Quick Determination of Medium Wave Coverage – Application Note 15

Predicting medium wave groundwave coverage is a complex matter, involving such factors as power, antenna height, soil conditions, atmospheric noise and mutual interference from other stations. Often it is desired to approximate coverage in a simplified way to assist in making budget choices, or otherwise judging feasibility between available options. This note describes such a simple system which can be applied with no complex mathematics. Of course, final judgments of coverage should be based on a full engineering review of all relevant propagation models and detailed station design information.

Table I relates the drop-off of field intensity with distance to frequency and soil conductivity. It gives the percentage of unattenuated field at one mile available at the desired distance. This table is the basis of the medium wave coverage estimation system discussed here.

To use, first determine the unattenuated field at one mile for the proposed power and antenna system. Typical values are:

 Second, identify the type ground conductivity in the direction desired. Typical values are:

Third, multiply the unattenuated field by the field strength factor for the desired distance and frequency in Table I. This gives an approximation of the coverage that can be expected from a proposed power, frequency, site and antenna configuration. By computing several scenarios at one reception point, various alternatives can be evaluated. Typical field intensities required for good groundwave coverage in the absence of interference from other stations are:

Example:

It is desired to compare the coverage of a 1000 watt station at 790 kHz and 1/4 wave antenna with that of a 5000 watt station at 1380 kHz, also with 1/4 wave antenna. The signal at 50 miles is required. The path is hilly with assumed conductivity of 5 mS.

From this analysis, we can conclude that the 1000 watt facility would be superior to 5000 watts, and that neither would provide dependable service at 50 miles, unless in an area with very low natural noise, such as in the northern U.S. in wintertime. To provide a quality service at this distance, approximately 5000 watts would be required at 790 kHz, and 75,000 watt at 1380 kHz!

Download Printable Table 1