Broadcast/SMR (FCC Part 73) Propagation Module
The Broadcast/SMR module of SoftWright’s Terrain Analysis Package (TAP) uses topographic elevation information to compute height above average terrain (HAAT) from a specified site. This information is used to compute field strength from the f(50,50) and f(50,10) curves for FM and TV from Part 73 of the FCC Rules. Distances to a specified signal contour are calculated for SMR (specialized mobile radio) by appropriate deration of the Part 73 curves. It also calculates the f(50,90) field or contour for digital television filings in accordance with OET Bulletin No. 69, “Longley-Rice Methodology for Evaluating TV Coverage and Interference.”
The Broadcast (Part 73) and Carey (Part 22) propagation models are based on the pertinent sections of the U.S. FCC Rules and Regulations. Both of these methods are essentially simplified statistical methods of estimating field strength and coverage based only on a station’s effective radiated power (ERP) and height above average terrain (HAAT). Since the terrain information is averaged, neither model takes into account specific individual localized obstructions or shadowing. Also, since the average used for these models only includes the terrain between three (3) and sixteen (16) kilometers from the transmitter site, terrain obstructions outside of this range are ignored. This means that identical results will be calculated whether or not a transmitting antenna has clear line of sight or complete blockage by an obstruction in the first 3 kilometer portion of a path. Likewise, any terrain obstructions beyond 16 kilometers that block the line of sight to a more distant receiving antenna are ignored. The main use for either of these models is for license applications or other submissions to the FCC which specifically require the use of the methods described in Part 73 or Part 22 of the FCC Rules, or other administrative requirements, such as certain frequency coordination procedures.
On the other hand, Bullington, Okumura, and Longley-Rice are more analytical models that consider a number of other factors, such as individual obstructions (either terrain or manmade), terrain roughness, etc. Okumura is often used in urban environments and includes correction factors for various area types, such as urban, suburban, etc. Bullington considers individual obstructions and computes losses, for example, for terrain obstructions, ridges, etc. Longley-Rice is a general model that considers radio horizons and various environmental conditions.