Research on Testing Method for Shielding Effectiveness of Irregular Cavity Based on Field Distribution Characteristics

Abstract

A method of measuring the shielding effectiveness of the irregular cavity is proposed in this paper. An electromagnetic topology model of the irregular cavity is established according to the characteristics of the irregular cavity, and the electromagnetic field distribution characteristics inside the irregular cavity are obtained based on the simulation of the field distribution of the irregular cavity. Combined with the test method of regular cavity shielding effectiveness, the characterization and testing methods of the shielding effectiveness of the irregular cavity are given comprehensively, which compared with the conventional testing method, verifies how effectively the shielding effectiveness of the irregular cavity is tested.

1. Introduction

With the electromagnetic environment growing increasingly complex, the electromagnetic environmental effects are receiving more and more attention, and electronic equipment is susceptible to electromagnetic interference or even failure [1]. In order to ensure the reliable operation of the equipment, protective measures against electromagnetic interference are indispensable, and electromagnetic shielding (SE) is one of the most important methods to suppress electromagnetic interference. Usually, a metal cavity is used outside for electromagnetic shielding and protection. Considering the special installation or fixing requirements of some equipment and the arrangement requirements of the shielding cavity, the cavity structure is not a regular cuboid or cylinder, and there may be irregular structures such as protrusions, plates, steps, and blocks. In actual applications, there will be some apertures in the metal cavity due to the requirements of ventilation, heat dissipation, and the wiring of the internal equipment. These apertures will reduce the SE of the cavity because of electromagnetic field coupling. Therefore, research on the shielding effectiveness of irregular cavities with apertures is of great significance to the electromagnetic protection of electronic equipment.

For the research on the testing method of the shielding effectiveness of the irregular cavity, the classification of the irregular cavity is defined firstly. Secondly, an electromagnetic topology model to analyze the shielding effectiveness of the irregular cavity is built, and then the characteristics of the electromagnetic field in the cavity are obtained, and the characterization method of the shielding effectiveness of the irregular cavity is given. After these steps, the testing method for the shielding effectiveness of the irregular cavity is proposed.

There are two main methods for calculating the shielding effectiveness of the cavity with apertures: numerical and analytical ones. Complex structures can be simulated through numerical methods [2,3,4,5,6], but a lot of computing time and memory are usually needed to build a model with sufficient accuracy. At the same time, they are not conducive to the regularity analysis of the effects of parameters. By contrast, the physical meaning obtained using the analytical algorithm [7,8,9,10], with a fast calculation speed, is more explicit than the one established by the actual model. At present, there are some analytical methods for the regular of shielding cavities with apertures [11,12,13,14,15,16], but further simplified ones are still needed, and the calculation of the shielding effectiveness of actual irregular cavities should be summarized and studied. In this paper, the electromagnetic topology model in the analytical method is used to establish the calculation model of the shielding effectiveness of the irregular cavity, and the irregular structure is classified.

For the field distribution research of the shielded cavity, most previous studies chose the center position of the shielding body or the position around the center for analysis and calculation. A series of related research has been conducted in this field. For example, an approximate statistical model of the internal field distribution of the complex cavity was deduced by the research team of Lehman and Hill of the Philip Laboratory of the US Air Force using the analytical method [17]. In addition, the team used the central limit theorem or the principle of maximum entropy for the complex cavity’s internal field to study the complex cavity’s internal field and established the distribution characteristics of the electromagnetic field and the statistical model of the electromagnetic field distribution and correlation coefficient [18,19]. Anlage [20,21] used the S-parameter matrix of the test port to evaluate the electromagnetic loss characteristics of the cavity and combined it with the Random Matrix Theory (RMT) to verify Hill’s description of the statistical distribution law of the internal field of a complex cavity. The Federal Aviation Administration (FAA) concluded that the use of mode stirring could effectively test the electromagnetic attenuation coefficient of the aircraft cavity after testing the airworthiness of various types of aircraft [22].

