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AM Broadcasting - The New Cellular Design Factor – Technical Note 108

folded unipole system used to solve reradiation problems

As cellular systems have sprouted towers across the landscape, a seemingly unlikely conflict has arisen between cellular operators and old-fashioned AM broadcast stations. Awareness of this important, but obscure, situation has just been heightened by the Federal Communications Commission (FCC).

In Public Notice #4396, dated Aug. 11, 1987, the FCC sent a message to cellular operators that they must avoid RF interference with all AM broadcast stations. Furthermore, cellular operators must now prove to the FCC that they have corrected all such problems. This must be done at their own expense.

This edict has puzzled many cellular operators. After all, how can a cellular facility operating above 800MHz interfere with an AM broadcast station operating in the vicinity of 1MHz? As it turns out, it is not the radio waves from cellular transmitters that cause the interference, but the effect of their transmitting towers on the AM radiation.

Unfortunately, typical cellular towers are just the right height to reradiate signals transmitted from AM broadcast stations. In effect, each cellular tower operates like a “mini” AM station, rebroadcasting interfering signals. Unlike the signals from cellular antennas that are affected little by metallic objects several feet or more away, AM broadcast antenna signals can be affected seriously by structures as much as two miles away, particularly if the AM has a multitower, directional antenna systems.

The rapid increase in the number of cellular towers, coupled with a profusion of AM stations in many metropolitan areas, has given rise to significant conflicts between the two facilities. Cellular system planners often have ignored this interference problem, which is compounded because AM stations are engineered and licensed to tight specifications, which the FCC requires them to adjust, prove out and maintain.

Many AM station radiation patterns are controlled to within 0.5dB or less of a design pattern, which is much more precise than the control on cellular antennas. Thus, correcting distortions caused by the proximity of cellular towers to AM antennas can be extremely expensive. Engineering costs of $25,000 to $100,000 to tune an AM antenna system are not unusual today. Because sole responsibility for correction is on the cellular permittee, there is an obvious need for AM awareness and enlightened design to avoid major FCC and fiscal liability.

The FCC rules

Having presented some background on the problem, let’s look at the specific requirements of the FCC Public Notice.

First of all, it’s important to understand that the FCC has made the protection of AM stations mandatory for all licensees and permittees planning to construct or to modify a tower within two miles of a directional AM array, or within one-half mile of a non-directional AM tower. Significantly, the FCC includes not only new construction, but modification of existing towers. Under certain circumstances, even moving or changing the configuration of cellular transmission lines and antennas can have a significant effect on a nearby AM station. So, the FCC policy is broadly applicable, and must be considered whenever any cellular tower or antenna work is done. Some specific procedures are required by the FCC to ensure that proper steps are taken to protect the AM licensee.

If construction is within one-half mile of a non-directional AM tower, the cellular licensee should, prior to construction of the proposed tower, notify the AM station so that the station may determine operating power by the indirect method. According to the FCC, the cellular licensee shall be responsible for the installation and continued maintenance of any detuning apparatus necessary to prevent adverse effects upon the radiation pattern of the AM station .

Measurements shall be taken both before the construction of the tower and after the installation of all related appurtenances. After construction is complete, an antenna impedance measurement of the AM station shall be made and sufficient field strength measurements taken at a minimum of 10 locations along each of eight equally spaced radials to establish that the AM radiation pattern is essentially omnidirectional. The results of the field strength and impedance measurements should be reported to the commission in an application for the AM station to return to the direct method of power determination. This application is filed on FCC Form 302.

If construction is within 2 miles of a directional AM array, the cellular licensee should notify the AM station so that, if necessary, the AM station may determine operating power by the indirect method and request temporary authority from the FCC in Washington, DC, to operate with parameters at variance in order to maintain monitoring point field strengths within authorized limits. [See Section 73.51(e) and (f).] Again, the cellular licensee shall be responsible for the installation and continued maintenance of all detuning skirt necessary to prevent adverse effects on the radiation pattern of the AM station.

Both before construction of the tower and after the installation of all appurtenances thereon, it is required that a partial proof of performance, as defined by Section 73.154(a) of the commission’s rules, shall be conducted to establish that the AM array has not been affected adversely. Prior to or simultaneous with the filing of the FCC 489 notification, the results of the measurements must be submitted to the commission.

