Alternate Propagation Models

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Now under the Skytower decision, released in 2010, the Commission has liberalized the use of supplemental methods. Gone is the delta-h requirement. It has been replaced with a simple demonstration that the 70dBu contour distance exceeds the predicted standard distance by at least 10 percent. It is important to note that supplemental showings pertain only to city-grade field strengths, and by extension apply only to prediction of coverage over the city of license or compliance with the main studio rule. The protected contours for the various classes of FM facilities must still be determined through the use of the standard method.

At some point in the not-too-distant future, the Commission is expected to become better equipped to handle supplemental showings without referral to the OET. What that means for future use of alternate methods is somewhat murky, but the hope is that a standard set of study parameters will be released to the engineering community so everybody winds up on the same page. This will of course reduce the necessity of conflicting interpretations, thereby conserving the limited resources available to the staff.

As I previously mentioned, Longley-Rice lends itself to some fairly wide variations in how the model can be tweaked. For instance, at what height should the receive antenna be considered? The curves were developed with a receive antenna height of 9.1 meters, or 30 feet AGL. Obviously a varying receive heights will yield different field strengths. Similarly, what receive antenna gain should be considered? Is 0dBd appropriate, or is a different value, perhaps more representative of mean antenna gain, a better illustration?

By the same token, how should we address the impacts of localized groundcover? Without delving very deeply into the esoteric mathematics of the model, one can intuitively understand that in a very dense urban environment, the field strength would be expected to be lower due to many more objects "soaking" up the signal. Finally, even the atmospheric conditions in a particular locale will have a bearing on the predicted coverage. Coverage in desert environments such as Phoenix is treated differently than in Chicago, with the latter environment resulting in a larger footprint.

I'll share two coverage examples illustrating a comparison between the FCC standard method and Longley-Rice. The first illustrates the coverage of one of the FM stations in Chicago. The colored shading underneath illustrates the Longley-Rice predicted signal levels, with FCC derived contours overlaid. The second illustration demonstrates coverage for a station in the area of the Rockies. The effect of terrain is quite obvious in this instance.

Chicago plot
Rockies plot

Comparison of the FCC standard method and Longley-Rice for a station in Chicago (top) and in the Rockies.

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