752.5 MHz and 1172.5 MHz. The frequency  of electric fields in the model at frequencies 752.5 MHz and 1172.5 MHz. The frequency 752.5 MHz 752.5 MHz responds to the lowest resonance mode of the cavity (theoretically at frequency 752.94 MHz).  responds to the lowest resonance mode of the cavity (theoretically at frequency 752.94 MHz). Figure 5 Figure 5 represents a sectional view of the model. There can be seen its slices in the  x‐y plane and    represents a sectional view of the model. There can be seen its slices in the x-y plane and z-x z‐x plane, respectively.  plane, respectively. There are visible minima and maxima of electric field inside the cavity; if the field is changing in  any direction, it responds to resonance mode 1 (or higher) in the same direction. One change from  minimum  to  maximum  and  again  back  to  minimum  (or  inverse)  represents  one  index  of  the  resonance  mode.  No  change  of  electric  field  responds  to  zero  resonance  mode  indexes.  The  first  resonance mode TE110 is at the frequency 752.5 MHz, (x‐y‐z axes, dimensions 289 mm × 275 mm ×  241 mm). A 3D view of the same solution is shown in Figure 6. 

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   Figure 5. Distribution of the electric field E [V/m] in the model: (a) Frequency 752.5 MHz, x‐y plane;  Figure 5. Distribution of the electric field E [V/m] in the model: (a) Frequency 752.5 MHz, x-y Figure 5. Distribution of the electric field E [V/m] in the model: (a) Frequency 752.5 MHz, x‐y plane;  (b) frequency 752.5 MHz, z‐x plane; (c) frequency 1172.5 MHz, x‐y plane; and (d) frequency 1172.5  plane; (b) frequency 752.5 MHz, z-x plane; (c) frequency 1172.5 MHz, x-y plane; and (d) frequency (b) frequency 752.5 MHz, z‐x plane; (c) frequency 1172.5 MHz, x‐y plane; and (d) frequency 1172.5  MHz, z‐x plane.  1172.5 MHz, z-x plane. MHz, z‐x plane. 

   Figure 6. Distribution of electric field E [V/m] in the 3D model, frequency 752.5 MHz.  Figure 6. Distribution of electric field E [V/m] in the 3D model, frequency 752.5 MHz.  Figure 6. Distribution of electric field E [V/m] in the 3D model, frequency 752.5 MHz.

Figure 7 shows the dependence of simulated GSE on frequency and theoretical SE of circular  Figure 7 shows the dependence of simulated GSE on frequency and theoretical SE of circular  Thereplaced  are visible minimashielding  and maxima ofthat  electric field insidefrom  the cavity; if the field is changing in aperture  in  ideally  plate  is  calculated  Equation  (6).  There  are  visible  aperture  placed  in  ideally  shielding  plate  that  is  calculated  from  Equation  (6).  There  are  visible  any direction, it responds to resonance mode 1 (or higher) in the same direction. One change from influences of cavity resonances on SE—drops on the GSE trace, which represent decreases of GSE.  influences of cavity resonances on SE—drops on the GSE trace, which represent decreases of GSE.  minimum to maximum and again back to minimum (or inverse) represents one index of the resonance Each drop responds to one TE resonance mode of the cavity. Of course, no drops of SE occur in the  Each drop responds to one TE resonance mode of the cavity. Of course, no drops of SE occur in the  mode. No change of electric field responds to zero resonance mode indexes. The first resonance mode theoretical dependence.  theoretical dependence.  TE110 is at the frequency 752.5 MHz, (x-y-z axes, dimensions 289 mm ˆ 275 mm ˆ 241 mm). A 3D view of the same solution is shown in Figure 6. Figure 7 shows the dependence of simulated GSE on frequency and theoretical SE of circular aperture placed in ideally shielding plate that is calculated from Equation (6). There are visible influences of cavity resonances on SE—drops on the GSE trace, which represent decreases of GSE. Each drop responds to one TE resonance mode of the cavity. Of course, no drops of SE occur in the theoretical dependence.

