Tailoring Antenna Focal Plane Characteristics for a Compact Free-Space Microwave Complex Dielectric Permittivity Measurement Setup

Jayaram Kizhekke Pakkathillam,Jayaprakash Poojali,C. V. Krishnamurthy,Balamurugan T. Sivaprakasam,Kavitha Arunachalam

doi:10.1109/tim.2020.3013997

Abstract

This article presents a compact precision free-space microwave measurement setup with a choice of three dielectric lenses to tailor the antenna focal plane characteristics for extracting complex dielectric permittivity of small samples. Custom designed spot-focusing horn antenna pairs were used to achieve a compact setup with antenna separation distance, 2 f l :4λ c -8λ c and focal spot size, f s :1λ c -1.5λ c , where λ c is the wavelength at center frequency. Using the compact free-space setup, relative complex permittivity (e' - je'') was extracted over 8-12 GHz for lowand high-loss dielectrics with lateral dimensions, 3.3λ c × 3.3λ c and 10λ c × 10λ c . For large materials under test (MUTs), i.e., 10λ c × 10λ c , measurement accuracy in dielectric constant, Δe'% was <; 0.65% and <; 1.14% for low- and high-loss dielectrics, respectively. For smaller MUTs (3.3λ c × 3.3λ c ), Ae'% was <; 0.89% and <; 2.29% for low- and high-loss MUTs, respectively. The error in loss tangent (Δtanδ) varied over 0.002-0.016 and 0.015-0.056 for large (10λ c × 10λ c ) and small MUTs (3.3λ c × 3.3λ c ), respectively. For large MUTs, biconvex lens pair with the smallest f s (1λ c ) and f l (4λ c ) among the three lenses yielded the best accuracy in dielectric constant (e') due to tight field focusing at the focal plane. The plano-convex lens pair yielded the best accuracy in loss tangent (tanδ = e''/e') for large MUTs due to slow variation in the phase of the local plane wave. By tailoring antenna focal plane characteristics, a compact free-space setup that is 6×-10× smaller than the classical setup for handling MUTs that are 1/5th of the size used in classical setup is demonstrated without compromising the measurement accuracy.

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