Electromagnetic field analysis technology inside the cavity has gradually been phased in for practical use in laboratory research, and there are a few types of research on the irregular cavity. Meanwhile, the testing standards applicable for SE in different situations have been widely used and are given in [23,24,25], but sometimes these methods are not applicable for the SE testing of irregular cavity. In this paper, the effects of irregular structures on the cavity field distribution are analyzed, with the shielding effectiveness characterization and testing methods obtained based on the field distribution characteristics. The CST (3D EM Simulation Software) is used to simulate the field distribution in the irregular cavity.

2. Electromagnetic Topology Calculation Model of Shielding Effectiveness of Irregular Cavity

2.1. Electromagnetic Topology Modle

The physical model is shown of the irregular shielded cavity irradiated by plane waves in Figure 1. Based on the Robinson algorithm, the free space is equivalent to a transmission line with a characteristic impedance of Z₀, the front part and the back part of the cavity are equivalent to Z_g1 and Z_g2, respectively, and both are rectangular waveguides, and the apertures and irregular structures are equal to two-port networks in the model.

Based on the electromagnetic topology theory, the above physical model is reconstructed as an equivalent transmission line network with tubes and nodes. The equivalent circuit diagram and the corresponding signal flow diagram are shown in Figure 2 and Figure 3, respectively:

Based on the BLT (Baum–Liu–Tesche) matrix equation of the transmission line network, the BLT equation to solve the node voltage response is as follows:

$$V=\left[U+S\right]\cdot {\left[\Gamma -S\right]}^{-1}\cdot E$$

(1)

-

$V$ is the voltage vector.

$$V={\left[V_{1,1},V_{1,2},V_{2,2},V_{2,3},V_{3,3},V_{3,4},V_{4,4},V_{4,5},V_{5,5},V_{5,6}\right]}^T$$

(2)

The vector element $V_{i,j}=V_{i,j}{}^{inc}+V_{i,j}{}^{ref}$ is the full voltage at each node.

-

$U$ is the unit hypermatrix.

-

S is the scattering matrix.

$$S=diag\left({\rho }_1,S_2,S_3,S_4,S_5,{\rho }_6\right)$$

(3)

Because of the reflection coefficient of matching node $J_1$, ${\rho }_1=0$. $J_6$ is the reflection coefficient of the short-circuit node, and ${\rho }_6=-1$. The scattering matrices of observation points $P_1\left(S_3\right)$ and $P_2\left(S_5\right)$ are $S_3=S_5=\left[\begin{array}{cc}0& 1\\ 1& 0\end{array}\right]$.

The equivalent two ports of aperture structure node are T-type ones (Figure 4), and scattering matrix $S_2$ is:

$$S_2=\left[\begin{array}{cc}\frac{Y_1-Y_2-Y_a}{Y_1+Y_2+Y_a}& \frac{2Y_2}{Y_1+Y_2+Y_a}\\ \frac{2Y_1}{Y_1+Y_2+Y_a}& \frac{Y_2-Y_1-Y_a}{Y_1+Y_2+Y_a}\end{array}\right]$$

(4)

The equivalent two-ports of the special-shaped structure node are Π-type ones (Figure 5), and scattering matrix $S_4$ is:

$$S_4=\left[\begin{array}{cc}\frac{Y_1{}^2-2Y_a{Y}_b-Y_a{}^2}{\left(Y_1+Y_a\right)\left(Y_1+Y_a+2Y_b\right)}& \frac{2Y_1{Y}_b}{\left(Y_1+Y_a\right)\left(Y_1+Y_a+2Y_b\right)}\\ \frac{2Y_1{Y}_b}{\left(Y_1+Y_a\right)\left(Y_1+Y_a+2Y_b\right)}& \frac{Y_1{}^2-2Y_a{Y}_b-Y_a{}^2}{\left(Y_1+Y_a\right)\left(Y_1+Y_a+2Y_b\right)}\end{array}\right]$$

(5)

-

$\Gamma$ is the propagation matrix.

$$\mathsf{\Gamma }=diag\left({\mathsf{\Gamma }}_1,{\mathsf{\Gamma }}_2,{\mathsf{\Gamma }}_3,{\mathsf{\Gamma }}_4,{\mathsf{\Gamma }}_5\right)$$

(6)

In the formula, $\mathsf{\Gamma }i=\left[\begin{array}{cc}0& e^{{\mathsf{\gamma }}_i{\mathrm{d}}_i}\\ e^{{\mathsf{\gamma }}_i{\mathrm{d}}_i}& 0\end{array}\right]$.

