## 4 Measurement Results  
Figure 4 presents the results of an experimental study of the resonant properties of CEBs with a double-folded slot antenna and coplanar lines. To develop a resonant receiving system at given frequencies, it is necessary to adjust the elements of the system, in particular, the capacity of the SIN junction in CEB and the length L = 2 \(\cdot\) (L\({}_{1}\) + L\({}_{2}\) + L\({}_{3}\)) of the central coplanar line (see Figure 1). During the experiments, the effect of changes in these system parameters on the operating frequency was tested (Figure 4). We have also analyzed various designs of coplanar lines with rectangular turns as in Figure 3 (solid curves) and with rounded turns (dashed curves). It can be seen that by selecting the capacitance and lengths of the coplanar lines, the system can be tuned to the specified frequency range. It is seen that all characteristics have a clearly defined peak at different frequencies with a bandwidth of around 15 GHz. So, the resulting bandwidth of 6-9% practically meets the requirements for the LSPE experiment receiving system. Out of band peaks can be attributed both to peaks of YBCO amplitude-frequency characteristic and to the methodic of CEB sample irradiation. Due to the necessity to test the first on-chip filter prototypes we have placed up to 6 samples with various antennas on one chip and certain receivers are shifted from the chip center and standing waves differently affect various antennas. These issues will be corrected in future designs of both YBCO sources with more suitable broadband antennas and new designs of CEBs with on-chip filters with only one antenna at the chip center. Nevertheless, our experimental setup where the generator lens was mounted close to the receiver lens separated by infrared filters, allowed significantly reducing spurious resonances, which inevitably arose in experiments with room temperature sources of GHz radiation outside the cryostat [15] due to additional reflections from cryostat shields and much larger power. Figure 3: The experimental setup view, its schematic representation, and samples design. The experimental center frequencies are in some cases shifted from the calculated values (see Figure 4, calculated center frequencies are given in the legend). This difference is most probably caused by the influence of the standing waves in a substrate, which were not taken into account in the initial simulations. Another possible reason for the frequency shift, found when designing the system [7], was the effect of dc wires, which will be precisely calculated in future designs. However, the most critical reason for the frequency shift is the fact that at low capacitance of the order of 10 fF, a small increase in capacitance due to overexposure of a tunnel junction may significantly shift the resonance to lower frequencies. Besides, at frequencies of several hundred GHz and at low temperatures it may be necessary to take into account the surface impedance of gold, which differs from the one at low frequencies and room temperature, and is known to be frequency-dependent. Also, as follows from the experimental results, the samples with rectangular coplanar lines demonstrate a stronger effect of standing waves leading to sharper satellite spikes. The simulation results, taking into account reflections from the substrate edges, are shown by crosses for the parameters close to the experiment for green solid curve with diamonds and qualitative agreement with the experiment is observed. Finally, among all the data we have found a sample (black dashed curve), which has a smooth shape and is centered exactly at the required 240 GHz. These sample parameters will be scaled in future designs to form a pair of antennas for 220/240 GHz channel of LSPE. Figure 5 presents the comparison of CEB output voltage noise in the absence and in the presence of YBCO GHz signal.