The influence of the electromagnetic field on the Romanian seaside area.

Samoilescu, Gheorghe ; Sotir, Alexandru ; Constantinescu, Mircea 等

Abstract: The paper analyzes the influence of the electromagnetic field and its implications on the seaside area. It also shows details about the construction of the screens and the materials used to protect against the propagation of the fields. The authors have taken a closer look at the screening of the static, quasi-stationary and non-stationary fields along the shore area of the Black Sea. The paper studies the penetration of the electromagnetic fields within insulating materials, semiconductors and conductors.

Keywords: electromagnetic pollution, electromagnetic field, screening, protection, di-electrics.

1. INTRODUCTION

The paper aims at shaping a strategy of intervention at a regional level, choosing an optimal intervention technology against the electromagnetic field, which pollutes the environment leading to the apparition of certain diseases. Reaching this goal implies an interdisciplinary participation on the part of several specialists, taking into account that the explanation of certain phenomena is of a physical-chemical nature. The communication network form the seaside area represents the main source of electromagnetic field. The special complexity and the heterogeneous nature of the actual transmission systems, generated among others by the great diversity of the transmission devices currently in use, requires an ample analysis of the interoperation possibilities of the latter. The combined usage of wire transmissions, radio, radio-relay, troposphere, cosmic require an accurate software, technical compatibility necessary to acquire quality connections, with minimal errors. The main issues regarding the polluting influence of the electromagnetic field are : a). the prevailing technical-organizational procedure is the radio network with a great number of correspondents which leads to a great emission time, a reduced traffic density per subscriber, high vulnerability, and high likeness of interference; b) reduced reliability of the radio wires and radio-relay due to low operating probabilities of the previous generation means; c) great differences in the characteristics of certain means which regulate an optimal compatibility: the frequency range, operating mode, non-adjustable power, omni directional antennae; d) constructive incompatibilities of some radio electronic means; e) lack in measurement and control gear in order to determine field values, harmonics and interferences and to set risk boundaries around the antennae and the radio electronic means etc.

2. EXPERIMENTAL PART

2.1 General Aspects Regarding the Propagation of the Electromagnetic Field

In the case of a homogenous, isotropic medium--as a physical structure, immobile, non-polarized, deprived of hereditary properties, linear and without electrical charges, Maxwell's equations have the form :

rot[bar.E] = -[mu][partial derivative][bar.H]/[partial derivative]t; rot[bar.H] = [sigma][bar.E] + [sigma] [partial derivative][bar.E]/[partial derivative]t (1)

div[bar.B] = 0; div[bar.D] = 0 (2)

These are completed with the material equations:

[bar.B] = [mu][bar.H]; [bar.D] = [epsilon][bar.E]; [bar.J] = [sigma][bar.E] (3)

In general, the field has a harmonic variation:

[??](x,y,z,t) = [bar.E](x,y,z) x [e.sup.j[omega]t] (4)

[??](x,y,z,t) = [bar.H](x,y,z) x [e.sup.j[omega]t] (5)

The equations (1) become within the complex:

rot[??] = - j[omega][mu][??]; rot[??] = ([sigma] + j[omega][epsilon])[??] (6)

The relations (1) and (2) are valid for the open air (charge deprived) as well as for metals, such as screens. Consequently, applying the rotor operator to the relation (6) and taking into account that

rot(rot[bar.A]) = gard(div[bar.A]) - [DELTA]A (7)

it results that:

[DELTA][bar.E] = j[omega][mu]([sigma] + j[omega][mu])[bar.E]; [DELTA][bar.H] = j[omega][mu]([sigma] + j[omega][sigma])[bar.H] (8)

The equations (8) represent the propagation equations of the field in a medium with constitutive parameters [sigma],[mu],[epsilon]. We will note with [GAMMA] the propagation constant:

[[GAMMA].sup.2] = j[omega][mu]([sigma] + j[omega][epsilon]) (9)

[[GAMMA].sup.2] may be separated into a real part and an imaginary part:

[GAMMA] = [alpha] + j[beta] = [alpha] + j[[GAMMA].sup.o] = [alpha] + j[k.sub.o] (10)

where: [alpha]--attenuation constant; [beta],([[GAMMA].sup.o], [k.sub.o])--the phase constant (wave number).

As a result, the propagation equations, also callers Helmholtz's equations, become:

[DELTA][bar.E] = [[GAMMA].sup.2][bar.E]; [DELTA][bar.H] = [[GAMMA].sup.2][bar.H] (11)

The way to solve the screening problems reside in solving the vectorial equation devised by Helmholtz inside, outside and within the screen wall, given the limitation conditions, and in determining the screening factor and the attenuation under these circumstances. In order to simplify this path leading to the solution we have opted for numerical methods.

