Nonlinear junction detector

The non-linear junction detector, or an NLJD, is a device that illuminates a small region of space with high-frequency RF energy.

In its basic form, an NLJD can also detect things that are not themselves electronic in nature, so the use of the device requires a modicum of skill and experience.

As a countermeasure against an NLJD, professional covert listening devices (bugs) of the Central Intelligence Agency were equipped from 1968 onwards with a so-called isolator.

A means to hinder isolating a non linear junction is to add inexpensive diodes to places where the NLJD is expected to sweep.

Thousands of diodes were mixed by the Soviets into the building's structural concrete, making detection and removal of the true listening devices by its American occupants nearly impossible.