## 6 Conclusions  
The concept was presented. The measurements verifying IMD generation at the input of several transistor amplifiers (including a real DVB-T receiver) were conducted. The value of frequency conversion efficiency referred to input varied with input power. However, the most important conclusion is that the power level of the IMD products, referring to the amplifier's input, reached values sufficient to be received in the case of back-radiation from a PCL. Acquired data allowed the calculation and simulation of a C-PCL system to be performed. Assuming typical real antenna parameters, the detection range of a PCL system versus the transmitted power was estimated. It was found out that it is possible to remotely induce the intermodulation distortions in the input part of a low-noise amplifier. These IMD products may be received with the use of a dedicated narrowband receiver. The bandwidth of this receiver may be very narrow in the case of stable CW sounding frequencies; however, the possible Doppler shift of IMD frequencies (when the C-PCL system is installed on a drone) should be considered.  
The detection range depends mainly on the level of the sounding signals' transmitted power. There is also obvious influence of antenna gains, especially in the case of the PCL receiving antenna, where a circular array of dipoles may be used. The equivalent transmitting beam of the PCL dipole array with respect to the generated IMD wave has directivity, which depends on the IMD frequency and the sounding signals' angle of arrival.  
This is the reason why the additional link-loss depending on situational configuration should be considered. In the case when a PCL system contains separate reference antenna, its gain is typically high, which improves the PCL detectability.  
The main conclusion is that such an advanced \"silent\" system as a PCL may be tracked and detected with the use of novel and specialized methods exploiting the knowledge from nonlinear circuit theory and methodology of UAV application, including artificial intelligence for mission planning and scanning scenario or a multistatic approach. The further conclusion is that the use of a nonlinear method of detection allows new solutions for an anti-radar missile to be developed.  
The topic of the real implementation of a C-PCL system should also contain an analysis of possible counter-action from further improved versions of PCL systems. For example, a countermeasure in the form of detecting the sounding signals by a PCL and switching off the power supply of the system (going to standby mode) does not protect a PCL from being detected. Then, the bias point of LNA will be changed but the transistor still keeps its nonlinear behavior allowing it to be detected, likely giving a substantially lower power of IMD signals, but still with a non-zero value. The other possibility of countermeasure is to deploy a kind of decoy device that would mimic a real PCL system. These kinds of topics are under development and are out of the scope of this paper.  
Conceptualization, Z.S.; methodology, Z.S.; software, M.K.; validation, Z.S.; formal analysis, Z.S. and M.K.; investigation, T.R.; resources, Z.S., M.K. and T.R.; data curation, M.K. and T.R.; writing--original draft preparation, Z.S.; writing--review and editing, Z.S. and M.K.; visualization, M.K.; supervision, Z.S.; project administration, Z.S.; funding acquisition, Z.S., M.K. All authors have read and agreed to the published version of the manuscript.  
This work was financed by the Military University of Technology under research project UGB 22-849 in terms of state-of-the-art analysis, resources provisioning, measurements performing and publication costs. This work was co-financed by the European Social Fund under the \"Operational Programme Knowledge Education Development 2014-2020\" regarding programming skills training for data processing purpose.  
The authors declare no conflict of interest.