Where ${\gamma }_1=j{k}_0$, ${\gamma }_2={\gamma }_3=j{k}_{g1}$, ${\gamma }_4={\gamma }_5=j{k}_{g2}$, and $d_i$ is the length of each tube.

-

$E$ is the source vector.

$$\mathrm{E}={\left[\frac{1}{2}{V}_s{e}^{{\mathsf{\gamma }}_1{\mathrm{d}}_{11}}-\frac{1}{2}{V}_s{e}^{{\mathsf{\gamma }}_1{\mathrm{d}}_{12}} 0 0 0 0 0 0 0 0\right]}^T$$

(7)

2.2. Irregular Cavity Classification

Based on the classification of irregular cavities in the electromagnetic topology theory, irregular cavities are typically divided into six types, as shown in Figure 6. When the electric field is perpendicular to the long side of the aperture, the most common irregular structures or a combination of the above six structures are covered.

The equivalent two-port types of six irregular structures are different. The structure of the step and the sheet is a T-type one, and the thick block structure is the Π-type one. In addition, the types of components in the equivalent two-port network of each irregular structure are also different. The parameters of equivalent circuit of irregular structure are as follows:

1.

Capacitive step

In Figure 6a, the characteristic admittance of anterior chamber $Y_g=\sqrt{1-{\left(\lambda /2a\right)}^2}/Z_0$, and $Y_g^{\prime }=Y_{g}b/b^{\prime }$ is the characteristic admittance of the rear cavity.$b^{\prime }$ is the height of the rear cavity.

$$\frac{X}{Y_g}\approx \frac{2a}{{\lambda }_g}\left[ln\left(\frac{1-{\alpha }^2}{4\alpha }\right){\left(\frac{1+\alpha }{1-\alpha }\right)}^{\frac{1}{2}\left(\alpha +\frac{1}{\alpha }\right)}+\frac{2}{A}\right]$$

(8)

where, ${\lambda }_g={\lambda }_0/\sqrt{1-{\left(\lambda /2a\right)}^2}$, $\alpha =b^{\prime }/b$, $A={\left(\frac{1+\alpha }{1-\alpha }\right)}^{2\alpha }\frac{1+\sqrt{1-{\left(2b/{\lambda }_g\right)}^2}}{1-\sqrt{1-{\left(2b/{\lambda }_g\right)}^2}}-\frac{1+3{\alpha }^2}{1-{\alpha }^2}$.

2.

Inductive step

$$\{\begin{array}{l}\frac{X}{Y_g}\approx \frac{2a}{{\lambda }_g}2.33{\alpha }^2\left(1+1.56{\alpha }^2\right)\left(1+6.75{\alpha }^{2}Q\right),\alpha \ll 1\\ \frac{Y_g}{X}\approx \frac{{\lambda }_g}{2a}\frac{{\beta }^2\left(1+\beta \right)ln\frac{2}{\beta }}{1-\frac{\beta }{2}}\left(1-\frac{27}{8}\frac{Q+Q^{\prime }}{1+8ln\frac{2}{\beta }}\right),\beta \ll 1\end{array}$$

(9)

where $\alpha =a^{\prime }/a=1-\beta$, $Q=1-\sqrt{1-{\left(2a/3\lambda \right)}^2}$, $Q^{\prime }=1-\sqrt{1-{\left(2a^{\prime }/3\lambda \right)}^2}$, ${\lambda }_g=\lambda /\sqrt{1-{\left(\lambda /2a\right)}^2}$, and ${{\lambda }_g}^{\prime }=\lambda /\sqrt{1-{\left(\lambda /2a^{\prime }\right)}^2}$. $a^{\prime }$ is the width of the rear cavity.