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40 40 35 35 30 30 25 25 20

SE /SE GSE [dB][dB] / GSE

Theoretical SE Simulated GSE Theoretical SE Simulated GSE

20 15 15 10 105

50 500 1000 1500 2000 2500 f [MHz] 0 500 1000 1500 2000 2500  f [MHz]   compared  Figure 7. Simulation  result  of  global  shielding  effectiveness  (GSE)  from  3D  FEM  model  Figure 7. Simulation result of global shielding effectiveness (GSE) from 3D FEM model compared with theoretical calculation of SE for a circular aperture. Drops in the simulated results are caused by  Figure 7. Simulation  result  of  global  shielding  effectiveness  (GSE)  from  3D  FEM  model  compared  with theoretical calculation of SE for a circular aperture. Drops in the simulated results are caused by cavity resonances.  with theoretical calculation of SE for a circular aperture. Drops in the simulated results are caused by 

cavity resonances.

cavity resonances. 

4. Measurement Methods 

4. Measurement Methods

4. Measurement Methods  The shielding effectiveness is defined by Equations (1), (2) and the measurement methods are  The shielding effectiveness is defined by Equations (1), (2) and the measurement methods are based on the following principle; there is measured electric (or magnetic) field at a point of the space  The shielding effectiveness is defined by Equations (1), (2) and the measurement methods are  with  and following without  the  shielding  enclosure  [14,15].  One  measurement  measuring  based on the principle; there is measured electric (or magnetic)method  field atwith  a point of the space based on the following principle; there is measured electric (or magnetic) field at a point of the space  antenna is shown in Figure 8.  withwith  and without the shielding enclosure [14,15]. One measurement method with measuring antenna and  without  the  shielding  enclosure  [14,15].  One  measurement  method  with  measuring 

is shown in Figure 8. antenna is shown in Figure 8.   

 

  Figure 8. Experimental shielding  effectiveness  measurement  in  an  anechoic  chamber.  This  method    uses transmitting and receiving antennas, while the control and evaluation of the measured data is  Figure 8. Experimental shielding  effectiveness  measurement measurement  in  chamber.  This  method  Figure 8. Experimental shielding effectiveness inan  ananechoic  anechoic chamber. This method provided by a personal computer.  uses transmitting and receiving antennas, while the control and evaluation of the measured data is  uses transmitting and receiving antennas, while the control and evaluation of the measured data is provided by a personal computer.  There is no problem with shielding effectiveness measurement of large enclosures—there are 

provided by a personal computer. standards for their measurement [1,16,17]. However, SE measurement methods are not unified [18].  On  There is no problem with shielding effectiveness measurement of large enclosures—there are  the  other  hand,  shielding  effectiveness  measurement  of  small  enclosures  is  quite  problematic.  There is no problem with shielding effectiveness measurement of large enclosures—there are standards for their measurement [1,16,17]. However, SE measurement methods are not unified [18].  The  dimensions  of  the  measurement  antenna  or  electromagnetic  field  probe  are  not  standards their measurement [1,16,17]. However, SE measurement methods are not unified [18]. On  the for other  hand,  shielding  effectiveness  measurement  of  small  enclosures  is  quite  problematic.  negligible—which depends on the wavelength—and this parameter is limiting factor for shielding  On the other hand, shielding effectiveness measurement of small enclosures is quite problematic. The  dimensions  of  the  measurement  antenna  or  electromagnetic  field  probe  are  not The effectiveness measurement of small enclosures, where it is necessary to place the measuring antenna  negligible—which depends on the wavelength—and this parameter is limiting factor for shielding  dimensions of the measurement antenna or electromagnetic field probe are not negligible—which inside  the  shielding  enclosures.  An  antenna  inside  the  enclosure  represents  another  problem;  the  effectiveness measurement of small enclosures, where it is necessary to place the measuring antenna  antenna represents filling of the cavity and it can influence the electromagnetic field inside it. The  depends on the wavelength—and this parameter is limiting factor for shielding effectiveness inside  the  shielding  enclosures.  An  antenna  the  enclosure  represents  another  problem;  the  the cavity filling influences space inside it and the resonance frequencies can be shifted.  measurement of small enclosures, where it is inside  necessary to place the measuring antenna inside antenna represents filling of the cavity and it can influence the electromagnetic field inside it. The  The  measurement  should  be  performed  in  the  far  field  region,  where problem; the  characteristic  shielding enclosures. An antenna inside the enclosure represents another the antenna cavity filling influences space inside it and the resonance frequencies can be shifted.  impedance is conclusively defined; the theoretic distance is given by the expression:  represents filling of the cavity and it can influence the electromagnetic field inside it. The cavity filling The  measurement  should  be  performed  in  the  far  field  region,  where  the  characteristic  λ influences space inside it and the resonance frequencies can be shifted. r [m] (14) impedance is conclusively defined; the theoretic distance is given by the expression:  2π The measurement should be performed in the far field  region, where the characteristic impedance λ by the expression: is conclusively defined; the theoretic distance isr given  [m] where r represents the transition between near and far fields and λ represents the wavelength.  (14) 2π  

