The Rise of Wireless Internet Systems on AM Broadcast Towers

The demand for efficient and reliable internet connectivity continues to grow, particularly in underserved regions. One innovative solution emerging from this need is the establishment of Wireless Internet Service Providers (WISPs) operating from AM broadcast towers. An intriguing project currently underway at one client’s AM site involves converting an AM tower into a WISP, providing much-needed internet access to the surrounding community. This article aims to explore the intricacies of this venture, detailing the technical challenges and opportunities it poses.

Understanding AM Towers

To appreciate the project, it’s essential to understand the fundamental mechanisms of AM broadcasters, which operate using medium wave frequencies. The entirety of an AM tower functions as a transmitting antenna, and there are primarily two types of these structures: series-excited and shunt-excited.

A series-excited tower contains a base insulator that separates the tower from the ground, while a shunt-excited tower employs a skirt of wires that extend towards the top of the tower and is grounded at the base. The design of shunt-excited towers offers distinct advantages for co-location opportunities because the grounding simplifies the installation of additional antennas.

Although antennas can also be fitted on series-excited towers, this often necessitates the use of isolation coils, adding complexity to the setup. Regardless of the tower type selected, conducting a structural study is crucial to ensure that the additional antennas and associated equipment do not compromise the integrity of the tower.

Implementation of a Wireless Internet System

The ongoing initiative aims to configure a sectorized wireless internet system utilizing four 90-degree panels, each equipped with three access points. To facilitate communication between the access points and the ground-mounted router, a tower-mounted sixteen-port switch is installed behind the panel antennas. This configuration utilizes two fiber optic cables for effective data transmission. A 54-volt DC supply powers the switch, access points, and point-to-point radios, with one fiber run dedicated to subscriber traffic and the other for radio management. All components are sourced from Ubiquiti, noted for providing a cost-effective solution in the networking space.

Challenges with Equipment and Installation

When considering Ubiquiti equipment, it’s crucial to balance the benefits of affordability against potential reliability issues that can arise, particularly after firmware updates. Although past experiences with the gear might have presented challenges, the current project being a new operation acts as a proof of concept, suggesting that the equipment will suffice under the circumstances.

The installation process has progressed swiftly, with the tower crew efficiently completing the setup of the sectorized access points. Climbing the tower reveals a network of fiber optic cables, the 54 VDC power cable, and a backup Ethernet cable. All Ethernet jumper cables used to connect the access points to the switch are UV-rated, shielded Cat 5e cables with shielded connectors, which are critically important when dealing with the high RF fields present on an AM tower due to the skin effect.

Protection Against Interference

At the tower’s base, lightning protection measures are integrated. Both the DC power and Ethernet cables are routed through high-quality lightning protection units (LPUs), specifically Transtector models designed for outdoor use. This precaution is vital even though the equipment is rated for higher voltages. For instance, although we are using a 54-volt supply, the LPUs rated for 48 volts are compatible and function safely within specified limits.

In this system, a service loop was created on the DC cable to function as an RF choke, mitigating RF interference that could affect the input terminals of the LPU. This loop consists of several turns of cable, effectively keeping induced RF levels low. The backup Ethernet cable similarly employs protective measures to ensure quality connection, crucial especially in high-power AM stations operating at 1 KW.

Considerations for High-Power Stations

High-power AM stations often require specialized cabling, such as Superior Essex’s armored shielded cable known as BBDG (a rebranded version called EnduraGain OSP). This robust cable features a copper shield alongside optional aluminum spiral armor, providing enhanced protection against RF interference and physical damage. For installations on series-excited towers, fiber optic cables can navigate around the insulator without issue, given the absence of metal components.

For the DC power, a “Tower Lighting Choke” is employed. This device utilizes a coil to allow DC power to cross the base insulator while preventing AM RF from following the DC line to ground. While these are effective, it’s still essential to have lightning protection devices in place prior to the DC power supply to ensure comprehensive safety measures.

Conclusion

The establishment of a WISP on an AM broadcast tower exemplifies innovative thinking in addressing community connectivity needs. By effectively utilizing existing infrastructure, the project not only enhances internet access but also represents a sustainable model for future developments. As technology in networking infrastructure continues to evolve, adapting principles from broadcasting, like those employed in this project, will likely become more common. Through careful planning and execution, operational challenges can be navigated, demonstrating the importance of a thoughtful approach in deploying wireless systems.

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