No grandfathering

Two important points should be noted. Because the FCC is operating under a “first in” policy, there is no grandfathering. Even though a cellular facility was constructed before the date of the Public Notice, and the interference is not detected until after the Public Notice, the cellular operator remains fully responsible for its cleanup. On the other hand, if an AM facility is constructed or significantly modified after the cellular facility is built, it would be the responsibility of the AM licensee to adapt the construction to the presence of the cellular facility. Nevertheless, there are circumstances where the cellular operator could later be liable for changes or additions to his facility. Thus, even in these circumstances, close coordination between the cellular operation and the AM station is advisable.

Avoiding interference with AM stations involves a determination of the location of all AM facilities relative to all existing or planned cellular facilities. The relative electrical and physical parameters of all these facilities are then considered in an analysis that identifies any expected interactions. This usually involves the use of a sophisticated computer program that models the complete structural environment of the involved AM and cellular towers.

Depending on the extent of predicted interaction, it may or may not be necessary to treat the cellular tower to make it non-radiating at AM frequencies. Where reradiation is a predicted factor, it will be necessary to design appropriate isolation or detuning systems for the tower that will permit its cellular transmission function while appearing “transparent” to AM broadcast signals.

As noted above, the FCC requires a series of precise actions on the part of cellular licensees to protect AM stations during cellular construction and to verify that the proper tower isolation has been carried out. These are critical steps, requiring competent execution. Improperly done, it may be impossible, after construction, to prove that an AM antenna system has been restored to proper operation. This is particularly true in an environment with multiple non-cellular structures. Without adequate baseline data, it may be impossible for the cellular licensee to make a distinction between the influence of his antenna tower and those of other nearby structures. Do-it-yourself efforts to effect tower detuning are a well-traveled road to grief and despair.

If you are not familiar with AM broadcast facilities, you should engage a qualified broadcast technical consulting firm to carry out the necessary research, detuning and testing of cellular antenna systems. The importance of such professional advice on this specialized technology from the beginning of the project cannot be over emphasized.

Solving interference problems

Having reviewed the technical background of the cellular-AM interference problem, some approaches to solutions, and the FCC Rules; here are some practical considerations in problem avoidance as well as pros and cons of tower detuning methods.

The solution to interference problems is often complex. Because the AM band at 540kHz to 16OOkHz is so far removed from cellular frequencies, non-AM broadcast technical personnel are often not conversant with the demands of lower frequency technology.

AM broadcasters are licensed to maintain very specific radiated field intensities from their antenna systems. This is true of both directional and non-directional systems. The extensive interference range of AM stations, coupled with crowded band conditions, makes the AM allocation problem a complex one. Tight radiation pattern tolerances on the order of 0.5dB are not uncommon.

The extensive near-field of an AM antenna further complicates the problem. Near-field effects may extend to two miles or more, compared to a few feet at UHF, and measurements used to determine the station antenna pattern may extend out as far as 20 miles. To further complicate matters, tower heights typically used in cellular applications are a significant portion of a typical AM broadcast wavelength. Thus, they are all too frequently excellent reradiators of the AM signal.

For this and other technical reasons, it is rarely feasible for the owner of an interfering tower to solve a reradiation problem by readjusting the AM station antenna pattern. It is much more feasible to install a device on the cellular tower to “detune” it.

Detuning is the technique of adjusting current flow to a minimum on a reradiating object, thus reducing or eliminating its effect on an AM station. By incorporating an appropriate detuning device during tower construction, costs are typically only a few thousand dollars, and future licensing delays and costs can be avoided.

How may detuning be accomplished? Several approaches are available. If the cellular tower is short with respect to the AM frequency (generally less than one-fourth wavelength), all guy wires can be insulated, and the tower base put on an insulator. This breaks the current flow in the tower so that it is not a significant radiating object. However, all lighting conduits, transmission lines and other conductive paths leading to the tower must be isolated from ground at the AM broadcast frequency.

This approach is generally cumbersome. It requires lighting chokes and . It also introduces added loss in cellular systems and conducts  damaging lightning surges to attached equipment.

If the tower is over one-fourth wave-length at the AM frequency, then special problems arise. It becomes not only necessary to isolate the tower base, but it may be necessary to install insulators at various levels on the tower in order to sectionize it, and to install special tuning networks to control each of those sections. Unwieldy and costly with guyed towers, the required insulation may be cost-prohibitive for large self-support towers.

Folded unipoles

detuning skirt applied to a towerAlthough tower insulation techniques will always work (given a large enough budget), a far less cumbersome and expensive alternative is available for most situations. It is a technique that makes use of the current control capabilities of wire skirts attached to towers. In AM broadcast parlance, when used for transmitting, these configurations are known as “folded unipoles.”

