Propagation Methodology

FCC Propagation Curves

The FCC curves were created through a combination of the free-space equations and actual measurements, which augmented the equations with real world experience. Initially, the curves were available only as a set of graphs. However, with the advent of computers, the Commission employed its staff to translate the curves to a set of digitally stored tables, which could be interpolated by machine. With the input of desired signal level, radiated power, and effective antenna height the curves will give the user an accurate estimate of the distance from the antenna where the signal will exist. The curves can also be used to determine signal level at a distance with the input of power, antenna height and distance from the antenna. Proper use of the curves requires that the input variable "antenna height" be calculated to represent the antenna's height above "average terrain". The FCC specifies certain methods for determining this value. When topographic maps are employed, the Commission requires that at least 50 points be taken from 3.16 to 16 kilometers (FM) and then averaged to produce the height above average terrain. The computer implementation of the curves will generally take terrain samples at one/tenth kilometer intervals. The FCC's method is excellent at representing coverage over somewhat smooth or rolling terrain, however the methods tend to break down in places where the terrain is rugged. Since the method simply averages the terrain elevations, inaccuracies are introduced when the terrain varies widely or when it varies significantly at points beyond the method's 16-kilometer cutoff.

Longley-Rice Model

In the mid-sixties, the National Bureau of Standards published Technical Note 101. P. L. Rice, A. G. Longley, A. Norton and A. P. Barsis authored this two-volume propagation treatise in the course of their work at the Institute for telecommunications Sciences and Aeronomy at Boulder, Colorado. The concepts expressed in these documents were incorporated into a series of computer routines that came to be known as the "Longley-Rice Model". This model has recently been employed by the Commission to determine the new DTV allocation scheme. It has now become the standard alternative prediction method. Going well beyond the FCC curves, the Longley-Rice method considers atmospheric absorption including absorption by water vapor and Oxygen, loss due to sky-noise temperature and attenuation caused by rain and clouds. It considers terrain roughness, knife-edge, (with and without ground-reflections), loss due to isolated obstacles, diffraction, forward scatter and long-term power fading. The model and our V-Soft Communications implementation require the following inputs for analysis based on multiple point-to-point paths:

Frequency (20 - 20,000 MHz)

Transmitter antenna parameters:

Transmitter antenna height (above mean sea level - meters.) Transmitter antenna height (above ground - meters.) Transmitter power. Transmitter antenna pattern.

Receiver antenna height (above ground - meters)

System antenna polarization (vertical or horizontal)

System Ground Conductivity (mhoS/m)

.001 = Poor Ground
.005 = Average ground
.020 = Good ground
5.000 = Sea water
.010 = Fresh Water

System dielectric constant (Permitivity)

4.0 = Poor ground
15.0 = Average ground
25.0 = Good ground
81.0 = Sea and fresh water

System minimum monthly mean surface refractivity (Adjusted to sea level.)

200 to 450 (available from map, 301 N-units is default.)

Climate Code:

1 = Equatorial
2 = Continental sub-tropical
3 = Maritime Subtropical
4 = Desert
5 = Continental temperate (default for U.S. continent)
6 = Maritime temperate
7 = Maritime temperate overseas
Qt = (Time variability) The percentage of time the actual path loss is equal or less than the predicted path loss (Standard broadcast coverage = 50%)
Ql = (Location Variability) The percentage of paths (all with similar characteristics) whose actual path loss is less than or equal to the predicted path loss. (Used with area mode only.)
Qc = (Prediction Confidence or "Quality") The percentage of the measured data values the model is based on that are within the predicted path loss. (Standard broadcast = 50%, DTV = 90%.)

V-Soft Communication's implementation of Longley-Rice predicts received signal strength level at some 264,000 points. Our programs Probe and Terrain-3D allow instantaneous manipulation of these points to produce numerous graphic representations of the coverage pattern. The user can choose any of the pre-defined signal level representations or input a user-defined signal level. Costal features, cities, political boundaries and streets to the individual road level are available for plotting.

Probe 3 - Terrain 3-D