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Misconceptions About Computer Modeling of AM Directional Arrays

Making the AM Band a Better Place to Be

Back in September, after a 19-year wait, the FCC approved the much-anticipated rules that will permit computer modeling of AM directional arrays as a means of directional antenna performance verification. The NPRM came out early last spring and generated a lot of hubbub at the NAB spring show and through the summer. Now, the rules have been approved by the FCC and are slated to become effective in February.

This is a very good thing for AM broadcasters, and yet there is much misunderstanding and misinformation out there.

One misconception is that all directional AM stations will have to model their arrays. Not true.

Antenna modeling simply represents an option for broadcasters. Stations can continue operating under the terms of their existing licenses as before. But if there is a condition at a station (such as an out-of-tolerance monitor point or tower work above the base insulator) that would otherwise require pattern 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.

X-Y-Z Graphical Display of a Three-tower Top-loaded Antenna Model Using the MBPRO Modeling Program. Top-loaded arrays are eligible for modeling under the new FCC rules. In my view, that beats the heck out of walking and driving all over the countryside making proof measurements! But if a traditional proof is a more comfortable way to go for a particular station owner or engineer, that remains an option as well.

Another misconception is that arrays with unequal height towers are not eligible for the modeling option. The new rules provide for modeling of arrays with unequal height towers, provided that they are series fed.


I should note here that 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.

Still another bit of misinformation that I have heard from several places is that arrays using top-loaded towers cannot be modeled. Untrue. Top-loaded towers are very much eligible for the modeling option, again provided that they are series fed.

The new rules require reference field intensity measurements at three points on each of the null and lobe radials. Some have misunderstood this to mean that arrays licensed pursuant to the modeling option will still have monitor points. Again, not true. Monitor points will be a thing of the past for such stations. The reference field intensity measurements are filed along with the model, but they do not have licensed maximum values as monitor points do.

A requirement of the new rules is that modeled arrays have their sample systems recertified every 24 months. This requirement has produced no little angst among those who do not have a good understanding of what it entails.

A network analyzer, while a handy (and way cool) tool to use for this purpose, is not required. All you have to do is check the current/voltage/phase linearity of the base sample devices (TCTs) or the consistency of the impedance of the sample loops, and then check the sample lines for electrical length and loss. All that can be done with a bridge, oscillator and detector. 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.”

And you don’t have 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.

Besides doing away with monitor points, perhaps the biggest advantage of the modeling option for AM station owners and engineers is the cost savings that this 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.

The new modeling option also does away with 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 FCC fine and clearing up interference caused by the out-of-adjustment directional pattern. With a low-cost way to make their out-of-adjustment arrays compliant and clean up interference, I believe that a lot of AM station owners will be willing to make the relatively small investment to model and retune. This, I think, will make the AM band a much better place to be.