Research On The Near-Field Holography Measurements Of Slant-Axis Terahertz Antennas

Terahertz wave has an important and irreplaceable role in the field of astronomical research because of its advantages in studying the cosmic microwave background radiation,interstellar dust and medium,and the fine molecular and atomic spectral lines.The strong absorption of terahertz waves by the Earth's atmosphere limits the excellent terahertz atmospheric window to the rare dry areas of high altitude and extremely low temperature on the ground.It is a key technology with significant challenges to achieve the surface accuracy requirements of terahertz telescopes up to micrometers under harsh site conditions.Near-field radio holography,which features the advantages of high measurement accuracy and convenience,is one of the most commonly used surface alignment methods for millimeter and submillimeter wave radio telescopes.The slant-axis mount is more adaptable to the extreme site environment and can provide better protection for the overall mechanism of the telescope.However,the unique rotation mechanism of the slant-axis terahertz antenna will introduce additional error factors in the process of holography measurements.In order to achieve the surface measurement accuracy of micrometer-level required by terahertz antennas,it is necessary to develop high-precision measurement theory and error correction methods for the slant-axis antennas.Gives the facts mentioned above,in this paper we study the near-field radio holography of slant-axis terahertz antenna,the main results of this study are as follows:1.A high-precision near field to aperture field integral transformation is adopted,and the vector method of three-dimensional rotation matrix is proposed to accurately correct the residual path length and parallax effect;2.The non-uniform sampling and polarization mismatch of the holographic measurement system caused by the beam rotation of the slant-axis antenna has been analyzed in detail,and the calculation formulas of power loss and phase error due to polarization mismatch are derived;3.A W-band wide beam horn has been designed to provide illumination for the antenna under test with an edge taper of less than-6 dB within a range of ±64°.A far-field measurement system is constructed for accurate characterization of the phase pattern of the horn;4.The main-focus and Cassegrain-focus near-field holographic measurement experiments has been conducted,with a repeatability accuracy of better than 2.0 ?m RMS.The surface measurement results of the same antenna with both near-field radio holography and photogrammetry are in good agreement with each other.The near-field holography has been successfully applied to the surface measurement and alignment of the slant-axis terahertz antenna.The research results of this paper are expected to be directly applied to the high-precision surface alignment of the 5 m Dome A Terahertz Explorer for Antarctica.And it can provide key technical foundation reserves for the construction of larger-diameter millimeter and sub-millimeter wave telescopes in the future.