Back-Projected Cylindrical Modal Filtering for Digital Holographic Reconstruction Phase-Less Near-Field Measurement

In this letter, the cylindrical modal filtering is presented for digital holographic reconstruction (DHR) phase-less near-field (NF) measurement. The DHR phase-less NF facility can reconstruct the complex field of the antenna under test (AUT) by measuring only the intensity of the interference field on the AUT and reference antenna in one NF scanning plane. In addition to the retrieved AUT field, some unwanted items also exist in the reconstructed field. Hence, advanced filtering method is essential to extract the retrieved AUT field from the reconstructed field. In previous work, it is achieved by the back-projected (BP) filtering. In this work, the BP modal filtering is utilized to further improve the accuracy of the DHR phase-less NF measurement. To enhance the adequate modal separation between the desired and unwanted terms, the reconstructed field is firstly BP to the aperture plane, then represented with orthogonal cylindrical modes. Finally, the AUT field is extracted by twice modal filtering to remove the disturbances on two orthogonal directions. The simulation and experiment results indicate that compared with the results by the BP filtering, the accuracy of the DHR phase-less NF measurement has been improved 50% in valid angle by adopting the BP modal filtering.