Coverage Results Question

Coverage Results Question

Q: When I run a coverage study with TAP, the results don’t look like what I expect. What should I do?

A: A coverage study involves numerous settings in TAP that can have dramatic effects on the computed results. TAP gives you the flexibility to adjust these various parameters in order to model the actual conditions based on your engineering judgment.

The process of setting up and plotting a TAP Coverage study is discussed in other FAQ articles. Be sure to review those to better understand these suggestions.

Terrain Elevation Data

Be sure you are using the highest resolution topographic elevation data available for your area. Low resolution data (such as 30-second or even lower resolution) does not include the detail of elevation values (especially in rough terrain). The elevation values between the known points in the topographic data must be interpolated, and high and low elevations in the actual terrain can be missed, leading to inaccurate field strength calculations.

Site Elevation

If you are specifying a site elevation for the transmitter site, be sure the value is accurate and consistent with the topographic data. If you use the “Interpolated” option for the site elevation it will be estimated from the available elevation data. But if you enter a “Specified” value that is radically different from the surrounding elevation, that value will be used and can adversely affect the field strength calculations. If the specified site elevation is much higher than the surrounding terrain elevation data values, the computed field strength information will be too high. If the specified site elevation is significantly below the surrounding terrain values, the field calculations will be too low.

Transmitter Antenna Height

Be sure the transmitter antenna center of radiation is at the correct value. Using an incorrect site elevation (described above) and/or an incorrect antenna height above ground level (AGL) can actually result in an antenna center below some of the surrounding terrain, severely affecting the field calculations.

Transmitter site location

Be sure to specify the correct coordinates and datum for the transmitter site. If the actual site is at the top of a hill, but the coordinates you specify locate the site down the side of the hill, the resulting field calculations will be lower than expected.

Mobile Antenna Height

Be sure the mobile antenna height is correctly specified. (An antenna height of zero will cause a warning message to be displayed.) Entering the wrong units (such as 1foot above ground instead of 1meter) will affect the computed results.

Effective Radiated Power

Be sure you specify the correct Effective Radiated Power (ERP). If you change the antenna specification to a different pattern, the ERP value may not be automatically updated for the major lobe gain of the new antenna (depending on how the antenna is added to the facility information.) Be sure the ERP is correct for the Transmitter Power Output (TPO), antenna gain, and various losses associated with the transmitter facility.

Antenna Gain

If you use the antenna gain to compute the ERP, be sure the gain is specified in the correct units (dBd, dBi, or Relative Field). Different antenna units are discussed in the article "Field Intensity Units."

Antenna Pattern

Don’t expect the coverage area to be exactly the same shape as the antenna pattern. This is discussed in an FAQ article "Coverage with Directional Antenna."

Required Field Settings

Be sure you have the correct settings for the Required Field Strength value when you set up a contour calculation or plot a radial or tile study. If the field strength value to be shown is too low, the coverage area will be unusually large. If the field strength value is too high, the area will be too restricted. Very low field strength values may be too low to be of actual use with the mobile equipment. The process of determining and using appropriate required field values is discussed in FAQs articles "Required Field Values" and "Required Field Values FAQ."

Field Strength Units

Be sure you understand and use the correct field strength units. TAP field strength values are computed in dBu, which are different from dBm, uV, or other units. Field strength and receiver sensitivity units are discussed in the article "Field Intensity Units."

Contour Calculation Distance

Be sure the “Distance from Transmitter” values include the area of interest for your study for a contour calculation. If the distance values are too low the contour may be shown as a small circle around the transmitter site. This is discussed in an FAQ article "Circular Coverage Contours."

The circular contour may also be a symptom of Required Field Value used for the contour value set too low.

Contour Calculation Percentage

The contour calculation percentage is discussed in an FAQ article "Contour Interpolation Calculation." If the contour percentage value is set too high, the computed contour will be unrealistically restricted. For example, a contour value of 99.9% will result in a contour that extends only as far as the field strength values that meet the specified required field value on each radial. This results in useable coverage areas that are not included in the contour (as described in the FAQ).

Propagation Model Parameters

Be sure the parameters for the propagation model you have selected are appropriate for your application. The various propagation models are described in an FAQ article "Propagation Comparison."

For example, setting the variability percentages for Longley-Rice calculation at very high values may significantly lower the predicted field strength values. These settings for the Longley-Rice model are described in an FAQ article "Longley-Rice Variability Values."

Support

If you still have questions about the results of a coverage study you have run with TAP, you can send the Task files to SoftWright support for a more detailed analysis. The files will be in the WORK folder under the folder where TAP is installed, and the files associated with a particular Task will include the Task ID (assigned by the program when the Task was set up) in the file names, such as:

T0000001.INI

A0000001.INI

E0000001.INI

R0000001.DBF

S0000001.TXT

The actual files that are created depend on the type of study and other parameters.

Search SoftWright Website