The types of input values that generate a circular coverage plot can create either a very small circle or a large circle. (TAP performs numerous checks of the values you enter to try to warn you of unusual or incorrect parameters, but checking the following information is a good idea if you suspect the contour computed as a circle is not correct.)

Small Contours

If the coverage contour you computed is plotted on a coverage map as a very small circle right around the transmitter site, it indicates that the computed field values are very low, and the distance to the contour is therefore very small. Check the following input parameters:

• Transmitter ERP and units: Be sure the Effective Radiated Power is entered correctly, and 100Watts (100W) is not set as 100milliWatts (100mW), etc.
• Mobile facility antenna height: Be sure the Mobile Facility Database record used for the study has the antenna height correctly set. For example, if the correct height is 6ft AGL (Above Ground Level) and the mobile height is set as 6ft MSL (above Mean Sea Level), the mobile will be 'buried" in most parts of the country.
• Mobile facility required field value: The contour level computed is based on the "Required Field" value for the mobile facility. This value can be computed in the Mobile Facility Lookup, or from the Utilities menu in TAP. This is the field value in dBu that produces the manufacturer's specified receiver input (typically in dBm, uV, etc.) under the circumstances you specify (receiver antenna gain, line loss, body or building loss, etc.) If this value is abnormally high, the resulting contour will be very small, showing that only a small area receives this high signal level.

Large Contours

If the coverage contour you computed is plotted on a coverage map as a large circle, typically out to the distance of the radials you specified, check the following input parameters.

• Transmitter ERP and units: Be sure the Effective Radiated Power is entered correctly, and 100Watts (100W) is not set as 100kiloWatts (100kW), etc.
• Mobile facility antenna height: Be sure the Mobile Facility Database record used for the study has the antenna height correctly set. For example, if the correct height is 6ft AGL and the mobile height is set as 600ft the computed field will be unusually high, since the clearance of the signal path will be above most of the terrain.
• Mobile facility required field value: The contour level computed is based on the "Required Field" value for the mobile facility. This value can be computed in the Mobile Facility Lookup, or from the Utilities menu in TAP. This is the field value in dBu that produces the manufacturer's specified receiver input (typically in dBm, uV, etc.) under the circumstances you specify (receiver antenna gain, line loss, body or building loss, etc.) If this value is abnormally low, the resulting contour will be very large, showing that only a large area receives this low signal level.
• Radial Length: For the analytical propagation models that use terrain elevation values (Bullington, Longley-Rice, Okumura, etc.) and not just the height above average terrain (HAAT) along the paths (Broadcast, Carey, etc.), the contour is computed by interpolating the computed field strength values along each radial. If the contour value is beyond the end of a radial, the distance is set to the radial length. If this happens on all of the radials, a circular contour plot results. For example, suppose you want to plot the 16dBu contour (assuming you use the Required Field function described above to be sure this is the correct value). If you compute the field values along each radial out to 60km, but the computed values along the radial out to that distance never fall below 40dBu (because of the power and good site location of the fixed facility), the program will not find the 16dBu value along the radials, so the contour will be placed at the end of each radial, or 60km. This would indicate that the coverage for the fixed and mobile specifications you used covers the entire 60km radius around the transmitter site. However, if you need to see the actual predicted limits of the contour, you can run the study again and specify a larger distance value out from the transmitter site. You can use the TAP Single Point Field program to compute field values from the fixed facility on individual radials to get an idea of the distance range of the field level of the contour. In this example, using the Single Point Field program to compute the field at 80km, 100km, 120km, etc., on both the radial with the best expected coverage would you an idea of the range of the study you want.
• Contour percentage value: As described above, the contour value is interpolated from the field strength points along each radial. The "percentage" value specified for the calculation affects the distance computed for the contour along each radial. For example, if a percentage of 95% is used, then a few points along the radial below 16dBu (in the example) can be offset by a large number of higher field level points. This is typically a more realistic representation of the coverage using a contour. If the contour is drawn at the first point along the radial where the field drops below the specified value (such as behind an isolated hill, or in a small valley along the radial), the contour distance on that radial will be unrealistically restricted if numerous points beyond that hill or valley have a much higher value. Using this percentage value gives you control over the interpolation process. However, if you set the percentage value too low (such as 50%, or 10%), the interpolation becomes almost meaningless, since the contour can be placed much farther than is realistic. Suppose the computed field at 30km is 16dBu, and every point beyond that distance is much lower than 16dBu. If the percentage value is set at 90%, or 95%, the contour would be placed within a few points of the 30km distance. But if the percentage is set at 50%, the contour could be placed as far as 60km out on the radial, since 50% of the points (in the first 30km) are above the value, and 50% (from 30 to 60km) are lower. Typically, percentage values for the contour calculation should be 90% or above. Special requirements for interference contours or other reasons to set lower percentage values should be accomplished using the reliability and confidence settings in the particular model, such as Longley-Rice or Okumura.

Copyright 2001 by SoftWright LLC