It is crucial to correctly predict the S-matrix with plasma and set the optimal impedance matching device in the ion cyclotron resonance heating (ICRH) antenna system design. In this paper, a hybrid circuit/3DLHDAP code to verify the S-matrix measurements in the presence of plasma and optimize the load-resilient conjugate-T circuit for Large Helical Device (LHD) ICRH antennas has been developed and benchmarked. The variation of S-matrices for handshake form (HAS) and field-aligned-impedance-transforming (FAIT) antenna systems with density, magnetic field and coupling distance during heating obtained by the code's simulations agrees with that of with the LHD ICRH experiments. The mutual coupling of toroidally aligned HAS antennas is larger than that of poloidally aligned FAIT antennas over a wide range of densities. When the density and coupling distance increase, under a magnetic field on the magnetic axis of 2.75 T and 1.0 T, within a certain density change range, at the minimum voltage position with vacuum injection, the change rule of the antennas' Sa_minV_11 and Sa_minV_22 with density is opposite to that with coupling distance, which means that under certain conditions, adjusting the coupling distance may make up for the S-parameters changes caused by plasma density variation, keeping the minimum voltage position fixed, and may make impedance matching easier to achieve during long-pulse operation. Based on obtaining the S parameters, conjugate-T circuits for the HAS and FAIT antennas are designed with the hybrid circuit/3DLHDAP code, which can keep the reflection coefficients low without controlling impedance matching device over a wide range of plasma parameters region. The related results in this paper may provide some guidance for the high-power long-pulse operation of the ICRH antenna system on the fusion device.