Planar near-field measurements of specular and diffuse reflection of millimeter-wave absorbers

TL;DR

This paper presents a planar near-field measurement method for accurately characterizing both specular and diffuse millimeter-wave reflections of absorber materials, crucial for reducing far sidelobes in cosmic microwave background telescopes.

Contribution

The authors developed a novel planar near-field measurement technique to assess diffuse and specular reflections of millimeter-wave absorbers, improving precision over traditional methods.

Findings

Measured reflectance of five absorber samples from 70 GHz to 110 GHz.

Demonstrated higher precision and reduced standing wave effects compared to horn-to-horn methods.

Identified differences in angular response and polarization effects among materials.

Abstract

Mitigating the far sidelobes of a wide field-of-view telescope is one of the critical issues for polarization observation of the cosmic microwave background. Since even small reflections of stray light at the millimeter-wave absorbers inside the telescope may create nonnegligible far sidelobes, we have developed a method to measure the reflectance of millimeter-wave absorbers, including diffuse reflections. By applying the planar near-field measurement method to the absorbers, we have enabled two-dimensional diffuse-reflection measurements, in addition to characterizing specular reflection. We have measured the reflectance of five samples (TK RAM Large and Small Tiles and Eccosorb AN-72, HR-10, and LS-22) at two angles of incidence in the frequency range from 70 GHz to 110 GHz. Compared with conventional horn-to-horn measurements, we obtained a consistent specular reflectance with a higher precision, less affected by standing waves. We have demonstrated that the angular response and diffuse-to-specular reflectance ratio differ among various materials. The measurements also imply that some absorbers may affect the polarization direction when reflecting the incident waves.