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International Journal of Antennas and Propagation
Volume 2012 (2012), Article ID 897074, 6 pages
http://dx.doi.org/10.1155/2012/897074
Research Article

High-Frequency Electromagnetic Field Coupling to Small Antennae in a Rectangular Resonator

Otto-von-Guericke-University Magdeburg, IGET, Universitätsplatz 2, D-39106 Magdeburg, Germany

Received 9 September 2011; Accepted 1 December 2011

Academic Editor: Dragan Poljak

Copyright © 2012 Sergey Tkachenko et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The integral-differential equation for the current of an electrically small antenna, inside a resonator, which is induced by given sources, is approximately solved by the so-called “Method of Small Antenna,” both for dipole and loop antennas. The current induced in the antenna is evaluated using the scattering characteristics of small antennas in free space and regularized Green’s function of resonator. As example of application of the theory, a transfer function (“external field→induced voltage”) for the coupling through aperture is calculated.

1. Introduction

Investigation of the coupling of high-frequency electromagnetic fields caused by intentional electromagnetic interferences to linear structures becomes an actual topic. Usually the corresponding test experiments and simulation models are applied to devices in free space [1]. However, in reality, electronic equipment is enclosed in different kinds of resonator-like shells: cabinets of computers, airframes, frames of cars, and so forth. These enclosures change the interaction of electromagnetic fields with the scatterers remarkably due to rereflections of electromagnetic fields inside the resonator [2].

It was shown in many experiments that the main mechanism of such interaction in free space is electromagnetic coupling to interconnections of different scales. Often these interconnections are electrically small (printed circuit boards, chips, etc.) but can have own resonances. Currents and voltages induced in such objects in free space can be evaluated by a method, which includes two simple models: a model of a small near-resonance linear antenna to describe the common mode and a model of a small loop to describe the differential mode [3].

In our papers, [4, 5], we proposed a method to analyze the coupling to an electrically short dipole (or monopole) antenna in a resonator (Method of Small Antenna, MSA) by consequently using the scattering theory. The MSA is based on the analysis of the integro-differential equation describing the induced current in the neighborhood of the antenna. This approach gives the possibility to analytically express the solution for the scattered current in the small antenna inside the resonator from the free space solution and the regularized cavity Green’s function. One can also investigate the input impedance of the small antenna, the current transfer ratio for two small antennas, and so forth.

In the present, work we expand our method for the case of electromagnetic coupling to an electrically small loop in the resonator. Again, using an approximate solution for the induced current in free space and a regularized cavity Green’s function, we derive an equation for the induced current in an electrically small loop in a resonator. The solution looks like the one for free space but contains a so-called “resonator impedance,” instead the radiation resistance of free space. This “resonator impedance” depends on the parameters of the antenna, the parameters of the resonator, and also the coordinates of the loop.

The developed theory was applied to investigate an important practical problem: the calculation of the transfer function (external electromagnetic field → scattered current) for electrically small wiring objects (printed circuit, chip, etc.) inside a rectangular resonator. It is assumed that electromagnetic waves penetrate into the cavity through a small aperture and excite the scattering objects. We show that the analytical evaluations are in good agreement with results of numerical simulation.

The paper is organized as follows: in the Section 2, we shortly repeat main results of MSA for the small electrical dipole antenna in a resonator. In the Section 3, we present new results of MSA for the small loop inside the resonator. In the Section 4, we calculate transfer functions for both type of scatterers and compare our results with numerical calculations. In the short conclusion, we outline directions of future investigations.

2. Small Electrical Dipole Antenna in a Rectangular Resonator

We consider a thin-wire (electrical) dipole antenna inside a rectangular cavity with well-conducting walls, see Figure 1, which is excited by an electric field 𝐸0(𝑟).

897074.fig.001
Figure 1: Electrical and magnetic dipoles in a resonator.

