Why Standard Two-Way Radios Fail Underground
A handheld radio on the surface relies on radio waves travelling through open air, bouncing off buildings and terrain, but fundamentally moving unobstructed most of the time. Take that same radio underground and the physics change completely. Rock absorbs and reflects radio frequency energy far more aggressively than air, and the mineral content of the rock itself can make this worse depending on what's being mined.
Tunnels and drives compound the problem with geometry. A radio signal travelling in a straight line hits a bend in the tunnel and simply doesn't follow it around the corner. Fit a single antenna at the tunnel portal and coverage might reach fifty metres in before dropping to nothing, regardless of how much transmit power the radio has.
Where This Matters Most
Underground metalliferous mines, coal mines, and tunnelling operations for infrastructure like road and rail projects all face this same coverage problem. Anywhere workers go out of direct line of sight from a surface-mounted antenna is a potential dead zone unless the system has been specifically designed to follow them in.
What Is a Leaky Feeder System and How Does It Work
A leaky feeder is a specially constructed coaxial cable with small, deliberate gaps or slots in its outer shielding. Instead of containing the radio frequency signal entirely, as a normal coaxial cable does, it "leaks" a controlled amount of signal along its entire length. Run that cable the length of a tunnel and you effectively turn the whole tunnel into one continuous antenna, rather than relying on a single fixed point.
Because the cable itself follows the tunnel, it goes around every bend, junction, and incline the tunnel has. There's no line-of-sight requirement, since the "antenna" is physically routed through the space workers are actually in.
Amplifiers Counter Signal Loss
Signal strength naturally drops off over distance along any cable. In-line amplifiers, or boosters, are spaced at intervals along the leaky feeder run to bring the signal back up before it drops below a usable level. This is what allows leaky feeder systems to extend for kilometres underground rather than being limited to a few hundred metres.
- Radios communicate with the leaky feeder cable itself, not a distant fixed antenna
- Coverage follows the tunnel route exactly, including bends and branches
- Amplifier spacing is calculated based on cable loss and required signal margin, not a fixed universal distance
Leaky Feeder vs Repeaters vs DAS: Choosing the Right Approach
Leaky feeder isn't the only way to extend radio coverage underground, and it isn't always the right tool for every job. The right choice depends on the shape of the space being covered.
| Approach | How It Works | Best Suited To |
|---|---|---|
| Leaky Feeder | Radiating cable run along the full tunnel route, leaking signal continuously | Long, narrow, winding tunnels and drives where continuous coverage without gaps matters most |
| Repeaters | Fixed relay stations that receive and retransmit a signal at higher power | Larger open underground chambers, or extending range between separate work areas |
| Distributed Antenna System (DAS) | Multiple fixed point antennas connected back to a shared signal source | Wider underground spaces where line-of-sight gaps between antenna points are manageable |
Many real underground sites use a combination rather than picking one approach exclusively, leaky feeder along the main drives where continuous coverage is critical, with repeaters bridging between separate sections. Tait's TB7310 base station repeater and the TB7300 transportable repeater are both genuine options for extending coverage between zones, with the transportable unit suited to sites where the working face itself keeps moving.
NZ Mining and Tunnelling Safety Requirements
New Zealand's underground mining and tunnelling health and safety regime sits under the Health and Safety at Work (Mining Operations and Quarrying Operations) Regulations 2016, enforced by WorkSafe. These apply to underground and opencast coal and metalliferous mines, and to tunnelling operations, including infrastructure tunnels for road and rail projects that meet the regulations' scope.
The regulations require mining operations to maintain a documented health and safety management system, including principal hazard management plans for significant risks and arrangements for emergency response. WorkSafe's own guidance material, including the Underground Mines Emergency Protocol, sets out how a level 3 underground emergency should be managed. None of this specifies a particular communications technology by name, but reliable, continuous underground communications is a practical prerequisite for meeting these obligations in practice, not a nice-to-have layered on top.
Why This Matters for Equipment Choice
A hazard management plan that assumes workers can always be reached, or can always call out, only holds up if the communications system underneath it actually reaches every part of the working. This is exactly where coverage gaps in a legacy or poorly designed system become a genuine compliance and safety issue, not just an operational annoyance.
Planning an Underground Communications System
A genuine underground communications design starts with the physical layout of the workings, not a product catalogue. The length, width, and number of bends in each drive, the presence of any hazardous atmosphere requiring intrinsically safe equipment, and how far workings are expected to extend all shape the right combination of leaky feeder, repeaters, and radio hardware.
Intrinsically Safe Equipment
Where flammable gas or combustible dust is present, every radio, battery, and accessory in the system needs matching intrinsic safety certification, not just the cable and amplifiers. The Tait TP9361 IS is a genuinely IECEx and ATEX certified DMR portable radio built for exactly this kind of hazardous underground environment.
- Map the actual tunnel or drive layout before specifying cable length or amplifier count
- Confirm whether any section of the site requires intrinsically safe radios and accessories
- Plan for future extension of the workings, not just the current footprint
- Test coverage at the working face itself, not just partway down the drive
