Bruce Whisner

Measurements of medium frequency propagation characteristics of a transmission line in an underground coal mine

In underground coal mines, medium frequency (MF) communication systems couple their signals to metal infrastructures such as ac power cables and wire-based telephone lines, which guide the signals to propagate for a long distance. To better understand the propagation characteristics of MF signals, an easy-to-use measurement method was recently developed at the National Institute for Occupational Safety and Health. The method will be introduced along with an equivalent transmission line model for a long metallic infrastructure in underground coal mines. The model serves as the fundamental driver for the method development. Propagation measurements on a twisted pair of telephone lines in an underground mine were made using this method. The measurements confirmed the low MF signal attenuation rate and the dependence of the propagation characteristics of the line on the electrical properties of surrounding coal and rock as theoretical studies predicted.

Measurements of Medium-Frequency Propagation Characteristics of a Transmission Line in an Underground Coal Mine

In underground coal mines, medium frequency (MF) communication systems couple their signals to metal infrastructures such as AC power cables and wire-based telephone lines, which guide the signals to propagate for a long distance. To better understand the propagation characteristics of MF signals, an easy-to-use measurement method was recently developed at the National Institute for Occupational Safety and Health. The method will be introduced along with an equivalent transmission line model for a long metallic infrastructure in underground coal mines. The model serves as the fundamental driver for the method development. Propagation measurements on a twisted pair of telephone lines in an underground mine were made using this method. The measurements confirmed the low MF signal attenuation rate and the dependence of the propagation characteristics of the line on the electrical properties of surrounding coal and rock as theoretical studies predicted.

AN INTRODUCTION TO A MEDIUM FREQUENCY PROPAGATION CHARACTERISTIC MEASUREMENT METHOD OF A TRANSMISSION LINE IN UNDERGROUND COAL MINES

An underground coal mine medium frequency (MF) communication system generally couples its electromagnetic signals to a long conductor in a tunnel, which acts as a transmission line, and exchanges signals with transceivers along the line. The propagation characteristics of the transmission line, which is usually the longest signal path for an MF communication system, play a major role in determining the system performance. To measure the MF propagation characteristics of transmission lines in coal mine tunnels, a method was developed based on a basic transmission line model. The method will be presented in this paper along with the propagation measurements on a transmission line system in a coal mine using the method. The measurements conflrmed a low MF signal power loss rate, and showed the in∞uence of the electrical properties of surrounding coal and rock on the MF propagation characteristics of the line.

Simulation and measurement of through-the-earth (TTE), extremely low-frequency signals using copper-clad, steel ground rods

The underground mining environment can greatly affect radio signal propagation. Understanding how the earth affects signal propagation is a key to evaluating communications systems used during a mine emergency. One type of communication system is through-the-earth, which can utilize extremely low frequencies (ELF). This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating current injection at ELF, and in particular, ground contact impedance. Measurements were taken at an outside surface testing location. The results obtained from modeling and measurement are characterized by electrode impedance, and the voltage received between two distant electrodes. This paper concludes with a discussion of design considerations found to affect low-frequency communication systems utilizing ground rods to inject a current into the earth.

Comparison of medium frequency propagation characteristics of a transmission line measured from both ends in a coal mine entry

An underground coal mine medium frequency (MF) communication system generally couples its electromagnetic signals to a long conductor in a tunnel (entry). The entry plus the conductor acts as a transmission line (TL). Transceivers exchange signals along the line. The propagation characteristics of the TL, which is generally the longest signal path, play a major role in determining the system performance. In this paper, we present MF measurements of TL characteristics as measured from both ends of the TL in a coal mine entry. The characteristic impedances from both ends have some significant differences, but the other propagation parameters are very similar.