λ 2π

where r represents the transition between near and far fields and λ represents the wavelength.  r “ rms

where r represents the transition between near and far fields and λ represents the wavelength.

(14)

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In practice, the dimensions of the receiving antenna are often larger or comparable with the wavelength, and the distance between the near and far fields is defined by the Rayleigh criterion Energies 2016, 9, 129  8 of 12  [19]: 2 In  practice,  the  dimensions  of  the  receiving  are  often  larger  or  comparable  with  the  2Dantenna  (15) r “ rms wavelength, and the distance between the near and far fields is defined by the Rayleigh criterion [19]:  λ 2

2 D receiving antenna. where D represents the characteristic dimension r of the [m] λ

(15)

 

5. Experimental Measurements where D represents the characteristic dimension of the receiving antenna.  The experimental measurements were performed in the semi-anechoic chamber at the University 5. Experimental Measurements  of West Bohemia that is constructed for measurement at 3 m distance between the antenna and The  experimental  measurements  were  performed  semi‐anechoic  chamber  field at  the  device under testing (DUT). This 3 m distance providesin a the  uniform electromagnetic in the University  of from West 26 Bohemia  that to is 18 constructed  for maximum measurement  at  3 of m electric distance field between  the  on frequency range MHz up GHz. The value depends antenna and device under testing (DUT). This 3 m distance provides a uniform electromagnetic field  the high-frequency power amplifier; for the 3 m distance we are able to produce 10 V/m in the in the frequency range from 26 MHz up to 18 GHz. The maximum value of electric field depends on  frequency range from 80 MHz up to 3 GHz. Two types of HF power amplifiers are used: a FLH-200B the  high‐frequency  power  amplifier;  for  the  3  m  distance  we  are  able  to  produce  10  V/m  in  the  (frequency range from 20 MHz to 1 GHz, maximum output power 200 W) and a FLG-30C (1–3 GHz, frequency range from 80 MHz up to 3 GHz. Two types of HF power amplifiers are used: a FLH‐200B  30 W)(frequency range from 20 MHz to 1 GHz, maximum output power 200 W) and a FLG‐30C (1–3 GHz,  and two types of antennas—BTA-M log periodic antenna (frequencies from 80 MHz to 2.5 GHz) and a30  BBHA 9120E (500 MHz–6log  GHz). For the measurement of from  electric W)  and  two  horn types antenna of  antennas—BTA‐M  periodic  antenna  (frequencies  80  field MHz  an to   ETS HI-6005 electric field probe was used. 2.5 GHz) and a BBHA 9120E horn antenna (500 MHz–6 GHz). For the measurement of electric field  an ETS HI‐6005 electric field probe was used.  The measurement method described in chapter Measurement Methods was used in this case, but The measurement method described in chapter Measurement Methods was used in this case,  the ETS HI-6005 electric field probe was replaced by a receiving antenna (see Figure 8). The BTA-M log but  the  ETS  field  from probe amplifiers. was  replaced  by first a  receiving  antenna  (see performed Figure  8).  The  periodic antennaHI‐6005  radiateselectric  the power The measurement was without BTA‐M  log  periodic  antenna  radiates  the  power  from  amplifiers.  The  first  measurement  the shielding enclosure; the electric field strength was measured with the E-field probe. Thewas  second performed without the shielding enclosure; the electric field strength was measured with the E‐field  measurement was performed with the shielding enclosure; the E-field probe was inserted inside the probe. The second measurement was performed with the shielding enclosure; the E‐field probe was  box. The shielding effectiveness was calculated from measured values using Equation (1). inserted  inside  the  box.  