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International Journal of Antennas and Propagation

Volume 2012 (2012), Article ID 730145, 18 pages

http://dx.doi.org/10.1155/2012/730145

## Electromagnetic Field Coupling to Overhead Wire Configurations: Antenna Model versus Transmission Line Approach

^{1}Department of Electronics, University of Split, Split, Croatia^{2}Pascal Institute, Blaise Pascal University, Clermont-Ferrand, France

Received 14 August 2011; Accepted 3 December 2011

Academic Editor: Sergey V. Tkachenko

Copyright © 2012 Dragan Poljak and Khalil El Khamlichi Drissi. 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 paper deals with two different approaches for the analysis of electromagnetic field coupling to finite length overhead wire: the wire antenna theory (AT) and the transmission line (TL) method. The analysis is carried out in the frequency and time domain, respectively. Within the frequency domain analysis the wire antenna formulation deals with the corresponding set of Pocklington integrodifferential equation, while the transmission line model uses the telegrapher's equations. The set of Pocklington equations is solved via the Galerkin-Bubnov scheme of the Indirect Boundary Element Method (GB-IBEM), while the telegrapher’s equations are treated using the chain matrix method and the modal equation to derive per-unit-length parameters. For the case of the time domain analysis AT model uses the space-time Hallen integral equation set, while TL approach deals with the time domain version of the telegrapher’s equations. Hallen equations are handled via time domain version of GB-IBEM, while time domain telegrapher’s equations are solved by using Finite Difference Time Domain (FDTD) method. Many illustrative computational examples for the frequency and time domain response, respectively, for several configurations of overhead wires, obtained via different approaches, are given in this paper.

#### 1. Introduction

The electromagnetic field coupling to overhead wires is of great practical interest for many EMC applications [1–11], such as transient excitation of antennas, power, or communications cables. The electromagnetic field coupling to finite length overhead wires can be determined by means of the transmission line model or the thin wire antenna theory in either frequency or time domain [1]. In particular, the transient response of a wire configuration of interest can be computed directly, by solving the related time domain equations or by the indirect approach, that is, by solving their frequency domain counterpart. When the indirect approach is used the frequency spectrum has to be calculated, and then the transient response is computed by means of the Inverse Fourier Transform (IFT).

Many practical engineering problems dealing with electromagnetic field coupling to thin wires can be analyzed by using the Transmission Line (TL) models [1–6]. These models include the analysis of incident electromagnetic field exciting the line and the propagation of induced currents and voltages along the line.

The TL models yield valid results if the line length is significantly larger than the separation between the wires and also larger than the actual height above ground [8].

On the other hand the TL approximation cannot provide a complete solution for the excitation of a given wire configuration by an incident field if the wavelength of the electromagnetic field exciting a wire structure is comparable to or less than the transverse electrical dimensions of the structure. Namely, the TL model fails to predict resonances and accounts for the presence of a lossy ground only approximately [1]. One of the serious problems with TL approach occurs due to the fact that current grows to infinity at resonant points as there are no losses and radiation resistance to limit its flow [8]. The full wave approach, based on the wire antenna theory and related integral equations, is more rigorous and should be used whenever the above-ground transmission lines of the finite length are considered. However, a serious drawback of AT approach is rather long computational time required for the calculations pertaining to long lines.

This paper deals with the analysis of electromagnetic field coupling to overhead wires in either frequency or time domain by using both antenna model and transmission line approach, respectively.

A number of illustrative computational examples regarding electromagnetic coupling to overhead wires are given in the paper.

The aboveground wires are subjected to electromagnetic fields arriving from a distant source and inducing current to flow along the wires. The key to understanding the behaviour of induced fields is the knowledge of current distribution induced along the wires. These currents generate scattered fields propagating away from the equipment.

The paper is organized as follows: Section 2 deals with the frequency domain analysis followed by related numerical solution methods for overhead wires. Section 2 ends up with many illustrative examples related to the aboveground lines and PLC (power line communications) systems.

