## 5 Discussion  
The detection range may be extended when the sounding frequencies are lower, i.e., closer to the lower edge of the DVB-T band as the free-space losses decrease. The real design of a dedicated C-PCL system should contain an analysis of real background noise, including a distributed power density from DVB-T channels, depending on the DVB-T transmitter's physical localization in a given area. The presence of such background signals can lower the SNR value and deteriorate the C-PCL detection range. It can be overcome with digital signal processing, meanings, i.e., by the design of a sounding waveform with a modulation orthogonal to DVB-T's one and by applying a matched detection in the C-PCL receiver. The digital processing gain should compensate a decrease in the SNR value caused by DVB-T transmitters. On the other hand, the use of wider band signals requires a wider band of the C-PCL receiver which allows more noise power to enter into the signal processing. Therefore, the opposite method is to use a bandpass filter with as narrow a transmission band as possible. The third possible approach is to use a kind of adaptive signal cancelling designed for DVB-T background signals entering into a C-PCL receiver. Taking into account thermal noise only, with the assumption of a 10 dB signal-to-noise ratio in the C-PCL receiver, the possibility of the application of such a system was shown. Although the SNR value was chosen conveniently in terms of detectability, a reverse propagation analysis proved that this condition can be met with a reasonable magnitude of C-PCL transmitter power. What is more, the C-PCL system antennas may be chosen or optimized for higher values of gain than modelled. In the best of cases considered in the analysis, using the conversion losses measured for typical amplifiers, the detection range of 1 km is reachable with the use of a sounding power of about 100 W at 750 MHz. The harmonic radar approach is known and used in order to detect objects containing electronic circuits. However, the most developed topic is the detection of specially prepared circuits, called nonlinear transponders [19]. The design of such a transponder covers the design of antennas for sounding and harmonic signals, and optimized and matched nonlinear circuits, most frequently consisting of a Schottky diode. In that case the parameters of a nonlinear circuit are known and the transponder antennas and operation frequency are to be optimized. Moreover, commonly the one harmonic is chosen, e.g., the second, and the remote exciter, i.e., a sounding and reading device, and the sounding signal form may be easily optimized. Such transponder circuits with appropriate interrogating circuits are used, for example, to track insects [20]. In the case of detection of hidden PCLs, there is no knowledge a priori about the parameters of the \"transponder\". It should be assumed \"the worst case\", i.e., the lack of knowledge about the exact parameters of the antennas and LNA in PCL. That is why the authors performed an analysis that covers a range of possible values of these parameters. Some of the initial guesses concerning the PCL structure resulted from the literature survey and knowledge of engineering trends. This work concerns the real case of a radar that is commonly regarded to be completely undetectable. Several devices, that may be used as an LNA for PCL, were tested in order to gain knowledge about typical IMD behavior. The real-world laboratory tests of these devices allowed the analysis to be performed and a C-PCL system performance to be estimated. Further works may concern wideband and frequency-modulated sounding signals in order to suppress the influence of surrounding opportunity signals or obtain matched detection and signal compression. Another issue is the prediction of frequency- and direction-dependent variation of the PCL antenna directive gain when it serves as the transmitter of IMD signals.