A conventional communications tower with insulated guy wires is used in the folded monopole approach. No base or sectionalizing insulators are required. The bottom of the tower is grounded and all feedlines and lighting conduits are attached to the tower in a normal manner. An array of vertical wires is then arranged symmetrically about the tower, a few feet from the tower face. At appropriate points, insulators and tuning arrangements are installed in the vertical downleads. The number and spacing of these downleads, the placement of insulated segments and the configuration of tuning components are chosen by the designer to provide the desired attenuation to AM tower currents and thus signal reradiation from the tower.

Figure 1 is a sketch showing a typical 2-section commercial implementation of this approach. The wire skirt, supports and associated monitoring and control equipment are furnished as a complete system, with standard versions available for installation on guyed cellular and microwave towers up to 600 feet high. Custom implementations are used for tall and self-supporting towers. Adjustments to systems using the wire skirt approach are usually straightforward. Immediately before tower construction, field intensity measurements are made on the AM station at several dozen locations chosen to document the AM broadcast antenna’s normal performance. The tower is then erected and the detuning system is installed. Its components are adjusted as recommended by the manufacturer, while AM broadcast field intensities at the reference points measured prior to construction are observed.

Adjustments to detuning components and foldwire configurations are made until measurements indicate that the cellular tower no longer has any significant effect on the AM pattern. When adjustments are completed, an additional set of AM field intensity readings are taken at all points previously recorded. This data is then analyzed and documented for FCC filing and for future maintenance reference.

Progress in these adjustments is conveniently observed by monitoring AM current flow at suitable points on the tower being detuned. Custom implementations generally include a detuning control center, mounted at the base of the tower, and RF sample loops and cables for installation at various points on the tower. These cables are terminated at the control center for convenience of monitoring the adjustment procedure, and for periodic maintenance checks. There are also tuning coils and capacitors in the control center that are connected remotely to tower-mounted portions of the detuning assembly with phasing cables. Remote control of tower-mounted tuning components and remote monitoring eliminate the need for extensive on-tower adjustments and current probing, minimizing the need for steeplejack services.

 

Detuning points

Whatever approach is used, some general comments on detuning are in order. In particular, the importance of stability cannot be overemphasized. Once the tower is detuned, it must stay that way. Reaching that happy state requires measures not normally encountered in typical communications practices. Some of the more important areas where detuning systems tend to fail, and selected solutions, are:

• Inadequate bonding of tower structure. On welded towers, each joint between sections on one leg from top to bottom should be tack-welded. On field-assembled towers, all joints should be solid metal-to-metal, no paint, with one leg welded all the way down.
• Inadequate grounding. Typically, a 12’x12' ground mesh with long driven ground rods on its edges and corners should be placed directly under the tower. The tower should be bonded to this copper strap at least two inches wide.
• Poorly bonded antennas and transmission lines. Everything that goes on the tower must be securely bonded to it. Grounding kits must be used to establish metal-to-metal bonds between all transmission lines and conduits and the tower at intervals no greater than 75 feet.
• 
Inadequate control. The same high standards mentioned above must be observed whenever equipment is added to or deleted from the tower. It is possible for a single, sloppily installed transmission line to cause more interference to an AM station’s broadcast pattern than the tower alone, without detuning.
• Neglected maintenance. Periodic checks of the detuning system are necessary. Windstorms break connections, and lightning damages components. Also, a regular program of checking and recording monitor loop reading is essential. Semiannual checks should be adequate, with additional checks after all tower work.

 

Of course, there are other paybacks to a well-installed tower detuning system. The bonding and grounding techniques recommended will reduce lightning damage incidents and may eliminate some land mobile intermodulation  problems. Good practices and records will also ensure maintenance of good relations with the AM station licensee, and protect the cellular licensee from false accusations and liability if the AM station suffers subsequent pattern problems from causes unrelated to the cellular operation.

For many cellular operators, outside assistance may be necessary to identify and resolve AM reradiation problems. AM broadcast practices and FCC regulations pertaining to these problems are quite different from those that most cellular technical people are familiar with. As pointed out above, errors can be costly and impose a high risk on do-it-yourself efforts.

For hardware and technical advice, consultants and detuning equipment manufacturers can be of considerable assistance, and the services of a consulting engineer versed in broadcast engineering situations are essential for unique and complex situations. Tower manufacturers also may be consulted for advice on structural aspects of interfacing detuning equipment with tower structures.

An FCC condition on your new license, or an AM broadcast tower found to be near a new or desirable cell site, need not cause despair. Help and action options are available to solve almost any AM reradiation problem.

 

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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.

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