The  shielding  effectiveness  was  calculated  from  measured  values  using  The frequency range of experimental measurement for the enclosure dimensions (Figure 2) is Equation (1).  from 500 MHz to 2.5 GHz. The lowest frequency was set bellow the first cavity resonance of the The frequency range of experimental measurement for the enclosure dimensions (Figure 2) is  enclosure. From theto  dimension of the cavity and Equation the the  lowest frequency from  500  MHz  2.5  GHz.  The  lowest  frequency  was  set (5), bellow  first cavity cavity resonance resonance  of  the  (and enclosure. From the dimension of the cavity and Equation (5), the lowest cavity resonance frequency  the first transversal electric mode TE110) is 752.94 MHz. (and the first transversal electric mode TE110) is 752.94 MHz.  Figure 9 shows the experimental measurement in the semi-anechoic chamber. The shielding 9  shows  the  experimental  measurement  in  the are semi‐anechoic  chamber.  The  shielding  enclosureFigure  and optical cable from the electric field probe visible in foreground (the paper box enclosure  and  optical  cable  from  the  electric  field  probe  are  visible  in  foreground  (the  paper  box  below the enclosure is the stand); the transmitting antenna and absorbers are visible in background. below the enclosure is the stand); the transmitting antenna and absorbers are visible in background.  The optical cable was pulled through the cable gland representing a waveguide and the SE is not The optical cable was pulled through the cable gland representing a waveguide and the SE is not  influenced by the next aperture in the shielding enclosure (the critical frequency of the waveguide is influenced by the next aperture in the shielding enclosure (the critical frequency of the waveguide is  approximately 8.79 GHz). approximately 8.79 GHz). 

  Figure 9. Experimental measurement in the anechoic chamber. The shielding enclosure is placed on  Figure 9. Experimental measurement in the anechoic chamber. The shielding enclosure is placed on the paper stand (set 1.5 m above ground in the axis of the transmitting antenna) in the foreground of  the paper stand (set 1.5 m above ground in the axis of the transmitting antenna) in the foreground of the picture, the transmitting log‐per antenna is placed in the background of the picture. 

the picture, the transmitting log-per antenna is placed in the background of the picture.

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6. Results Comparison  6. Results Comparison 6. Results Comparison  The  simulated GSE GSE  dependence  on  frequency  of  model the  3D  model  compared  the  SE  The simulated dependence on frequency of the 3D compared with the SE with  experimental The  simulated  GSE  dependence  on  frequency  of  the  3D  model  compared  with  the  SE  experimental measurement is shown in Figure 10. There is visible a good agreement of the resonance  measurement is shown in Figure 10. There is visible a good agreement of the resonance frequencies. experimental measurement is shown in Figure 10. There is visible a good agreement of the resonance  frequencies.  These are frequencies  are bepossible  to  be  calculated  using  (5).  Small between differences  These frequencies possible to calculated using Equation (5). Equation  Small differences the frequencies.  These  frequencies  are  possible  to  be  calculated  using  Equation  (5).  Small  differences  between the calculated and measured (or simulated) frequencies are caused by the 2.5 MHz steps of  calculated and measured (or simulated) frequencies are caused by the 2.5 MHz steps of measurement between the calculated and measured (or simulated) frequencies are caused by the 2.5 MHz steps of  measurement (or simulation).  (or simulation). measurement (or simulation).  40 40

