High frequency (HF) electromagnetics is concerned with the generation and propagation of electromagnetic energy in free space, together with its interaction with dielectric or metallic media. The numerical simulation of HF electromagnetics involves solving Maxwell's equations for the electric and magnetic vector fields. What differentiates HF from low-frequency (LF) electromagnetics is the fact the HF radiation propagates in free space, and not via the material volume of a conductive material. The term HF is misleading, and should be more correctly referred to as full wave analysis, as all of Maxwell's equations are solved. The equations solved for LF electromagnetics are the same, but with some simplification.
Generally speaking HF simulations are required when the wavelength is similar or smaller that the geometric dimensions of the structure. Or, as a very rough guide, HF electromagnetic simulations typically cover frequencies above 500 kilo Hertz (kHz), i.e. radio, microwave, Infra red and optical frequencies (and higher!).
Low Frequency Electromagnetics can be either a static or quasi-static approximation of High Frequency electromagnetics. LF electromagnetic simulations solve simplified forms of Maxwell’s equations (e.g. neglecting displacement current). The approximations are true for situations when the working frequency is either, zero (statics) or, when the wavelength is much larger than the geometric dimensions of structure. There is no definite cross over frequency between LF and HF, considering a solid conductor, at static or low frequencies the current flows in the bulk of the material, but as the frequency increases the current travels closer to the conductors surface and into the surrounding dielectric region. At high frequencies, such as microwave, only a small fraction of the energy travels in the volume of the conductor. At this stage the conductor is often referred to as a waveguide. It is technically more rigorous to define LF Emag as either Static or Quasi-static.