AM Tower Collocation: Money on the Table?

AM Tower Collocation Introduction

AM radio broadcast towers are rapidly becoming the new “hot topic” in the wireless communications industry. The name of the game today for wireless carriers and tower owners alike is “colocation, collocation, co-location” 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, new site opportunities are becoming stressed, leaving AM towers as strategic, or often, the only possible locations for new site opportunities. Even where open sites exist for new towers, local zoning and planning authorities often require that all collocation options be exhausted before “Greenfield” towers are permitted.

This places AM tower owners directly in the path of economic opportunity. How much opportunity? In many locations, cellular and PCS antenna locations rent for about $2500 per month. If four tenants can be attracted to an AM tower, that’s potentially a $10,000 per month revenue stream. Even better, the wireless carriers typically pick up the costs of tower modification and on-going maintenance!

To tap this pot of gold, it’s important to understand how AM tower colocation works from a carrier perspective, and what you, the AM broadcaster, must consider to present a viable site opportunity to the wireless industry.

After a bit of orientation, some specific operational and technical concerns will be presented.

Why Not AM Tower Collocation?

Historically, the wireless industry has been warned to stay clear of AM radio stations at all costs. This not only had to do with attaching to them, but also with reference to locating nearby and interfering with their broadcast patterns. 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. This has left underserved “holes” around many AM’s, often in prime suburban areas.

In the past, AM towers were considered unsuitable for antenna attachment by most cellular and PCS wireless carriers 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 and broadcasters alike. AM station owners have also wanted assurance that the tower collocation methods proposed by the wireless operator were reliable, proven, and 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. Few broadcast engineers and consultants have experience in tower collocation, except for the odd STL or FM standby antenna. 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.

Current Technology for AM Tower Collocation

Today, equipment is available from several AM antenna system manufacturers which will provide the ability to co-locate communications transmission systems on both single tower and multiple tower AM antenna systems. Companies such as LBA Technology Inc. and Kintronics Labs have equipment which once installed, insures that the 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. Moreover, additional antennas and transmission lines can be added to the tower in the future without reengineering the AM isolation. This means the owner can lease additional space to other wireless carriers, limited only by the tower structural capability.

On non-directional towers, an advanced folded unipole isolation system is typically used. This results in direct grounding of the AM tower. Wireless antennas and transmission lines are mounted and bonded directly onto the structure. The folded unipole uses a unique wire cage impedance transformer. In the LBA Technology implementation, lower portions of the cage are heavily insulated and spaced away from the tower to allow ready operational access to the wireless antenna system by tower climbers, even while “hot.” Such folded unipole tower collocation systems benefit the AM station with improved efficiency, "air sound", and lightning protection, thus enhancing the tower collocation experience for the station. Conversion to an advanced folded unipole type tower collocation system may entail replacing the station’s antenna tuning units (ATU's)  and some “on tower” construction.

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 collocation. The cost-effective approach to this end is to employ specially designed isolation systems between the base station equipment and the AM tower. The manufacturers mentioned earlier, along with a qualified Consulting Engineer, can assist in this design.

AM Tower Collocation Planning

Planning for AM tower 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 benefits 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 tower 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 co-located 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” can be employed 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, carrier equipment grounding, 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 to understand that the wireless carrier will essentially drive the installation and will demand the highest quality hardware, detailed planning, and full documentation. These are not always well understood or appreciated by AM operators and engineers.

It is important that the AM tower 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 tower collocation candidates and avoid unneeded detuning situations.

Successful Outcomes of AM Tower Collocation

In summary, professionally managed AM tower 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 Corp., AT&T Mobility and Verizon Wireless. With careful planning, competent project management, and the use of quality hardware integrated into the overall AM tower 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 tower collocation relationship.

Appendix A - Planning and Operational Issues

Wireless Carrier Culture
AM broadcasting and wireless carrier operations are two very different cultures. Neither typically knows or understands much about the other. Part of the art of tower collocation is successfully blending the two.

Carriers are highly organized and follow rigid procedures on planning, construction, and regulatory matters. They use the best materials, and are highly time and cost driven on projects. They design for 24/7 reliability, and have the technical depth to back it up. Usually, they will insist on strict control of the entire tower collocation project.

AM stations need to cover themselves with a fair, but protective lease with appropriate indemnities. AM tower collocation is first a business and legal project. Once the project is engineered and agreed upon, the AM local technical resources typically have only a minor role in the carrier’s execution. The AM must ensure that the carrier has engineering resources who understand AM to oversee its successful completion. The use of specialist broadcast business attorneys in these negotiations is recommended.