For simplicity, we assume that the antenna is parallel to the 𝑧-axis. The current induced in the antenna satisfies the electric field integral equation (EFIE), which contains the tensor (dyad) Green’s function of the cavity [4, 5]. The same EFIE equation for the current induced in the excited antenna, can be written in free space. Of course, in this case it contains the Green’s function for free space. For an electrically small antenna the EFIE equation in free space can be solved by the approximation of a small near-resonance (electrical) dipole antenna [3], see Figure 2𝐽𝐸(𝑗𝜔,𝑙)𝐸𝑧𝐾𝐸(𝑗𝜔)𝑓(𝑙)=𝜀𝐸,e𝑌𝐸𝑌(𝑗𝜔)𝑓(𝑙),𝐸1(𝑗𝜔)=1/𝑗𝜔𝐶𝑎+𝑗𝜔𝐿𝑎+𝑅𝑎.(1) For a thin symmetric dipole antenna [3], 𝜔𝑎𝜋𝑐𝐿,𝐶𝑎=𝜋𝜀0𝐿/2,𝐿ln(2𝐿/𝑑)2𝑎=1𝐶𝑎𝜔2𝑎.(2) Here the quantities 𝐿, 𝑑 are the antenna length and diameter, respectively, 𝜔𝑎 is the first resonance frequency of the antenna, 𝐶𝑎 and 𝐿𝑎 are the antenna capacitance and inductance, respectively, and 𝑅𝑎 is the antennaload.

fig2
Figure 2: Model of a small near-resonance electrical dipole antenna.

The effective electromotive force 𝜀𝐸,e and the current distribution function 𝑓(𝑙) for a passive antenna with a distributed uniform excitation 𝐸𝑧 (𝑅𝑎𝐿𝑎/𝐶𝑎), see Figure 2(a) are given by (3) and (4), respectively [35], 𝜀𝐸,e=𝐸𝑧𝐿2,(3)𝑓(𝑙)14𝑙2𝐿2.(4) By the analysis of the EFIE for the small antenna in a resonator, it can be shown that the induced current is described by a simple equation like (1), which, however, contains together with the load resistance 𝑅𝑎 also the “impedance of the resonator” 𝑍RES. This value is calculated using the resonator Green’s function, regularized for coinciding arguments, 𝐺𝐸RES, and it depends on both the antenna and resonator parameters. For the passive dipole (distributed excitation), one obtains (detailed derivation and explanation of expression (5) the reader can find in [3, 4]) 𝐽𝐸(𝑙)𝐸𝑧𝐾𝐸(𝑗𝜔)𝑓(𝑙)=0.5𝐸𝑧𝐿𝑌𝐸(𝑗𝜔)𝑓.𝑌(𝑙),𝐸=11/𝑗𝜔𝐶𝑎+𝑗𝜔𝐿𝑎+𝑅load+𝑍𝐸RES,𝑍(𝑗𝜔)𝐸RES(𝑗𝜔)=𝐺𝐸RES𝑍𝑍𝐹𝑑.𝑒.𝐿2,𝐹𝑑.𝑒.=𝐿/2𝐿/2𝑓𝑑.𝑒.2(𝑙)𝑑𝑙=,3𝐿𝐺𝐸RES𝑍𝑍𝜂04𝐽𝐾𝑎𝑏𝛾𝛾max𝑛𝑥,𝑛𝑦=1𝑘𝜈𝑥2+𝑘𝜈𝑦2×sin2𝑘𝜈𝑦𝑥sin2𝑘𝜈𝑦𝑦×𝛾𝑐𝜈/𝑧𝑐(𝛾)+1𝛾𝑠(𝛾)4𝜋𝛾max𝑘2𝛾3max3,𝑘𝜈𝑥=𝜋𝑛𝑥𝑎,𝑘𝜈𝑦=𝜋𝑛𝑦𝑏,𝛾𝜈=𝑘2𝑥+𝑘2𝑦𝑘2,𝜔𝑘=𝑐1𝑗,||𝛾2𝑄max||2.5𝑘.(5) An analogous simple equation for the induced current can be derived for the lumped excitation [5].

The input impedance of a small dipole antenna in a rectangular resonator was also calculated in [6] to investigate statistical properties. In this work, a different split of the resonator’s Green’s function was used, taking advantage of the Coulomb gauge for the vector and scalar potential. This was combined with the assumption of a fixed linear free-space current distribution in the antenna for lumped excitation.

3. Small Magnetic Dipole (Loop) Antenna in Rectangular Resonator

Consider an electrically small loop inside a rectangular cavity (see Figure 1) with well-conducting walls. For simplicity we assume that the plane of the loop is normal to the 𝑧-axis. The loop interacts with the magnetic field which is excited in the resonator by one or another way (an additional internal radiating antenna, penetration of a field into the cavity through apertures, etc.). This field induces a current 𝐽 in the loop, which, in turn, serves as a source of the scattered electromagnetic field. This small loop is equivalent to a magnetic dipole with dipole moment𝑀=𝐽𝑆, where 𝑆 is the equivalent area of the loop.

