Vegetation fires are slightly ionised gaseous medium. Omnipresent alkali
metal species in plant's organic structure are the main source of thermally
produced electrons in the fires. In the flames, electron-neutral particle
collisions dominate other modes of particle interaction. The collision
regime absorbs some of the incident energy when the fire is illuminated with
electromagnetic waves. The rate of electromagnetic wave absorption in the
vegetation fires has implications on the safety of fire-fighters. During
wildfire suppression, radio communication blackout at vhf/uhf has been
experienced. This may be partly due to thermal ionisation in the fire. In
the experiment, the extent of ionisation in vegetation fires is measured
using a 2-port vector network analyser. X-band microwaves are caused to
propagate combustion zones of eucalyptus bark and guinea grass fires with
maximum temperatures of 1114 and 1054 K respectively. Alkali content in the
vegetation fuel was different. Measurements show maximum ionisation in
flames produced from guinea grass, which had almost twice much potassium as
that of eucalyptus bark, to be 2.63 × 1016 m−3 while that produced
in eucalyptus bark flame was 1.46 × 1016 m−3.