3.

Capacitive obstacles with a zero thickness value

$$\{\begin{array}{c}\frac{X}{Y_g}\approx \frac{4b}{{\lambda }_g}\left\{ln\left(\frac{2b}{\pi d}\right)+\frac{1}{6}{\left(\frac{\pi d}{2b}\right)}^2+\frac{1}{2}{\left(\frac{b}{{\lambda }_g}\right)}^2{\left[1-\frac{1}{2}{\left(\frac{\pi d}{2b}\right)}^2\right]}^4\right\},\frac{d}{b}\ll 1\\ \frac{X}{Y_g}\approx \frac{2b}{{\lambda }_g}\left[{\left(\frac{\pi d^{\prime }}{2b}\right)}^2+\frac{1}{6}{\left(\frac{\pi d^{\prime }}{2b}\right)}^4+\frac{3}{2}{\left(\frac{b}{{\lambda }_g}\right)}^2{\left(\frac{\pi d^{\prime }}{2b}\right)}^4\right],\frac{d^{\prime }}{b}\ll 1\end{array}$$

(10)

where $d^{\prime }$ is the height of two sheets, and d is the distance between two sheets.

4.

Inductive obstacles with a zero thickness value

$$\{\begin{array}{c}\frac{X}{Y_g}\approx \frac{a}{{\lambda }_g}ta{n}^2\left\{\frac{\pi d}{2a}\left[1+\frac{1}{6}{\left(\frac{\pi d}{{\lambda }_0}\right)}^2\right]\right\},\frac{d}{a}\ll 1\\ \frac{X}{Y_g}\approx \frac{a}{{\lambda }_g}co{t}^2\left\{\frac{\pi d^{\prime }}{a}\left[1+\frac{2}{3}{\left(\frac{\pi d^{\prime }}{{\lambda }_0}\right)}^2\right]\right\},\frac{d^{\prime }}{a}\ll 1\end{array}$$

(11)

where $d^{\prime }$ is the height of two sheets, and $d$ is the distance between two sheets.

5.

Capacitive obstacles with finite thickness

$$\{\begin{array}{c}\frac{X_a}{Y_g}=\frac{X}{Y_g}+\frac{b}{d}tan\frac{\pi l}{{\lambda }_g}\\ \frac{X_b}{Y_g}=\frac{b}{d}csc\frac{2\pi l}{{\lambda }_g}\end{array}$$

(12)

where $\frac{X}{Y_g}=\frac{2b}{{\lambda }_g}\left[ln\frac{e}{{\lambda }_g}+\frac{{\alpha }^2}{3}+\frac{1}{2}{\left(\frac{b}{{\lambda }_g}\right)}^2{\left(1-{\alpha }^2\right)}^4\right]$, $\alpha =b^{\prime }/b$, and $b$ is the height of two blocks, and $b^{\prime }$ is the distance between two blocks. $l$ is the thickness of two blocks.

6.

Inductive obstacles with finite thickness

$$\{\begin{array}{c}\frac{X_a}{Y_g}=\frac{X}{Y_g}-\frac{b}{d}tan\frac{\pi l}{{\lambda }_g}\\ \frac{X_b}{Y_g}=\frac{b}{d}csc\frac{2\pi l}{{\lambda }_g}\end{array}$$

(13)

where $\frac{X}{Y_g}=\frac{2b}{{\lambda }_g}\left[ln\frac{e}{{\lambda }_g}+\frac{{\alpha }^2}{3}+\frac{1}{2}{\left(\frac{b}{{\lambda }_g}\right)}^2{\left(1-{\alpha }^2\right)}^4\right]$, $\alpha =b^{\prime }/b$, and $b$ is the height of two blocks and $b^{\prime }$ is the distance between two blocks. $l$ is the thickness of two blocks.

