Am Detuning – If Its Broke, You Gotta’ Fix It!

Over the years, thousands of wireless towers nearby the United States have been detuned to nearby Am stations. They have been detuned because the Federal Communications Commission rules need wireless carriers and other tower users to safe Am broadcast station antenna patterns.

The same Fcc regulations need “the prolonged maintenance” of these Am protection systems. It is an Fcc violation not to do so.

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Regrettably, many operators of detuned towers have missed this part of the rules, and many Am detuning systems are in a sad state of repair. This not only violates the Fcc rules, but jeopardizes the Am station functioning, and can even lead to civil liability!

Wireless engineers are well versed in cellular systems; any way Am engineering concepts are often not included in their study or experience. Thus, it’s easy to overlook detuning maintenance. Am detuning systems are entirely passive, have no failure bells and whistles, and their failure in no way affects the wireless carrier operation.  The systems are just some more wires and cables among many on a tower.

When the Am detuning system is installed, it is a “set and forget” procedure. Left undisturbed, that introductory setting is likely good for many years. Unfortunately, that happy state is rarely achieved on a communications tower.

An Am detuning system consists of a wire cage about a tower, often to the top, and a tuning box with components to resonate the system to the Am frequency. While deceptively simple, the devil is in the details, and practically any disturbance to the mechanical or electrical configuration of the system can cancel out the Am detuning effects.

A few years ago, an Lba engineer did a study of over 100 cell sites and found that only 25% were in Fcc compliance! Amazingly, some sites were improperly installed, or used the wrong components, and could never have been detuned.

How do Am detuning systems fail? Lba has been responsible for deploying and rebuilding hundreds of these systems. Here are some of the major failure modes our engineers see:

• Lightning strikes - Often invisible on the exterior, lightning fries components and shatters insulators inside the detuning cabinet. It is also known to destroy system standoff insulators on the tower.
• Wind - May disrupt detuning cables, or displace other tower mounted antennas and mounts to touch detuning wires.
• Poor building - Too many systems are of the “cheapo” variety, often using light construction, unsuited to the robust utility grade requirements of tower survival. Like the proverbial “One Horse Shay”, these systems often just naturally fall apart after a few years.
• Tower climbers - Not knowing their function, it is just too tempting to get those detuning wires out of the way by wrapping them nearby climbing pegs, or clamping them to the tower with a new antenna platform.
• Environmental - Often vendors have ignored incompatible metals, important to corrosion on connections. Some have used detuning boxes that are not weather tight, important to water and insect damage.
• Tweakers - Only great engineers should be permitted to adjust the detuning system. When unqualified persons try to adjust the system, they most often leave it non-functional, but there is no physical evidence in their wake! Only skilled tests can then find the qoute and set things right.

There is no required inspection duration for Am detuning systems in the Fcc rules. However, it is generally conception that a two year inspection and recalibration interval is appropriate, and Lba concurs with that. Of course, a special inspection would be thorough if irregularities are noted in between times, major tower modification or antenna building is undertaken, heavy ice or wind occurs, or the site is vandalized.

We also propose several good practices to minimize the requirement for detuning system fix or replacement:

• Site warnings - Prominently post the site as detuned, and need that any issues be discussed with site authority or the detuning system vendor.
• Contractor briefs - Site contractors should be briefed on avoidance of detuning damage and the requirement to engage a great detuning seller to support them if modifications are needed.
• Durable building - Only systems using “utility grade” building from a proven seller should be installed. Inferior systems should be substituted at first opportunity.
• Site log - Any inspection, passage to detuning controls or system changes should be part of a permanent site log with date, activity performed, inspection results, and approving authority.

Am Detuning – If Its Broke, You Gotta’ Fix It!

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