Interference between land mobile or microwave towers and standard broadcast (AM) radio antenna systems is becoming increasingly common. The solution to these problems is often complex and, since the AM band at 540 to 1600 kHz is so far removed from land mobile frequencies, non-AM broadcast technical personnel are often not familiar with the demands of lower frequency technology.
In the AM broadcast band, stations 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, make the AM allocation problem a complex one. Very tight radiation pattern tolerances on the order of 0.5 dB 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 dozens of feet at VHF, and measurements used to determine the station antenna pattern may extend out as far as 20 miles. To compound matters, tower heights typically used in land mobile and microwave are a significant portion of a typical AM broadcast wavelength. Thus, they are all too frequently excellent reradiators of the AM signal.
Recognizing this potential for problems, the FCC normally includes conditions with construction permits that propose new towers within two miles of an AM station to make certain that they do not interfere with the AM station operation. Even if a land mobile license doesn’t bear such conditions, the licensee is responsible for correcting problems arising after tower construction. The FCC takes the attitude that the first station in is to be protected, placing the burden upon the land mobile or microwave licensee to resolve any detrimental interaction his station installation may have with the AM broadcast station.
A word about economics. The adjustment and maintenance of an AM broadcast directional antenna system is complex and costly. The cost of initial adjustments may range from $20,000 to $100,000 or more. 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. Much more feasible is the installation of a device on the communications 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 AM detuning device during tower construction, costs are typically only a few thousand dollars, and future licensing delays and costs can be avoided.
How may AM detuning be accomplished? Several approaches are available. If the communications tower is short with respect to the AM frequency (generally under ¼ wavelength), any 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 lightning conduits, transmission lines, and other conductive paths leading to the tower must be isolated at the AM broadcast frequency. This approach is generally cumbersome. It requires lighting chokes and isocouplers, introduces added loss in communications systems, and conducts damaging lightning surge current to attached equipment. If the tower is a significant portion of a wavelength at the AM frequency, then special problems are presented. 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 sectionalize it, and to install special tuning networks controlling each of those sections. Unwieldy and costly even with guyed towers, the required insulation may be cost-prohibitive for large self-support towers.
Although tower insulation techniques will always work (given a large enough budget), a far less cumbersome and expensive alternative is available for most situations. This technique 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 this approach. No base or sectionalizing insulators are required. The bottom of the tower is grounded. 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 any insulated segments and the configuration of tuning components are chosen by the designer to provide the desired attenuation to AM signal reradiation from the tower.
The wire skirt, supports, and associated monitoring and control equipment are furnished as a complete kit, with standard versions available for installation on guyed towers up to 600 feet high. Custom implementations are used for very tall and self-supporting towers.
Adjustment of systems utilizing the wire skirt approach is usually straight forward. Immediately prior to tower construction, field intensity measurements are made on the AM station at appropriate locations chosen to record the AM broadcast antenna’s normal performance. The AM detuning system is then 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 tower has no significant effect on the AM pattern.
Progress in these adjustments is conveniently observed by monitoring AM current flow at suitable points on the tower being detuned. Folded Detunipole kits include a detuning control center, mounted at the base of the tower, and 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. The Folded Detunipole also has tuning coils and capacitors mounted in the control center and connected remotely to tower-mounted portions of the detuning assembly with remote control 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 steeple jack services.
Whatever approach is used, some general comments on AM detuning are in order. In particular, the importance of stability cannot be over emphasized. Once the tower is detuned, it should stay that way! Reaching that happy state requires a number of measures that are not normally encountered in typical communications installations. Some of the more important areas where AM detuning systems tend to fail and suggested solutions are:
(1) 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 spot-welded all the way down.
(2) Inadequate grounding. Typically, a 12′ x 12′ ground mesh with driven ground rods on its edges and corners should be placed directly under the tower. The tower should be bonded to this with a copper strap at least two inches wide.
(3) Poorly bonded antennas and transmission lines. Every thing that mounts on the tower must be securely bonded to it. Grounding kits should be used to establish metal-to-metal bonds from all transmission lines and conduits to the tower at least every 75 feet.
(4) Inadequate control. The same high standards pointed out 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.
(5) Neglected maintenance. Periodic checks of the AM detuning system are necessary. Windstorms break the connections, and lightning damages components.
Of course, there are other paybacks to a well installed tower detuning system. The bonding and grounding techniques required reduce lightning damage incidents, and may well eliminate some land mobile intermodulation problems. Good practices and records also will maintain good relations with the AM station licensee, and protect the radio communications licensee from false accusations if the AM station suffers subsequent pattern problems.
For many land mobile and microwave operators, outside assistance may be necessary in identifying and resolving AM reradiation problems. AM broadcast practices and FCC regulation are quite different from those familiar to most two-way and microwave technical people. As pointed out above, errors can be very costly.
For hardware and technical advice, consultants and detuning equipment manufacturers can be of considerable assistance. For unique and complex situations, the services of a consulting engineer versed in broadcast engineering are recommended. Tower manufacturers also may be consulted for advice on any structural aspects of interfacing detuning equipment with the tower structure. So, an FCC condition that appears on a new license, or an AM broadcast tower found to be near a new tower site, need not cause despair. Help is available to solve a reradiation problem affecting a nearby AM station.