The shielding effectiveness of the observation points can be obtained by solving the BLT matrix equation of the node voltage response [26]. The calculation results are in good agreement with the simulation results when the frequency exceeds the first resonant frequency of the cavity. The solution takes only a few seconds, which is much faster than the simulation time required by the CST simulation. It also has a good prediction of the resonance frequency point of the irregular structure, and a relatively systematic analysis model of the shielding effectiveness of the irregular structure is presented.

3. Simulation Analysis of Field Distribution in Irregular Cavity

3.1. Simulation Parameters of Irregular Cavity Field Distribution

Since the field distribution in the cavity is affected by the size, shape, and other structural characteristics of the irregular cavity, the electromagnetic field distribution characteristics are mainly studied in the irregular cavity, with apertures in the frequency range of 20 MHz–1 GHz. In this process, the effects of the special-shaped structure on the distribution law of the electromagnetic field in the cavity are comprehensively analyzed.

The objects simulated are six primary, irregular cavities, with the size of the regular part set being 300 × 120 × 360 mm³, the thickness of the cavity being 1 mm, and the size of the rectangular aperture being 40 × 20 mm².

The specific parameters of the six irregular cavities simulated by CST are shown in

Table 1. The parameters of six irregular structures.

Numbering	Type	Heteromorphic Structure Parameters
1	Capacitive step	300 × 20 × 160 mm3
2	Inductive step	30 × 120 × 160 mm3
3	Capacitive obstacles with a zero thickness value	300 × 20 × 1 mm3, d2 +d3 = 180 mm
4	Inductive obstacles with a zero thickness value	25 × 120 × 1 mm3, d2 +d3 = 180 mm
5	Capacitive obstacles with a finite thickness	300 × 10 × 40 mm3, d2 +d3 = 200 mm
6	Inductive obstacles with a finite thickness	20 × 120 × 40 mm3, d2 +d3 = 200 mm

. The number of the steps or obstacles is two in each irregular cavity. d2 and d3 are the parameters shown in Figure 1.

3.2. Simulation Results of Field Distribution in the Irregular Cavity

The field distribution of six irregular cavities in the typical transmission mode Mode303 is simulated to study the effects of the irregular structure on the field distribution.

3.2.1. Basic Model Field Distribution

The basic model field in Mode303 is shown in Figure 7.

The corresponding transmission modes are displayed on the cross-sections in all of the directions in the cavity when the frequency is in Mode = 303, with three half-wave transmission modes in the x-direction and the z-direction, which is in line with Mode303.

3.2.2. Field Distributions of Capacitive Step and Inductive Step

Mode303 field distribution diagrams of the two step structures are shown in Figure 8 and Figure 9. Since the capacitive step changes the size in the y-direction, where there is no mode, the field distribution in the cavity shows no significant changes, while the inductive step changes the size in the x-direction, so the modes in the second half of the step structure are mixed, and the latter two modes are blended.

3.2.3. Field Distributions of Capacitive and Inductive obstacles with a Finite Thickness

Mode303 field distributions of the two thick block structures are shown in Figure 10 and Figure 11.

The field distribution of capacitive obstacles with a finite thickness is more in line with the basic model than the inductive thick block cavity is, which is slightly different and can be seen more clearly from the y section view. In contrast, the distribution of the modes becomes blurred due to the cross-coupling of the modes in the x-direction and z-directions.

3.2.4. Field Distributions of Capacitive and Inductive Obstacles with Zero Thickness

Mode303 field distributions of the two obstacles with a zero thickness value are shown in Figure 12 and Figure 13. The field distribution of the capacitive obstacles with a zero thickness value is in line with the basic model. The changes to both of them in the cavity are relatively consistent with the cross-coupling.

The field distribution of the capacitive obstacles with a zero thickness value is aligned well with the basic model with similar sections, and it has only a small effect on the field distribution. In contrast, there are some changes in the field distribution of the inductive obstacles with a zero thickness value on the y section, mainly reflected in more independent modes and fewer changes in the overall field distribution due to the intervention of the irregular structures that separate the previously connected modes.

