101+ things to do with a LimeSDR

30 A Nonlinear Junction Detector (Bug Detector)

I saw this https://hackaday.com/2017/09/20/spy-tech-nonlinear-junction-detectors/ and thought “I wonder if a LimeSDR could do that”. So a quick speed read through one patent from 1971 ( US3631484, there are lots of patents in this area) and it looks like it could possibly work maybe ? It would not be perfect, because you want the TX frequency to be as high as possible so you are limited to one third of 3800MHz (ideally the frequency used should be in a unlicensed ISM/Short Range Devices band). The reason for higher frequency is the larger signal strength returned by smaller devices. Sadly because the two RX channels in a limeSDR can not tune further apart than 61.44MHz this means that only one RX channel can be used, and that it must hop between the 2nd and 3rd harmonic frequency for analysis which is not optimal, but it should still work. It might be slower, but the end result should be the same. Ultimately it just means that you can’t use a Triplexer (LPF,BPF,HPF) to perform the 2nd and 3rd harmonic analysis in real time.

Basic analysis is as follows:

  • If there are no 2nd or 3rd order harmonics present, there is probably little of interest in front of the antenna.
  • If the 2nd harmonic is smaller than the 3rd harmonic, it is probably some metal in front of the antenna.
  • If the 2nd harmonic is larger than the 3rd harmonic, it is probably a nonlinear junction in front of the antenna.

The actual frequency used should not be critical in detecting a NLJ, but higher is going to be a tiny bit more sensitive for extremely small devices.

Frequency to use, and some random numbers for an appropriate diplexer:

Region 1
ISM band
carrier frequency 433.92 MHz (2nd harmonic 867.84 MHz ; 3rd harmonic 1301.76 MHz)
Diplexer: TX 433.92 MHz LPF Cutoff Frequency & RX 867.8 MHz HPF Cutoff Frequency
SRD860 (limited to 25–100 mW ERP )
carrier frequency 866.6 (2nd harmonic 1733.2 MHz ; 3rd harmonic 2599.8 MHz)
Diplexer: TX 866.6 MHz LPF Cutoff Frequency & RX 1733.2 MHz HPF Cutoff Frequency

Region 2
ISM band
carrier frequency 915 MHz (2nd harmonic 1830 MHz ; 3rd harmonic 2745 MHz)
Diplexer: TX 915 MHz LPF Cutoff Frequency & RX 1830 MHz HPF Cutoff Frequency

It is not too hard to design your own Butterworth Lowpass Filters and Butterworth Highpass Filter for a custom made Diplexer.