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A Quad Beam Variable Height Antenna System for Reliable Underground Mine Tunnel Communications

Sihua Shao, Charles Fent, Matthew Dodd, Atef Elsherbeni

arXiv:2610.22125v1eess.SYcs.SYeess.SP

TL;DR

Tunnel multimode propagation creates severe spatial fading, complicating reliable underground mesh communications. QuadBeam uses independently positionable patch antennas to address the different fading profiles of multiple links. In mine measurements, height adjustment recovered up to 22 dB of received power, while repositioning neighboring antennas caused no noticeable change in the active link.

  • Problem

    Multimode tunnel propagation causes severe spatial fading, and a common displacement of antennas on a multi-link relay can improve one connection while degrading another.

    Source: §I · p. 2

  • Method

    QuadBeam is a compact hub with four independently positionable 2.4 GHz patches and a 900 MHz monopole, evaluated with a preliminary prototype and mine measurements.

    Source: §I · p. 2

  • Results

    Vertical repositioning over approximately 20 cm yielded up to 22 dB recovery at an LOS site, while peak-to-null RSS variations across five carriers were 19 dB to 26 dB.

    Source: 1) Spatial fading along the tunnel · p. 4, 2) Received-power recovery through… · p. 5

  • Takeaways & Limitations

    The measured height maximizing RSS varied with frequency and location, providing experimental motivation for independently optimizing antennas associated with different mesh links.

    Source: §IV · p. 5

    The results are preliminary because only one patch mount is motor-driven.

    Source: §IV · p. 5

Abstract

from arXiv · show

Mine accidents can damage the wired backbone supporting underground wireless networks, forcing rescue teams to deploy temporary mesh nodes to maintain communication with the surface. Although higher carrier frequencies can support bandwidth-intensive applications such as live video, multimode propagation within mine tunnels produces severe, spatially localized fading. Movable antennas can mitigate these fades by adjusting antenna position; however, moving all antennas on a multi-link relay node simultaneously may improve one link at the expense of another. This paper introduces QuadBeam, a compact mesh node antenna system comprising four independently positionable 2.4~GHz patch antennas mounted on the side faces of a central hub, together with a 900~MHz monopole for backup voice communication. A preliminary prototype incorporates one motor-driven patch mount and three manually adjustable mounts. Bench measurements show interelement coupling below $-46$~dB between patches on adjacent faces. Measurements at five carrier frequencies from 2.40~GHz to 2.48~GHz in the Edgar Experimental Mine reveal 19~dB to 26~dB received-power variations across transmitter and receiver separations of 8.3~m to 18.3~m. Adjusting the receiver height over approximately 20~cm yields received-power improvements of up to 22~dB. Repositioning neighboring patches and removing or reinstalling the monopole produce no noticeable change in the active link. These results demonstrate the potential of independently positioned antennas to alleviate spatial fading in underground mesh communications.

I. INTRODUCTION

Mine accidents can disrupt wired and wireless communication infrastructure, so rescue teams deploy temporary mesh nodes to relay information between tunnels and surface command. QuadBeam applies independently adjustable antennas to address tunnel fading while avoiding the trade-off of moving all antennas on a multi-link relay together.

I. INTRODUCTION

  • Rescue teams deploy battery-powered mesh nodes along tunnels to relay voice, messaging, and personnel-tracking information to a surface command center.Mine accidents can disrupt communication infrastructure, including wireless systems that depend on a wired backbone.Source: §I · p. 1
  • Higher frequencies can support bandwidth-intensive applications, but multimode propagation creates severe spatial fading that depends on both antenna positions.An unfavorable deployment location may experience a persistent deep fade.Source: §I · p. 1
  • Experimental validation of movable antennas in underground mines had not been reported, and common antenna displacement on a multi-link relay may improve one link while degrading another.Prior work included indoor experiments and underground simulation with a reconfigurable intelligent surface.Source: §I · p. 1
  • QuadBeam provides four independently adjustable 2.4 GHz patches on a compact hub, plus a 900 MHz monopole intended for backup voice service.The preliminary prototype has one motor-driven patch mount and three manually adjustable mounts; contributions include coupling, mine fading, and height-repositioning measurements.Source: §I · p. 2

II. QUADBEAM ANTENNA PROTOTYPE

The QuadBeam prototype combines four 2.4 GHz patches and a 900 MHz monopole on a compact hub. Its patch design uses truncated corners for right-hand circular polarization.

