Is This Cell Tower a “Hot” AM Tower?
A quick look at cellular collocation on AM towers
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There are some 10,000 AM broadcast towers in the United States. Most are in ideal locations and just the right height to host cell antennas. Yet, very few AM towers are also used for cell towers. What’s going on here?
It has been remarked that AM broadcasting and cellular communications are “one hundred years and 1000 megahertz apart”. Why? Because AM broadcasting is a technology that emerged almost 100 years ago, operating between 0.53 and 1.71 MHZ (530 to 1710 kHz), and is little changed from its origins. By contrast, cellular and “wireless” systems mostly operate from 700 to 2700 MHZ and are even now evolving new and more useful forms.
Why is this important? Because of two main factors: power and antenna size. AM requires antennas a thousand times larger than does cellular. AM may cover hundreds of miles, compared to a few miles for a cell site. That takes much more power than is required for cell coverage, leading to a perception of safety hazards.
The differences between AM and cellular are so great that few engineers and technicians crossover between the technologies, and there is virtually no crossover between business interest. In short, they don’t talk to one another. But, they should!
Many AM towers have been at the same locations for 50 years or more, often on once rural sites, now grown up into dense suburban areas. Such areas are notably difficult for siting new wireless towers, yet the AM towers are “part of the community”, and often relatively non-controversial for wireless collocation. That makes them very attractive to the wireless community – except for their reputation as “hot” and untouchable. AM towers are a resource whose utilization for cellular colocation sites can serve the wireless industry while bringing new revenues to hard pressed AM stations.
And, as we shall see, being “hot” is not necessarily an impediment to cellular colocation!
How are AM antennas different from Cell Antennas?
Unlike a cell tower, which is only a support for the attached cell antennas, the entire AM tower structure radiates signal as the AM radio station’s antenna. Since the tower structure is radiating the AM signal, an AM tower is often referred to as “hot”.
LBA uses two primary methods to facilitate the installation of wireless users on hot AM towers. One method involves isolating the collocated transmission lines from the AM tower to prevent disruption of the AM transmission, accomplished through isolators such as a ColoCoil™. The other method involves the installation of a folded unipole or skirt cable feed, such as a ColoPole™, on the tower.
With the increasing use of remote radio heads (RRH’s), special isolators such as the ColoCoil RRH™ with fiber optic coupling have been introduced to enable their placement on AM towers.
In addition, some older installations use a cluster of units called capacitive isocouplers. These are becoming functionally obsolete since most are unable to handle current 4g LTE technology. These “isocoupler” systems are also highly vulnerable to lightning, as this Verizon case study relates.
We’ll explain both principal approaches and generally what to look for to indicate that a cell tower may be a “hot” AM tower.
What does a “hot” AM tower site look like?
There are several clues to look for that will help you recognize an AM site:
- Are call letters like WXYZ-AM displayed on the site?
- Does the tower have a ceramic base insulator?
- If guyed, are there insulators in the guy wires?
- Is the tower within a large cleared area?
- Is the tower base fenced, often with wood pickets?
- Are there multiple towers several hundred feet apart in the same field?
- Are isolation devices, such as a ColoCoil™ or ColoPole™, installed?
- Does a portable radio, placed near the tower base, overload on one or more frequencies?
If you see any of these things, you should investigate further before approaching the tower! Be aware that the power of AM stations in the US may be as high as 50,000 watts, which can cause serious damage or death. Fortunately, most AM colocations are on towers with 10,000 watts or less, but even at lower powers safety requires positive identification of hot AM towers.
If you are still uncertain about the status of a tower, you can use the free LBA online AM screening tool for help. It will tell you all the AM sites near a geographical coordinate. All you need are the tower coordinates. For detailed technical information about the AM operation, you can also go to the FCC AM Query site.
What are the safety concerns while working on or near a “hot” AM tower?
There are significant safety and operational issues which must be carefully addressed concerning the installation and maintenance of wireless equipment on a “hot” AM tower. Fortunately, these RF concerns can be managed and are not a significant problem. 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 collocated antenna equipment. Both FCC and OSHA permit work on “hot” AM towers with proper power levels and precautions. FCC “hot” tower guidance is found in publication OET-65.
As the chart shows, it is considered safe to climb a hot tower at certain power levels. OET-65 also contains charts which allow a determination of the safe distance from a “hot” tower to stay within Maximum Permissible Exposure (MPE) limits. These determinations vary for each station and are best evaluated by an RF engineer. As a generalization, for a typical quarter wave tower, a safe distance is about one meter for 1000 watts, or two meters up to 10,000 watts.
