Abstract: The experimental results show that flat sample cell not only can be used to measure ESR spectrum of aqueous sample in a TE₀₁₁ mode cylindrical cavity,but also gives a good cavity quality factor Q and high quality ESR signals. The observed signal height was 3.2 times as large as the maximum signal height that can be obtained when a capillary tube with optimum size was used in the same cavity, and 3 times as large as the signal height when a flat cell was used in a TE₁₀₂ mode rectangular cavity with waveguide size. In this paper,a theoretical analysis is given.

Key words: electron spin resonance(ESR), sensitivity of ESR spectrometer, TE₀₁₁ mode cylindrical cavity, flat sample cell