II. QUADBEAM ANTENNA PROTOTYPE

  • The prototype combines four nominally identical 2.4 GHz patch antennas with a 900 MHz monopole, and includes an extra patch for use as a measurement transmitter.The monopole is mounted above the compact hub.Source: §II · p. 2

1) Patch antennas:

  • Two truncated corners on each single-feed square patch produce right-hand circular polarization, reducing sensitivity to uncontrolled node orientation.A circularly polarized antenna receives a linearly polarized field with an ideal 3 dB mismatch; orthogonal linear antennas can incur larger penalties.Source: 1) Patch antennas · p. 2
  • The patch has a 28.6 mm side, 2 mm corner truncations, and feed offsets of 5.89 mm and 0.98 mm, and was iterated after a measured resonance shift.It uses a copper-clad FR4 board with copper foil forming the radiating patch.Source: 1) Patch antennas · p. 2

2) Monopole:

The monopole is mounted above the QuadBeam hub on a copper-clad ground plane. The hub shields its electronics and allows vertical patch repositioning, though only one mount is motorized in the prototype.

2) Monopole:

  • A commercial 900 MHz monopole connects through an SMA adapter to an 18 × 18 cm copper-clad ground plane mounted above the hub.The ground-plane side length is slightly greater than half a free-space wavelength at 915 MHz.Source: 2) Monopole · p. 2

3) Hub and positioning mechanism:

  • The 10 cm-tall printed hub uses copper-foil side walls for shielding, and its four patch mounts move vertically; one is motorized and three are manual.The intended configuration uses an actuator for each patch, unlike the current single-actuator prototype.Source: 3) Hub and positioning mechanism · p. 2

B. Antenna Characterization

Bench characterization measured antenna impedance and patch radiation properties. The representative patch has a single main lobe, while its axial ratio increases toward the upper end of the test band.

1) Reflection coefficient:

  • Patches 1 and 2 cover measured −10 dB impedance bands of 2350–2474 MHz and 2350–2482 MHz, respectively; the monopole covers 892–933 MHz.At the five test frequencies, patch reflection coefficients range approximately from −9 dB to −16 dB.Source: 1) Reflection coefficient · p. 2

2) Radiation properties:

  • At 2.40 GHz, the representative patch has an approximately 50° half-power beamwidth; its axial ratio rises from 1.2 dB at 2.40 GHz to about 6 dB at 2.46–2.48 GHz.Boresight axial ratio is below 3 dB from 2.30 GHz to 2.42 GHz, indicating near-circular polarization at lower frequencies and elliptical polarization toward the upper band.Source: 2) Radiation properties · p. 2

3) Interelement coupling:

Adjacent-face patch coupling was very low in bench measurements, while the mine campaign measured received signal strength across distance and receiver height using a prototype with only one motor-driven mount.

3) Interelement coupling:

  • With patches 1 and 2 on adjacent hub faces, measured |S41| was approximately −46.1 dB at 2.39 GHz and below −50 dB above 2.43 GHz, consistent with less than 0.003% coupled power at highest measured coupling.Source: 3) Interelement coupling · p. 2
  • Opposite-face and patch-to-monopole coupling were assessed only indirectly, because the bench measurements were outside an anechoic chamber and omitted the monopole.Source: 3) Interelement coupling · p. 3

A. Measurement Setup and Procedure

  • A nonmetallic cart, telescoping mast, lab jack, and 1 mm-resolution draw-wire sensor supported manual receiver-height adjustments in the mine drift.The drift is approximately 4.5 m wide and 3.3 m high, with rough rock surfaces, wire mesh, gravel, and mining equipment.Source: A. Measurement Setup and Procedure · pp. 3–4
  • Measurements included LOS separations of approximately 15 m and 22 m and an obstructed NLOS location 6 m from the transmitter, with receiver height swept approximately 1.00–1.20 m.The distance sweep ran from 8.3 m to 18.3 m in 0.1 m increments; five frequencies from 2.40 to 2.48 GHz were used, and 1,115 RSS readings were recorded.Source: A. Measurement Setup and Procedure · pp. 3–4

1) Spatial fading along the tunnel:

Received power varied sharply with position and frequency over the measured tunnel distance, without a clear monotonic decline as separation increased.

