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AM Revitalization Efforts, Part Three

We now find ourselves a couple months downstream from the release of the Third Report and Order

AM revitalization continues to move ahead. The First Report and Order is barely two years old, and now we find ourselves a couple months downstream from the release of the Third Report and Order.

The initial document was wide in its scope and caused some heartburn among broadcasters, as should have been expected. The recent release is notable for what it addressed�� and for the cans kicked further down the road.

In the First Report, the FCC posited several questions concerning the use of MoM modeling for skirt fed towers. Some engineers expressed reservations due to the potential need for an expanded set of procedures and limitations. Due to their physical construction, skirted towers can result in odd current distributions, which may pose a problem in a directional setting. Others were bullish on the use of MoM for skirts and convinced that sufficient knowledge existed to permit modeling such arrays.
Credit: Scott Fybush

The commission notes this in the second footnote. Items still to be resolved are the most contentious of the original release. Namely, the proposal to change protections based on class; the rolling back of the nighttime RSS calculation methodology to that used before 1991; and the expanded band authorization surrender. These proposals may be addressed, and as the commission indicates, other items may be attached to the ongoing effort.

In 2001, the commission redefined what constituted a partial proof of performance. Prior to that time, all radials contained in the latest full proof of performances were considered. But the burden was reduced to require only the radials with a monitor point; and if a pattern has fewer than four monitor points, then the radials adjacent to the monitored radials required measurement.

The Third Report reduces the burden further by requiring only monitored radials to be measured. This change will certainly reduce the cost associated with a partial, but the lack of measurements in lobes, often not monitored, may allow array efficiency issues to go unnoticed. Thus, it is good practice to spot check these areas when field measurements are performed.

If your antenna system was proofed by MoM, then you should be familiar with the annual recertification process. Under the Third Report, that requirement disappears, unless changes to sampling system components are performed. This change makes sense and should have been made years ago.

With the implementation of a MoM proof, reference field strength measurements are required. These field strength measurements essentially function as de facto monitor points, and as my friend of many years, Mike McCarthy, correctly pointed out, these measurements are crucial in diagnosing array health.

It used to be that monitor points, phase monitor readings and base current ratios were a three-way cross check on array health. With base current meters no longer required, the reference field strength measurements were the only way to identify an apparent problem as real world or sample system related. Just as locks keep honest people honest, the public availability of reference field strength measurements tends to make for good neighbors. Moving forward, only the initial set will be required.

One interesting change was the elimination of the survey requirements when converting an existing array to a MoM proof, if no modification of the tower quantity or geometry occurred.

The logic behind this was that it was an existing array, and so the towers are where they are supposed to be. Maybe they are � not only do we find tower coordinates to be incorrect in many cases, but I have also run across a couple of arrays where the towers were not at the correct spacing and orientation.

In one instance years ago, the owner used a compass and measuring tape to stake out the locations of the two towers. The resulting error in the tower azimuth and array spacing screwed up the null location, and necessitated a last minute CP modification. Field strength measurements played a large part in finding this problem.

Four additional items under MoM proofing were addressed, with results on adoption. A necessary clarification to the description of shunt capacitance in modeling the tower base region was made.

The adoption column contained changes to the MoM rules regarding re-proofing after antenna addition and elimination of current distribution measurements for certain configurations when MoM is used. In the case of the former, housekeeping tweaks were made so that changes that do not influence modeled values will not require re-proofing, while the latter eliminated current distribution measurements for top loaded systems. The proposal not adopted was the use of modeling for skirted towers.

In the First Report, the commission posited several questions concerning the use of modeling for skirt fed towers at paragraph 74. Specifically, the commission sought comments pertaining to constraints on skirt modeling, the use of specific software packages to model them, requirements for sampling and if any costs would arise from these proposals.

When a highly theoretical question is posed to a group of engineers, one should expect a collection of varying answers, which ultimately can be distilled down to one side or other of a central argument. That�s exactly what happened.

Reservations were expressed about using MoM to model skirted towers due to the potential need for an expanded set of procedures and limitations. Additionally, due to their physical construction, skirted towers can result in odd current distributions, which may pose a problem in a directional setting, despite the potential workarounds. In the other camp, some were bullish about the use of MoM for skirts and convinced that sufficient knowledge exists to permit modeling such arrays. No doubt due to conflicting opinions between industry heavyweights, the commission elected to shove this issue aside.

The last item addressed in the Third Order pertains to the Main Studio Rule.

Two years ago under different leadership, the commission expressed reluctance to eliminate main studio requirements entirely due to concerns over ensuring compliance with local service obligations. While expressing this reluctance, it was also acknowledged in the First Report that the existence of a main studio for some AM stations especially in difficult economic climates, could represent a financial hardship. And so, at that time, comments were sought as to what extent the rules should be relaxed.

Now in late 2017, the commission has voted to eliminate main studio requirements, other than the maintenance of a local telephone number within the community of license, or alternatively, a toll-free number. This expected change is tied to a separate proceeding, which superseded the First Order proposal, and as a result, main studio rule changes have been dropped from AM revitalization as of the Third Order.

Certainly, the 2015 First Report was ambitious in scope and represented the first major changes to the AM-related rules, other than translators, in a quarter century. The latest report continues to chip away at the long list of proposals, but most of the items addressed this time around are mostly housekeeping in nature. I, for one, am pleased to see the commission taking an interest in AM, as the senior band is where broadcasting began.

While the number of points measured in a partial proof could have a minor economic impact to an AM station, how much revitalization is really taking place?

Revitalization imbues something with new life, energy or activity. While the proposals so far appear to have a positive impact on the AM service, are the efforts bold enough?

Perhaps one of the forthcoming proposals hinted at in the Third Report will be the long-awaited start of the transition to all-digital. That would be true AM revitalization.

Ruck is the principal engineer of Jeremy Ruck and Associates, Canton, Ill.