Zoning and Permitting Applies to You
AM antenna towers are attractive to wireless carriers for several reasons. High among them are often favorable zoning and permitting biases that reduce red tape and community opposition to new cell antennas. The carriers have entire departments and legal staff dedicated to zoning and permitting, and so will vet these issues early in the negotiation process. Broadcasters tend to have a “do it yourself” approach to such things, and more than one tower collocation project has ended up on the rocks because of broadcaster end-run attempts on zoning. It is important to have a specialist site advisor in carrier zoning issues on your side. This is something that must be done formally and by the book!

Tower Replacement or Reinforcement
Often AM towers are inadequate to support cellular antenna systems and lines. Frequently, if the tower is already zoned and permitted for additional antennas, the cellular company or tower company that leases the AM tower will structurally upgrade the tower, or even replace it. While this is good, the plans must be carefully drawn with AM RF issues in mind. More than one replacement tower has gone up without insulators!

Protect Ongoing Operations
An ongoing operational agreement should be put in place. It should include a protocol for coordination between parties on maintenance with procedures and designated authorities on each behalf to ensure compliance with RF safety plans and to protect operational integrity of both carrier and AM facilities.

The AM must recognize that there will be considerable access traffic to the tower and compound. Not just tower crews, but electronics maintenance techs, generator techs, generator fuel trucks, and yard maintenance crews, for instance. Almost never is an AM tower visited, in the experience of most stations. This means that the AM may have to beef up general site security and inspect its transmitter site more often.

Provide for Adequate Carrier Space
Successful tower collocation sites require adequate carrier ground space at the tower base, and proper access for roads and utilities. Carriers spend a good deal of A&E (architect and engineer) money to design the tower base environment, to say nothing of building it out. Expect the AM tower base and ATU to be in a separate fenced compound. The tower collocation devices should be located at that boundary so the “hot” part is inside the fence, and carrier techs can safely access coax ports on their side for testing. Tower mounted isolation devices can be a safety hazard, and, as such, are not as much favored.

Carrier equipment may be housed in shelter buildings or stand alone environmental cabinets, all of which must be specially shielded, or properly situated to avoid RF ingress. Unless your AM tower is very close to a building, there is little chance that building space will be required. Of course, 365/24/7 access is a given, imposing special challenges upon the use of a tower in swampy or inhospitable locations.

RF Hazards Need Attention
Contrary to general belief, it is possible to work on “hot” AM towers. FCC guidelines permit low power operation without RF hazard strictures at antenna power ranging from about 3000 watts at 540 kHz to 300 watts at 1700 kHz. Appropriate engineering investigation can guide tower selection and/or work practices to facilitate carrier tenant maintenance.

Generally speaking, the emissions from most cell carrier installations are insignificant at ground level and won’t be a problem for AM stations. However, a proper RF safety plan is essential to ensure safe and FCC compliant on-tower work. Particular care in evaluation must be exercised if the tower is to also accommodate higher power services such as FM, terrestrial satellite repeaters or mobile TV systems. Furthermore, many local authorities are now requiring formal proof of compliance with FCC RF hazard rules. 

How Much AM Power is Too Much?
As power on a tower goes up, potential carrier collocation issues increase. The feasibility of tower collocation is a matter of budget and operational convenience for both the AM and carrier. The choice of carrier isolation technology is an important factor in the analysis. In a directional array, the important factor is the power in the tower under consideration, not the licensed power. A thorough engineering investigation is necessary in all but the lowest power situations. It has been our experience that successful collocates can be done on 10,000 watt towers, but usually “lower is better”!

AM Directional Compatibility
Collocation on one or more towers of a directional array is often quite practical. In fact, in terms of maintenance flexibility, such installations can offer advantages over nondirectional tower collocations. A good directional array collocation requires a balance between carrier operational logistics and AM technical factors.

The number one factor in tower selection from a carrier perspective is convenience for fast and safe maintenance and/or system modifications. This suggests the choice of a tower that is inactive in one mode (preferably daytime) and that uses low power. At the same time, that tower should be low impedance and stable for minimum perturbation of array operation. Part of the equation is accessibility for roads and underground utilities without unreasonable disruption of AM transmission line and ground system elements.

For the protection of both the AM and the carrier, it is very important that coordination of RF grounding, equipment placement, interim operation and RF safety plans be accomplished and memorialized before construction begins.

Construction will inevitably result in some AM array disruption, and plans should be in place for appropriate FAA notices, FCC STA’s, power reduction, pattern changes, or other actions to facilitate the construction phase. Of course, associated costs should be factored into the lease agreement.