Section 3 outlines the time domain analysis and related method of solutions of governing equations. Some computational examples pertaining to the multiconductor aboveground lines are given. Finally, the conclusion summarizes what has been discussed throughout this work.

#### 2. Frequency Domain Models and Methods

This section deals with the wire antenna theory and transmission line (TL) approximation, respectively, for the analysis of electromagnetic field coupling to overhead lines of finite length in the frequency domain. The formulation arising from the wire antenna theory is based on the set of coupled Pocklington integrodifferential equation for half-space problems. The effect of a two-media configuration is taken into account by means of the reflection coefficient approximation [12]. The resulting integro-differential expressions are numerically handled via the frequency domain Galerkin-Bubnov scheme of the Indirect Boundary Element Method (GB-IBEM) [8].

Transmission line model in the frequency domain is based on the corresponding telegrapher's equations which are handled by using the chain matrix method [10].

##### 2.1. Antenna Theory Approach: Set of Coupled Pocklington Equations

Modeling of arbitrarily shaped wires located at different heights above a lossy ground is an important task in both antenna and electromagnetic compatibility (EMC) studies [1].

This section firstly deals with an assessment of the current induced along multiple wire configurations above a lossy ground. Once the currents along the wire array have been obtained, the radiated field components could be determined.

The set of Pocklington equations for a configuration of overhead wires can be obtained as an extension of the Pocklington integro-differential equation for a single wire of arbitrary shape. The Pocklington equation for a single wire above a lossy ground can be derived by enforcing the continuity conditions for the tangential components of the electric field along the perfectly conducting (PEC) wire surface. First, a single wire of arbitrary shape, insulated in free space, as shown in Figure 1 is considered.

For the PEC wire the total field composed from the excitation field and scattered field vanishes [1, 8]: Starting from Maxwell’s equations and Lorentz gauge the scattered electric field can be expressed in terms of the vector potential : The vector potential is defined by the particular integral over a given path C (considered conductive wire structure): where is the induced current along the line and denotes the lossless medium Green function: and is the distance from the source point to the observation point, respectively, while the propagation constant of the homogeneous medium is given by Inserting (3) into (2) gives the relation for the scattered electric field: Combining (6) and (1) results in the Pocklington integral equation for the unknown current distribution along the wire of arbitrary shape insulated in free space: where denotes the tangential component of the electric field illuminating the wire.

Now the case of curved wire located above an imperfectly conducting ground can be analyzed by extending integro-differential equation (7) using the reflection coefficient approach [12]. The geometry of an arbitrary wire and its image, respectively, is shown in Figure 2.

The excitation function is now composed from incident and reflected field, respectively, Performing certain mathematical manipulations the Pocklington integro-differential equation for a curved wire above a lossy ground becomes [12] where is the unit vector normal to the incident plane, while arises from the image theory and is given by and is the distance from the image source point to the observation point, respectively.

An extension to the case of multiple curved wires is straightforward, that is, it follows [12]
where is the total number of wires and is the unknown current distribution induced on the *n*th wire. Furthermore, and are the Green functions of the form
where and are distances from the source point and from the corresponding image, respectively, to the observation point of interest.

The influence of a lossy half-space is taken into account via the Fresnel plane wave reflection coefficient (RC) for TM and TE polarization, respectively [12],
where * θ′* is the angle of incidence and is given by,
and is the complex permittivity of the ground.

For the special case of single horizontal straight wire above a lossy half-space, Figure 3 integro-differential equation (9) simplifies into where is the induced current along the horizontal wire and denotes the Green’s function given by

Furthermore, if an array of multiple horizontal wires is considered (Figure 4), system of (11) becomes
where is the unknown current distribution induced along *n*th wire, is the known excitation field tangential to the *j*th wire surface, and is the corresponding Green function:
It is worth noting that a trade-off between the rigorous Sommerfeld integral approach and approximate RC approach is presented in [12]. Although reflection coefficient approximation causes certain error (up to 10%) it takes a significantly less computational effort then a rigorous Sommerfeld approach [8].

The total electric field irradiated by configuration of multiple wires of arbitrary shape is given by [13, 14]
where
Note that index *0* and *i* are related to the source and image wire, respectively.

