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The New World of AM DA MoM

SBE Asked to Develop Training Programs; MoM Not Without Critics

WASHINGTON New rules adopted by the Federal Communications Commission will soon allow most AM broadcasters in the United States to verify the performance of their AM directional antenna by modern computerized methods.

With the new rules, the FCC modified the means of verifying AM directional antenna performance to include antenna pattern-prediction modeling known as Method-of-Moments (MoM) computer modeling. The new rules had not gone into effect as of late October; they were still pending approval by the Office of Management and Budget and publication in the Federal Register.

There are approximately 1,900 directional AM stations currently licensed in the United States, according to the commission.

The rule changes should save broadcasters money and time, industry watchers said. Previously, broadcasters were required to conduct magnetic field-strength measurements for AM DA verification. MoM computer modeling is considered by most as a simpler and less time-consuming task.

It’s possible that as many as half of the AM directional arrays in the United States are out of compliance in one way or another, and in many cases the cost of bringing them back into compliance has effectively prevented owners from doing so, several industry experts said.

The FCC is permitting the use of the new computerized verification method for AM stations using series-fed radiators. However, towers that are shunt fed or top loaded are ineligible. The commission is requiring verification of the antenna monitoring system every two years.

Nearly every large AM broadcast group in the country supported the rulemaking request, including CBS Radio, Clear Channel Radio, Citadel Broadcast Company, Cumulus Media Inc., Entercom Communications Corp. and Cox Radio. The groups, along with broadcast consulting engineers and equipment manufacturers, formed a coalition to support MoM adoption.

For the new antenna certification rules to affect a broadcaster, the individual must file an FCC 302 application for a new or modified application. If the application is filed specifying MoM, the new rules will apply and magnetic field measurements will no longer be required when doing a full or partial proof, said Ray Benedict, chair of the AM Directional Antenna Performance Verification Coalition and director of spectrum engineering for CBS.

“The coalition is satisfied. We have asked SBE to develop educational programs to help explain the new rules to station engineers and to assist them in gaining the knowledge necessary to comply with the new rules.”

Benedict said he expects some radio stations and broadcast groups to have the internal expertise to do computer-modeling work in house.

Less expensive

“It will save money and it is a simpler, less time consuming procedure,” Benedict said.

Even with MoM certification as an option, the FCC retains the right to ask AM broadcasters for a complete magnetic field measurement if changes are made or parts are replaced in an antenna system, Benedict added.

Barry Thomas, president of SBE and vice president of engineering for Lincoln Financial Media, said, “Licensing under MoM rules will eliminate monitor point readings and will greatly simplify and reduce the cost of all subsequent changes to the array. I believe it would be prudent for all stations eligible for MoM licensing to do so because of the benefits over time.

“As long as there is significant care in the accuracy of the sampling mechanisms, computer modeling is at a level of sophistication such that models can be used more accurately to predict performance of an antenna system than limited field measurements.”

Training broadcast engineers and broadcast engineer consultants on computer modeling, which uses Numerical Electromagnetic Code-4 or MININEC, will be a crucial step in the adoption process, said Jack Sellmeyer, president of Sellmeyer Engineering.

“The process requires qualified engineers trained in the use of Method of Moments calculations. The effort required is not trivial and requires certain skills to obtain accurate results which will stand up to scrutiny,” Sellmeyer said.

The FCC said in its MoM decision that a minority of commenters dissented to the proposal in comments for MM Docket No. 93-177, “An Inquiry Into the Commission’s Policies and Rules Regarding AM Radio Service Directional Antenna Performance Verification.”

Greater Media was the most notable broadcaster to dissent.

In its public comments, Greater Media and Charles A. Hecht & Associates, a broadcast engineering consulting firm, wrote, “while the techniques specified in the coalition’s proposal produce a directional antenna pattern that is ‘reasonably close’ to the authorized pattern, the coalition’s procedures are not adequate in themselves for antenna performance verification. Greater Media advocates adoption of a hybrid method consisting of modeling and a set of field-strength measurements similar to that required for a partial proof.”

Immediate savings?

Despite reports that MoM will result in significant cost savings for AM broadcasters, at least some remain unconvinced that immediate savings will be evident.

“We’ve heard of some compelling estimates because of the hours and hours saved because the time-consuming field measurements are not required,” one industry observer admitted. “Time will tell for sure. The real savings will likely be over time as the total cost of proof of compliance will be much reduced with a properly licensed MoM array.”

Mark Mueller, president of Mueller Broadcast Design, a broadcast engineering consulting firm, said while modeling can help avoid lengthy field tuning and measurements, the field-intensity measurements themselves are not that difficult and should not cost as much as some charge to take them.

“This new method to verify performance introduces additional costs upfront, as well as ongoing certification costs. The big savings seem to be a mirage,” Mueller said.

Mueller, who has worked with directional arrays since 1983, specifically questions the new costs likely to be associated with the re-certification of the antenna monitoring system every two years.

“The station will have to go off the air to re-certify the sample system. This will also most likely require consultant fees and won’t be cheap — probably in the several thousand dollars range,” Mueller said.

“At the end of the day, all that is really saved is the cost of the field intensity measurements, which for most smaller arrays only take a few days under the current rules. MoM antenna modeling is not foolproof.

“A qualified, trained engineer will still have to visit the site with the proper test gear, verify overall construction, verify the antenna sample system, measure the self-impedance of the towers and generate the mutual impedance input data necessary for accurate modeling. Consulting engineer visits are usually expensive and this won’t be an exception.”

Yet at least one consulting engineer disputed that view on cost savings.

“Saving days and sometimes weeks of field measurements is a significant savings for a broadcaster. Not only that, but the validity of internal array measurement techniques for a proof is undoubtedly better than relying on magnetic field measurements in a perturbed environment,” said Ben Dawson, managing partner of Hatfield & Dawson Consulting Engineers.

Dawson said the requirement for sample system measurements every two years is trivial.

“It’s a few hours of work at the most. A reasonable percentage of the techs and contract service providers and virtually all consulting engineers know how to use an impedance bridge and an oscillator/detector,” Dawson said.