Numerical Accuracy and Airbox Size: A Comparison of FEM to MoM and Analytical Results for Dipole Antennas

When simulating antenna problems using the finite element method (FEM), it is necessary to truncate the three-dimensional open region surrounding the antenna (so-called airbox), and apply boundary conditions at the edge of the computational domain such that the problem resembles that of radiation into free space. Generally, it is advised that the distance between a radiating surface and the simulation boundary be some minimum fraction of a wavelength, depending on the type of boundary condition being used. While per-forming FEM simulations of electrically small antennas, we found that the radiation pattern and efficiency can vary significantly with airbox size, even when this size met or exceeded the recommended minimum, and even when a very fine mesh density was maintained in the vicinity of the antenna. Further investigation has shown that this is the case for conventional antennas as well.