## 3 Experimental Results
The AFC of the YBCO antenna has recently been measured in [52] from the height of Shapiro step in the frequency range from 240 to 350 GHz with the same BWO as a radiation source. The maximum response has been observed from 250 to 280 GHz, which corresponds to the first harmonics of the YBCO JJ dipole antenna. Figure 6 shows the CEBs response (dependent on the absorbed power) for several values of the external magnetic field. For convenience, the curves starting from the second one are shifted up by  
Figure 4: CEBs response depending on YBCO voltage (oscillation frequency). Comparison of the CEBs signal from BWO and YBCO sources. Signatures of YBCO and YBCO FFO correspond to the case of zero magnetic field and case of magnetic field variation. Figure 5: The experimental \(IV\) curves of bicrystal junction \(L\) = 50 \(\upmu\)m correspond to the increasing magnetic field (from top to bottom). The color indicates the level of CEBs response. 0.5 mV relative to the previous one. To characterize this picture, let us smooth out the dependences (grey dashed curves in Figure 6) and find the voltage of the local maximum of the response \(V_{mr}\) and the value of response \(S_{mr}\). It can be seen that for larger magnetic fields, the position of maxima is shifted towards higher voltages as it must in the traveling wave regime, see also Figure 5.  
An array of fluxons, which for a high magnetic fields transforms into the travelling wave, propagates along the junction with the phase velocity \(u=V_{dc}/(d\mu_{0}H_{e})\)[46], where \(V_{dc}\) is the average voltage across the junction, \(H_{e}\) is the external magnetic field applied in the direction perpendicular to the grain boundary, \(d\) is the effective magnetic thickness of the barrier and \(\mu_{0}\) is the magnetic constant. The voltage at the top of the flux-flow step on the IV characteristic is determined by the VM condition: the velocity \(u\) approaches the Swihart velocity \(\bar{c}\) and, hence, \(V_{vm}\approx\bar{c}d\mu_{0}H_{e}=\bar{c}dB_{e}\). This ratio is confirmed experimentally: the voltage position of the step for larger fields depends linearly on the applied field [46,51]. The voltage range from 0.2 to 1.1 mV corresponds to Josephson frequencies ranging from about 100 to 550 GHz. The VM step becomes smaller and wider as the voltage is increased. This is expected from the increasing surface losses at higher frequencies [38]. The same picture is observed for the received power (CEBs response): the applied magnetic field allows to increase the response voltage \(V_{mr}\) in a wider frequency range. To find out what the maximum power is possible for a given voltage (frequency) in the travelling wave regime, it is necessary to investigate the \(S_{mr}(V_{mr})\) characteristic (red dashed curve in Figure 7), i.e., the envelope of the maximums of each of smoothed curves from Figure 6. Each point corresponds to a different magnetic field value. The dependence of the CEBs response on the YBCO voltage \(S_{mr}(V_{mr})\) has a flat maximum at the frequency range \(\sim\)350-500 GHz. The form of this curve also depends on surface losses, the antenna system, impedance matching, etc. It is interesting to compare the power from the JJ, operating in the phase rotation regime, when the external magnetic field is zero and the frequency is tuned by the bias current variation only (blue solid curve in Figure 7), and YBCO FFO, operating in the traveling wave regime with the frequency tuned by both bias current and magnetic field variation. One can see that in the latter case the oscillation power increases and can be observed in a much wider frequency range. Therefore, this experimental result is a confirmation of recent theoretical observation that the oscillation power in the case of lag-synchronization (occurring also in the travelling wave regime) can be several orders of magnitude larger than for perfect in-phase synchronization of chain elements [53].