Fig. 2: Schematic view of the parallel detector.

a One channel of the parallel detector, the sample is inserted into a solenoid coil, which is tuned and matched (T& M), and connected to the transmitter (Tx) or receiver (Rx) via a switch. RF denotes radio frequency, PA denotes the power amplifier in the Tx path, LNA labels the low-noise amplifier in the Rx path. b The geometry of the 4-coil array simulation in COMSOL, the 4 samples are shown in blue. c Circuit model of the parallel detector with S-parameters, the amplifiers are substituted by matched loads assuming that all the amplifiers are tuned and matched to the cable’s characteristic impedance (50Ω). Free induction decays are denoted by FID. The symbols a and b indicate the forward and reverse waves respectively. S_C, S_M, S_CM denote the S-matrices of the coil array, the T& M network, and their combination, respectively.