3. RESULTS AND DISCUSSIONS

Because [[epsilon].sub.o] = 1/36[pi] x [10.sup.9] F/m; [[mu].sub.o] = 4[pi] x [10.sup.-7] H/m and in Metals [sigma] = [10.sup.5] / [10.sup.8] S/m for every interest wavelength we may approximate in the screen wall:

[sigma] >>[omega][epsilon] (12)

Which means that in metals we may neglect the displacement current as compared to the conduction one. As a result, we may approximate that:

[[GAMMA].sup.2] = j[omega][sigma][mu] (13).

When increasing the frequency, due to the pellicle effect there is no regular reapparition of the current density within the conductor. The distance within the conductor for which the field applied on the surface decreases "e" times (e=2,71 is the basis of natural logarithms), is called penetration depth and is noted [delta]:

[delta] = [square root of 2/[omega][sigma][mu]][m] (14).

It results that:

[GAMMA] = [square root of j[omega][sigma][mu]] = [square root of 2]j/[delta] = 1 + j/[delta] = [alpha] + j[alpha] (15)

In a dielectric medium (air) [sigma] = 0 (medium with no losses):

[[GAMMA].sup.2] = j[omega][mu]([sigma] + j[omega][epsilon]) = [j.sup.2][[omega].sup.2][epsilon][mu]; [GAMMA] = j[omega] [square root of [epsilon][mu]] = j [omega]/v = j 2[pi]/[lambda] (16)

Noting [k.sup.2.sub.o] = [[omega].sup.2][epsilon][mu], the wave equations become, in a free space:

[DELTA][bar.H] = -[k.sup.2.sub.o][bar.H]; [DELTA][bar.E] = -[k.sup.2.sub.o][bar.E] (17)

In a non-stationary regime (high frequency) the displacement current cannot be neglected any more and [GAMMA] will be given by the relation (16). In the case of homogenous plane waves (uniform and regular)--the phase planes are still planes of constant amplitude. Yet, the field sources have finite dimensions. Still, at a great distance from the source, a small portion from the wave layer may be considered plane. Such waves may exist only in finite, homogenous mediums, being produced by sources placed at the infinite.

Having a variable magnetic field, in the case of metallic screening shelter, which produce a "shading" effect, there is great necessity of grounding (it is more a galvanic by-pass--to the ground--rather than a real screen). Avoiding interstitials is also important (it reduces the induced currents).

The attenuation increases with the value of the induced currents, respectively with the screen conductivity. Together with the increase in frequency, the attenuation tends to the infinity and using non-ferromagnetic metals is more advantageous, always bearing in mind that these materials have no screening effect for the magneto-static fields (f = 0), which do not induce currents.

In the case of a perfect conductor ([sigma] = [infinity]) and infinite frequency (f [right arrow] [infinity]), the attenuation factor of the screen for tangential fields tends to the infinite: [a.sub.t] = [H.sub.t2]/[H.sub.t1] = [infinity] where [H.sub.t2] = [J.sub.s] is a density of a surface current (A/[m.sup.2]) in the dielectric neighboring the screen (due to the pellicle effect of the induced currents); [H.sub.t1] = 0 is the magnetic field tangential to the screen material. When there is increase in frequency, the quasi-stationary regime is no longer valid because there appear displacement currents whose magnetic field cannot be neglected any more. In this case, the screens are in the remote fields of the sources, in which the electric and the magnetic fields are coupled together.

4. CONCLUSIONS

The seaside area is the most exposed to electromagnetic fields due to navigation and the social-economical development. Each field type must be studied and various ways of reducing their effects must be designed and forwarded.

a). the electrostatic fields are easily screened with the help of metallic carcasses with grounding or not. Grounding is recommended for the protection of personnel when touching screens, as well as for the case of plane, shield-like screens. Similarly, the screens made of thick walls with high permissiveness have a certain screening effect as to the fields (dielectric materials)

b). magneto static fields may be screened through ferromagnetic coatings with high magnetic permissiveness, having thick walls (the copper screen of the coaxial cables has no screening effect on the magneto static fields)

c). the electric field of low frequency is easily screened, reflection being the main attenuation mechanism

d). the magnetic field of low frequency is painfully screened by reflection (low [Z.sub.s], low [Z.sub.uH], a low difference [Z.sub.uH] - [Z.sub.s], good adaptation, insignificant reflection).

In exchange, the magnetic field is attenuated by absorption at these frequencies, absorption becoming the main attenuation mechanism in this case.

5. REFERENCES

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* * * ICNIRP Standard 2002.

* * * A review of the potential health risks of radiofrequency fields from wireless telecommunications devices, Raport prepared by the Royal Society of Canada Ottawa, 1999.