[[Image:fdtd_lec1_14_gridplane.png|center]]
==1.7 Defining Project Observables==
[[Image:fdtd_lec1_15_fieldprobe.png|right]] Project observables are output quantities that you would like to compute at the end of an FDTD simulation. By default, an FDTD time marching scheme does not generate any output data unless you define one or more project observables before you start a simulation.
[[Image:fdtd_lec1_16_fieldsensor.png|right]] [[Image:fdtd_lec1_17_radiationpattern.png|left]] Field Probes
The simplest observable is a Field Probe, which is used to record the field values as a function of time at a specific point inside the computational domain. To define a field probe, right click on the <b>Field Probes </b> item in the “Observables” section of the navigation Tree and selec <b>Insert New Observable… </b> In the Field Probe Dialog, select X from the dropdown list labeled Direction. This means that your probe will record the X component of electric and magnetic fields. Enter the point (5, 5, 75) as the <b>Coordinates </b> of the field probe. Click the OK button of the dialog to accept the changes
<b>Field Sensors </b>
Field sensors are are used to visualize the near fields of your structure on a plane parallel to one of the three principal planes. The field sensor planes extend across the entire computational domain. To define a field sensor, right click on the <b>Field Sensors </b> item in the “Observables” section of the Navigation Tree and select <b>Insert New Observable… </b> In the Field Sensor Dialog, enter the point (0, 0, 0) for <b>Coordinates </b> and select <b>X </b> from the dropdown list labeled Direction. This means that your field sensor plane will be the YZ plane, which passes through the dipole antenna. We would like to display the fields in the frequency domain at 1GHz. Accept the other default settings in the dialog box, and select <b>OK </b> to continue. A new entry Sensor_1 is added to the Navigation Tree, and the field sensor is now represented in the project workspace by a purple plane across the computational domain.
<b>Radiation Patterns</b>
To plot the radiation patterns of a radiating structure, you need to define a far field observable. A radiation box has to be established that encloses all the radiating objects. The electric and magnetic fields on the surface of this box are used to calculate the far field. By default, the radiation box is defined 0.1 free-space wavelength away from the bounding box of the geometry. To define a far field observable, right click on the <b>Far Fields </b> item in the <b>Observables </b> section of the Navigation Tree, and select <b>Insert New Radiation Pattern… </b> In general, you can accept the default values, unless a special case is being analyzed. The radiation box appears as a cyan or light blue box around your physical structure.
<b>Port Definition</b>
For calculating the port characteristics of the dipole antenna such as S parameters and input impedance, you need to set up a port. To do so, right click on <b>Port Definitions </b> under the <b>Observables </b> section of the Navigation Tree, and select <b>Insert New Port Definition… </b> By default, since you have only one source, it is assigned as Port 1. Accept the default values for PORT_1 and click <b>OK </b> to accept these values.
[[Image:fdtd_lec1_18_portdefinition.png|center]]