About Output Requests

Far-Field Observables

Most key performance indicators (KPIs) available in Antenna Placement are far-field observables. For more information about far-field observables, see Farfield Calculation Overview in the CST Studio Suite help documentation. Additional KPIs available in Antenna Placement are described below.

Return Loss

The default Antenna Placement settings do not return output for the return loss. The return loss KPI is derived from the reflection coefficient of port 1, which could fail if more than one port exists.

The return loss describes the amount of power that is reflected at the antenna input because of mismatches between the antenna and the transmission line. A high return loss can reduce the total efficiency or radiation efficiency, or it can deteriorate signal detection. The app calculates the return loss as follows:

R e t u r n L o s s = - 20 \log_{10} (| S 1, 1 |),

where $S 1, 1$ is the reflection coefficient.

Magnitude of Input Impedance

The default Antenna Placement settings do not return output for the magnitude of input impedance. The magnitude of input impedance KPI is derived from the reflection coefficient of port 1, which could fail if more than one port exists.

The input impedance describes an antenna as a load of a transmission line with a given reference impedance. If there are no reflections, the input impedance of the antenna is $Z_{r e f}$ (that is, the reference impedance of the antenna's feeding port). The app calculates the input impedance as follows:

I n p u t I m p e d a n c e = [Z_{r e f} \frac{1 + S 1, 1}{1 - S 1, 1}],

where $S 1, 1$ is the reflection coefficient.

Coverage Efficiency

Coverage efficiency is the percentage of the far-field sphere around an installed antenna that has a gain above a threshold you specify. Antenna Placement determines the coverage efficiency by first simulating an installed antenna's gain (that is, the strength) in the spherical coordinate system. Then, the app uses these results to calculate the coverage efficiency. For example, the polar plot below shows an installed antenna's gain in a section plane of its far-field sphere. The black dots represent where the far-field sphere surpasses the threshold specified (in this case, 0 dB). Sectors of the far-field sphere that exceed the threshold are shown in green; all other sectors are shown in red.

You calculate the coverage efficiency in the section plane using the following equation:

C o v e r a g e E f f i c i e n c y = 100 \times \frac{Θ}{Θ + Φ},

where $Θ$ is the angle of the green sector and $Φ$ is the angle of the red sector. In the example above, $Θ$ is the 120° angle starting at 300° and sweeping to 60°; $Φ$ is the 240° angle starting at 60° and sweeping to 300°. This equation applies to a section plane of the far-field sphere; the app calculates the 3D coverage efficiency by extrapolating this concept.

Antenna Placement can calculate the coverage efficiency of an installed antenna for a sector of the far-field sphere that you specify. This capability is particularly helpful if there are regions that are not of interest to your analysis.

When you request the coverage efficiency, Antenna Placement adds it to the list of key performance indicators (KPIs) that it computes during a simulation.