3.3. Analysis of Field Distribution in Special-Shaped Cavity

The field distribution of the irregular cavity has been simulated and analyzed before. The typical resonance frequency has been selected for simulation research, mainly for the field distribution of six basic, irregular cavities. However, this analysis concerns on the effects of the irregular structures on the field distribution and the selection of observation points in the shielding effectiveness testing method.

1.

The effects of irregular structures.

The inductive, irregular structure has a more significant impact on the field distribution than the capacitive one does. As the electric field polarization direction is the height direction in the simulated plane wave, the field distribution of the cavity may be more sensitive to the size or structure changes perpendicular to the plane wave polarization direction.

2.

Selection of observation points.

The overall change in the intensity of the electric field and the position where the higher field intensity is likely to occur are analyzed to select the appropriate points for observation based on the test requirements on the ground and the field distribution characteristics of the irregular cavity.

For irregular cavities, the overall distribution characteristics are in line with those of the regular cavities only, with the following differences:

• Irregular cavities mean that the distributions of the front and back parts are different, with the cavity of the step structure being the most obvious part. As the step structure changes the size of the front and back parts of the cavity, and the change in the size of the capacitive step is the height direction, the mode transmission has only a small effect on the overall field distribution since there is no mode transmission in the height direction of the back part of the cavity in the Mode303. However, there are changes in the length direction of the back part of the cavity in the inductive steps, resulting in the coupling of the modes in the front and back parts of cavities.
• The regular cavity is similar to the irregular cavity of the thick block type on the whole, which is mainly reflected in the cross-coupling of the inductive thick block appearance mode.
• As there is a small volume in the cavity that the thin plate takes up, the capacitive thin plate has only a small effect on the field distribution of the cavity. In contrast, the inductive thin plate has more significant effects, which is similar to the inductive thick block.

4. Characterization and Test Method of Shielding Effectiveness of Irregular Cavity

4.1. Characterization of Shielding Effectiveness of Irregular Cavity

Shielded cavities have a wide range of types and applications, and testing standards applicable in different situations can be found for different cavities [24,25,26]. Among these standards, IEEE 299-2006 and IEEE 299.1-2013 are the most widely used ones, and the shielding effectiveness test methods for cabins that are above 2 m and 0.1–2 m cabins are given, respectively.

The number of modes and mode distribution of the field distribution inside the cavity change with frequency based on the research on the field distribution of the irregular cavity. The field strength obtained from relatively fewer observation points may not be the maximum field strength on the ground in the cavity. Therefore, it is necessary to reconsider the position of the field strength monitoring points applicable to the shielding effectiveness of the irregular cavities.

On the one hand, the field distribution in the shielding cavity is affected by the irregular structure. In the low-frequency band with few modes in the cavity, the appropriate observation node position can be selected during the test based on the mode distribution. Generally, the center position of the cavity can be selected. Since this position is at the center of the length, height, and width directions, modes are more likely to appear at the center of the length and height directions or keep a fixed distance from the wall in the width direction, such as 0.3 m, which is given in the standard. In the high-frequency band with more modes in the height direction, the maximum field strength in the cavity may not be measured in a precise way by the original observation points, a particular movement in the height direction is taken into account, or two center positions can be selected that are bisected in the height direction.

On the other hand, as the internal space of the shielding cavity is limited, the test antenna’s volume needs to be considered from the actual testing perspective. It may not be possible to increase the observation points, which partly explains why the standard only provides observation points at a fixed distance from the wall, as one or two observation points need be added in a specific direction of a particular frequency band in the large cavity.

4.2. Test Method for Shielding Effectiveness of Irregular Cavity

For the irregular cavity, the antenna size cannot be measured by the regular test method, including the biconical antenna between 20 MHz and 100 MHz specified in the standard and half-wave dipole antennas or logarithmic periodic antennas between 100 MHz and 1 GHz because of their large size. In addition, as the polarization direction of the antenna needs to be considered, the space for the antenna must be provided in both of the directions, which is not applicable in the cavity with limited space because the coupling between the antenna and the wall will make the measured results inaccurate.

