Accurate permittivity characterization has attracted a lot of attention in various areas. Resonant characterization methods are well-known for their accuracy, but they are restricted in very narrow frequency ranges, and thus, they are normally not recommended to be used for dispersive or high-loss materials. Transmission line characterization techniques are outstanding for being inexpensive, accurate, and broadband, but the algorithms are often complex to perform. This paper proposes a fast, simple, and accurate broadband permittivity characterization algorithm, which is mainly suitable for millimeter-wave applications. It combines a general line–line method and a closed-form algorithm, extracting the complex permittivity of the material under test (MUT) without the need for calculating any intermediate parameters. Validation measurements on de-ionized water in the frequency range from 140 to 220 GHz are in very good agreement with the literature data, which successfully indicates that the proposed algorithm is reliable and accurate for millimeter wave permittivity characterization.

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Though microwave ablation (MWA) technology has shown obvious advantages, it has the problem that the ablation zone is not sufficiently localized. As a result, normal tissue might be inevitably damaged during the ablation process. To solve this problem, the comprehensive MWA theory is firstly analyzed in this paper, and then a frequency-adaptive ablation approach is proposed and verified by simulations. Accordingly, a frequency-adaptive MWA system for optimizing the ablation zone is developed. As the tissue's electromagnetic properties change with the increasing of temperature during the ablation, the proposed system is designed to monitor the working performance of the antenna in real-time and to automatically select the optimum working frequency, simultaneously. The ablationcontrolling system is designed as an integrated controller, so that the overall size and the fabrication cost are obviously reduced. Preliminary validation experiments are performed on the system, which shows that the proposed system is capable of delivering stable power output from 0.5 GHz to 3 GHz and creating a highly localized ablation zone. The proposed MWA system is expected to be a dedicated device for the treatment of tumors.

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