SoftWright LLC software

The Okumura model computes basic median field strength and then adjusts for factors such as terrain, type of area (urban, suburban, open, etc.), terrain slope, etc. from the families of curves, as described in “Field Strength and its Variability in VHF and UHF Land-Mobile Radio Service”, Review of the Electrical Communications Laboratory, Vol. 16, Numbers 9-10, Sep.-Oct, 1968, by Yoshihisa Okumura, et. al. The Okumura adjustments for area type, terrain slope, street orientation, etc., can be included or excluded individually from calculations. Facility and path parameters (transmitter power, directional antenna, beam tilt, earth curvature correction, antenna heights, etc.) are all user-specified to provide maximum flexibility in system design.

The Hata/Davidson model uses developed empirical formulas derived from the many Okumura families of curves. The calculations depend upon which type of environment is present in the coverage area – urban, large city, small/medium size city, suburban or open area. Although Hata’s model requires the inclusion of transmitting and receiving antenna heights it does not contain any of the path-specific corrections included in the more detailed Okumura Propagation model.

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 Egli module is an implementation of John Egli’s methodology published in October, 1957 in the Proceedings of the IRE. This model is derived from measured data out to a distance of approximately 50 miles and over gently rolling terrain with average hill heights of approximately 50 feet. It relies on frequency, distance and heights of transmitting and receiving antennas. It does not rely on localized path elevation data.

In the Bullington model, computed free space field strength is adjusted for obstruction attenuation for knife-edge diffraction based on the method described in “Radio Propagation for Vehicular Communications” by Kenneth Bullington (IEEE Transactions on Vehicular Technology, November 1977). Facility and path parameters (transmitter power, directional antenna, beam tilt, earth curvature correction, antenna heights, etc.) are all user-specified to provide maximum flexibility in system design. Bullington is widely used for link and land mobile coverage and is the best model for air to ground prediction and also in rugged terrain. It computes field intensity values at specified radial, tile or individual locations, based on terrain data for a site and path, transmitter power, directional antenna, etc., using free space field and the Bullington obstruction loss method.

The Rounded Obstacle model is based on Section 7 (“Diffraction Over a Single Isolated Obstacle”) of Tech Note 101(Transmission Loss Predictions for Tropospheric Communication Circuit, 1967, NTIS), treating path obstructions as rounded obstacles as described in Section 7.2. In addition, methods from Section 9 (“Forward Scatter”) are used to compute tropospheric scatter losses. The two loss components may be combined in any of several user-selected methods. Some of the source code for the model was adapted from the program QZGBT used for protecting the RF noise floor at the National Radio Astronomy Observatory (Green Bank, WV) radio astronomy site.

The Shadow Map Module identifies locations that:  1) are shadowed (obscured) by terrain or obstructions;  2) have line of sight, but obstructions within the specified (e.g., 0.5 first) Fresnel zone for a specific frequency, and 3) have clear line of sight with no obstructions within the specified Fresnel zone.  For specified transmit and receive antenna heights, the shadowed areas around a site are clearly identified.

The Carey module uses topographic elevation information to compute height above average terrain (HAATs) from a specified site. This information is used to compute field strength from the f(50,50) and f(50,10) Carey curves from Part 22 of the FCC Rules. Distance to service contours for Cellular Geographic Service Area (CGSA) are also calculated, as well as the calculations described in FCC Report and Order 94-201. The Carey Module also allows calculation of the f(50,90) values based on f(50,50) adjusted according to Figure 5 from R-6406.

The VHF/UHF Reliability module provides a flexible, easy-to-use tool for designing and evaluating link reliability in both UHF and VHF bands. It provides calculations of received signal level and fade margin. Using the flexibility of the RF facilities database in TAP, transmit and receive sites can be evaluated in different combinations of locations and hardware configurations to find the best design for each site and the best overall route for multiple site installations. Several propagation models calculate received field strength and the user can select the calculation that provides the most realistic prediction for the path. Printable summaries of various site equipment configurations and path parameters are available to document the design process and the predicted fade margin of the proposed facilities.

The AutoPath module enables TAP to automate setting up path specifications eliminating manual data entry. The program can then plot a path profile for each specified path, as well as compute the field strength value at the end of the path. Path information can be defined from a single transmitter site, or for a different transmitter site for every path. The path information (end point coordinates, antenna heights, earth curvature, transmitter ERP, etc.) can be entered manually into TAP, or imported from another source, such as a spreadsheet or database.