## 2 Experimental Setup  
CEB represents SINIS (SIN--superconductor-insulator-normal metal) junctions Al/Al\({}_{2}\)O\({}_{3}\)/Al-Fe/Al\({}_{2}\)O\({}_{3}\)/Al with a normal metal tiny nanoabsorber made of aluminum with suppressed superconductivity due to underlayer of Fe. The saturation power for one CEB is about 0.5 pW. Taking into account the expected power load, the 2-dimensional array of CEBs [49] has been realized as a meander-type structure [3; 12]. The receiving system represents a single pixel for the Olimpo balloon experiment, optimized for power loads up to 100 pW. The unit cell of dipoles is a modification of the dense rectangular grid, distributed in a more sparse way over the unit cell [49]. The serial and parallel DC connections of CEBs are optimized for impedance matching and minimal overall system noise [3]. The pairs of \(\lambda/2\) dipoles sensitive to both orthogonal linear polarizations are connected to each other by dc lines with 1 \(\upmu\)m width. The impedance of each bolometer comes from a series connection of a 30 Ohm resistance and a 55 fF capacitance. In difference with the original design [3; 12], the unetched 280 \(\upmu\)m substrate has been used to match the bolometer bandwidth with the YBCO oscillator antenna bandwidth in the range 240-280 GHz.  
The long grain boundary Josephson junctions (GBJs) were fabricated by on-axis dc magnetron sputtering [50] of YBa\({}_{2}\)Cu\({}_{3}\)O\({}_{7-\delta}\) thin films on the surface of 24\({}^{\circ}\)[001]-tilt Zr\({}_{1-x}\)Y\({}_{x}\)O\({}_{2}\) bicrystal substrates and further etching. The length of the junctions \(L\) along the grain boundary (sample photo in Figure 1) was 50 \(\upmu\)m, the thickness was 0.3 \(\upmu\)m. We have obtained critical current density 370 kA/cm\({}^{2}\) and the \(l_{c}R_{n}\) product of 1.54 mV for \(T=6\) K. The junctions were very long since their lengths are much larger than the Josephson penetration depth \(\lambda_{I}=\sqrt{\Phi_{0}/(2\pi\mu_{0}J_{c}d)}=0.6-0.9\)\(\mu\)m (\(T\)\(\sim\)6 K), which determines the size of a fluxon in the junction. Based on the analysis of the transport properties, the best structures were selected and located at the center of the Si lens for effective radiation. The perpendicular to the grain boundary magnetic field \(B_{e}\) of the order of tenths Gauss was produced by a copper wire coil with many adjacent loops (solenoid). The coil inner diameter of 10 mm was at least one order of magnitude larger than the junction length. Therefore, the magnetic field in the vicinity of the junction was assumed to be nearly uniform [50]. We have checked that the detector itself has no response to the magnetic field through the Josephson oscillator coil.  
In Figure 1 the scheme of the experimental setup is presented. The distance between the detector (CEBs) and the source (YBCO) is 3 cm. The YSZ substrate with the YBCO GJJ was attached to the Si hyperhemisphere lens with 4 mm diameter at the 2.7K plate of 3 He refrigerator. The radiation from YBCO goes after the lens through the copper horn to the Si substrate with the CEBs from the bottom side. The receiving detector consists of an array of 192 CEBs, connected in 4 parallel rows to match with JFET amplifier impedance [3]. The array is surrounded by a copper shield [14] to avoid radiation from other directions and is attached to the 0.3 K cryostat plate.