Discussion About the Ratio Method for Measuring Millimeter Wave Absorption by Biological Entities

An attempt to measure millimeter wave absorption spectra of E. coli was performed with a commonly used ratio method. The influences of water, which absorbs millimeter wave strongly, and the thickness of a sample holder were greatly reduced in our measurement compared with former reports. Some resonance-like absorption spectra were observed, but the resonance was found not from the absorption by E. coli after carefully examining the measurement. The reasons giving rise to the resonance-like artifacts were discussed. Proposals for making better use of the ratio method are given.