a low profile antenna array

A Low Profile Antenna Array for Underground Mine Communication

Rong-Yu Chao and Kah-Seng Chung School of Electrical and Electronic Engineering

Curtin University of Technology

GPO Box U1987, Perth 6001

Western Australia

Abstract The special environment in underground mines which features severe multipath fading and high attenuation rate in UHF band re-

quires the antenna to have low profile, sim- ple structure, polarisation diversity, directiv- ity and high gain. A unique small antenna array consisting of low profile wire elements is designed for this particular application. Opti- misation of various array parameters such as interelement spacing and element length with respect to array gain and frequency bandwidth is undertaken by using Moment Method. Ef- fect to the array performance of a rectangular

conducting box is also investigated. Measurement in a building corridor has ver- ified the effectiveness of this antenna array for underground communication.

1 INTRODUCTION Good communication system in underground mines can largely increase safety and production output. Unfortunately, it has been a neglected area in the mining industry mainly due to the difficulties of im- plementing such system and the high cost involved. Expensive leaky cables or low-capacity conducting

wires are often used to provide a mobile communica, tion environment in underground mines[l][Z]. How- ever, for increased safety and instantaneous com-

munication to each person, extensive coverage and constant remote-controlled monitoring on wme haz- ardous areas are necessary. These demands favour the application of UHF radio systems[3]. Because of the special environment in underground mines which features severe multipath fading and high attenuation rate in UHF band[2][4], it is impor- tant for antennas to have low profile, simplestructure, polarisation diversity, unidirectional pattern and high gain. This paper presents the design of an antenna array using low profile inverted-F wire element. 2 UHF PROPAGATION FEATURES IN TUNNELS AND THE PRO- POSED ARRAY In UHF band, hard rock and coal bounding an un- derground mine tunnel act as relatively low loss di- electrics with dielectric constants in the range 5-10. Radio wave transmission takes the form of waveg- uide propagation, because the wavelength is much smaller than the tunnel dimensions. Wave attenu- ation through a tunnel is mainly caused by propaga- tion loss into the surrounding materials, ohmic loss is negligible at such a high frequency. The attenu- ation of the wave (in dB) increases nearly linearly with increasing distance which is a characteristic of wave guide propagation. Due to the geometrical di- mension of most underground mine tunnels, where the height is smaller than width, horizontal polarisa- tion produces significantly lower transmission loss at a given distance than does vertical polarisation in a line-of-sight path. However complete signal depolari- sation will happen around corners. Cross polarisation produces a loss intermediate between horizontal and vertical[4][5][6]. Because of the above mentioned features of UHF propagation in tunnels, an antenna with simple struc- ture, horizontal or cross polarisation and high gain will be an ideal one. If a monopole is used, it has to mounted on the side walls of the tunnel to provide a horizontally polarised field. This would not be a good location from concerns of personnel safety and the antenna vulnerability to damage. Hence ceiling- mount is preferred. Another important supporting

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Authorized licensed use limited to: CHINA UNIVERSITY OF MINING AND TECHNOLOGY. Downloaded on December 2, 2009 at 22:43 from IEEE Xplore. Restrictions apply. Gh D2 Y Figure 1: Three-element inverted-F array factor for this mounting pwition is that the ceiling

in most mine tunnels is reinforced with metals which provide a good ground for antennas.

The inverted-F antenna (IFA) chosen here has a low profile compared with a quarter wavelength monopole antenna, and yet has a similar directiv- ity gain[I. In additional to its radiation pattern of vertical electrical field which is equivalent to that of a monopole, it also has a horizontally polarised r%

diation pattern due to its horizontal element. Be cause of this polarisation diversity, it has been found to have a better performance than a quarter wave- length monopole in environment where strong multi- path fading effect exists[8]. As shown in Fig.1, three such antennas are used to form an array to achieve high gain and unidirection- ality. The array is driven through the central element while the elements at each side of the central element act as a director and reflector, respectively. Since the two vertical wires of each IFA are both connected to the ground except the central IFA, there is virtually no effect if we remove one vertical wire. Feed point to the central IFA is different from the conventional IFA in order to achieve resonance and impedance match- ing. 3 ARRAY PARAMETERS OPTIMI- SATION Array parameters such as interelement spacing, ele- ment length and height are critical for good perfor- mance. In optimising these parameters with respect 12 I 2 4 6 6' 0 D1 (cm) D1 (cm) Figure 2: Array gains and bandwidth vs D1 to directivity gain and bandwidth, moment method is used to include the interelement coupling effect which strongly affects the current distribution on the array elements because they are relatively small compared to the wavelength. Simulation has also been done when this array is mounted on a rectangular metal box. Wire grid and small patch models are used to enable us to calculate the current distribution on the array and the box using moment method[9]. When the array is mounted on an infinitely large ground plane, the simulation results of gain and band- width with respect to the distance D1 between #1 and #2 elements are shown in Fig.2. As the distance increases from l.Ocm to B.Ocm, the horizontal gain Gh is always decreasing, while the vertical gain G,