Antenna Simulation — Gain, Radiation Pattern & Impedance Matching

How does FEM or FDTD compute the radiation pattern of an antenna?

Solve the near-field EM problem on a finite computational domain, then apply a near-field to far-field (NFTF) transformation. Equivalent surface currents on a closed surface enclosing the antenna are projected to the far field via radiation integrals, giving gain versus angle without extending the mesh to far-field distances.

Why is antenna simulation more complex than circuit analysis?

Antennas interact strongly with their environment — a handset antenna performs differently near a human head; a vehicular antenna is affected by the car body. The radiation boundary must be handled carefully to avoid reflections. Accurate simulation requires full 3D modeling of the antenna plus nearby structures.