Measured SE Measured GSE SE Simulated Simulated GSE

/ GSE[dB] [dB] SESE/ GSE

30 30 20 20 10 10 0 0 -10 -10 -20 500 -20 500

1000 1000

1500 2000 2500 1500 2000 2500 f [MHz]   f [MHz]   Figure 10. Comparison of results between measured SE and simulated GSE of 3D model.  Figure 10. Comparison of results between measured SE and simulated GSE of 3D model. Figure 10. Comparison of results between measured SE and simulated GSE of 3D model. 

The values of GSE are almost better than the measured values; it could be caused by using of  The values of GSE are almost better than the measured values; it could be caused by using of  The values of GSE are almost better than the measured values; it could be caused by using of the the PEC boundary condition for the shielding enclosure.  the PEC boundary condition for the shielding enclosure.  PEC boundary condition for the shielding enclosure. The simulation results for frequencies higher 2 GHz are tentative, because the convergence of  The simulation results for frequencies higher 2 GHz are tentative, because the convergence of  The simulation results for frequencies higher 2 GHz are tentative, because the convergence of the the model is not guaranteed (see Figure 4). The mean values are 23.39 dB for GSE and 23.59 dB for SE  the model is not guaranteed (see Figure 4). The mean values are 23.39 dB for GSE and 23.59 dB for SE  model is not guaranteed (see Figure 4). The mean values are 23.39 dB for GSE and 23.59 dB for SE (the (the mean error being 0.2 dB).  (the mean error being 0.2 dB).  mean error being 0.2 dB). The 2D model definitions are shown in Figure 11. The source representing an electric current  The 2D model definitions are shown in Figure 11. The source representing an electric current  The 2D model definitions are shown in Figure 11. The source representing an electric current dipole with current  I0 = 1 A is placed on the left. Absorbing boundary condition (ABC) was applied  dipole with current  dipole with current II00 = 1 A is placed on the left. Absorbing boundary condition (ABC) was applied  = 1 A is placed on the left. Absorbing boundary condition (ABC) was applied along the model boundary, which represents a free space surrounding model. The shielding box is  along the model boundary, which represents a free space surrounding model. The shielding box is  along the model boundary, which represents a free space surrounding model. The shielding box is approximated by a PEC.  approximated by a PEC.  approximated by a PEC.

   

Figure  11.  Area  of  the  2D  FEM  model,  including  dimension  in  millimeters  and  setting  of  the  Figure  11. 11.  Area  setting  of of  the the  Figure Area of  of the  the 2D  2D FEM  FEM model,  model, including  including dimension  dimension in  in millimeters  millimeters and  and setting boundary conditions.  boundary conditions.  boundary conditions.

The  comparison  between  the  2D and  3D  simulations  of  the  GSE dependence on  frequency  is  The  comparison  between  the  2D and  3D  simulations  of  the  GSE dependence on  frequency  is  The comparison between the 2D and 3D simulations of the GSE dependence on frequency is shown in Figure 12. The results from one 2D simulation are shown here, while other results of 2D  shown in Figure 12. The results from one 2D simulation are shown here, while other results of 2D  shown in Figure 12. The results fromhpone 2D simulation shown here, while other results of ‐adaptivity  models are were  described  in  [8].  These  results  simulations  including  results  of  the  simulations  including  results  of  the  hp‐adaptivity  models  were  described  in  [8].  These  results  2D simulations including results of the hp-adaptivity models were described in [8]. These results correspond with enclosure dimension 289 mm × 275 mm (it represents a slice of the 3D model in the  correspond with enclosure dimension 289 mm × 275 mm (it represents a slice of the 3D model in the  with dimension 289 2D  mmsimulations;  ˆ 275 mm (it represents a slice of zthe 3Dcannot  model be  in zcorrespond ‐axis).  There  are enclosure only  TExx0  modes  for  higher  modes  in  the  ‐axis  z‐axis).  There  are  only  TExx0  modes  for  2D  simulations;  higher  modes  in  the  z‐axis  cannot  be  calculated.  Another  simulation  (model  which  represents  a  y‐axis  slice  of  the  3D  model)  must  be  calculated.  Another  simulation  (model  which  represents  a  y‐axis  slice  of  the  3D  model)  must  be  solved for determining of all TE modes.  solved for determining of all TE modes. 