In the frequency band from 20 MHz to 1 GHz, the laboratory’s self-developed receiving spherical dual antenna is used for the shielding effectiveness test of the irregular cavity. The plan for the test is given based on the shielding effectiveness characterization and selecting the location and number of observation points, as shown in Figure 12 and Figure 13. The transmitting antenna is the biconical antenna (20 MHz~100 MHz) or logarithmic period antenna (100 MHz~1 GHz).

The layout of a certain observation point is shown in Figure 14 and Figure 15. The position and number of observation points can be selected based on the analysis above of the shielding effectiveness of the irregular cavity in

Table 2. Location and number of observation points.

Test Frequency Band	Location and Number of Observation Points
20 MHz~0.8 fr	Single point: Cavity center position
0.8 fr~1 GHz	Multi-point: the center position of the x-direction, the y-direction or the center position of the multi-segment division, and the center position of the multi-segment division in the z-direction

.

In the table, f_r is the first resonance frequency of the cavity. At 20 MHz~0.8 f_r, no transmission mode appears in the cavity, so the center position of the cavity can be used during the test; at 0.8 f_r~1 GHz, the mode is formed in the cavity. During the test, selecting multiple equally divided center positions as observation points in all of the directions is necessary.

The antenna polarization direction is arranged based on the test requirements. The test is generally carried out at horizontal and vertical polarizations if no particular need exists.

5. Measurement for Shielding Effectiveness of Irregular Cavity

The self-developed, spherical dipole receiving antenna is used to test the shielding effectiveness of the irregular cavity based on the proposed test method for the shielding effectiveness of the irregular cavity.

5.1. Test Parameters

The test object is selected as the existing aluminum cavity in the laboratory, with a size of 470 mm × 400 mm × 610 mm; the test irregular structure is chosen as an aluminum tube, with a size of 100 mm × 100 mm × 100 mm, which is placed in the middle of the bottom of the cavity as a step-like structure; the frequency band is 20 MHz~1 GHz; the signal source is placed on the outside during the test.

5.2. Test Layout

5.2.1. Reference Test Arrangement

The actual test setup for the test (use the test setup of the horizontal polarization of the biconical antenna in the 20 MHz~100 MHz frequency band as an example) is shown in Figure 16. The layout in the 100 MHz~1 GHz frequency band is the same, substituting the biconical antenna for a periodic log antenna.

5.2.2. Shield Test Arrangement

The internal and external layout of the cavity (taking vertical polarization as an example) during the shielding test of the cavity shielding effectiveness is shown in Figure 17, where the aluminum cube is placed in the middle of the bottom of the cavity. The overall setup of the test system is similar to that of the reference test shown in Figure 16.

5.3. Test Results

During the reference test, the output power of the signal source is set as −10 dBm, and a scatter test is carried out within 20 MHz~1 GHz.

During the shielding test, the output power of the signal source remains unchanged, and the first resonant frequency f_r is 550.4 MHz based on the size of the tested cavity: 470 mm × 400 mm × 610 mm. Since the shape of the aperture of the tested cavity is circular, and the length size is similar to the height, the patterns in the two directions that may appear in the interior are unlikely to have different parities.

The following observation points are selected based on Table 2, and a schematic diagram of the location of observation points is shown in Figure 16. For the observation points between 20 MHz and 440 MHz (0.8 f_r), a single point is selected, which is the No. 1 point in the center of the cavity. For observation points between 440 MHz (0.8 f_r) and 1 GHz, single or multiple points can be selected for testing, where single points at the center position of cavity No. 1 and multi-points of the bisecting point in the length direction and height direction are chosen for comparative testing. In addition, confined by the aluminum cube structure, the width direction is only selected as the center position, with four observation points from No. 2 to No. 5. A schematic diagram of the location of observation points is shown in Figure 18.

