Suppose you have computed two contours as shown below, and the existing blue contour must not be overlapped by your proposed 22dBu contour shown in red:

If your (red) contour was computed with an ERP of 100 Watts, and there is clearly prohibited overlap, you may want to consider reducing the ERP of your facility toward that contour (either by an actual power reduction, use of a directional antenna, etc.).

First, you can use TAP to determine the "worst case" of the overlap, such as the azimuth from your site where the overlap is greatest with the protected contour.

You can use the "Calculate distance between two points" button for this:

When you click the button, a form is displayed to select the points.

Use the Fac Lookup button on the first point to get the coordinates of the facility under consideration (the one where the ERP is to be reduced).

Then double-click the latitude or longitude box for the second point so you can locate the point on the map with the mouse. Click a point on the protected (red) contour where the overlap appears the worst. Then click the Draw Path button on the TAP Graphic Window Calc form to draw a line marking the overlap.

Now click the Preview Field Strength button. The Field Strength Preview form is displayed:

Be sure the Fixed Facility information is for the facility you are using, and the propagation model parameters are set to the model you want to use (the same model used to compute the contour overlap).

Then click the map at the point where the proposed contour must not overlap the protected contour:

The program will take a few seconds to compute the field value, and a red "X" or a green asterisk will be displayed (as explained in the Field Strength Preview FAQ). To see the actual field value computed, click the "More" button on the Preview form:

The form will expand to show the details of the calculation, including the field strength value near the bottom of the listing.

In this example, the value at the protected contour is 40.49dBu, almost 19dB higher than the permitted 22dBu. Now the question is, how much must the ERP be reduced to ensure that the 22dBu contour does not overlap the protected contour.

Since we need 19dB reduction, the power P reduced from the original 100 Watts using the formula:

-19 = 10 LOG (P / 100)

Solving this gives the value for P of approximately 1.25 Watts.

Recomputing the contour using that value demonstrates that the 1.25 Watt contour (shown in green) does not overlap the protected contour.

This example included several examples of rounding the intermediate results to be conservative and ensure the new (green) contour would not overlap the protected (blue) contour.

In come cases it may be important to repeat this exercise for several overlap points to be sure you avoid the overlap while maintaining your desired coverage.