Dielectric properties Dielectrics property Dust Engines Finite element method Gasoline Gasoline particulate filter Mass determination Microwave cavity perturbations Mixing rules Particulate filters Perturbation techniques Radiofrequencies Radio‐frequency Soot Soot mass determination Substrates dielectric properties finite element method (FEM) gasoline particulate filter (GPF) microwave cavity perturbation mixing rule radio‐frequency (RF) soot mass determination