## 2 Materials and Methods: The Original Version of the Ex-PdBI W-Band Receiver  
The ex-PdBI receiver consists of a single cryogenically cooled cryostat equipped with two frequency channels: one for the 84-116 GHz band and the other for the 210-248 GHz band [28]. Since the SRT was designed to cover all frequencies up to 116 GHz, only the 84-116 GHz channel is considered. The original version of the system permits the detection of only linearly polarised signals. Figure 1a shows a schematic of the original version of the receiver. The feed system consists of a circular corrugated horn that has a flare angle of 7.42 degrees, an aperture diameter of 12.4 mm, and a length of 39.2 mm [28; 29]. A phase error at the aperture of approximately one-tenth of the wavelength (i.e., approximately 3 mm) and a beam waist close to the aperture is consequently generated by the feed. A rectangular-to-circular guide step transformer is installed downstream from the feed horn. An optical system composed of a high-density polyethylene (HDPE) lens is designed and placed at the aperture of the horn to compensate for the phase error and to optimally position the beam waist. Owing to this form of optics, the antenna beam for the receiver is adapted for the optics of the PdBI telescopes, guaranteeing a sub-reflector edge taper of approximately \(-12\) dB with an aperture angle of \(\pm 5.6\) degrees. Further details about the HDPE optical system and the feed system are reported in [28; 29]. A polarisation grid is installed on the receiver to allow only one linear polarisation (i.e., the horizontal or vertical contribution) to enter the feed, depending on the orientation of its wires. In this manner, the receiver provides a linearly polarised electromagnetic signal at the back end. The ex-PdBI receiver utilises a down-conversion system, with a mixer as the first active element of the chain. The weak 3 mm wave signal \(\
u_{\mathrm{RF}}\) from the radio astronomy source (in the 84-116 GHz range) is added to a relatively strong local oscillator (LO) monochromatic signal \(\
u_{\mathrm{LO}}\) produced locally and coupled into a non-linear element, a superconductor-insulator-superconductor (SIS) mixer (see Figure 1a,b). The output signal at the frequency difference \(\
u_{\mathrm{IF}}=\,\
u_{\mathrm{LO}}-\
u_{\mathrm{RF}}\,\), named the intermediate frequency (IF), preserves the amplitude and phase of the original RF signal. The down-converted IF signal falls in the 1.2-1.8 GHz range. This IF range falls within the 0.1-2.1 GHz baseband of the SRT. The down-conversion operation is carried out using a backshort-tuned single side band (SSB) SIS mixer similar to the one illustrated in Figure 2b [32; 33], which operates at the cryogenic temperature of 4 K. The SSB SIS mixer design includes a rectangular waveguide with a single adjustable non-contacting backshort for the tuning, which achieves a good matching of the SIS junction impedance in the signal band (typically lower side band, LSB, \(\
u_{\mathrm{LSB}}=\
u_{\mathrm{LO}}-\
u_{\mathrm{IF}}\)) in any portion of the 84-116 GHz receiver band, while rejecting the image sideband (typically upper side band, USB, \(\
u_{\mathrm{USB}}=\
u_{\mathrm{LO}}+\
u_{\mathrm{IF}}\)). The achieved image sideband rejection is about 15 dB at the centre of the IF band (around 1.5 GHz). The SIS junction consists of two layers of superconducting metal (niobium) separated by a thin insulator layer of aluminium oxide a few nanometres thick. Typically, the SIS junctions have a size of the order of a few \(\upmu\)m\({}^{2}\) and their integrated thin-film superconducting tuning circuitry sits on top of a quartz substrate, which is itself mounted across a waveguide [31]. Regarding the local oscillator, the receiver was developed to use the Gunn-based local oscillator from IRAM telescope, which exploits the performances of a semiconductor device (Gunn diode) to achieve oscillation at a precise frequency [34].