"Hot Towers" Becoming Hot Topic For Wireless Industry

AM collocation at WCCM in Boston handles dozens of antennas and cable runs for AT&T Wireless and OmnipoiBy Lawrence Behr
As reprinted from Site Management & Technology Magazine, Spring 2002 Edition

AM (“hot”) radio broadcast towers are rapidly becoming the new “hot topic” in the wireless collocation industry.  The name of the game today for wireless carriers and tower owners alike is “collocation, collocation, collocation” on existing structures.  To date this has not necessarily included attachment to an existing AM radio broadcast tower, even though there are an estimated 10,000 AM towers in existence in the United States.  In many locales, collocation opportunities are becoming very scarce, leaving AM towers as the strategic, or often, the only possible location for new site opportunities.  

Why not AM collocation?

Historically, the industry has been warned to stay clear of any AM radio stations at all costs.  This not only had to do with attaching to them, but also with reference to interfering with their broadcast patterns. And, indeed, wireless carriers must prove to the Federal Communications Commission that they have considered and corrected all such problems when constructing or modifying any tower within three kilometers of an AM station.

In the past, AM towers were considered unsuitable for antenna attachment by most wireless carriers (cellular, PCS, SMR) due to presumed grounding difficulties,  interference and safety considerations.  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 collocation methods proposed by the wireless operator were reliable, proven, acceptable to the FCC and would not harm their signal coverage pattern. In the past, these outcomes could not be readily or easily assured.  Many wireless system designers, and AM tower owners themselves, are yet unaware that new technologies are now available to solve these problems and efficiently integrate wireless and AM systems at reasonable costs.

In the AM band, the tower itself is the radiating element without a need for attached antennas. However, wireless antennas and coaxial cables are self-contained systems that merely attach to their support structures. Achieving compatibility with the AM tower through electrical integration or isolation of wireless antennas is a challenging engineering exercise.  Since AM broadcasting is a specialized field, many wireless system designers and constructors, not being conversant with lower frequency technology, have been unaware of the techniques available to make wireless compatible with AM. Thus, many potential AM tower collocations have been avoided as technically impossible or prohibitively expensive.  Sometimes large additional costs have even been incurred for detuning the resulting new tower near the avoided AM station!

The solution to these problems is actually straightforward in most cases, and can be readily implemented at reasonable cost, by using a qualified consultant and the latest hardware solutions.

A new approach to AM collocation

New LBA Technology CoLoCoil isocoupler allows multiple wireless coaxial cables.One of the more recent new technological approaches to AM collocation is CoLoSite™. This technology has been developed through the collaboration of two LBA Group companies having over 35 years of experience in AM broadcast and wireless industries.  This system is based on patented, proprietary hardware by LBA Technology, Inc. with engineering and integration systems implemented by Lawrence Behr Associates, Inc. The resulting CoLoSite™ systems approach overcomes the traditional obstacles to collocation while permitting virtually unlimited wireless users on most AM towers.

With CoLoSite™, collocation is practical for both single tower and multiple tower AM antenna systems. Once a CoLoSite™ system is installed, wireless antenna and coaxial cable installations have virtually no effect on the host AM tower(s), and the AM signal has no effect on the wireless antenna.  The hardware components are permanently integrated into the AM system and designed for "utility-grade" life. Lifecycle monitoring, alarming and maintenance capabilities are available for integration into carrier platforms. Moreover, additional antennas and transmission lines can be added to the tower in the future without the use of additional isolation devices.  This means the owner can lease additional space to other wireless carriers, limited only by the tower structural capability.

On non-directional towers, the CoLoPole™ isolation system is typically used. The CoLoPole™ results in direct grounding of the AM tower. Wireless antennas and transmission lines are mounted and bonded directly onto the structure. The CoLoPole™ uses a unique wire cage impedance transformer. 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 collocation 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 collocation. The cost-effective approach to this end is to employ specially designed isolation systems between the base station equipment and the AM tower. LBA has developed the CoLoCoil™ for this purpose. It effectively prevents the wireless transmission lines from impacting the operating parameters of the directional AM towers. CoLoCoils™ are modular, accommodating orderly future expansion of wireless user requirements with minimal impact on AM host facilities or the carrier’s compound.

AM collocation planning

Planning for AM collocation begins with an analysis of the station facility. While all AM stations may theoretically be used for wireless collocations, practical factors may make some facilities economically or technically unattractive 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, at several power levels. 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, but benefit the wireless installation with superior lightning protection!

Because AM towers operate "hot" at high RF voltages, proper selection of candidate towers is very important to cost effective and operationally supportable collocation. 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.  For instance, it is not true that AM stations must always be shut down for installation and maintenance of collocated antenna equipment. Both the FCC and OSHA permit work on "hot" AM towers with proper power levels and precautions. Alternate operating modes and temporarily deployable AM towers similar to the familiar wireless “COW” are also available should shutdown be needed. With a high level of expertise employed in the planning phase, these safety and operational concerns can be addressed.

The location of the wireless equipment shelter or pad must also be carefully chosen to minimize AM interactions, and appropriate shielding and filtering may be needed. Electromagnetic field modeling techniques allow experienced designers to specify exact locations and outfitting for equipment packages to minimize interactions.

It is important that the AM collocation integrator be involved in the process at the site acquisition stage. There are many subtleties to negotiation of a satisfactory lease or acquisition agreement for which expert input is mandatory. Advance screening of all site candidates can also reveal possible AM collocation candidates and avoid unneeded detuning situations.

Successful outcomes

In summary, professionally managed AM collocation is not only possible, it has been successfully accomplished throughout the country for tower service providers and for wireless carriers such as Sprint, Nextel, AT&T and Omnipoint. With careful planning, competent project management, and the use of quality hardware integrated into the overall AM collocation site development process, success is certain. Furthermore, professional interaction with the AM host makes the station a willing and positive partner in the long-term collocation relationship.


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About LBA

LBA Group companies serve technical infrastructure needs related to the broadcast, wireless, electromagnetic compatibility and safety sectors worldwide. We provide consulting, training and other telecommunications industry services. We also produce and market hardware for radio transmission, RF shielding, safety and testing.