E-fields of electrode-based through-the-earth (TTE) communication

An electrode-based through-the-earth (TTE), or linear TTE, communication system sends its signal directly through the earth overburden of a mine by driving an AC current into the earth. The resultant current, present at the receiver, is detected as a voltage and communication is established. The electrode-based TTE system may achieve a considerably large transmission range >305 m (1000 ft). This kind of system may be implemented for emergency communication close to an area such as a refuge alternative or other strategic location to establish communication between underground miners and the surface. Since it does not rely on extensive infrastructure underground, this communication would provide an alternate path out of the mine. Given the attenuation characteristics of the earth-which is the major factor determining the performance of TTE systems-the receiver sensitivity, transmitter antenna length and orientation, working frequency, and transmitted current required for communication can be estimated and determined. In an effort to estimate the earth attenuation, the analytic solution for the electrical field distribution of an electrode-based TTE communication system in a homogenous half-space is derived. A prototype system was built and tested at a mine site. The test data is compared with the modeling results, and a close agreement is found between them.

Mathematical modeling and measurement of electric fields of electrode-based through-the-earth (TTE) communication

There are two types of through-the-earth (TTE) wireless communication in the mining industry: magnetic loop TTE and electrode-based (or linear) TTE. While the magnetic loop systems send signal through magnetic fields, the transmitter of an electrode-based TTE system sends signal directly through the mine overburden by driving an extremely low frequency (ELF) or ultralow frequency (ULF) AC current into the earth. The receiver at the other end (underground or surface) detects the resultant current and receives it as a voltage. A wireless communication link between surface and underground is then established. For electrode-based TTE communications, the signal is transmitted through the established electric field and is received as a voltage detected at the receiver. It is important to understand the electric field distribution within the mine overburden for the purpose of designing and improving the performance of the electrode-based TTE systems. In this paper, a complete explicit solution for all three electric field components for the electrode-based TTE communication was developed. An experiment was conducted using a prototype electrode-based TTE system developed by National Institute for Occupational Safety and Health. The mathematical model was then compared and validated with test data. A reasonable agreement was found between them.

Simulation and Measurement of Through-the-Earth, Extremely Low-Frequency Signals Using Copper-Clad Steel Ground Rods

The underground mining environment can greatly affect radio signal propagation. Understanding how the earth affects signal propagation is a key to evaluating communications systems used during a mine emergency. One type of communication system is through the earth (TTE) that can utilize extremely low frequencies (ELF). This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating current injection at ELF , and in particular ground contact impedance. Measurements were taken at a surface testing location, outside. The results obtained from modeling and measurement are characterized by electrode impedance and the voltage received between two distant electrodes. The paper concludes with a discussion of design considerations found to affect low-frequency communication systems utilizing ground rods to inject a current into the earth.

Medium-Frequency Signal Propagation Characteristics of a Lifeline as a Transmission Line in Underground Coal Mines

Underground coal mines in the United States of America are required to install lifeline (LL) cable inside escapeways to guide miners out of a mine when visibility becomes poor due to heavy smoke. Some LLs consist of single or multiple steel conductors covered with a protective plastic outer layer. Research has shown that this type of LL can be a good conductor to guide a medium-frequency (MF) communication system signal to travel over large distances. To understand the MF propagation characteristics of an LL, National Institute for Occupational Safety and Health researchers took measurements on a section of LL in a coal mine, and obtained propagation parameters for analysis. The measurement data show that MF signals have a low attenuation which can enable the use of an LL for communication throughout a mine. The propagation parameters measured are presented in this paper.

Medium frequency signal propagation characteristics of a lifeline as a transmission line in underground coal mines

Underground coal mines in the United States of America are required to install lifeline (LL) cable inside escapeways to guide miners out of a mine when visibility becomes poor due to heavy smoke. Some LLs consist of single or multiple steel conductors covered with a protective plastic outer layer. Research has shown that this type of LL can be a good conductor to guide a medium-frequency (MF) communication system signal to travel over large distances. To understand the MF propagation characteristics of an LL, National Institute for Occupational Safety and Health researchers took measurements on a section of LL in a coal mine, and obtained propagation parameters for analysis. The measurement data show that MF signals have a low attenuation which can enable the use of an LL for communication throughout a mine. The propagation parameters measured are presented in this paper.