Abstract
This article reports on an investigation into data filtering techniques for material characterization using a commercially available WR-15 (50-75 GHz) material characterization kit (MCK). The MCK uses a guided free-space method (operating like the conventional quasi-optical focused beam system) that enables measurement of S-parameters of dielectrics using a vector network analyzer (VNA). Multiple reflections and resonances exist in the MCK and they cannot be readily eliminated by calibration. To minimize these effects on extracting dielectric constant and loss tangent from the measured S-parameters, data filtering techniques (e.g., Savitzky-Golay filter and time-gating) can be adopted. A comparison of the measurement results using these two techniques was carried out on five common dielectric materials. Generally, the results obtained using these two filtering techniques are consistent with each other, and the Savitzky-Golay filter offers better performance in the scenario where the specimen quality is not ideal. Three specimens were also measured using an open resonator (operating at 36 GHz). There is generally good agreement between the results from the MCK and the open resonator.
Highlights
T HE millimeter-wave spectrum has been actively exploited by a range of applications including backhaul and fronthaul for 5G communications, space-borne radiometers for earth remote sensing, automotive radar sensors, and so on.This has driven the demand for accurate characterization of material properties, that is, dielectric constant, ε, and loss tangent, tan δ
There is a good agreement between the results of Savitzky–Golay filter and time-gating, for thicker samples
There is systematic variation with frequency for the time-gating results, for the acetal co-polymer sample, and this does not agree with the expected physical behavior (i.e., ε of homogeneous and noncomposite dielectric materials should exhibit a linear response to frequency [2])
Summary
T HE millimeter-wave spectrum has been actively exploited by a range of applications including backhaul and fronthaul for 5G communications, space-borne radiometers for earth remote sensing, automotive radar sensors, and so on. The open resonators and free-space methods have been most widely used for measurements at millimeter-wave frequencies. Open resonators provide the most accurate measurement of lowloss dielectrics at millimeter-wave frequencies, due to the ultrahigh unloaded Q-factor of the resonator (greater than 200 000 at >100 GHz [2]) This method requires the specimen thickness to be approximately an integral number of half-wavelengths (in the medium of the dielectric) and the specimen diameter to be greater than 5 times the radius of the Gaussian beam, used during measurement, at the beam’s waist. An open resonator (the most accurate method at millimeter-wave frequencies) has been used, for the first time, for benchmarking the measurement results obtained using the MCK. The MCK has low insertion loss (∼1.3 dB, averaged across the waveguide band); there are considerable resonances in both the S11 (or S22) and S21 (or S12) responses These resonances make a direct impact on subsequent measurements of the S-parameters of the samples. The use of time-gating can be undesirable for some measurement scenarios (e.g., when propagation of measurement uncertainty is of interest)
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