Dielectric heating in insect control has been studied for almost a century and there is still a gap between the lab research and industrial adoption. In order to refine current techniques, we have presented some studies on physical issues which had long been overlooked in previous studies. Followed our research, we demonstrated a novel approach in microwave insect control.
We built a microwave applicator system for uniform irradiation of small samples at the relatively high, and rarely exploited, ISM frequency of 24 GHz. It features an anechoic chamber, a 24-GHz extended interaction oscillator as microwave source, and a linear-to-circular polarization converter. Simulated and observed radiation patterns as well as preliminary heating test indicate its functionality for our planned insect control experiments and related physics studies. This type of applicator may also be a suitable tool for microwave-assisted research for which irradiation uniformity, controllability, and in situ diagnostics are desired.
We heated insect sample with the applicator systems of 3 different frequencies, including the rarely-used 24 GHz. In compares with well-studied frequencies (2.45 GHz and 27 MHz), 24 GHz heats much faster and thus present its advantages at power saving.
In other words, high-frequencies like 24 GHz are more likely to kill insect in extremely short time scale, which might save the energy cost by 1 to 2 order of magnitude. The shallow penetration of 24 GHz heating is also suitable for treating small object.
We also studied and quantified some of the major causes for heating non-uniformity.
The polarization-charge shielding effect, a major inherent cause of heating non-uniformity, were identified to contribute 18.1% of percentage temperature spread in
microwave heating. Our experiment demonstrated the anechoic chamber and circularly-polarized wave can act as remedies to non-uniformity.
We have also demonstrated the first direct evidence of differential (selective) heating in microwave insect control followed by a quantitative study on the physics involved.
While it occurs much greater at extremely fast heating, differential heating can probably provide an alternative approach to overcome the damage of farm products from heating non-uniformity.
Since these issues are of a common nature in microwave heating, the in-depth understanding of these most persistent difficulty and advantages encountered in RF and microwave heating can hopefully be a useful reference for a wide range of microwave heating applications, particularly in insect control, and our new approach can hopefully fill the gap between the lab research and industrial adoption in a near future.
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Appendix
Appendix. The industrial, scientific, and medical (ISM) radio band
Frequency range Center frequency Bandwidth Wavelength (cm)
6.765 – 6.795 MHz 6.78 MHz 0.3 MHz 4424.78
13.553 – 13.567 MHz 13.56 MHz 0.14 MHz 2212.39
26.957 – 27.283 MHz 27.12 MHz 0.326 MHz 1106.19
40.66 – 40.7 MHz 40.68 MHz 0.4 MHz 737.46
433.05 – 434.79 MHz 433.92 MHz 1.74 MHz 69.14
902 – 928 MHz 915 MHz 26 MHz 32.75
2.4 – 2.5 GHz 2.45 GHz 0.1 GHz 12.24
5.725 – 5.875 GHz 5.8 GHz 0.15 GHz 5.17
24 – 24.25 GHz 24.125 GHz 0.25 GHz 1.36
61 – 61.5 GHz 61.25 GHz 0.5 GHz 0.54
122 – 123 GHz 122.5 GHz 1 GHz 0.27
244 – 246 GHz 245 GHz 2 GHz 0.14
ISM bands follow ITU Radio Regulations, see Wikipedia: http://en.wikipedia.org/wiki/ISM_band