Chenming Zhou

The Equivalence of the Ray Tracing and Modal Methods for Modeling Radio Propagation in Lossy Rectangular Tunnels

Ray tracing and modal methods are widely used for modeling RF propagation in tunnels. In this letter, we show that the two methods are mathematically equivalent and yield the same results. We also compare the theoretical predictions from both methods to field measurements at different frequencies (455, 915, 2450, and 5800 MHz) and find good agreement.

RF Propagation in Mines and Tunnels: Extensive measurements for vertically, horizontally, and cross-polarized signals in mines and tunnels.

This article reports measurement results on radiofrequency (RF) propagation in tunnels and mines for vertically, horizontally, and cross-polarized signals. Extensive measurements have been made in concrete tunnels, coal mines with rock dust and shotcrete, with and without conductive mesh, and hard-rock mines. The measurements include power attenuation over distances with and without line of sight (LOS) at four frequencies (455, 915, 2,450, and 5,800 MHz) that are common to underground radios, as well as swept wideband-frequency measurements from 30 MHz to 1 GHz at fixed separation distances to investigate the attenuation behavior as the waveguide cutoff frequency is approached. In addition, power loss associated with radio propagation around a 90? corner was measured for the four frequencies and two polarizations in an operational coal mine.

Modeling RF propagation in tunnels

As mandated by the 2006 Mine Improvement and New Emergency Response (MINER) Act, many underground coal mines have installed UHF radio systems to provide communications between personnel on the surface and underground. In an effort to better understand UHF signal propagation in tunnel environments, we made radio signal attenuation measurements in concrete tunnels at frequencies of 455 MHz, 915 MHz, 2.45 GHz, and 5.8 GHz. In this paper, a ray tracing method is used to model the channel. Although the tunnel has an arched roof, the model describes the propagation behavior very well using a rectangular tunnel with the same width. The measured data verify the ability of the ray tracing model to predict radio frequency propagation at all the four frequencies. In addition, the propagation constants for the different frequencies are investigated and shown to be consistent with the measured values.

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.

Modeling and measurement of the influence of antenna transversal location on tunnel propagation

The ray tracing and modal methods are applied to model the influence of antenna location (within the tunnel cross section) on tunnel propagation. Measurement results in a railroad tunnel are provided to validate the models. Simulation results are shown to agree with measurement results well. Both simulation and measurement results suggest that more receive power can be obtained by placing the transmitter or receiver antenna close to the center location of the tunnel cross section.

Modeling the influence of wall roughness on tunnel propagation

At the ultra-high frequencies (UHF) common to portable radios, a mine tunnel is often modeled as a hollow dielectric waveguide. The roughness condition of the tunnel walls has an influence on radio propagation and therefore should be taken into account when accurate power predictions are required. In this paper, we derive a general analytical formula for modeling the influence of the wall roughness. The formula can model practical tunnels formed by four dielectric walls, with each having an independent roughness condition. It is found that different modes are attenuated by the same wall roughness in a different way, with higher order modes being significantly more attenuated compared to the dominant mode. The derivation and findings are verified by numerical results based on both ray tracing and modal methods.

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.

Measurement of RF propagation in tunnels

Radio signal propagation in tunnels exhibits significant differences from outdoor or indoor multipath environments (radio signals reflected off walls, floors, or other objects). This research measures the propagation of UHF/SHF radio signals from 455 MHz up to 5.8 GHz in frequency bands commonly used for underground wireless communications and electronic tracking. The paper focuses on the experimental methods used to collect vertically and horizontally polarized signal propagation data in tunnels as a function of separation distance between the transmitter and receiver antennas, and presents measurement results. The data show the dependence of the propagation characteristics on frequency, polarization, and tunnel dimensions.

Modeling and measurement of wireless channels for underground mines

This paper investigates wireless channel modeling for underground mines. The ray tracing and modal methods, which have been widely used for modeling radio propagation in tunnels, are applied to model wireless channels in underground mines. In addition, propagation measurements are taken in an underground hard rock mine at three different frequencies (455 MHz, 915 MHz, and 2.45 GHz). Simulation results based on the ray tracing and modal methods are compared to measurement results and show agreement. Challenges for modeling wireless channels in mines are discussed.

Simulation and measurement of medium frequency signals coupling from a line to a loop antenna

The underground-mining environment can affect radio-signal propagation in various ways. Understanding these effects is especially critical in evaluating communications systems used during normal mining operations and during mine emergencies. One of these types of communications systems relies on medium-frequency (MF) radio frequencies. This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating MF coupling between a transmission line (TL) and a loop antenna in an underground coal mine. Two different types of measurements were completed: 1) line-current distribution and 2) line-to-antenna coupling. Measurements were taken underground in an experimental coal mine and on a specially designed surface test area. The results of these tests are characterized by current along a TL and voltage induced in the loop from a line. This paper concludes with a discussion of issues for MF TLs. These include electromagnetic fields at the ends of the TL, connection of the ends of the TL, the effect of other conductors underground, and the proximity of coal or earth. These results could help operators by providing examples of these challenges that may be experienced underground and a method by which to measure voltage induced by a line.