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the z-axis). There are only TExx0 modes for 2D simulations; higher modes in the z-axis cannot be calculated. Another simulation (model which represents a y-axis slice of the 3D model) must be solved for determining of all TE modes. Energies 2016, 9, 129  10 of 12  40

Simulated GSE, 3D Simulated GSE, 2D

30

GSE [dB]

20 10 0 -10 -20 500

1000

1500 f [MHz]

2000

2500

 

Figure  Figure 12.  12. Comparison  Comparison of  of results  results between  between global  global shielding  shielding effectiveness  effectiveness of  of the  the 2D  2D and  and 3D  3D FEM  FEM model simulations.  model simulations.

7. Conclusions  7. Conclusions This paper describes the problems with determining the shielding effectiveness of perforated  This paper describes the problems with determining the shielding effectiveness of perforated shielding enclosures. General shielding effectiveness theory was defined for a perfect environment  shielding enclosures. General shielding effectiveness theory was defined for a perfect environment and and  conditions,  but  this  theory  does  not  correspond  real  cases—these  are  by influenced  by  conditions, but this theory does not correspond with real with  cases—these are influenced dimensions vs .  spot  observation  points  in  general  SE  theory).  For  this  reason  a  new  dimensions  of  antennas  ( of antennas (vs. spot observation points in general SE theory). For this reason a new definition definition  called  global effectiveness shielding  effectiveness  (GSE)  was  Even established.  Even exist when  there  exist  called global shielding (GSE) was established. when there standards for standards  for  measurement  of  the  SE,  these  are  applicable  only  in  certain  cases,  where  large  measurement of the SE, these are applicable only in certain cases, where large enclosures are examined, very  linear small  enclosures  are  examined,  but  are these  methods  not  applicable  but these measurement methods notmeasurement  applicable for very small are  enclosures having thefor  smallest enclosures having the smallest linear dimension smaller than 2.0 m.  dimension smaller than 2.0 m. Numerical methods can be helpful for determining of SE. There are lots of numerical methods  Numerical methods can be helpful for determining of SE. There are lots of numerical methods  etc.) which are used for SE calculations. The paper presents the FEM model of  (FEM, FDTD, MoM, (FEM, FDTD, MoM, etc.) which are used for SE calculations. The paper presents the FEM model of the the perforated shielding enclosure, various definitions and geometries are discussed. The 3D model  perforated shielding enclosure, various definitions and geometries are discussed. The 3D model is is supplemented with the complete set of results, while the 2D model only with a part of results. The  supplemented with the complete set of results, while the 2D model only with a part of results. The disadvantage of 3D models is large hardware requirement and long solution time. 2D models are not  disadvantage of 3D models is large hardware requirement and long solution time. 2D models are not ‐adaptivity of FE mesh can be used for 2D models, in this case it is guaranteed  time consuming. Also  time consuming. Also hp hp-adaptivity of FE mesh can be used for 2D models, in this case it is guaranteed the correctness of results (the convergence of the model from the mathematical viewpoint).  the correctness of results (the convergence of the model from the mathematical viewpoint). The verification of the numerical results using the total electric energy inside the model is an  The verification of the numerical results using the total electric energy inside the model is an important part of the paper. The dependence of the total electric energy on DOF shows the solution  important part of the paper. The dependence of the total electric energy on DOF shows the solution accuracy; the model with more DOF must be solved for high frequencies (with the same accuracy as  accuracy; the model with more DOF must be solved for high frequencies (with the same accuracy as for for  frequencies  lower  of number  of  DOF).  This  solution,  however,  requires  longer  low low  frequencies with awith  lowera number DOF). This solution, however, requires longer computational results  were  verified Our by  computational  and hardware. more  powerful  hardware.  The  simulation  time and more time  powerful The simulation results were verified by measurements. measurements.  Our  measurement  method  uses  a  small  electric  field  probe  instead  of  receiving  measurement method uses a small electric field probe instead of receiving antenna, the dimensions antenna,  the correspond dimensions toof  the  probe  correspond  a  cube  with  70 measurement mm  edge.  The  experimental  of the probe a cube with 70 mm edge.to  The experimental was performed measurement  was  performed  in  a  semi‐anechoic  chamber  and  it  was  based  on  general  SE  theory  in a semi-anechoic chamber and it was based on general SE theory (however, the dimensions of the (however, the dimensions of the electric field probe are not negligible—it is not a dot observation  electric field probe are not negligible—it is not a dot observation point). The probe dimensions allow point). The probe dimensions allow SE measurement of a small shielding enclosure with dimensions  SE measurement of a small shielding enclosure with dimensions of less than 2.0 m. of less than 2.0 m.  The miniaturization of electronic devices provides a space for SE numerical calculations—there The miniaturization of electronic devices provides a space for SE numerical calculations—there  are no possibilities for SE measurement (or very limited). The importance of numerical calculations for are no possibilities for SE measurement (or very limited). The importance of numerical calculations  shielding effectiveness in the future, therefore, will grow. The future research will be focused on SE for shielding effectiveness in the future, therefore, will grow. The future research will be focused on  SE  simulation  and  measurement  of  the  complex  perforated  shielding  enclosures,  including  ventilation structures or cable glands.  Acknowledgments: This research has been supported by the Ministry of Education, Youth and Sports of the  Czech  Republic  under  the  RICE—New  Technologies  and Concepts  for Smart  Industrial  Systems,  project  No. 