For a more simple description of the electromagnetic field of a small loop using the mathematical apparatus of Green’s functions, it is convenient to formally introduce a magnetic charge𝐽𝑀 and a magnetic current𝜌𝑀 (see, for example, [7]). The dipole moment of the small loop is connected with the integral of magnetic current by 𝐽𝑀𝑟𝑑𝑉=𝑗𝜔𝑀.(6) Then the electrical and magnetic field in the resonator can be written as𝐴𝐸=rot𝑀,(7a)1𝐻=𝑗𝜔𝜇0𝐴graddiv𝑀+𝑘2𝐴𝑀,(7b) where 𝐴𝑀=𝐺𝐴,𝑀𝑟,𝑟𝐽𝑀𝑟𝑑3𝑟(7c)is the vector potential caused by the magnetic current; 𝐺𝐴,𝑀(𝑟,𝑟) is the tensor Green’s function for the vector potential. This Green’s function has a simple diagonal form in free space 𝐺𝐴,𝑀𝑓𝑠𝑝.𝑟,𝑟=𝜇0||4𝜋exp𝑗𝑘𝑟,𝑟||||𝑟,𝑟||𝐼.(8) In the resonator, this Green’s function can be calculated by a different way. Here, as in our early research of the small electrical dipole [4], it is convenient to use the resonator Green’s function in double sum representation (expansion into transverse waveguide modes of the cavity [7]). It is a diagonal tensor, whose 𝑧𝑧-component is (the other components can be obtained by cyclic rearrangement) given by 𝐺𝐴,𝑀RES𝑧𝑧=𝜇0𝑎𝑏𝑛𝑥,𝑛𝑦=0𝜀0,𝑛𝑥𝜀0,𝑛𝑦𝑘cos𝑣𝑥𝑥𝑘×cos𝑣𝑥𝑥𝑘cos𝑣𝑦𝑦𝑘cos𝑣𝑦𝑦×1𝛾𝜈𝛾sinh𝜈×𝛾sinh𝜈𝑧𝛾sinh𝜈𝑧𝛾,𝑧𝑧sinh𝜈𝛾𝑧sinh𝜈(𝑧),𝑧𝑧(9) with 𝑘𝜈𝑥=𝜋𝑛𝑥𝑎,𝑘𝜈𝑦=𝜋𝑛𝑦𝑏,𝛾𝜈=𝑘𝜈𝑥2+𝑘𝜈𝑦2𝑘2,𝜀0,𝑛=1for𝑛=0,=2for𝑛0,andsoon.(10) The magnetic and electrical field can be obtained in an analogous way after integration with the Green’s function for the magnetic and electrical fields, respectively, using (8) or (9) and applying (7a) and (7b).

Let us now investigate properties of the Green’s functions in the neighborhood of the small scatterer. To make our method clearer, we begin from the case in free space. The 𝑧𝑧-component of the Green’s function for the magnetic field from (8) and (9) is approximated by𝐺𝐻,𝑀𝑓.𝑠𝑝.𝑧𝑧||||𝑥=𝑥1𝑦=𝑦1𝑘|𝑧𝑧|1114𝜋2𝑗𝜔||𝑧𝑧||3+𝑘20||𝑧𝑧||223𝑗𝑘30=𝐺𝐻,𝑀sing.𝑧𝑧𝑧𝑧+𝐺𝐻,𝑀𝑧𝑧.(11) As one can see, the Green’s function is decomposed into two parts. The first, singular part (first and second term in the brackets) represents the near field and contains electrostatic and magnetostatic energy. The second, regular part (third term in the bracket) is constant in the neighborhood of the scatterer and is responsible for the far field (radiation for the case of free space, see below).

