One might assume that in the high technology age in which we presently live, the prediction of the area served (coverage) by an FM station or FM translator is highly accurate.  Not so.  While not as bad as weather prediction, the data available has quite a bit of uncertainty.

    The Federal Communications Commission in cooperation with industry has developed two sets of charts which serve as guide lines in predicting the expected area to be served.  These charts are used in applying for a construction permit (CP) for an FM Broadcast Station.  They are not required for an FM Translator Station applying for a CP but can still be used to predict translator coverage. A slider is used with the charts to predict coverage.

    One chart (FCC 73.333 Fig. 1) estimates the field strength expected at 50 % of the potential receiver locations to be served 50% of the time. The other chart (FCC 73.333 Fig. la) estimates the field strength expected at 50% of the potential receiver locations for at least 10% of the time. For reasonable results it is required that all points to be served are equal or lower than the average ground elevation of the translator location. For example if there is a mountain or other obstacle between the translator location and the area to be served, the charts are meaningless.

    The best way to show the use of the charts is to go through a few examples.  For these purposes we will use 5 element and 10 element yagi antennas.  A typical 5 element yagi antenna has a power gain over a dipole of  7.5 dB and a 10 element yagi 9.5 dB.  7.5 dB corresponds to a power gain of 5.6 and a 10 element to a power gain of 8.9.

    This information tells us how much the antenna will boost the radiated power of the translator.  In the case of a 1 Watt translator, the power will be 1 X 5.6 = 5.6 Watts ERP (effective radiated power) for a 5 element yagi and 1 X 8.9 = 8.9 watts for a 10 element yagi.  For a 10 Watt translator the power will be 10 X 5.6 = 56 Watts for a 5 element yagi and 10 X 8.9 = 89 Watts for a 10 element yagi.  It is not quite this good in practice because part of the power will be lost going up the transmission line from the translator to the antenna.  For this discussion we will assume that we are radiating 8 Watts for a 1 watt and 80 Watts for a 10 Watt translator.

Example 1 (See Fig. 2)

    The height of the antenna above average terrain is 200 feet.  The effective radiated power is 80 Watts.  We wish to know how far away from the translator location we can pick up 20 mV/M (microvolts/meter) of signal.

    Line up the right side of slider with the 200 foot line.  Move the slider vertically until 80 Watts (left side of slider) aligns horizontally with the 40 dB mark (left vertical axis on graph) (This shown in fig. 2 as line AA).

    Line BB is the 20 microvolts/meter field strength.  On the right edge of scale 20 uV/M is about 30 miles.  This is the signal one could expect at 50 percent of the locations 50 percent of the time providing the receiving antenna had a clear shot to the translator transmitting antenna.  This is not an uncommon situation for a ranch home that is not in a populated area.  20 uV/M is a good signal.

    For a community it would be wiser to use 100 uV/M.  The reason for the need for a higher signal is the attenuation caused by trees, buildings, etc.  This value is shown by line CC.  The distance has been reduced to approximately 16 miles.

Example  2 (See Fig. 3)

    Conditions are the same except the antenna height has been increased to 500 feet above average terrain.  Move the slider so that the right edge lines up with the 500 foot mark.  Move the scale vertically until 80 Watts lines up with 40 dB on the left.  Again this is represented by line AA in Fig. 3. Line BB indicates that the distance to the 20 uV/M point is now approximately 41 miles and the distance to the 100 uV/M point is approximately 23 miles.

Example 3 (See Fig. 4)

Antenna height 500 feet above average terrain.

Radiated power 8 Watts.

Line AA indicates 8 Watts to be in line with 40 dB mark.

Line BB shows the 20 uV/M field strength range to be approximately 27 miles and line CC shows the 100 uV/M field strength to be approximately 14 miles.

50-10 Charts

    The same procedure applies to the 50-10 charts.  In most cases these charts have little value except to predict what could happen.  Such would apply to a cochannel interference study.

Conclusions

    As near as we know the preceding is the best information currently available for predicting coverage.  The charts clearly demonstrate that antenna height is more important than translator power. Strong cochannel signals can also reduce the range of the translator especially with the cheap AM/FM "boom boxes" since they are susceptible to overloading which generates intermodulation components.

    Useful information on FM contours calculations can also be found at the FCC site: http://www.fcc.gov/mmb/asd/bickel/curves.html.
 
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