A chemically etched D-band waveguide orthomode transducer for CMB measurements

This study presents a prototype D-band waveguide orthomode transducer (OMT) fabricated using chemically etched brass platelets. This method offers a fast, cost-effective, and scalable approach for producing waveguide OMTs above 100 GHz, making it well-suited for current and future Cosmic Microwave Background polarization experiments, where large focal planes with thousands of receivers are required to detect the faint primordial B -modes. Chemical etching has already demonstrated its effectiveness in manufacturing corrugated feedhorn arrays with state-of-the-art performance up to 150 GHz. Here, we evaluate its applicability to more complex structures, such as OMTs. We designed a single OMT prototype operating in the 130–170 GHz range, fabricated by chemically etching 0.15 mm-thick brass plates, which were then stacked, aligned, and mechanically clamped. Simulations based on metrological measurements of the OMT profile predict return losses below -10 dB, isolation better than -30 dB, and transmission around -0.5 dB. The measured transmission and isolation, however, is around -1.5-2 dB and <- 20 dB, respectively. Further simulations show that the degradation in the transmission is related to defects and roughness along the etched profile ( RMS ≃ 3 μ m), which is a typical and unavoidable effect of chemical etching. The discrepancy in isolation, instead, could arise from a slight rotation ( ~3 °) of the polarization angle within the measurement chain. Our results show that chemical etching is a fast, low-cost, and scalable technique for producing waveguide OMTs with state-of-the-art performance in terms of return loss and isolation. However, at frequencies above 100\,GHz the transmission coefficient may degrade due to the mechanical precision limitations of chemical etching.