For the special case of single horizontal straight wire above a lossy half-space (Figure 3), it follows [15] For the case of multiple horizontal wires the expressions for electric field are given by [15] The radiated magnetic field of the curved wire system can be written as follows [13, 14]: where The reduction to the case of a single straight wire or straight wire array is straightforward, as in the case of electric field given by (22)–(27).

##### 2.2. Numerical Solution

The set of Pocklington integro-differential equations (11) has been solved by using the Galerkin-Bubnov scheme of the Indirect Boundary Element Method (GB-IBEM). An outline of the method is given here, for the sake of completeness while the method has been presented in detail elsewhere, for example, in [8].

Performing the Galerkin-Bubnov scheme of (GB-IBEM) in the frequency domain the set of coupled integro-differential equations (11) is transformed into the following matrix equation [13] where the mutual impedance matrix is given by [13]: while the voltage vector is given by [13] Once the current distribution is obtained, the radiated field can be obtained applying the similar BEM formalism [13]. Thus, the total field is given by where the field components due to a wire segment radiation are given by The total magnetic field is given by [13] while the magnetic field components are given by [13]

The reduction to the case of a single straight wire or straight wire array is straightforward and can be found elsewhere, for example, in [8].

##### 2.3. Transmission Line Approximation: Telegrapher’s Equations in the Frequency Domain

Voltages and currents along the multiconductor transmission line shown in Figure 4 induced by an external field excitation can be obtained using the field-to-transmission line matrix equations in the frequency domain [10]: where the longitudinal impedance matrix is given by and the transversal admittance matrix can be written as where [] is the per-unit-length longitudinal inductance matrix for a perfect soil [] and [] are the per-unit length transverse capacitance and conductance matrix of the multiconductor line, respectively. Furthermore, is the per-unit length internal impedance matrix of the conductors, and is the per-unit length ground impedance matrix. Finally, and are sources vectors expressed in terms of the incident magnetic and electric field, respectively [1, 8].

##### 2.4. Computational Examples

The first computational example is related to the analysis of an overhead wire (Figure 3) of length , radius located at height m above PEC ground and illuminated by the plane wave. The amplitude of the electric field is and it is parallel to *x*-axis. Figure 5 shows the frequency response at the center of the line. The results computed via GB-IBEM and TL are compared to the results obtained via NEC using RC and Sommerfeld integral approach, respectively, to account for the presence of a lossy half-space. The agreement between the results obtained via the different approaches is found to be satisfactory.

Figure 6 shows the frequency response for the same line located above an imperfectly conducting half-space for various values of ground conductivity mS/m. The results calculated via different approaches agree satisfactorily again.

Next computational example is related to a simple Power Line Communications (PLCs) system. PLC technology aims to provide users with necessary communication means by using the already existing and widely distributed power line network and electrical installations in houses and buildings. However, one of the principal drawbacks of this technology is related to electromagnetic interference (EMI) problems, as overhead power lines at the PLC frequency range (1 MHz to 30 MHz) act as transmitting or receiving antennas, respectively [13].

Figure 7 shows the geometry of a simple PLC system consisting of two conductors placed in parallel above each other at the distance . The conductors are suspended between two poles of equal height, thus heaving the shape of the catenary.

The geometry of a catenary is fully defined by such parameters as the distance between the points of suspension, *, *the sag of the conductor, , and the height of the suspension point, , as shown in Figure 7. The imperfectly conducting ground is characterized with electrical permeability *ε** _{r}* and conductivity

*.*

*σ*The conductors are modeled as thin wire antennas excited by the voltage generator at one end and terminated by the load impedance at the other end.

The influence of the load impedance is taken into account by modifying continuity condition for the tangential components of the electric field at the wire surface: where is the corresponding conductor per length impedance of the conductor.

The modified Pocklington equation for the wire containing the load impedance is now given by Set of integral equations (41) is numerically solved using via GB-IBEM.

