Standards of FM Allocation and Interference


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In the reserved part of the band, contour protection under Section 73.509 is king. There, whatever can be shoehorned in while meeting the coverage, directional antenna, and contour protection requirements can in practice be authorized. This allows for many more stations to exist in this portion of the band, but also gave rise to some bizarre theoretical directional patterns in the 2007 filing window. However, as you approach the demarcation point between the two band segments, both situations may need to be considered. Within three channels of 92.1MHz, reserved band facilities have to meet spacing requirements to facilities on 92.1MHz and higher. To their fellow NCE facilities below 92.1MHz, contour protection must be employed.

But there's more

The other main piece of the puzzle is the pesky intermediate frequency situation. To make things simpler, facilities separated by 53 or 54 channels must meet minimum spacing requirements. There are some very limited grandfathered exceptions to this; however, in general if you have an ERP of greater than 99W, you must take into account facilities on these channels.

Although you can use alternate propagation methods to demonstrate coverage in a more broad sense following the Skytower decision, all of this interference and overlap stuff is still based on the standard FCC model, the mechanics of which we have previously discussed. Perhaps that may someday change, however, by limiting showings to the standard model, much uncertainty and interpretation can be eliminated, thus making the job of the staff easier.

One other allocation item of interest worth discussing is the Raleigh Waiver. This concept, available only to NCE facilities, allows a station to receive contour overlap, while not causing it. It applies only on second- and third-adjacent channel situations where the interference area is vastly smaller than the gain in the coverage area. While this can obviously enhance coverage of your facility, be aware it is not a one-sided deal, and the other station can also potentially overlap, so be sure to have your consulting engineer consider that situation before heading in that direction.

To round out the discussion from this month, two examples are provided. The first illustrates a situation where two actual fully spaced facilities wind up resulting in prohibited contour overlap. The second illustrates a contrived contour overlap situation, and how the interference area by the FCC model is not necessarily the same as the overlap area.

Two fully spaced facilities can have actual interference overlap

Two fully spaced facilities can have actual interference overlap


Actual contour overlap does not match the FCC model

Actual contour overlap does not match the FCC model





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