FAQ on wireless underground connectivity: part 2

Exploring the issue of “tunnel radio” and its solution using distributed antennas at fixed locations has led to another interesting solution known informally as the “leaky cable” or “leaky feeder” system.

Q: What is the formal name for this approach?
A: This approach is technically known as a radiating coaxial-cable system, which combines wired and wireless features. When properly designed and installed, these systems offer reliable, full-coverage, portable two-way radio communication in confined spaces. Some advanced systems even support RFID tags for personnel tracking within a specific range.

Q: What is the topology of such a system?
A: The leaky feeder cable is designed to emit radio signals along its entire length, similar to a sprinkler hose. This coaxial cable serves as the backbone of the communication system across the site.

Q: What physical elements are needed?
A: The system starts with a headend power amplifier and a receiver for the relevant frequencies. These components connect to the leaky cable to facilitate communication.

Q: Are boosters required along the way?
A: In most cases, yes. The maximum cable run is typically 300 meters, and amplifiers can be installed along the cable to enhance the RF signal strength for additional cable runs and wireless linking to nearby radios.

Q: Is this system only suitable for mines and underground environments?
A: No, this system is commonly used to provide Wi-Fi connectivity in subways and other enclosed settings.

Q: What are some challenges in subway and tunnel environments?
A: Traditional installations face difficulties due to the limited space in underground tunnels, where wall antennas cannot fit alongside the tracks. Therefore, a leaky cable running parallel to the tracks is a more practical approach.

Understanding the Leaky Cable

Q: What exactly is a leaky coaxial cable?
A: Unlike standard coaxial cables that prevent RF energy from escaping, a leaky coaxial cable is designed to allow controlled leakage of radio signals along its length. The shielding of the cable is tailored to permit the desired amount of RF energy to escape, as depicted in Figure 1.

For standard coaxial cables, the level of shielding varies with the frequency and construction of the cable. Looser braiding in lower-cost cables allows more RF penetration, while tighter braiding in higher-quality cables offers better shielding. It is essential for leaky cables to have carefully designed slots or openings that match the frequencies of interest to control the amount of RF leakage.

Q: Is it as simple as using cheaper coaxial cables that leak RF?
A: No, the design is more complex. The shielding of the coaxial cable must be engineered to have specific slots or openings for controlled RF leakage. Cheaper cables with inadequate shielding may compromise performance. The proper design of these slots is crucial for the cable’s effectiveness.

Q: What are some of the challenges in designing leaky cables?
A: Several technologies are employed to create the required slots or openings in the cable, ranging from loose braid to milled leaky cables. The spacing of these apertures determines whether the cable operates in coupled mode or radiating mode, as illustrated in Figure 2.

Q: What are the implications of these modes?
A: Coupled-mode radiation functions as a surface wave with power flow parallel to the cable axis, providing coverage within a limited region around the cable. In contrast, radiating-mode cables direct energy perpendicular to the cable axis, offering more efficient transmission but with stronger frequency dependence.

Q: How is the performance of leaky cables characterized?
A: Coupling loss, measured in decibels, indicates the power received by a dipole at a fixed distance from the cable relative to the power inside the cable. The 50% coupling loss is a common metric, with values ranging from 50 dB to 80 dB over a distance of 20 feet from the cable.

While coupling loss is crucial, other factors like cable attenuation also impact performance. Leaky cables require proper installation and maintenance to ensure optimal functionality.