The actual example is related to the simple PLC circuit shown in Figure 7. The distance between poles is , with the radii of wires . The wires are suspended on the poles at heights and . The maximum sag of the conductor is assumed to be . Ground parameters are and *σ*. The power of the applied voltage generator is 2.5 *μ*W (minimum power required for the PLC system operation) and operating frequency is chosen to be 14 MHz. The value of the terminating load is 500 Ω. Figure 8 shows the current distribution along the simple PLC system for different values of sag.

Radiated electric and magnetic fields at the distance of 30 m from the wires and 10 m above ground are shown at the Figures 9 and 10, respectively.

Analysis of the radiated field distributions shows that the conductor sag does not influence the far-field region significantly while the near-field distribution is mainly determined by the conductor geometry. Finally, the power of the applied voltage generator is changed to 1 mW (average power used at the actual PLC systems) and operating frequency is varied between 1 and 30 MHz. The values of the terminating load are chosen to be 50 Ω, 500 Ω, 5000 Ω, thus simulating different conditions within the power grid. The maximum values of the radiated electric field at the distance of 30 m for different arrangements are shown in Table 1.

According to the available international standards [16, 17], radiated electric fields should not excide level of 30 *μ*V/m at the distance of 30 m. Obviously, the radiated field levels are at best case more than 10 times higher than the proposed limit. The spatial distributions of the radiated electric field have been calculated for the number of frequencies in the frequency range from 1 to 30 MHz. Maximum levels of the calculated electric fields values are shown to excide the limits defined by the standard for the disturbances caused by information technology equipment.

#### 3. Time Domain Models and Methods

This section deals with direct time domain analysis of transient electromagnetic field coupling to straight overhead wires using the wire antenna theory and the transmission line method, respectively. The time domain antenna theory formulation is based on a set of the space-time Hallen integral equations. The transmission line approximation is based on the corresponding time domain Telegrapher's equations. The space-time integral equations arising from the wire antenna theory are handled by the time domain scheme of GB-IBEM. The time domain Telegrapher's equations are solved using the Finite Difference Time Domain (FDTD) method. Time domain numerical results obtained with both approaches are compared to the results computed via NEC 2 code combined with Inverse Fourier Transform procedure. Some illustrative comparisons of results obtained by means of antenna theory and transmission line approach are presented in this section.

It is worth mentioning that, for the sake of simplicity, only straight wires are analyzed in this paper.

##### 3.1. The Antenna Theory Model

Generally, a direct time-domain analysis of thin wire in the presence of a lossy half-space can be carried out via the appropriate space-time integral equations of either Pocklington or Hallen type [1, 8]. When applied to the solution of the Hallen integral equation the Galerkin-Bubnov Indirect Boundary Element Method (GB-IBEM) [8] results in relatively complex procedures compared to various procedures for the solution of Pocklington equations, but, at the same time, it is proven to be highly efficient, accurate, and unconditionally stable [8, 18, 19]. On the other hand, the implementation of GB-IBEM to the solution of the Pocklington-type equation is relatively simple, but suffers from serious numerical instabilities. The origin of these instabilities is the discretization of space-time differential operator [8]. The GB-IBEM solution of the Pocklington equation in free space for certain values of time domain integration parameters has been presented elsewhere, for example, in [19], while the Hallen integral equation solution by means of GB-IBEM has been obtained for thin wire structures in the presence of a dielectric half-space, for example, in [11]. In both cases, the influence of imperfect ground has been taken into account via the corresponding reflection coefficient. The numerical solution was mostly limited to scenarios in which the finite conductivity of the ground could be ignored. This approximation involves cases where the wires are sufficiently far from the two-media interface or where the ground conductivity is appreciably low or very high, that is, where the approximation of pure dielectric medium or perfect ground is applied. Through these approximations the time-dependent part of the reflection coefficient function vanishes, and the resulting matrix equation simplifies significantly.

However, for the cases where these approximations are not valid, modifications to the original methods are required in order to include the ground conductivity [8]. Namely, the related reflection coefficient is space-time dependent, and the resulting convolution integrals have to be included in the matrix system and numerically computed. This leads to a significant increase in the overall computational cost of the method and consequently requires several modifications.

