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The impedances of short electric dipole communication antenna in the conductive media

Keywords:

V.M. Olshanskiy – Dr. Sc. (Phys.-Math.), Senior Research Scientist, IPEE RAS. E-mail: vmolsh@yandex.ru
S.V. Volkov – Leading Engineer, IPEE RAS. E-mail: mendur@mail.ru
V.P. Rublev – Ph. D. (Eng.), Faculty of Instrument Making, FEFU. E-mail: lavr46@mail.ru
Xue Wei – Ph. D. (Eng.), Associate Professor, Head of Laboratory, Harbin Engineering University. E-mail: weihe@mail.ru


In this article we are talking about low-frequency conducting currents communication. We are talking about the case when the total depth of the transmitting and receiving antennas in a conducting medium is comparable with the distance between them, or even exceeds it. Electromagnetic signal from the source can not be represented as wave, since signal attenuation is very large. For the case the efficiency of the dipole antenna can be evaluated as the ability to provide the greatest possible signal at the remote locations. This efficiency is determined by the multiplication of the current in the antenna and the effective length of the antenna. If we will fix power in the antenna, the smaller the resistance, the greater the current, and the greater antenna effectiveness. Accordingly, when engineering development systems of underground and underwater communications, we should aim to achieve the lowest possible values of the complex resistance ( impedance) of the antenna. Total impedance of the antenna is determined by the sum of several components: Ze = Zech + Zw + Zdin + Zgr where: Zech – contribution to the impedance due to electrochemical processes occurring at the border «electrode−medium»; Zpr – impedance of the cables connecting the power supply to the electrodes of the antenna; Zdin – impedance associated with the interaction of current spreading in the environment and the currents in the wire connecting the antenna and the source; Zgr = Rgr – grounding resistance of electrode antenna (Olshansky, 1990, 2004). Depending on the specific problem and, more importantly, from the authorized dimensions of antennas, the ratio between the components of the impedance change significantly. If we are talking about antennas fixed on people (communication between divers or emergency communication in mines), the size of the electrodes equals to tens of centimeters. In this case we are talking about low frequencies of communication. Range can be tens or hundreds of Hz ( up to some kHz) for sea water or kHz to some MHz for underground communication. In the case of underwater and underground communication antenna impedance is determined mainly by grounding resistance. Although the formulas for calculating of the grounding electrodes resistance are well known, experimental measurements of impedance, especially for electrodes buried in the soil, are of great interest. Need for experimental verification in this case is dictated, for example, by the complex nature of dependence of electrical conductivity of ground with different content of moisture. When using conduction currents as carriers of signal we have to use very low frequencies to achieve relatively large distances. In the case of these frequencies electrochemical processes occurring at the border «electrode−environment» may significantly affect to the impedance value. This impedance components are determined by the material and dimensions of the electrodes, the nature of the medium (soil or water), current density. The lower the frequency and size and smaller the current density, the greater the contribution of electrochemical impedance components. Furthermore, the electrodes may be exposed to electrochemical corrosion. The smallest value of deposits give impedance electrodes made of stainless steel, carbon, brass. Especially promising are textile carbon materials. Frequency characteristics of the same cilindrical cell from the stainless steel filled by different types of soil (sand, loam, humus) are provided at different moisture content. Data on change of an impedance of the cell filled with different types of soil are provided, at additions of water and when drying. Experiments, on the underground communication, performed in field conditions, demonstrated that that reducing the antenna impedance is an effective way to improve link reliability. Recommendations about decrease in impedances of underground dipole antennas are made.
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