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simulation and measurement of the complex perforated shielding enclosures, including ventilation structures or cable glands. Acknowledgments: This research has been supported by the Ministry of Education, Youth and Sports of the Czech Republic under the RICE—New Technologies and Concepts for Smart Industrial Systems, project No. LO1607 and by the project SGS-2015-002. Author Contributions: Both authors contributed equally to this work. Both authors performed the measurement and designed the simulations, discussed the results and commented on the manuscript at all stages. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Abbreviations The following abbreviations are used in this manuscript: FEM GSE SE FDTD MoM ABC PEC DOF

Finite element method Global shielding effectiveness Shielding effectiveness Finite difference time domain method Method of moments Absorbing boundary condition Perfect electric conductor Degrees of freedom

References 1.

2.

3.

4. 5.

6.

7. 8.

9. 10.

Institute of Electrical and Electronics Engineers (IEEE). IEEE standard method for measuring the effectiveness of electromagnetic shielding enclosures. In IEEE Std 299–2006 (Revision of IEEE Std 299–1997); IEEE: New York, NY, USA, 2007; pp. 1–52. Robinson, M.P.; Benson, T.M.; Christopoulos, C.; Dawson, J.F.; Ganley, M.D.; Marvin, A.C.; Porter, S.J.; Thomas, D.W.P. Analytical formulation for the shielding effectiveness of enclosures with apertures. IEEE Trans. Electromagn. Compat. 1998, 40, 240–248. [CrossRef] Liu, E.; Du, P.-A.; Nie, B. An extended analytical formulation for fast prediction of shielding effectiveness of an enclosure at different observation points with an off-axis aperture. IEEE Trans. Electromagn. Compat. 2013, 56, 589–598. [CrossRef] Araneo, R.; Lovat, G.; Celozzi, S. Compact electromagnetic absorbers for frequencies below 1 GHz. Prog. Electromagn. Res. 2013, 143, 67–86. [CrossRef] Olyslager, F.; Laermans, E.; De Zutter, D.; Criel, S.; De Smedt, R.; Lietaert, N.; De Clercq, A. Numerical and experimental study of the shielding effectiveness of a metallic enclosure. IEEE Trans. Electromagn. Compat. 1999, 41, 202–213. [CrossRef] Lozano-Guerrero, A.J.; Diaz-Morcillo, A.; Balbastre-Tejedor, J.V. Resonance suppression in enclosures with a metallic-lossy dielectric layer by means of genetic algorithms. In Proceedings of the IEEE International Symposium on Electromagnetic Compatibility (EMC 2007), Honolulu, HI, USA, 9–13 July 2007; pp. 1–5. Li, M.; Nuebel, J.; Drewniak, J.L.; DuBroff, R.E.; Hubing, T.H.; Van Doren, T.P. EMI from cavity modes of shielding enclosures-FDTD modeling and measurements. IEEE Trans. Electromagn. Compat. 