Improved-Sensitivity Resonant Electric-Field Probes Based on Planar Spiral Stripline and Rectangular Plate Structure

The designs of two resonant electric-field probes with improved sensitivity for near-field measurements are proposed and measurements in weak radiated level are conducted. One probe is the resonant monopole probe and the other is named the resonant T-shaped probe with further improved sensitivity. Each of the two probes is fabricated on four-layer printed circuit board using high-performance and low-loss Rogers material (ε
_r
= 3.48 and tanδ = 0.0037), which consists of an LC resonator made up of a planar spiral stripline (inductor) and a rectangular plate (capacitor), a single-stub transformer, and a SubMiniature version A connector. The LC resonator can achieve a specific resonant frequency for a specific narrowband of interest with the pure input resistance. The single-stub transformer is used to transform the pure input resistance to the input impedance of measurement instruments to acquire maximum transmission power. Compared with the referenced broadband probe with the same physical size of tip, the measured |S12| of the resonant monopole probe is enhanced by 9 dB at the resonant frequency. The resonant T-shaped probe proposed has 3.3-dB sensitivity higher than the resonant monopole probe in terms of |S12|. In order to validate the improved sensitivity of the resonant probes, comparisons on the improvement of sensitivity with other probes are also introduced.