Hacker News Reader

Comment by weinzierl | Hacker News Reader

Doesn't mention the good old Nonlinear Junction Detector. (NLJD) [1] or The Broom as they call it. It can find any unshielded device containing semiconductors, no matter if the device is powered or not - which is pretty cool. It is possible to build one yourself which is even cooler. Although I have to say, while it is a WW II era device, DIY building is not a beginners project. The British documentary "The Spying Game - Walls Have Ears" has a good interview with its inventor that even includes a demo of the device by him in his wonderful old school lab[2].

[1] https://en.wikipedia.org/wiki/Nonlinear_junction_detector

[2] https://www.youtube.com/watch?v=4EWYI9zfF9Y#t=22m30s

View on HN

Ha! So we may not be able to scan for "life forms" ala Star Trek, but apparently we have a somewhat universal scan for semiconductors. That's amazing!

Well, not life forms as we know them, anyway.

Yet.

> It can find any unshielded device containing semiconductors, no matter if the device is powered or not - which is pretty cool.

From Wikipedia: “professional covert listening devices (bugs) of the Central Intelligence Agency were equipped from 1968 onwards with a so-called isolator.”

The pic of the bee in that Wikipedia article is amazing. From that linked article:

> A bumblebee with a transponder acting as a non-linear junction for tracking the animal by way of harmonic radar.

That looks like something out of a James Bond movie!

Can't a well build device mitigate this by just using up the energy provided by the NLJD before it is re-transmitted by the semiconductors as a harmonic of the original signal? As far as I my knowledge goes of these, it can detect the created harmonics of the original signal.

Using up all the energy would require a perfect impedance match, which would be very hard to achieve.

Even a 1 percent error would be easy to detect, since radio receivers can easily have 100 dB of dynamic range, and decibels is a log scale, so that's massive!

The linked wikipedia page describes a countermeasure a little like that described.

https://en.wikipedia.org/wiki/Nonlinear_junction_detector#Co...

Probably doesn't mention it because it's not a function of the gadget they're advertising. NLJDs cost several grand, after all.

That is super interesting! I wonder if there is an FPGA version of this for the filtering part of the circuit so that it's easier to make.

Reply on news.ycombinator.com