Glossary

The technical terms of the comparison, explained.

FDTD (finite-difference time-domain): Solves Maxwell's equations step by step in time on a grid of small cells. One run gives the response over a whole frequency band. Well suited to PCB antennas and broadband structures; memory grows with the volume of the grid.
FIT (finite integration technique): A time-domain method close to FDTD, used by the CST time-domain solver. Like FDTD it gives a broadband result from one run.
FEM (finite element method): Divides the volume into tetrahedra and solves at one frequency at a time. Adaptive meshing refines where the field changes fast. Very accurate for complex 3D shapes; a frequency sweep needs several solutions or interpolation.
MoM (method of moments): Only the metal surfaces (and sometimes the dielectric interfaces) are meshed, not the air around them. Efficient for metal antennas and large open structures.
2.5D MoM (planar): Method of moments for layered planar structures: the substrate layers are assumed infinite. Fast for circuits, but less accurate for an antenna whose ground plane or board edge matters.
MLFMM / FMM (fast multipole method): An acceleration of the method of moments that makes very large problems (an antenna on a car or an aircraft) solvable.
PO / SBR (physical optics, shooting and bouncing rays): Asymptotic high-frequency methods for structures much larger than the wavelength: radar cross-section, antenna placement.
Thin-wire MoM (NEC): Method of moments where the antenna is made of thin wires. Very fast, but no dielectric substrate: not for PCB antennas.
S11 (return loss): The part of the power sent to the antenna that is reflected back, in dB. Below -10 dB, less than 10 % is reflected: the usual target in the operating band.
VSWR (voltage standing wave ratio): Another way to express the mismatch: 1 is perfect, 2 corresponds to S11 ≈ -10 dB.
Input impedance: The complex impedance (R + jX) seen at the feed of the antenna. The matching network transforms it to 50 Ω.
Smith chart: A chart of the reflection coefficient on which impedances and the effect of series and shunt components are drawn. Used to design the matching network.
Matching network: Two or three inductors and capacitors (L, π or T network) between the radio and the antenna, so that the antenna presents 50 Ω at the operating frequency.
Bandwidth (-10 dB): The frequency range where S11 stays below -10 dB.
Radiation efficiency: Radiated power divided by the power accepted by the antenna. The rest is lost in copper and substrate. Total efficiency also includes the mismatch loss.
Gain and directivity: Directivity: how much the antenna concentrates power in its best direction compared with an isotropic antenna (dBi). Gain = directivity × radiation efficiency.
Radiation pattern: The gain in every direction, shown as 2D cuts or a 3D surface; computed from the near field by a near-to-far-field transformation.
Mesh: The division of the model into cells (FDTD) or elements (FEM, MoM). Too coarse: wrong results; too fine: long simulations. Automatic meshing is a key usability difference between tools.
PML / absorbing boundary: A boundary layer that absorbs outgoing waves so the simulated space behaves like open space.
IFA / MIFA (inverted-F antenna): The most common PCB antenna: a quarter-wave trace above the board edge, with a short to ground and a feed. MIFA meanders the trace to save space.
GPU acceleration: Running the solver on a graphics card. Time-domain methods (FDTD, FIT) gain the most, often 10 times or more.
Perpetual vs subscription licence: Perpetual: you own the version you bought, maintenance (updates) is optional. Subscription or annual lease: the tool stops when you stop paying.