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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:

Power 1\4 Wave 1/2 Wave
250 watts 95 mV/m 120mV/m
1 kW 190 240
5 kW 425 530
10 kW 600 750
25 kW 950 1190
50kW 1340 1680
100 kW 1900 2400
The above assume 120 radials, 1/4 wave long in the ground system.

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

5000 mS/m 
40 
10
Seawater
Loamy, organic soil, plains
Hilly farmlands
10 mS/m
4
1
Fresh water
Rolling uplands, forests
Mountains, sandy soils, rocky soils

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:

City business or factory areas
City residential areas
Rural areas - thunderstorm seasons
Rural areas - quiet seasons
Intermittent service
10-50 mV/m
2-10
1-3
0.25-1
0.1

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.

 

For 790 kHz: Unattenuated field is 
Table I factor @ 50 miles 
Approximate intensity @ 50 mi.
190 mV/m 
x .0024=
0.46 mV/m
For 1380 kHz: Unattenuated field is
Table I factor @ 50 miles
Approximate intensity @ 50 mi.
425 mV/m 
x .00061=
0.26 mV/m

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

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