This section deals with the transient analysis of multiple horizontal wires above a lossy ground using the Hallen integral equation approach.

The set of space-time Hallen's integral equations can be derived as an extension of the single wire case. First, a single wire insulated in free space is considered.

Thin wire antenna or scatterer of length and radius , oriented along the *x*-axis, is considered. The wire is assumed to be perfectly conducting and excited by a plane wave electric field. For the sake of simplicity, the analysis is restricted to the case of a normally incident electric field.

The tangential component of the total field vanishes on the PEC wire surface, that is, where is the incident and scattered field on the metallic wire surface. Starting from Maxwell equations and obeying the Lorentz gauge one obtains a time domain version of (2): where is the space-time-dependent vector potential.

According to the thin wire approximation, only the axial component of the vector potential exists, that is, (43) becomes
where *c* denotes the velocity of light.

The corresponding solution of (44) can be expressed in terms of a sum of the general solution of the homogeneous equation and the particular solution of the inhomogeneous equation:
The solution of the homogeneous wave equation is given as a superposition of incident and reflected wave [8]:
while the particular solution is given by the integral [8]:
where *L* denotes the total antenna length.

On the other hand, the magnetic vector potential on the PEC wire surface is given by the particular integral:

Combining (45)–(48) yields the space-time Hallen equation: where is the equivalent axial current to be determined, is the known tangential incident field, is the distance from the source point (the equivalent current in the antenna axis) to the observation point, and is the wave impedance of a free space.

The unknown functions and account for the multiple reflections of the current at the free ends of the wire.

A direct time formulation for a straight thin wire above a dissipative half-space can be obtained as the extension of the free-space Hallen equation (49).

The free space Hallen equation (49) is first transferred into the Laplace frequency domain: where is the Laplace variable.

According to the theory of images the free space integral equation (50) is extended by an additional term multiplying the Green function of the image source by space-frequency-dependent reflection coefficient for TM polarization. The integral equation in the frequency domain is given by where and is determined by the expression [1] where and are the lossy medium conductivity and permittivity, respectively, and.

The reflection coefficient (RC) approach is a satisfactory approximation in half-space calculations, as long as the field is calculated far away from the source, and the imperfect ground, respectively, to ensure [8].

Performing the convolution, the time domain counterpart of (52) is obtained in the form
where *r*() is the space-time reflection coefficient which, for convenience, can be written in the form [18]
where

Note that is the modified Bessel function of the first order and *n*th degree.

For the case of normal incidence, which is considered for the sake of simplicity, the excitation term is given by where .

The transient ground-reflected field is obtained as the convolution of the incident field and the space-time reflection coefficient for the angle of incidence (in accordance with the parallel incidence of the electric field), as is proposed in [20] and the integral equation (53) becomes

The unknown time functions , and can be obtained in the same manner, as in the case of free space, in terms of as auxilliary functions and [8]:
where
while and are the distances from the wire ends to the source point and *, * are the distances from the image wire ends to the image source point.

If the case of a perfect (ideal) dielectric half-space is considered, the Hallen equation (53) simplifies into Space-time integral equation (53) or (61), respectively, can be solved assuming the zero current at the free ends of the wire and with the initial conditions requiring the wire not to be excited before the certain instant [8].

The transient behavior of straight horizontal thin wires located at different heights above an infinite ground plane is determined by a set of the coupled space-time integral equations of the Hallen type [11]:
where denote the index of the observed and source wire, respectively. Furthermore, and are the lengths of the *s*th and *v*th wire, and *, * are the *x*-coordinates of the source and observation points on respective wires. The distances between observation point ) on the wire *v* and source point on the wire are given by
where asterisk is related to source points are located on the image wire.

Unknown time signals and account for the multiple reflections of transient currents at the wire open ends and can be written in the form while the auxiliary functions are defined, as follows: where and are distances from considered source point on each wire to a corresponding observation point at the ends of the wire : while and are distances between the source point at the image of the wire and observation point located at the ends of the wire :

The space-time reflection coefficient accounts for the influence of the interface and is given by [11] where

The angle is the angle between the source point on the image of the wire and the observation point on wire *v*.

