Underground Transmission: Exploring the Route of Signals Delivered to Motorists Through Tunnels
In the labyrinth of urban tunnels, maintaining radio signal reception is no small feat, especially considering the obstructions like rock, concrete, and metal that would normally hinder such communication. However, a clever solution known as leaky feeder cables has made this possible, enabling reliable two-way radio communication in deep underground environments.
Leaky feeder cables, coaxial cables with small gaps or slots that allow radio signals to "leak" in and out continuously along their length, are the unsung heroes of this technology. They create a controlled radio wave environment inside tunnels, effectively turning the cable itself into a long antenna. This allows for radio communication where direct radio signals cannot penetrate.
This system is crucial for safety and operational efficiency, enabling continuous communication between workers underground and surface control centers. For instance, the Maryland Transportation Authority installed a system to rebroadcast AM stations in Baltimore's Inner Harbor Tunnel in the mid-1970s, but it never worked well and was removed during a later tunnel refurbishment.
Fast forward to 2001, both the Lincoln and Holland tunnels received FM rebroadcast capabilities, and cities like New York, Boston, Seattle, Baltimore, Detroit-Windsor, and Pittsburgh have heavily trafficked tunnels that don't lose radio reception. Tunnels in urbanized areas like New York's Holland and Lincoln Tunnels keep the radio alive even underground, allowing AM and FM signals to reach motorists.
However, not all tunnels are so fortunate. The Pennsylvania Turnpike tunnels cause a complete loss of all radio signals. Similar signal loss might occur on either side of the Eisenhower Tunnel on Interstate 70 west of Denver.
The leaky feeder system's effectiveness is not limited to the United States. The George Washington Bridge used leaky feeder cables for communication with motorists on AM broadcast frequencies in 1940. In the late 1990s, Braverman's Radio Systems installed systems for Boston and Seattle's Sea-Tac Airport Tunnel.
The FCC has a specific subpart of its Part 15 rules for tunnel radio systems, and licenses are not required for these broadcasts. However, incidents like the outage of AM signals in the Hudson River tunnels that lasted for "months, possibly years" in 2018, as reported by WABC(TV), highlight the need for constant vigilance and maintenance.
In Boston, the Dewey Square Tunnel used an IGM Instacart system to override signals from local AM and FM stations for traffic announcements, PSAs, and paid commercials. Pittsburgh FM stations are received relatively well on the west side of one of the Pennsylvania Turnpike tunnels, but are absent on the eastern side.
In conclusion, leaky feeder systems provide a tailored radio environment inside tunnels, circumventing the need for traditional broadcasting licenses by limiting signal leakage to the enclosed tunnel area while maintaining clear communication. These systems have proved invaluable in ensuring safety and efficiency in urban tunnel networks.
- Leaky feeder cables, functioning by continuously allowing radio signals to leak in and out along their length, create a controlled radio wave environment within tunnels, effectively turning the cables into long antennas for reliable radio communication in deep underground areas.
- The Maryland Transportation Authority installed a system in the mid-1970s to rebroadcast AM stations in Baltimore's Inner Harbor Tunnel, but it never worked well and was later removed during a tunnel refurbishment.
- In 2001, both the Lincoln and Holland tunnels received FM rebroadcast capabilities, and cities like New York, Boston, Seattle, Baltimore, Detroit-Windsor, and Pittsburgh have heavily trafficked tunnels that don't lose radio reception.
- The effectiveness of leaky feeder systems is not limited to the United States; Braverman's Radio Systems installed systems for Boston and Seattle's Sea-Tac Airport Tunnel in the late 1990s, and the George Washington Bridge used leaky feeder cables for communication with motorists on AM broadcast frequencies in 1940.
