Radiowave Propagation Measurements and Prediction in Bushfires

Australian vegetation is fire-prone. Every year, wet and dry sclerophyll forests of Western Australia, southeastern Australia and grassland ecosystems of the northern part of the continent are subject to high intensity fires. The sclerophyll vegetation contains up to 2.71 % of the element potassium. The element exists in plants’ organic matrix: attached to the oxygencontaining and carboxyl functional groups; in aqueous form such as potassium (K) ions surrounded by water; and as discrete particles in dried plant parts. Theoretically, temperature in the conflagrations can be as high as 2000oC. During the high intensity bushfires, potassium atoms and salts are released from the plant structure as it crumbles into the combustion zone where the species are ionized. Up to 20 % of the potassium present in plants is ionized in a bushfire environment. During suppression of the threat, high frequency (HF) ultra high frequency (UHF) radio communications systems are in constant use by suppression crews in firegrounds. Despite their use, HF and very high frequency (VHF) radio communications are reported to be failing in extreme bushfire conditions. The reports of radio communication failure tend to be anecdotal and therefore warrant an investigation. This study aims at carrying out field and laboratory radio wave attenuation and phase shift measurements at HF to X-band frequencies in moderate intensity fires. Very high intensity bushfires often spread very fast and change direction rapidly therefore it is unsafe to set up equipment for measuring attenuation and phase shift in the fires. Consequently, numerical experiments were used to study radio wave propagation in very high intensity fires. Propagation measurement data at radio wave (HF-VHF) frequencies through fire are scarce and that which is available lacks precision. It is also the purpose of the study to produce attenuation and phase measurement data at these frequencies. The field and laboratory measurements were carried out using a Radio Wave Interferometer (RWI) and Vector Network Analyzer (VNA HP 8277C). RWI uses the same principles as Microwave interferometer (MWI) except that RWI works at radio frequencies. Electron density and momentum transfer collision frequency in moderate intensity bushfire plumes were estimated from the attenuation and phase shift measurements.