Review of HF and coupled problems

When starting the discussion of High Frequency (HF) problems, we first need to define what our scope of interest is. For example, 1 MHz is a high frequency for power engineers but a rather low frequency for microwave specialists. In electromagnetic simulations we will consider as HF those frequencies, for which the size of the analysed object is comparable to the wavelength. It is difficult to set up specific constraints for the size-to-wavelength ratio in HF, but a rough estimate would be above 0.05 (so that the quasi-static approach cannot be used) and below 200 (so that quasi-optical approach becomes inaccurate).