Almost from the outset, the development of radars was accompanied by the development of technologies to defeat them, or “electronic countermeasures (ECM)”. The first approach to defeating radars was to simply jam them.

There were two approaches to jamming, “passive” and “active”. Passive jamming involves dispensing lengths of metal, originally paper-backed strips of aluminum, called “chaff” that create radar echoes to hide the aircraft or ship throwing them out. Chaff has to be cut to a length appropriate to a particular radar — a half or quarter of the radar’s operating wavelength — and chaff designed for a radar operating at one wavelength doesn’t work well against a radar operating at a substantially different wavelength. Chaff also has to be dumped in substantial amounts to create an adequate screen against prying radars.

Incidentally, the terminology for chaff can be confusing. Originally, the British dispersed “window”, which were strips cut to lengths to jam relatively high-frequency radars. They then also dispersed “rope”, which were very long strips intended to jam low-frequency radars. The term “rope” still survives to an extent today, but for the most part the term “chaff” is universal.

Active jamming involves a transmitting device that attempts to interfere with the operation of adversary radars. There are two approaches to active jamming: the jammer can either dump out sheer noise and try to drown out the radar, in which case it is known as a “disruptive” jammer, or it can send back false returns, in which case it is known as a “deception” jammer. The simplest form of deception jammer is the “pulse repeater”, which sends back amplified copies of a radar return after a delay, to confuse a radar as to the exact location of a target. It can also generate a return copy with no delay to make the target appear to be bigger than it really is, for example making a small decoy drone aircraft look like a big bomber.