Substituting (70) into (62) yields
where is the space-time-dependent tangential electric field on the *v*th wire.

For the case of normal incidence the total excitation field is given as the sum of the incident field and the field reflected from the interface [11],

The time shift represents the time required for the wave to travel from the highest wire to the height of the observed *v*th wire. Assigning the highest wire with index , it can be written

The field reflected from the interface for the case of normal incidence is given by
where is the time needed for the wave to travel from observed *v*th wire to the interface.

For the case of PEC ground plane, the space-time reflection coefficient (54) simply becomes

Thus, the set of (72) simplifies into and the field reflected from PEC ground is simply given by

Given the dielectric constant of the medium and the known excitation , a system of coupled Hallen integral equations can be solved using time domain version of GB-IBEM and by taking into account appropriate boundary and initial conditions. Boundary conditions assume zero currents at the end of each wire, while initial conditions assume all the currents to be zero for .

##### 3.2. The Numerical Solution

First, numerical procedure for single wire Hallen equation is outlined. Applying the weighted residual approach in the spatial domain and GB-IBEM procedure [8], the following local matrix system is obtained:

The space-dependent local matrices representing the interaction between *i*th source and *j*th observation element are defined as follows:
where stands for the shape functions, while additional time dependent vectors are given by
where

Assembling the local matrices and vectors into the global ones the following global matrix system is formed: where

Applying the weighted residual approach in the time domain and using the Dirac impulses as weight functions, the time sampling is provided, and the following recurrent formula is obtained:
where is current for the *j*th space node at *k*th time instant, is total number of space segments, while the overbar indicates the absence of diagonal members.

It is worth noting that the numerical calculation of convolution integrals is rather tedious task leading to tremendously large computational time of the overall method. The main advantage of the method, on the other hand, is its unconditional stability.

Time domain GB-BEM procedure for the set of Hallen equations is undertaken in a similar manner as in the case of a single wire.

The solution of (72) and (77), respectively, is also carried out using the GB-IBEM technique.

Applying the boundary element discretisation to (72) and (77), respectively, leads to a local system of linear equations for the *v*th observed wire:
where denotes the index of the elements located on the *s*th source wire and the *v*th observed wire, respectively, with as the total number of space segments, while is the actual number of wires.

Finally, substituting (64)–(67) into (87), the following local matrix system is obtained: where denotes excitation vector and space-dependent matrices are of the form

Relations containing summation from to infinity pertain to the reflections of transient current from the wire ends. Note as the observed time interval is always finite, only a finite number of reflections occurs within a given observation interval. A shorter observed interval requires smaller number of summands and vice versa.

According to GB-IBEM, a global matrix system is assembled from the local matrix systems for all wires . Finally, the resulting global matrix system can be written as follows:

The time-domain solution on the *i*th boundary element is given by
where are unknown coefficients, are the linear time-domain shape functions, and is the total number of time samples.

Applying the weighted residual approach to (90) leads to the expression where denotes the set of time-domain weights.

Using the set of Dirac impulses for the test functions, time sampling is ensured and (92) becomes

If the space-time discretization is performed by satisfying the Courant condition, , the transient current for a *j*th space node and *k*th time node can be obtained from a recurrence formula. Separating the terms relating to the current induced at the instant in (93) yields
where overbar indicates the absence of diagonal terms.

The first term in (94) pertains to the current at the *j*th space node and *k*th time node, that is, the present instant. Other terms are related to all previous instants. Finally, the recurrence formula for the transient current at *j*th space node and *k*th time node is obtained in the forms

where is total number of space elements, is the index of the *k*th time instant.

##### 3.3. The Transmission Line Model

The time-domain field-to-transmission line coupling equations can be written in the matrix form [11] where “*” stands for the convolution product, is the transient inverse Fourier transform of the ground, conductors matrix , and is the Laplace variable.

and are the excitation terms, given by [11] where is the transverse voltage derived from the transverse incident field excitation [11], and