On the use of arbitrary radiation patterns as field-sources for FDTD simulations

For tasks like antenna integration on large platforms or electromagnetic compatibility engineering, accurate and efficient simulation of the interaction of electromagnetic radiators with a complex, electrically large environment is required. Using the finite difference time domain (FDTD) method, the radiating sources are often too complex to be modeled directly within a large scenario. Also, the internal geometry and the materials used inside commercial antennas are often unknown. Therefore, an equivalent expression for the source is required. We present a field source condition similar to the total/scattered field approach, which excites a radiator's (near) field, based on the field representation in terms of spherical waves. While the expansion coefficients for the spherical wave base can be determined most easily from the radiator's far field (given by, e.g., measurements or antenna data sheets), the spherical wave representation can be used to compute the radiator's field everywhere else. The application of this principle to the FDTD method is presented, and as an example, the radiation pattern of a circularly polarized wireless LAN antenna is used to obtain the antenna's near field, which in turn is applied as field source for an FDTD run. The radiation pattern computed numerically by the FDTD method utilizing the new field source compares well with the data-sheet pattern of the antenna.