In order to extract the divergence of the Green’s function inside the resonator in explicit form, we divide the summation domain of (9) into two parts [4] (see Figure 4): one corresponding to the values of wave number 𝛾𝑣 (less than some value 𝛾max) and the other part contains those values, which exceed 𝛾max. Then the summation may be approximated by integration (practical numerical calculation has shown that a good choice for 𝛾max is 2.5𝑘). As a result, we obtain the following expression for (𝐺𝐴,𝑀RES)𝑧𝑧 in the vicinity of the scatterer: 𝐺𝐻,𝑀RES𝑧𝑧||||𝑥=𝑥1𝑦=𝑦1𝑘|𝑧𝑧|1𝜇0𝑎𝑏𝛾𝑣𝛾max𝑛1,𝑛2=0𝜇+04𝜋exp𝛾max||𝑧𝑧||||𝑧𝑧||.(12) Now, substituting (12) into (7b), going to the limit 𝑘|𝑧𝑧|0 and keeping main terms, including constant ones, we obtain the desired decomposition of the resonator Green’s function for the magnetic field: 𝐺𝐻,𝑀RES𝑧𝑧||||𝑥=𝑥1𝑦=𝑦1𝑘|𝑧𝑧|11𝑗𝜔𝑎𝑏𝛾𝑣𝛾max𝑛𝑥,𝑛𝑦=0𝜀𝑜,𝑛𝑥𝜀𝑜,𝑛𝑦𝑘𝑣𝑥2+𝑘𝑣𝑦2×cos2𝑘𝑣𝑥𝑥cos2𝑘𝑣𝑦𝑦×𝛾sinh𝜈𝑧𝛾sinh𝜈(𝑧)𝛾𝜈𝛾sinh𝜈+114𝜋×2𝑗𝜔||𝑧𝑧||3+𝑘20||𝑧𝑧||2𝛾3max3𝑘2𝛾max.(13) Again, as for the case of free space, we can extract from (13) the singular near field part (the first and the second terms in the square brackets), which look like that one in free space and the regular far field part (the sum and the third and fourth terms in the square brackets, which take into account rereflection of the signal from the cavity walls and contain all information about system resonances).

The solution of the interaction problem with the singular part of the Green’s function can be obtained by the model of a small loop [3] (see Figures 3(a) and 3(b))𝐽𝐻(𝑗𝜔)𝜀𝐻,e𝑌𝐻(𝑗𝜔)=𝐾𝐻(𝑗𝜔)𝐻𝑧,𝜀(14a)𝐻,e=𝑗𝜔𝐻𝑧𝜇0𝑌𝑆,(14b)𝐻1(𝑗𝜔)=𝑗𝜔𝐿𝑎+𝑅𝑎.(14c)Here the quantities𝑆, 𝐿𝑎 are the antenna area and inductance, and 𝑅𝑎is the antenna load. For a thin circular loop antenna with radius 𝑅 and diameter of the wire 𝑑 [3],𝐿𝑎=𝜇0𝑅ln16𝑅𝑑2.(15) Let now the loop be under the action of an external magnetic field 𝐻𝑧. If we take into account only the singular part of the Green’s function (11) or (13), the induced current is defined by (14a), (14b) and (14c). But, if we would like to take into account the regular part of the Green’s function, we have to add to the magnetic field in (14a) an additional part 𝐻𝑧𝐻𝑧=𝐺𝐻,𝑀𝑧𝑧𝐽𝑚𝑧𝑟𝑑𝑉=𝐺𝐻,𝑀𝑧𝑧𝐽𝑚𝑧𝑟𝑑𝑉=𝐺𝐻,𝑀𝑧𝑧𝑗𝜔𝑀𝑧=𝐺𝐻,𝑀𝑧𝑧𝑗𝜔𝑆𝐽(16) Here we take into account (11), (6) and the connection of the magnetic moment with the electrical current. Then, from (14a), we have 𝐽𝐻=𝐻𝑧+𝐺𝐻,𝑀𝑧𝑧𝐽𝑗𝜔𝑆𝐻𝐾𝐻(𝑗𝜔).(17) This is a linear algebraic equation for the induced current𝐽𝐻 whose solution yields 𝐽𝐻=𝐾𝐻(𝑗𝜔)𝐺1𝐻,𝑀𝑧𝑧𝑗𝜔𝑆𝐾𝐻(𝐻𝑗𝜔)𝑧=𝑗𝜔𝜇0𝑆𝑗𝜔𝐿𝑎+𝑅𝑎𝜇0𝜔2𝑆2𝐺𝐻,𝑀𝑧𝑧𝐻𝑧=𝐾𝐻(𝑗𝜔)𝐻𝑧.(18) By this way, taking into account the regular part of the Green’s function yields the additional impedance 𝑍𝑎(𝑗𝜔) to the antenna resistance 𝑅𝑎 in (14b) 𝑍𝑎(𝑗𝜔)=𝜇0𝜔2𝑆2𝐺𝐻,𝑀𝑧𝑧.(19)

fig3
Figure 3: Model of a small loop.
897074.fig.004
Figure 4: Division of the summation domain.

For free space, with (𝐺.𝐻,𝑀)𝑧𝑧 from (11), (19) yields a well-known equation for the radiation resistance of the electrically small loop [3]: 𝑍freespace𝑎𝜂(𝑗𝜔)=0𝑘4𝑆26𝜋.(20) This agreement shows the correctness of our method for a small loop.

