The Talk-Back locations are typically shown with a plot of radial or tile studies. In these plots, the Talk-Back margin is computed internally, and the value is used to display the locations in the radial or tile study with Talk-Back coverage.

For contour coverage plots, you can use the Talk-Back Margin utility to determine the contour level that will show the area for Talk-Back in the system.

For example, suppose you compute the Talk-Out coverage for a particular Fixed Facility base station and Mobile Facility record. The study is defined from the Area Coverage Setup form:

When you run and plot the study, the contour for the talk-out area is shown:

The Talk-Out Margin function is selected from the Utility menu:

Talk-Back calculations are computed in TAP by comparing the "base TX to mobile RX" system with the "mobile TX to base RX" system. The net difference between the systems, or the "talk-back margin" is used to determine the talk-back area based on the computed values for the talk-out study.

The TalkBack System Margin form facilitates the margin calculations:

There are several options for filling in the values on the form:

• You can enter transmitter Effective Radiated Power (ERP) and the receiver Required Field values manually.
• You can use the Compute button for each value to display the forms to calculate the values. For the ERP values the forms enable you to enter Transmitter Power Output (TPO), and gains and losses to compute the ERP. For the Required Field values, you can enter receiver sensitivity and gains and losses to compute the field necessary for receiver operation.
• You can use the Facility Lookup buttons to create, edit, or select records from the Fixed Facility and Mobile Facility databases. The appropriate values for ERP and Required Field will be filled in the TalkBack System Margin form.

The Print button on the form can be used to create a simple ASCII document with the data from the form:

When the Talk-Back level is determined based on the ERP and Required Field values for the base station and mobile unit, a new contour can be computed to represent the Talk-Back area.

In the example above, the Talk-Back area will be represented by the 21dBu contour. You can use the Contour from Computed Field function in the Coverage menu.

The Contour from Computed Field function uses the radial field strength values already computed for the study to interpolate a new contour value.

You will be prompted to select the radial study to use for the new contour. Be sure to select the same study computed for the Talk-Out study.

Note that the Task will probably include the original radial study, as well as one or more contour studies previously computed from the radial values. Be sure to select the radial study to compute the new contour.

A warning message will be displayed if you select an contour study.

After the radial study is selected you will be prompted for the value of the contour to compute. In this example, you would enter 21.00 dBu for the new contour.

The contour will be interpolated in much less time than recomputing all the radial values.

You can then plot the new (Talk-Back) contour as any other TAP contour. Use the File-New Map menu:

In the Coverages section of the Map Plot Setup form, click the Add button, and select the Task and the contour to plot:

When the contours you want to plot are listed, click the Plot button:

The Talk-Out and Talk-Back contours can be plotted on the same map. In this example, the red contour represents the Talk-Out area, and the blue contour represents the Talk-Back area.

This plot illustrates a couple of interesting factors for the contour coverage calculated with TAP:

1. The blue (Talk-Back) contour includes a uniform arc to the north. This usually indicates that the specified contour and percent interpolation value went to the end of the computed radials. The actual coverage probably goes beyond the arc shown by the contour, but since the contour is interpolated from the computed radials, the length of the radials set in the Area Coverage Setup (in this example, 50km) is the maximum limit for the computed contour.
2. The blue (Talk-Back) contour covers a larger area than the red (Talk-Out) contour. This will depend on the values of the two systems. In this example, the mobile system losses (such as body or building losses) result in a very high mobile Required Field value. The very low required field value for the base station makes the Talk-Back system more efficient than the Talk-Out system. As a result, the Talk-Back coverage area is larger than the Talk-Out area.

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