Detecting Push to Talk radios using two tone intermodulation distortion

Detecting a Radio Frequency device such as a Push-to-Talk (PTT) radio has several military and civilian applications. Detecting PTT radios can be used in search and rescue applications and in providing early detection and warning of existing PTT radios that were previously unknown. This study analyzes the ability of using two tone intermodulation distortion as a way of detecting PTT radios, along with determining ways of improving or increasing this response. Two probing frequencies from a standoff transmitter arrive at the PTT radio and mix in the non-linear components of its radio receiver. As a result, a portion of this mixed response is reflected back. Our results have shown that this response can then be detected, in a semi-noisy environment, to determine the presence of this PTT radio receiver. We have seen that as the magnitude of the probing power increases the PTT radio can be detected from greater distances. However, we have also found that a limiting factor in our experiments is that as the probing power increases the passive intermodulation distortion of the testing system increases as well, therefore making it more difficult to distinguish the nonlinear response of the PTT radio from the non-linear response of the measuring equipment. Future work inlcudes using a coded probing signal which would allow for digital signal processing techniques to be used.