The shielding test results of the tested cavity’s shielding effectiveness are shown in

Table 3. Shielding effectiveness of point No. 1 in the center of the test cavity observation points.

Figure	Type of Transmitting Antenna	Polarization Direction	Shielding Effectiveness/dB
30	Biconical antenna	Horizontal	38.08
		Vertical	34.34
50	Biconical antenna	Horizontal	34.5
		Vertical	35.85
80	Biconical antenna	Horizontal	32.97
		Vertical	33.11
100	Biconical antenna	Horizontal	43.06
		Vertical	42.82
100	Periodic log antenna	Horizontal	46.23
		Vertical	42.63
200	Periodic log antenna	Horizontal	34.14
		Vertical	31.92
300	Periodic log antenna	Horizontal	26.54
		Vertical	31.49
500	Periodic log antenna	Horizontal	25.07
		Vertical	28.24
800	Periodic log antenna	Horizontal	18.21
		Vertical	19.2
1000	Periodic log antenna	Horizontal	14.26
		Vertical	16.68

and

Table 4. Shielding effectiveness of test chamber observation points from No.1 to No.5.

Frequency/MHz	Polarization Direction	No.1 SE/dB	No.2 SE/dB	No.3 SE/dB	No.4 SE/dB	No.5 SE/dB
500	Horizontal	25.07	24.79	26.59	24.83	24.25
	Vertical	28.24	28.72	27.19	28.78	27.7
800	Horizontal	18.21	17.88	18.12	18.89	18.03
	Vertical	19.2	20.85	18.65	20.03	19.07
1000	Horizontal	14.26	14.51	14.29	15.08	14.85
	Vertical	16.68	16.64	17.44	17.04	17.25

. While Table 3 shows the test results of shielding effectiveness at the No. 1 point in the center of the cavity in the frequency band of 20 MHz~1 GHz, Table 4 shows the comparison result of single-point and multi-point test data in the frequency band of 440 MHz (0.8 f_r)~1 GHz.

It can be seen from Table 3 that the single observation points test of the cavity shielding effectiveness is carried out smoothly. The results of different observation points in the 440 MHz (0.8 f_r)~1 GHz frequency band in Table 4 can be analyzed. The frequency points and the minimum shielding effectiveness observation points in the same antenna polarization directions are marked in Table 4.

Based on the test results, it can be found that when the frequency band is high, the modes in the cavity gradually increase. When only the center position of the cavity is used for testing, other locations with more immense field strengths are likely to be ignored, resulting in a more significant shielding effect.

When the shielding effectiveness of the irregular cavity is tested, more test points can undoubtedly improve the accuracy. Still, the time and labor that it takes will be significantly increased. If the characteristics of the body in the frequency band to be measured are analyzed in advance, selecting a small, but appropriate, number of observation points can effectively improve the validity of the test results and can significantly reduce the test time. In addition, the test accuracy can be effectively improved through the selection of multi-point observation points, which serve as a reference to each other when one is testing different observation points, avoiding the occurrence of invalid points with significant data deviations, and reducing the possibility of re-testing due to problems cropping up in operation.

6. Conclusions

In this paper, the method of selecting monitoring points and measuring the shielding effectiveness based on the field distribution is proposed. An electromagnetic topology model to analyze the shielding effectiveness is built first, which has a good prediction of the resonance frequency point of the irregular structure. Then, the appropriate points are selected for observation based on the test requirements on the ground and the field distribution characteristics of the irregular cavity. In the modeless range (20 MHz~0.8 f_r), the center position of the cavity can be used as the observation points during the test; in the under-mode range (0.8 f_r~1 GHz), multiple equally divided center positions are selected as observation points in all directions. The shielding effectiveness of irregular cavities can be tested accurately using this method. If the characteristics of the body in the frequency band to be measured are analyzed in advance, selecting a small, but appropriate, number of observation points can effectively improve the validity of the test results and can significantly reduce the test time.

This method can be used to test the shielding effectiveness of the irregular cavity, and the effectiveness of the method is verified in this paper. The following work was conducted to further simplify this testing method.