Q: What is the knife-edge loss value shown on the path profile window?

A: As you move the mouse cursor on a profile plot, the computed knife-edge loss at the location of the cursor is displayed at the bottom of the form.

For example, at the mouse location at the peak located at approximately 26 miles from the transmitter site on this profile, the computed loss is -3.97dB:

(Note that the vertical position of the mouse cursor has no effect on the knife-edge calculation. The program computes the horizontal location, finds any terrain obstacle or other obstruction at that distance, and does the calculation for the loss introduced by the path at that point for the line of sight and Fresnel zone shown.)

In this example, moving the mouse to the peak located at 33 miles shows a knife-edge loss of 4.10dB, and at 35 miles, 7.15dB. (You can see the correlation between the loss value and the Fresnel zone penetration and line of sight blockage.)

Note that as you move the cursor along the path, the value changes. The program considers each elevation point along the path as if it was a knife-edge. To get the best results, you should be sure the cursor is positioned on the peak elevation in a particular area to get the knife-edge calculation for that point.

The knife-edge loss calculation is based on the clearance of the line of sight, computed as a ratio of the first Fresnel zone, and then applied to Figure 10 of Bullington

The knife-edge loss calculations are base on a reflection coefficient of zero (0). The slight gain effect shown for clearance between 0.5 and 1.0 of the first Fresnel zone is ignored.

Copyright 1999 by SoftWright LLC