S-parameters of NaCl solutions ranging from 0 to 0.1 mol/L were measured over a frequency range of 1 to 4500 MHz using vector network analyzers and a five-pin SMA probe and analyzed using RF circuit simulators. An equivalent circuit of three parallel sub-circuits and R, C, and L components was synthesized. Each circuit component value was derived from the measured and simulated \({\hbox {S}_{11}}\) and Y. The measured \({\hbox {S}_{11}}\) and Y show a small circle plot outside the admittance grid above 2 GHz on a Smith chart, and the circle’s diameter decreases with NaCl concentration. The circular feature of the \({\hbox {S}_{11}}\) and Y above 2 GHz is due to serial resonant frequency between 2.4 and 2.7 GHz and parallel resonant frequency between 3.1 and 3.3 GHz. These values are simulated and calculated when the susceptance jB of the admittance Y equals zero. The measured, simulated, and calculated resonant frequencies are consistent. Measuring \({\hbox {S}_{11}}\) and Y higher than 2 GHz is essential to analyze the role of polar molecule \({\hbox {H}_{2}\hbox {O}}\) , \({\hbox {Na}^+}\) , and \({\hbox {Cl}^-}\) ions, and the validity of the equivalent circuit design, derived R, C, and L component values, and the highly sensitive five-pin SMA probe configuration.