2000, 42, 29–38. Kubik, Z.; Skala, J. Influence of the cavity resonance on shielding effectiveness of perforated shielding boxes. In Proceedings of the 8th International Conference on Compatibility and Power Electronics (CPE), Ljubljana, Slovenia, 5–7 June 2013; pp. 260–263. Kubík, Z.; Karban, P.; Pánek, D.; Skála, J. On the shielding effectiveness calculation. Computing 2013, 95, 111–121. Hromadka, M.; Kubik, Z. Suggestion for changes in shielding effectiveness measuring standard. In Proceedings of the 4th International Youth Conference on Energy (IYCE), Siófok, Hungary, 6–8 June 2013; pp. 1–4.

Energies 2016, 9, 129

11. 12. 13. 14.

15.

16.

17.

18.

19.

12 of 12

Celozzi, S. New figures of merit for the characterization of the performance of shielding enclosures. IEEE Trans. Electromagn. Compat. 2004, 46. [CrossRef] Tesche, F.M.; Ianoz, M.V.; Karlsson, T. EMC Analysys Methods and Computational Models; John Wiley & Sons Inc.: New York, NY, USA, 1992; ISBN: 0-471-15573-X. Chatterton, P.A.; Houlden, M.A. EMC Electromagnetic Theory to Practical Design; John Wiley & Sons Ltd.: Chichester, UK, 1992; ISBN: 0-471-92878-X. Holloway, C.L.; Ladburry, J.; Coder, J.; Koepke, G.; Hill, D.A. Measuring shielding effectiveness of small enclosures/cavities with a reverberation chamber. In Proceedings of the IEEE International Symposium on Electromagnetic Compatibility, Honolulu, HI, USA, 9–13 July 2007; pp. 1–5. Greco, S.; Sarto, M.S. Hybrid mode-stirring technique for shielding effectiveness measurement of enclosures using reverberation chambers. In Proceedings of the IEEE International Symposium on Electromagnetic Compatibility, Honolulu, HI, USA, 9–13 July 2007; pp. 1–6. Institute of Electrical and Electronics Engineers (IEEE). IEEE recommended practice for measurement of shielding effectiveness of high-performance shielding enclosures. In IEEE Std 299–1969; IEEE: New York, NY, USA, 1969. United States Military Standard MIL-STD-285. Method of Attenuation Measurements for Enclosures, Electromagnetic Shielding, for Electronic Test Purposes; U.S. Government Printing Office: Washington, DC, USA, 1956. Butler, J. Shielding effectiveness—why don’t we have a consensus industry standard? In Proceedings of the Professional Program Proceedings Electronics Industries Forum of New England, 1997, Boston, MA, USA, 6–8 May 1997; pp. 29–35. IEEE. IEEE standard definitions of terms for radio wave propagation. In IEEE Std 211–1997; IEEE: New York, NY, USA, 1998. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).