XF performs a surface integral over the fields near the boundary of the FDTD space and it applies a near to far field transformation in order to arrive at the radiated fields. This is an application of the surface equivalence theorem and the surface integral requires a reference point.
Consider the two projects in the first image. Project A has a half-wave dipole centered in the FDTD simulation space, and project B has a half-wave dipole offset λ/8 from the center. A 2-D far zone sensor is defined around the azimuth (XY) plane of the dipole in both projects and three simulations are run:
- Project A is simulated with the reference point set to the center of the space (co-located with dipole A).
- Project B is simulated with the reference point set to the center of the space (λ/8 offset from dipole B).
- Project B is simulated again with the reference point offset from the center of the space by λ/8 (co-located with dipole B).
The second image shows the far zone phase for each simulation. When the reference point is co-located with the dipole, a constant phase is computed, but when the reference point is offset λ/8, the phase has a difference of 45° (138.3° - 93.3° = 45°) at 90° (along the Y axis). At 0° (along the X axis) there is no difference in the far field phases.
The Far Zone Reference Settings are accessed by right-clicking on the Far Zone Sensors item in the Project Tree. The settings consist of an editor across the top of the Geometry window where users can select a Mode option, which applies to all of the Far Zone Sensors in the project.