DAB+ and FM Coverage in Tunnels: Radio Systems for Road and Highway Tunnels

Introduction

Radio coverage in road tunnels represents a fundamental technical challenge to ensure the safety and information of motorists. When we enter a tunnel, the FM and DAB+ radio signal is abruptly interrupted, creating not only discomfort for travelers, but above all a safety problem in case of emergencies.

FM and DAB+ Radio Frequencies: Technical Characteristics

FM (Frequency Modulation) Technology

FM radio operates in the frequency band between 87.5 and 108 MHz. This technology, used for decades, transmits the audio signal through frequency modulation. FM radio waves have specific characteristics:

• Linear propagation: the signal travels in a straight line
• Limited penetration: difficulty in crossing solid structures like mountains or tunnels
• Analog audio quality: subject to interference and signal degradation

The DAB+ (Digital Audio Broadcasting Plus) System

DAB+ in tunnels represents the digital evolution of radio. Operating primarily in the VHF III band (174-240 MHz), it offers significant advantages:

• Digital transmission: greater resistance to interference
• Spectral efficiency: more stations in the same bandwidth
• Superior audio quality: crystal-clear sound without static
• Additional services: real-time traffic information, weather and news

Why Radio Signals Don't Penetrate Tunnels

Highway tunnels act as a Faraday cage, effectively blocking external radio waves. The reinforced concrete structure and the presence of metal create an impenetrable barrier for FM and DAB signals. This physical phenomenon makes natural reception impossible inside tunnels, requiring specific technical solutions.

Radio Coverage Systems for Tunnels: How They Work

The Amplification and Distribution System

Radio coverage in road tunnels is based on an integrated system that includes:

1. External Reception Antennas

• Positioned outside the tunnel
• Capture FM and DAB signals available in the area
• Oriented to maximize reception

2. Signal Amplifier

• Receives the signal from external antennas
• Amplifies the signal while maintaining quality
• Distributes the signal inside the tunnel

3. Radiating Cable

• Carries the amplified signal along the entire tunnel
• Ensures uniform coverage for the entire length

The Radiating Cable (Leaky Feeder)

The radiating cable represents the most effective technology for propagating radio signals inside tunnels. This special coaxial cable features small openings that allow the signal to "leak out" in a controlled manner.

Advantages of Radiating Cable:

• Uniform distribution: the signal is radiated along the entire path
• Continuous coverage: no dead zones inside the tunnel
• Reduced maintenance: robust and reliable system
• Multiple compatibility: simultaneously supports FM, DAB+ and other services

System Installation

The radiating cable is installed along the tunnel walls, generally in the upper part of the structure. This optimal position is designed to ensure maximum coverage of the internal volume of the tunnel, while protecting the cable from possible accidental damage caused by passing vehicles. The high placement also facilitates maintenance operations and allows perfect integration with other systems present in the tunnel, such as lighting and ventilation.

Voice Break-In (VBI): Emergency Messages in Tunnels

A crucial functionality of modern DAB amplification systems in tunnels is the ability to transmit break-in messages in tunnels. The VBI system allows interrupting normal radio programming to broadcast urgent communications.

Types of VBI Messages

1. Traffic Information

• Queues or slowdowns at the tunnel exit
• Recommended alternative routes
• Updated travel times

2. Safety Warnings

• Adverse weather conditions
• Construction work in progress
• Presence of stopped vehicles

3. Real Emergencies

• Fires or accidents in the tunnel
• Evacuation instructions
• Directions for rescue services

How the VBI System Works

The break-in system operates through a combination of automatic and manually controlled technologies. Sensors distributed along the tunnel can automatically detect anomalous situations and activate pre-recorded messages. Control center operators can also intervene manually to handle specific situations with real-time messages. The system guarantees absolute priority to emergency messages, which override any ongoing transmission. For international tunnels, it's possible to configure multilingual messages to reach all users.

Regulations and Safety Standards

Radio coverage in highway tunnels must comply with precise European and national regulations. Tunnels over 500 meters must guarantee reliable emergency communications, radio coverage for rescue services, information systems for users, maintenance programs and periodic tests to ensure proper operation over time.

Advantages of a Professional Radio Coverage System

The implementation of a professional radio coverage system brings significant benefits on several fronts. From a safety perspective, it guarantees reliable communications in case of emergency, allows effective coordination of rescue services and can drastically reduce evacuation times.

Regarding user comfort, the continuity of FM and DAB radio service eliminates the annoying interruption that occurs when entering a tunnel. Motorists can receive real-time information about traffic and road conditions.

Future Technologies and Developments

The tunnel FM coverage and DAB+ tunnels sector is constantly evolving. Trends include:

• 5G Broadcast: integration with next-generation mobile networks
• IoT Integration: smart sensors for advanced monitoring
• AI and Machine Learning: predictive emergency management
• Energy Efficiency: low power consumption systems

System Maintenance and Reliability

A radio coverage system for tunnels requires:

• Periodic inspections: checking component integrity
• Functionality tests: verifying signal quality
• Software updates: for digital systems and VBI
• Maintenance: of radiating cable and antennas

Radio coverage in road tunnels is not just a matter of comfort, but an essential element for safety. Modern systems that integrate FM, DAB+ and break-in functionality represent the standard for safe and efficient tunnels. The implementation of these technologies requires specialized technical expertise and experience in the critical communications sector.

Do you need consultation for radio coverage in your tunnel?

Teleproject is a leader in the design and implementation of communication systems for critical environments. With years of experience in implementing radio systems for tunnels, we offer complete solutions to guarantee FM, DAB+ coverage and emergency communications in tunnels.

Our experts can support you in:

• Analysis and design of coverage systems
• Installation of amplifiers and radiating cables
• Implementation of VBI Break-IN systems for emergencies
• Maintenance and assistance