1) Spatial fading along the tunnel:

  • Fitted RSS-versus-distance slopes ranged from −0.45 to +0.26 dB/m, with no clear monotonic trend over the measured 10 m distance sweep, alongside pronounced position- and frequency-dependent fluctuations.Carrier medians lay between approximately −43 and −47 dBm.Source: 1) Spatial fading along the tunnel · p. 4
  • The reported peak-to-null RSS variation ranged from 19 dB to 26 dB, and a displacement of only 10 cm changed received power by as much as 14 dB.Table II’s footnote describes the range of the minimum RSS across all five carriers evaluated at each position; this need not equal the range of any one carrier.Source: 1) Spatial fading along the tunnel · pp. 4–5
  • At 10.0 m, received power was approximately −40 dBm at 2.42 GHz and −64 dBm at 2.48 GHz, and deep-fade positions shifted with carrier frequency.At 28 of 101 positions, at least one carrier was more than 10 dB below its own median.Source: 1) Spatial fading along the tunnel · p. 4

2) Received-power recovery through height adjustment:

Vertical repositioning over a modest height range produced substantial received-power recovery, but the favorable height depended on carrier and link geometry.

2) Received-power recovery through height adjustment:

  • 20 dB recovery occurred over 6.5 cm at the NLOS site, while height-dependent variations reached 22 dB at the 15 m LOS site and ranged from 4–12 dB at 22 m LOS.At 15 m LOS, 2.42 GHz RSS rose from −65 dBm at 1.038 m to −43 dBm at 1.203 m.Source: 2) Received-power recovery through… · p. 5
  • More than 90% of 5 mm height adjustments changed RSS by no more than 2 dB, but the strongest height varied with carrier and link geometry.The authors suggest millimeter-scale positioning resolution is adequate for the tested conditions; one height cannot be presumed optimal across carriers or links.Source: 2) Received-power recovery through… · p. 5
  • Optimizing the minimum carrier power improved it by 17 dB, 21 dB, and 10 dB at the NLOS 6 m, LOS 15 m, and LOS 22 m sites, respectively.Relative to the 1.10 m reference height, gains ranged from 6–10 dB; several LOS carriers were strongest near the top of the swept range.Source: 2) Received-power recovery through… · p. 5

3) Interaction among antennas:

Moving neighboring antennas or handling the monopole produced no noticeable change in the measured active-link RSS, though the observation was not a simultaneous multi-link throughput test.

3) Interaction among antennas:

IV. CONCLUSION AND FUTURE WORK

QuadBeam combines independently positionable patches with a backup voice antenna, and mine measurements show strong local fading and recovery from modest height changes. The prototype remains preliminary, with single-link received-power testing and substantial control and network evaluation left for future work.

IV. CONCLUSION AND FUTURE WORK

  • The preliminary prototype demonstrated adjacent-patch coupling below −46 dB and negligible observed disturbance of an active receiving link when neighboring antennas were repositioned.QuadBeam uses four 2.4 GHz patches and retains a 900 MHz monopole for backup voice communication.Source: §IV · p. 5
  • 19 dB to 26 dB peak-to-null RSS variations occurred across a 10 m drift segment, while approximately 20 cm of vertical repositioning recovered up to 20 dB at an NLOS site and 22 dB at an LOS site.The height maximizing RSS varied with frequency and measurement location.Source: §IV · p. 5
  • The results remain preliminary: only one mount is motor-driven, and the mine campaign primarily evaluated received power for a single active link.Future work will add closed-loop position control to all four antennas and increase their travel range.Source: §IV · pp. 5–6
  • Planned evaluations include longer-distance measurements in mine crosscuts and intersections, plus multi-node relaying and end-to-end throughput measurements.These tests are intended to evaluate fading outside the near zone and establish practical network-level benefits.Source: §IV · p. 6
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