For the loop inside the resonator, (19) gives an impedance value for the antenna (“impedance of loop-resonator system”) 𝑍RES𝑎, similar to the case of a small near-resonance dipole in Section 2. This value depends on both, the loop antenna and resonator parameters, and describes all resonances of the following system: 𝑍RES𝑎(𝑗𝜔)𝑗𝜂0𝑘𝑆21𝑎𝑏𝛾𝑣𝛾max𝑛𝑥,𝑛𝑦=0𝜀𝑜,𝑛𝑥𝜀𝑜,𝑛𝑦𝑘𝑣𝑥2+𝑘𝑣𝑦2×cos2𝑘𝑣𝑥𝑥cos2𝑘𝑣𝑦𝑦×𝛾sinh𝜈𝑧𝛾sinh𝑣(𝑧)𝛾𝜈𝛾sinh𝜈1𝛾4𝜋3max3𝑘2𝛾max.(21) Note, that the (18)–(21) in the present section describe, namely, the differential current mode in the loop. This mode prevails for small frequencies, much smaller than the frequency of the first resonance (for the circular loop 𝜔1𝑐/𝑅). For the higher frequencies to evaluate the total current, one has to take into account the common current mode, which can be obtained using the approach of small electrical dipole antenna described in previous section. Combination of these two modes (depending from the orientation of the loop) gives a total current, which can be compared, for example, with numerical calculations. Such comparison will be a subject of nearest investigations.

4. Application of the Developed Theory: Transfer Function “External Field-Induced Current”

Now it is assumed that an exterior field penetrates through a rectangular aperture into the cavity and excites a small dipole (see Figure 5). The dimensions of the resonator are 𝑎==0.79 m, 𝑏=0.534 m. The center of the 𝑧-directed unloaded dipole antenna with length 𝐿=0.25 m and radius 𝑟=1 mm is placed in the point with coordinates 𝑥=0.395 m, 𝑦=0.3 m, and 𝑧=0.295 m. The cavity is excited through the rectangular slit by the normally incident plane wave with amplitude 𝐸0=100V/m. The position of the center of the slit is 𝑥1=0.395 m, 𝑧1=5 mm; the dimensions of the slit are 𝑑𝑥=100 mm, 𝑑𝑧=5 mm.

897074.fig.005
Figure 5: Excitation of the internal dipole antenna through small aperture.

The aperture is modeled by two equivalent electric and magnetic dipoles placed in the center of the aperture [7]. For the considered case, only the 𝑥𝑥 component of the magnetic dipole moment 𝑀𝑥=𝛽𝑥𝑥𝐻sc𝑥 of the aperture is important, which can be calculated using the magnetic polarisability 𝛽𝑥𝑥=𝜋/24𝑑3𝑥(ln(4𝑑𝑥/𝑑𝑧)1)1. Here 𝐻sc𝑥is a short-circuit magnetic field because the plane of the aperture is assumed to be perfectly conducting when solving the external problem of diffraction. Now using the numerical data for the𝐻sc𝑥,we can calculate the magnetic moment of the aperture. Then we calculate the internal electrical field using (7a) and (7c), and (9) and, after that, calculate the current amplitude in the dipole antenna using the results of Section 2. Comparison of the MSA results with those of the well-known numerical code “CONCEPT” is presented in Figure 6. One can see a satisfactory agreement of the numerical and analytical results, especially in the frequency region up to the first resonance of the small antenna. The observed differences (especially the additional peaks near the main resonance) are due to the fact that in this example, taken from a practical HPM coupling problem, the conditions of applicability of the method of small antennas are not well satisfied: the size of the antenna is comparable with the wavelength and the cavity dimensions. Moreover, there may be additional resonances in the penetration of radiation through the slit.

897074.fig.006
Figure 6: Comparison of results of MSA with CONCEPT.

Note, that the calculation of MSA requires about one minute of calculation by the PC notebook (Processor T7250 2 GHz); at the same time direct CONCEPT calculation requires approximately one week of calculations by the PC cluster (6 PCs).

5. Conclusion

Interactions of high-frequency electromagnetic fields with small scatterers inside rectangular (resonators taking into account both common and differential modes) were investigated using the Method of Small Antenna. The results are important for the study of natural and intentional electromagnetic interferences with printed circuits, chips, and so forth inside racks and housings. Comparison of the results with numerical ones has shown a good agreement. In future, we intend to investigate effective damping of the interferences inside cavities using the generalization of the developed model for the multiscatterer case.

Acknowledgment

This work was supported by the German Research Foundation under the Contract NI 633/5-1.

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