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Compelling reasons to discount the 'bat avoidance of RF radiation sources' theory.

Posted by Leonard on 08 May 2008 at 04:57 GMT

After reading this paper, I have reservations about the efficacy of radio frequency (RF) field strengths (of the order of those quoted in the paper), being directly responsible for deterring bats through biomass heating. Firstly, RF frequencies in the order of 2 – 4 GHz are not optimum for heating liquids and other materials that have a high fluid content, even though this is the frequency range used by most if not all, domestic microwave ovens. Accordingly, we can infer that the impedance of flesh (with high water content) in this frequency range is significantly different from that of free space although I do concede that bat wings are likely to have significantly less water content than the body of a bat.

Using the time averaged RMS RF field strength threshold of 2 V/m quoted in the paper, if we assume (incorrectly, but as a worst case example) that the bat and its wings at the frequencies in question has the same RF impedance as free space and is, therefore, perfectly matched for maximum power transfer to the bat, further assume generously that the cross-sectional area (CSA) of a bat (with a 0.3m wing span and a chord of 0.1 m) is 0.03 sq m. and assume that this maximum CSA is orthogonal to the RF signal for maximum ensonification at all times, the average RF power absorbed by the bat is 0.32 mW. Neuweiler, in “The Biology of bats”, (2000), Oxford University Press, (P. 32), quotes data that indicate the power efficiency of flight in bats is somewhere in the range of 10% to 28%. For Plecotus auritus (one of the species used in this paper), Neuweiler provides a diagram (P. 30, Fig 1.12), which indicates that for a (nominal) flight speed of around 6 m/s, and also for hovering flight, the mechanical power required for flight is about 0.1 W. Assuming that flight power efficiency of this bat is (conveniently) 23.8% and that flight metabolic power losses are completely converted to heat, the excess heat power that must be dissipated to the air from the body and wings of the bat is 0.32 W. Recalling that the maximum (matched) heating effect of the threshold RF field strength on a 0.03 sq m bat, is 0.32 mW, it can be seen that the additional thermal load due to RF heating that must be dissipated to the atmosphere is about one–thousandth of that which the bat generates and dissipates during powered flight!

Even if we assume a much greater RMS RF field strength of 10 V/m, the RF power absorbed by the bat is still only 8 mW, or one-fortieth of the bat’s thermal load due to flight. As the RF impedance of the bat is not likely to be the same as that of free space at the frequencies in question, the power levels actually absorbed by the bat are likely to be significantly less than the maximum levels calculated here as an exercise!

Although the data and calculations I’ve provided cannot be directly related to the situation of any particular bat (including Plecotus auritus and the other species discussed in the paper), they provide a compelling reason to discount or at least seriously question, the efficacy of the “increased thermal loading” theory proposed by Nicholls and Racey to explain bat avoidance of RF radiation sources. Other mechanisms should be sought to explain the observed results.