Many AM stations employ more than one tower. Thus, it should be taken into account that AM stations may operate with different antenna combinations and/or different powers day and night. The collocation tower is often chosen to be at low power or unused during day hours to facilitate cell site maintenance.
Because the AM station can instantly go from a safe mode to a hazardous mode without warning, coordination of construction and maintenance activity with the station should be part of project planning. LBA’s AM colocation solutions are specifically designed to facilitate “hot” maintenance and to protect against unsafe conditions.
Another RF safety suggestion: Rigging an AM tower can pose safety risks to rigging cables as well as personnel handling any conductive objects elevated on the tower and ground. This is a consideration under OSHA crane standards, and this blog on rigging RF damage will give you a shocking example of why RF must not be ignored.
There is a substantial discussion about hazards around AM RF in the LBA paper “Is This Cell Tower Detuned?” Reading it is good preparation for working around any AM facility.
Sometimes it is difficult for someone who isn’t an AM engineer or a trained AM technician to determine if a tower is a “hot” AM tower or if the tower is only near an AM station and is detuned. If assistance is not locally available from the station, LBA personnel will be glad to assist you at 252-757-0279. We may request photos to help evaluate the tower’s status. This service is offered free and with no obligation.
More about AM collocation devices
In order to successfully collocate cellular antennas on an AM tower, the two signal bands must be segregated to prevent interaction between the AM and cellular signals. Typically, two basic approaches can be used, the “isolation” method, and the “adaptive” method.
The “isolation” method allows the AM station to continue to feed its RF energy to the tower just above a base insulator, while the wireless signals are fed through a device, such as a ColoCoil™, which allows the cell signals to pass while blocking the AM signals. Thus, the AM tower is “floating” for the AM RF.
The “adaptive” method uses a cage of wires called “foldwires” to feed the AM RF energy to near the top of the tower while the base of the tower is cold and grounded. The AM signal is specially tuned to excite the tower from the top so no isolation is needed for the cell antenna coaxes at the base of the tower. When coaxes and antennas are properly bonded to the tower there is no interaction between the AM and cellular signals. Because the foldwires are hot with AM, a reasonable spacing to work areas is desirable.
Identifying isolation method hardware – When this approach is utilized to collocate other communication infrastructure on a “hot” AM tower, then a special device or devices at or near the base of the AM tower will be in place through which the wireless feed lines pass for isolation. Typical is a ColoCoil™ isolator. The isolation system is packaged in a cubic cabinet module three feet per side. It normally has either 3 or 6 DIN connectors on both the input (cold) and output (hot) sides. The “hot” side can be recognized by the insulation panel through which the connectors are attached.
In the case of the ColoCoil™, multiple cabinets may be placed side-by-side or stacked on a pad or platform. While not disrupting the coaxial transmission line performance at wireless frequencies, the isolator method blocks the AM RF from the cellular base station equipment.
The photo above shows an isolation installation on a 5,000 watt AM tower in Hawaii. The ColoCoil™ isolators are feeding 18 coaxial lines to Verizon antennas on the tower.
Note the close spacing of all cell site equipment to the AM tower due to space restrictions. In such cases, it is important to power the AM transmission down to a safe level to ensure FCC MPE exposure compliance.
Another installation configuration is shown below. In this case the isolators are located side-by-side and behind a barrier fence which also serves to separate access to hot and cold sides. The fence is set at the proper distance that cellular personnel are not exposed to excessive MPE’s even while working around a fully powered AM tower. Note the snow shelter over the isolators and base equipment. This AM collocation site is in New Hampshire at a 10,000 watt radio station. The base insulator, so characteristic of AM “hot” towers can be clearly seen!
Other AM isolation method systems exist. As noted earlier, the hardware used for supporting remote radio heads (RRH’s) is fundamentally different than other isolation method approaches. Typically, one multiconductor cable, containing DC power, control circuits, and communications data signal is run up the tower.
The LBA ColoCoil RRH™ appears to be the only isolation system currently available to isolate these systems on AM towers. It uses a combination of inductive isolators and fiber optic isolators. Cabinet sizes vary with RRH support requirements, but the removable door side is always the AM “cold” side. The rear of the cabinet will have an insulator plate through which the “hot” RRH cables exit.
