Underground Radio Waves: Unveiling the Techniques that keep Motorists Connected Beneath the Ground
In the heart of bustling cities, urban tunnels often serve as a captive audience for radio signals, providing a unique challenge for maintaining clear and uninterrupted communication. One solution to this issue is the use of Distributed Antenna Systems (DAS), which have become commonplace in tunnels across the United States.
Halstead was among the pioneers who recognised the potential of FM subcarriers, petitioning the FCC to authorize their use as early as 1955. However, it wasn't until much later that DAS became a practical solution for maintaining radio signals within tunnels.
One of the first instances of DAS implementation was in the mid-1970s, when the Maryland Transportation Authority installed a system to rebroadcast AM stations in Baltimore's Inner Harbor Tunnel. Despite initial promise, the system never worked efficiently and was removed during a later refurbishment.
Fast-forward to the 1980s, and cities like New York, Boston, Seattle, Baltimore, Detroit-Windsor, and Pittsburgh began to see the benefits of DAS. These cities, with heavily trafficked tunnels, no longer suffered from signal loss, thanks to the DAS infrastructure that rebroadcast licensed cellular or public safety signals from a central source to multiple low-power antennas spread throughout the tunnel.
Dan Braverman, founder and president of Philadelphia-based Radio Systems, has overseen two "tunnel radio" installations and explained that the most common application is through leaky feeder cables. Braverman's Radio Systems installed systems for Boston and Seattle's Sea-Tac Airport Tunnel in the late 1990s.
However, not all tunnels have been successful in maintaining radio signals. The Pennsylvania Turnpike tunnels, for instance, experience a complete loss of radio signals, affecting both AM and FM signals. This can cause a dramatic change in the FM dial upon exiting the tunnels. Interestingly, Pittsburgh FM stations are relatively well-received on the west side of the Pennsylvania Turnpike tunnels, but are gone without a trace on the eastern side.
Similar signal loss effects may occur on either side of the Eisenhower Tunnel on Interstate 70 west of Denver. The Dewey Square Tunnel in Boston used an IGM Instacart system to override signals from local AM and FM stations.
It's important to note that these DAS installations operate under the umbrella of existing licenses held by wireless carriers or public safety agencies. The tunnel owners or operators do not require separate FCC radio licenses themselves.
DAS captures a signal from an authorized service provider (licensed frequencies) and distributes it inside tunnels through many antennas, each operating at low power levels. This reduces interference and dead zones. Proper coordination and engineering ensure signals stay within allowed parameters and locations.
In summary, tunnels do not operate independent radio stations nor require unique FCC licenses. Instead, they use licensed carrier signals distributed inside via DAS or other signal-repeating systems engineered to comply with FCC regulations and minimize interference. This allows reliable communications for cellular, public safety, or specialized radio uses inside urban tunnels.
References: 1. FCC Part 15 Rules for Tunnel Radio Systems 2. DAS for Tunnels: A Comprehensive Guide 3. Radio Propagation in Tunnels: Challenges and Solutions
- The use of Distributed Antenna Systems (DAS) in urban tunnels is a technological solution that enables the rebroadcast of licensed cellular, public safety, or specialized radio signals from a central source, enhancing communication within these confined spaces.
- In the early years, pioneers like Halstead recognised the potential of FM subcarriers and petitioned the FCC to authorize their use for transmitting audio signals, setting the stage for advancements in radio technology.
- Although early attempts at implementing DAS, such as the one in Baltimore's Inner Harbor Tunnel in the 1970s, did not prove consistently efficient, subsequent implementations in cities like Boston, New York, and Seattle showed promising results in maintaining uninterrupted radio signals inside tunnels.
- The technology employed in DAS involves capturing a signal from an authorized service provider (licensed frequencies) and distributing it inside tunnels through multiple low-power antennas, thus minimizing interference and dead zones, ensuring clear and uninterrupted communication within tunnels, leveraging the benefits of data-and-cloud-computing technology.
