Characterization of ALMA Band 2 Orthomode Transducers at Cryogenic Temperatures

We established a system to measure the insertion and return losses of Atacama large millimeter/submillimeter array (ALMA) Band 2 orthomode transducer (OMT) at cryogenic temperatures (4 K). The system comprises a measurement instrument at room temperature with a vector network analyzer (VNA) and two frequency extenders for the 67–116 GHz band, and a cryogenic system with vacuum feedthroughs and thermally insulated waveguides (TIWGs). To accurately characterize the OMTs at 4 K, we applied S-parameter calibration at the output ports of the TIWGs under cooling condition with multiple vacuum and cooling cycles. In this calibration, it is important to ensure the stability of the measurement system, including the drift of the extender, due to the multiple cycles required. Hence, we evaluated and improved the measurement repeatability of the transmission characteristics of the cryogenic system (feedthroughs and TIWGs) and performed the S-parameter calibration of the extenders before every cryogenic calibration and measurement. Thus, we could verify the accuracy of the measured insertion and return losses of the OMT at 4 K. The measurements also revealed a practical issue in OMTs fabricated using split block techniques. Our simulations suggest that the existence of a gap in the split planes significantly degrades the characteristics of the OMT; this gap is most likely due to thermal shrinkage at cryogenic temperatures. Our results show the importance of performing accurate cryogenic measurements and provide important knowledge for designing and producing OMTs for high-frequency bands.