Wideband measurement of the phase deviation and time-domain response of an open fire

This letter presents an extension of the free-space method for the dielectric characterization of Eucalypt litter fire using the measured transmission phase shift of a propagating signal over a frequency range of 5-40 GHz. This method is especially suitable for a broadband, routine, nonintrusive, and accurate evaluation of fire dielectric properties from the measured transmission coefficient (S₂₁). A 72 cm x 47 cm rectangular brick burning area prepared with Eucalypt ground litter was equipped with four K-type thermocouples at different heights to measure flame temperatures, which ranged from 283-865 K. The measured phase shift of an electromagnetic signal transmitted through the Eucalypt fire varied from 12.2 to -166.7° at 5.35 and 39.74 GHz. From this phase shift, the calculated real part of the relative permittivity of the fire was between 0.877 and 1, with the imaginary part from 0.02 to 0.09. The relative permittivity of less than unity confirms the formation of plasma in the combustion zone of fire, and shows good agreement with previous research. However, in this letter, the combustion zone region is defined with significantly higher accuracy, and characteristics are evaluated over a much broader frequency range. The extracted parameters are important for understanding of the reaction of electromagnetic waves in wildfire environments and can be used to design radar systems for forest fire detection.