In a long-awaited and much publicized action, the FCC recently enacted new rules that will allow AM broadcasters to use computer modeling for performance verification of many directional antenna systems.
The new rules give broadcasters some new options and provide for a much lower cost means of tuning and proofing an AM antenna. This represents a new paradigm in AM antenna work. To borrow a popular advertising phrase, “This is going to change everything.”
However, since the proposed rules first hit the radar screen of the trade press and message boards, there has been a good bit of misinformation floating around.
Even the release of the Second Report & Order back in September did not clear most of this up. As a result, there is much confusion and unnecessary worry throughout the AM broadcast community.
The AM Directional Antenna Performance Verification Coalition, which proposed the new rule, wishes to put these worries to rest by clarifying what the new rules and procedures mean for broadcast station owners, operators and engineers.
First off, antenna modeling represents an option for broadcasters. It is not mandatory for station owners to have their arrays modeled. Stations can continue operating under the terms of their existing licenses as before.
But if there exists a condition, such as an out-of-tolerance monitor point or tower work above the base insulator that would otherwise require adjustment and either a full or partial proof of the array, the option now exists to instead construct a model of the array, calibrate the sample system and adjust the array to the model-indicated parameters. In these days of high gas prices, that sure beats driving a bunch of radials! But if a traditional proof is a more comfortable route, that remains an option as well.
Only series-fed (i.e. insulated-base) towers are eligible for the modeling option. This rules out skirt-fed (“folded unipole”) elements and shunt-fed towers using a slant wire. Arrays using other than insulated base series-fed towers will have to stick with the old proof method.
Unequal height towers are eligible for the modeling option, provided that they are series fed.
Top-loaded towers are also eligible for the modeling option, again provided that they are series fed.
Monitor points will be a thing of the past for stations licensed pursuant to the modeling option. Instead, some reference field strength measurements are made with the modeling option. These measurements are filed along with the model, but they do not have licensed maximum values as monitor points do.
Recertification of the sample system is required every 24 months for stations employing the modeling option. This consists of the same measurements and tests made initially during the array tune-up, namely checking the current/voltage/phase linearity of the base sample devices (TCTs) or checking of the consistency of the impedance of the sample loops, and checking the sample lines for electrical length and loss. Also once every 24 months, the reference field strength measurements must be repeated. Again, these aren’t monitor points, so a higher field strength at a point than one filed with the license application does not constitute a “violation.”
Finally, it is not necessary to file an FCC Form 301 to employ the modeling option in most cases. The station license can be modified for eligible stations with a Form 302-AM.
Savings are real
AM station owners and engineers should recognize the cost savings that the modeling option represents.
Most if not all the variables associated with the old way of doing things can be eliminated, leaving a fast, fixed-cost means of tuning up and “proofing” a directional array.
Rather than days, weeks or even months of trial and error adjustments and measurements, the modeling and adjustment process can be completed in a couple of days. Instead of days or weeks of walking and driving radials and making field measurements, and instead of countless hours documenting the measurements, with the modeling option as soon as the array is adjusted to the model parameters and three field measurements are made on each pattern minima and maxima radial, you’re done. You can file the 302-AM and go home.
With this new option, gone are most of the excuses for having an out-of-adjustment array.
For a fixed sum, most arrays can be retuned using a model, eliminating the likelihood of a big fine and clearing up interference caused by the out-of-adjustment directional pattern.
That, we believe, will make the AM band a decidedly friendlier place.
The author is chair of the AM Directional Antenna Performance Verification Coalition, director of spectrum management for CBS and a past president of the Society of Broadcast Engineers.