AM Tower Colocation Benefits and Technical Details

Ellena Arroyo

Historically, wireless site acquisition teams have been warned… “if there’s ever been a place to stay clear of, an AM station is certainly it!” Today, there are an estimated 10,000 AM towers in the United States, and they are virtually untapped resources for wireless base station sites. And, notwithstanding the mythology, AM tower colocation is both practical and efficient.

The benefits of AM tower colocation in a wireless network system buildout are many. Typical benefits, which reduce time-to-market and result in cost efficiencies, are:

Available in desirable locations. In many locales, tower colocation opportunities are becoming scarce. Acquisition strategies that target AM towers bring immediate new site opportunities. AM towers have historically been located in highly urbanized areas, close to downtowns, where fill-in facilities are most needed. Often, an AM tower may be the only possibility where moratoria have been imposed.

Zoning delays are minimized. Local ordinances and zoning regulations increasingly restrict site acquisition opportunities. AM tower colocation usually requires little or no zoning action. Where zoning approval is needed, AM owners are typically influential allies within their communities and local communities typically attempt to accommodate AM stations’ requests. Furthermore, wireless antennas mounted on AM towers are relatively unobtrusive, and are being installed in an environment where residents are already used to the presence of towers. Thus, AM tower colocation fosters a good neighbor image with zoning boards.

AM Station owners are positive partners. Most AM station owners have looked at their tower systems as necessary evils. They become very supportive of AM tower colocation because it improves their facilities at little or no cost to them. It also generates a new, secure revenue stream.

AM tower colocation is economical. It often costs far less to implement AM tower colocation than it does to build a new site. Even with structural augmentation, completed cost may be below that of a “greenfields” site of comparable capacity and site rents may be favorably negotiated. When properly implemented, long-term operational constraints and costs are not significantly different from other tower colocation situations.

AM detuning problems are avoided. Network design imperatives increasingly place site requirements close to AM stations, triggering a requirement to protect the AM from signal interference with the new tower. This often requires detuning or other measures which increase the cost of site development. Wireless facilities located on AM towers do not require detuning, cost of which may actually exceed the cost of the AM tower colocation, saving on site budgets.

Why Have AM Sites Not Been Developed?

Wireless developers have stayed away from AM radio towers because of the traditional “Keep Away From AM!” adage, and not without reason. The ad hoc procedures, previously available for AM tower colocation, indeed did make the practice taboo for wireless carriers.

Coordinating construction between the vastly different AM and wireless cultures was frequently a slow and painful process. From an engineering perspective, the process of integration and demonstrating license compliance to the FCC often required tinkering, delays, and costs unacceptable to wireless carriers.

AM station owners have also wanted assurance that the tower colocation methods proposed by the wireless operator were reliable, proven, acceptable to the Federal Communications Commission and would not harm their signal coverage pattern. Those outcomes could not be assured with the old methods of AM tower colocation.

Many wireless system designers and constructors are yet unaware that a new technological capability can now efficiently and painlessly integrate wireless and AM systems.

CoLoSite Enables AM Tower Colocation

A new technological approach to AM tower colocation called CoLoSite has been developed. This tower colocation system is based in proprietary hardware and procedural systems, which overcome traditional obstacles to AM tower colocation.

The CoLoSite system permits virtually unlimited wireless users on most AM towers. Once the CoLoSite system is installed, additional users or antennas may be added with minimal additional cost. The hardware components are permanently integrated into the AM system, and designed for “utility-grade” life. CoLoSite recognizes fundamental differences of approach to non-directional and directional AM stations, and makes tower colocation practical in both cases.

In order to install users on conventional AM towers, the associated transmission lines must electrically be isolated from the AM tower to prevent disruption of the AM transmission. The CoLoSite technology used to accomplish this most efficiently differs depending upon whether the AM antenna system is a single, non-directional tower or a directional, multi-tower array.

On non-directional towers, the CoLoPole isolation system is used. The CoLoPole results in direct grounding of the AM tower. Thus, wireless antennas and transmission lines are mounted and bonded directly onto the structure. The CoLoPole uses a unique wire cage impedance transformer, derived from an AM-only design and proven in hundreds of installations. Lower portions of the cage are heavily insulated and spaced away from the tower to allow ready operational access to the wireless antenna system. The CoLoPole benefits the AM station with improved efficiency, “air sound”, and lightning protection, thus enhancing the tower colocation experience for the station.

Directional stations use multiple towers to form an FCC licensed radiation pattern crucial to protecting other stations from interference. This licensed pattern may not be disrupted by tower colocation. The cost-effective approach to this end is to employ specially designed isolation coils between the base station equipment and transmission line on the towers. The CoLoCoil was developed for this purpose. It effectively prevents the wireless transmission lines from impacting the operating parameters of the AM towers. CoLoCoils are modular, accommodating later expansion of wireless user requirements without significant impact on AM host facilities.

Notwithstanding the importance of the CoLoSite hardware to the AM tower colocation project, its overall success is primarily dependent on the complete project management of the effort. It is essential that this begins with initial coordination and negotiation with the AM station and extends to the final system testing and FCC reporting.

Planning for AM tower colocation begins with an analysis of the station facility. While all AM stations may theoretically be used for tower colocations, practical factors may make some facilities economically or technically impractical to develop. Where multiple towers exist, the most favorable of the towers must be chosen. A wrong choice may add thousands of dollars to project costs.

AM operations are at times complex, with different towers or even different sites, being used for day or night transmissions. This may impact costs and operational aspects of the project. For instance, selection of a tower used only at night could be a benefit to daytime construction and maintenance activities.

Normal site factors, such as access and construction convenience must be evaluated. Structural suitability of the tower and any required augmentation must be considered and viewed in terms of AM system parameter impacts. Further, each AM tower has beneath it a radial ground network of miles of copper wire. This is essential to proper AM operation and is mandated by the FCC. Special planning and construction precautions are needed to protect ground system integrity to avoid disruption and expensive replacement of the system.

Because AM towers operate “hot” at high RF voltages, proper selection of candidate towers is very important to cost effective and operationally supportable tower colocation. There are significant safety and operational issues, which must be carefully dealt with in installation and maintenance of wireless equipment near AM towers. Fortunately, these RF concerns can be managed and are not a significant problem in a CoLoSite installation. However, a high level of expertise is required in the planning phase to ensure that all safety and operational concerns are addressed.

It is not true that AM stations must be shut down for installation and maintenance of colocated antenna equipment. FCC and OSHA permit work on “hot” towers with proper power levels and precautions. The CoLoSite hardware is specifically designed to facilitate “hot” maintenance and to protect against unsafe conditions.

The location of the wireless equipment shelter or pad must be carefully chosen to minimize AM interactions, and appropriate shielding and filtering must be employed. Electromagnetic field modeling techniques allow experienced designers to specify exact locations for equipment packages to avoid interactions. As part of the CoLoSite solution, shielding systems that permit operation of sensitive equipment even in high AM RF fields are used.

In summary, AM tower colocation is not only possible, it is being accomplished successfully throughout the country today! When careful planning, competent project management and quality hardware are integrated into the overall AM colocation site development procedures, success is certain. Furthermore, professional interaction with the AM host makes the station a willing and positive partner in a long-term colocation relationship.

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