Tower Collocation on Multiplexed AM Systems
It is possible to colocate on a multiplexed AM tower. On a folded unipole system, no special provisions need be made. If multiple isocouplers are employed, they will generally not be AM frequency sensitive, but the combined shunt capacity may have significant impacts on the individual AM antenna impedances, which may compromise the multiplexer tuning. Devices like the LBA CoLoCoil® are AM frequency sensitive, but can be fabricated for multiplexer AM frequency operation with high isolation and minimal multiplexer impact. In each case, a careful engineering review of present and future system requirements is essential before selecting an isolation technology.

Appendix B - Technical Issues

FCC and FAA Matters
In general, no prior FCC or FCC authority is required, of the station, to add collocation antennas to an AM tower, so long as the height is not increased. On completion of construction, FCC Form 302 can be filed to recognize any changes beyond current license limits.

Impacts on Radiation Patterns
In most cases, adding carrier antennas to AM towers will not significantly change the vertical radiation pattern or efficiency. There will be somewhat greater changes to the antenna impedance because the electrical length of the tower is made a bit longer as the effective radius of the tower is increased by antennas and lines. However, this can be good news because the “Q” of the tower will often be decreased, improving bandwidth. Particularly in a critical directional array, these matters should receive engineering evaluation to verify acceptability.

Impedance Impact of Coaxial Lines
Many broadcasters are familiar with isolation involving a single coaxial cable as for an FM or STL. In carrier operations, multiple antennas are employed. It is not unusual to have 12, 24, or more cables and antennas on the tower. Not only can the physical presence of this hardware affect the impedance of the tower at AM frequencies, but the isolation devices may have large cumulative impacts on impedance. For instance, if 24 isocouplers with 20 pf capacity each to ground are arrayed, that is 480 pf total, or about 300 ohms shunt reactance at 1 MHz. That will transform an antenna impedance of 89 -j120 to about 45 –j96, a value perhaps inconvenient for the installed ATU!

Grounding and Bonding Practices
Carrier engineering departments have developed standardized grounding practices which are quite different and often conflicting with the mostly ad hoc practices employed in AM radio. Further, carrier practices are lightning, and not RF, oriented. Integration of these approaches is a significant issue to resolve that requires engineering knowledge of both. The protection, repair, or replacement of the AM buried radial ground system needs to be factored into early planning activities.

Providing for Coax Replacement, Additions
Rarely is a collocation tower dressed with lines and antennas, then forgotten. Rapid technology changes in carrier systems result in almost routine changes or additions of antennas and lines. The isolation method and infrastructure deployment must give maximum flexibility to the carrier, and keep tower crew workloads to a minimum. The old quarter wave stub isolation method, used for years for its simplicity in isolating a single coax, fails seriously when confronted by modern cable installation requirements, and is rarely, if ever, employed today.

Isolation Device Considerations
Modern carrier systems often employ tower top amplifiers and antenna positioning devises powered over the signal coaxial cable. Devices such as isocouplers do not pass DC or AC, and thus have limited utility. The isolation system should be designed for end-to-end DC and AC connectivity, for which devices like folded unipoles and ColoCoils™ are well suited.

Because of the wide range of frequencies employed by carrier systems (700 – 3600 MHz), and stringent VSWR specifications on the system, it is preferable to have no cable discontinuities. Devices, such as isocouplers, that are tuned to a pass frequency seriously limit broadband carrier installations. The most desirable installation is on a folded unipole tower where no interruptions in the coaxial cable are required at all.

Folded Unipoles can Improve Bandwidth
Folded unipoles are time tested in their ability to both match an AM tower, and make it “cold” at the same time. Towers with folded unipoles installed may be ready for tower collocation with little, if any work. In many cases, the folded unipole will need to be replaced with one that is built specifically for tower collocation. These systems employ wires that are spaced and insulated in such a way as to allow safe tower climber access. They are also built to accommodate clearance to multiple coaxial lines and ice bridges.

Non-directional series-fed towers can often be converted to folded unipoles with only replacement of the antenna tuning unit. The complexity and economics are a bit different with towers in a directional array, however. Since the folded unipole significantly alters the impedance and phase matching conditions at the base, you will likely need a new ATU to accommodate these parameters. This will usually require redesign by your consulting engineer, retuning of the array, and a partial proof of performance.

The upside of a folded unipole is not only isolation for unlimited coaxial lines, but better bandwidth and lightning protection. It also eliminates the need for tower lighting and sample loop isolation.