Identifying adaptive method hardware –The most often used approach here is the employment of a folded unipole AM feed system. A folded unipole often consists of a cage of 3 or 6 wires symmetrically arranged around the supporting tower. Standoff supports and insulators are used to keep the wires clear of the tower and appurtenances, and properly tensioned for stability of the AM operation. Coaxial cables are mounted and grounded directly to the tower, as are conduits for lighting and remote radio head cabling.
An example of such hardware is LBA’s patented ColoPole™ folded unipole. It can be recognized by the utility-grade #4 ACSR cable construction and tubular insulator/tensioning “Delta Units” at the base of each foldwire. Normally, folded unipoles have a ring connecting the foldwires just above the insulators and leading to an antenna tuning unit. The ring and lead wire should be considered hot.
Using this method permits the base of the AM tower to be grounded and allows the placement of wireless antennas and feed lines without further isolation. It is usually the preferred method of AM collocation on monopole and self-supporting towers where base insulators are expensive and unwieldy. Occasionally you may see a folded unipole on a tower with the base insulator bridged with copper strap. This is a conversion from a former base fed configuration in which the base insulator only remains for structural purposes, and the tower base is no longer hot.
To the left you see a ColoPole™ folded unipole installation on a self-supporting tower in Wyoming. In this case two 1000 watt AM stations on different frequencies share the American Tower Company tower with cellular tenants. You can clearly see the foldwires to the top of the tower, as well as the fiberglass insulators holding the wires well clear of the climbing area.
A caution is that folded unipole installations can look a lot like detuning systems. Recognition of and precautions for working around detuning systems are described in the LBA paper “Is This Cell Tower Detuned?”
In the photo above, LBA field rep Adam Carlson is installing a 5,000 watt AM antenna tuning unit (ATU) to feed the ColoPole™ folded unipole on the self-supporting tower behind him. ATU’s couple the AM transmitter signal to the tower, optimizing the “match” for best efficiency and audio bandwidth. These allow optimization of the AM signal, if needed, when cell system changes are made on the tower. Presence of an ATU cabinet is a good sign that an AM signal is present!
A few more comments on AM colocation systems
On non-directional towers, an adaptive system with a ColoPole™ or other folded unipole system is often used. The folded unipole system results in direct grounding of the AM tower. Thus, wireless antennas and transmission lines are mounted and bonded directly onto the structure with no other isolation needed.
The ColoPole ™ folded unipole uses a unique wire cage impedance transformer, derived from an AM-only design and proven in hundreds of installations around the world. Lower portions of the cage are heavily insulated and spaced away from the tower to allow ready operational access to the wireless antenna system.
A well-built, stable, folded unipole benefits the AM station with improved efficiency, better “air sound”, and direct-to-ground lightning protection, thus enhancing the colocation experience for the station.
However, the addition of a folded unipole does change certain AM operating parameters, and the wire rigging is vulnerable to damage. For that reason, tower companies and carriers often prefer to use an isolation method, such as the ColoCoil™ which little affects the established station operation, and remains under their control. Since the isolation equipment is at the base of the tower, it is less vulnerable to damage.
Directional AM stations use multiple towers carefully synchronized to form an FCC licensed radiation pattern. This pattern is crucial to protecting other stations from interference, and may change from day to night. This licensed pattern may have tolerances well under one decibel. Thus, ensuring that it not be disrupted by colocation is critical.
While the folded unipole adaptive method can technically be applied to a colocation tower in an AM array, it generally results in expensive change out of AM equipment and resynchronization of the AM towers.
The most cost-effective approach to collocation in a directional array is to employ specially designed isolation coils such as ColoCoil® isocouplers for this purpose. Placed between the base station equipment and transmission lines on the tower they effectively prevent the wireless transmission lines from impacting the operating parameters of the AM towers. Unique among isocoupler devices, LBA CoLoCoils ™ are compact and modular, accommodating seamless “bolt-on” expansion to wireless user requirements without significant impact on AM host facilities.
Selection of the proper method for any given collocation scenario on an AM tower requires an experienced analyst and the weighing of various factors attendant to both the carrier and the AM station. A detailed discussion of the AM collocation process can be found in LBA Tech Note 114: AM Colocation – It’s Real! and in the article AM Tower Colocation – Money On the Table?
If you encounter a question in the field about an AM colocation system, whether or not it is an LBA system, we will be glad to discuss it with you. Contact a technician at 252-757-0279 or firstname.lastname@example.org for this free assistance. If you are considering a new AM colocation site, we will be happy to provide services from initial consultation and support of your A&E team, to project management services.