Interference between land or microwave mobile towers and standard broadcast (AM) radio antenna systems is increasingly common. The solution to these problems is often complex, and because the AM band 540-1600 kHz is so far from land mobile frequencies, non-AM broadcast technical personnel are often not familiar with the requirements of frequency technology. lower.
In the AM broadcast band, stations are permitted to maintain very specific radiated field strengths from their antenna systems. This is true for directional and non-directional systems. The wide range of interference from AM stations, coupled with congested band conditions, make the AM allocation problem complex. Very tight radiation pattern tolerances on the order of 0.5 dB are not uncommon. The extended near field of an AM antenna further complicates the problem. Near field effects can extend to two miles or more, compared to tens of feet on VHF, and the measurements used to determine the station’s antenna pattern can extend up to 20 miles. To complicate matters, tower heights typically used in land mobiles and microwaves are a significant part of a typical AM broadcast wavelength. Thus, they are too often excellent reradiators of the AM signal.
Recognizing this potential for problems, the FCC normally includes conditions with building permits that propose new towers within two miles of an AM station to ensure they do not interfere with the operation of the AM station. . Even if a land mobile license does not include such conditions, the license holder is responsible for correcting any problems that arise after the tower is constructed. The FCC considers the first station to be protected, requiring the land or microwave mobile phone licensee to resolve any harmful interactions their station facility may have with the AM broadcasting station.
A word on the economy. Adjustment and maintenance of an AM broadcast directional antenna system is complex and expensive. The cost of the initial adjustments can range from $ 20,000 to $ 100,000 or more. For this and other technical reasons, it is seldom possible for the owner of an interfering tower to solve a re-radiation problem by readjusting the antenna pattern of the AM station. Installing a device on the communication tower to detune it is much more feasible. Detuning is the technique of adjusting the current flow to a minimum on an object emitting radiation, thereby reducing or eliminating its effect on an AM station. By incorporating an appropriate AM disagreement device during tower construction, costs are typically only a few thousand dollars, and future license delays and costs can be avoided.
How can AM disagreement be achieved? Several approaches are available. If the communication tower is short compared to the AM frequency (generally less than the ¼ wavelength), all guy wires can be insulated and the base of the tower placed on an insulator. This interrupts the flow of current through the tower so that it is not a large 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 additional loss into communication systems, and conducts damaging lightning surge current to connected equipment. If the tower represents a significant portion of a wavelength at the AM frequency, then special problems arise. It not only becomes necessary to insulate the base of the pylon, but it may be necessary to install insulators at different levels on the pylon in order to cut it, and to install special tuning networks controlling each of these sections. Heavy and expensive even with guyed pylons, the insulation required can be prohibitive for large freestanding pylons.
While tower insulation techniques still work (on a sufficiently large budget), a much less burdensome and expensive alternative is available for most situations. This technique uses the current control capabilities of metal skirts attached to towers. In AM broadcast jargon, when used for transmission, these configurations are called folded unipoles. A conventional communications tower with isolated guy wires is used in this approach. No basic insulation or isolation is required. The bottom of the tower is grounded. All power lines and lighting conduits are attached to the tower in the normal way. A network of vertical wires is then arranged symmetrically around the tower, a few feet from the face of the tower. At the appropriate points, isolators and tuning devices are installed in the vertical drop cables. The number and spacing of these taps, the placement of all isolated segments, and the configuration of the tuning components are chosen by the designer to provide the desired attenuation to the AM signal re-radiation from the tower.
The metal skirt, brackets and associated monitoring and control equipment are supplied 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, freestanding towers.
Adjustment of systems using the metal skirt approach is generally straightforward. Immediately prior to the construction of the tower, field strength measurements are made on the AM station at suitable locations chosen to record the normal performance of the AM broadcast antenna. The AM detuning system is then installed. Its components are adjusted according to the manufacturer’s recommendations, while the AM scattering field strengths at the reference points measured before construction are observed. Adjustments to the mismatch components and bent wire configurations are made until measurements indicate that the tower has no significant effect on the AM model.
The progress of these adjustments is conveniently observed by monitoring the AM current flow at appropriate points on the tower being disagreed. Folded Detunipole kits include a detuning control center, mounted at the base of the tower, and sample loops and cables to be installed at various points on the tower. These cables are terminated at the control center to facilitate monitoring of the tuning procedure and for periodic maintenance checks. The folded Detunipole also has tuning coils and capacitors mounted in the control center and remotely connected to the tower mounted parts of the detuning assembly with remote phasing cables. Remote control of tower-mounted tuning components and remote monitoring eliminate the need for extensive on-tower adjustments and current probes, thereby minimizing the need for bell tap services.
Whichever approach is used, some general comments on the AM disagreement are in order. In particular, the importance of stability cannot be overstated. Once the tower is out of tune, it should stay that way! Achieving this happy state requires a number of steps that are not normally encountered in typical communication installations. Some of the most important areas where AM disagreement systems tend to fail and the suggested solutions are:
(1) Inadequate bonding of the tower structure. On welded towers, each joint between the sections on a leg from top to bottom must be spot welded. On field-assembled towers, all joints must be solid metal-to-metal, unpainted, with one leg spot welded to the bottom.
(2) Inadequate earthing. As a general rule, a 12 ‘x 12’ ground trellis with grounding rods driven on its edges and corners should be placed directly under the tower. The tower should be tied to it with a copper strip at least two inches wide.
(3) Poorly linked antennas and transmission lines. Everything that mounts to the tower should be securely tied to it. Grounding kits should be used to make metal-to-metal bonds of all transmission lines and conduits to the tower at least every 75 feet.
(4) Insufficient control. The same high standards outlined above must be met whenever equipment is added or removed from the tower. It is possible that a single improperly installed transmission line causes more interference with an AM station’s broadcast pattern than the tower alone, without disagreement.
(5) Neglected maintenance. Periodic checks of the AM disagreement system are necessary. Windstorms break connections and lightning damages components.
Of course, there are other advantages to a properly installed tower detuning system. The required bonding and grounding techniques reduce lightning damage incidents and may well eliminate some land mobile intermodulation problems. Good practices and good records will also maintain a good relationship with the AM station licensee and protect the radiocommunication licensee from false accusations if the AM station experiences subsequent schematic problems.
For many land and wireless mobile operators, outside assistance may be required to identify and resolve AM re-radiation issues. AM broadcasting practices and FCC regulations are quite different from those experienced by most two-way and microwave technicians. As noted above, mistakes can be very costly.
For hardware and technical advice, consultants and disagreement equipment manufacturers can be of tremendous help. For unique and complex situations, the services of a consulting engineer with experience in broadcast engineering are recommended. Tower manufacturers can also be consulted for advice on all structural aspects of interfacing mismatch equipment with the tower structure. Thus, an FCC condition that appears on a new license, or an AM broadcast tower found near a new tower site, should not cause despair. Help is available to resolve a reradiation issue affecting a nearby AM station.