Radar countermeasures in the Second World War were the first large scale examples of electronic warfare as a deliberate operational tool. Two complementary approaches dominated Allied practice. The first used passive reflectors to create false radar returns, a technique codenamed Window by the British and chaff by the Americans. The second used active airborne jammers and tailored emissions to mask or confuse specific German radar sets. These methods were applied at scale during major operations and shaped a fast evolving measure countermeasure cycle that engineers and tacticians still study today.
At the hardware level Window consisted of many thousands of thin metallized strips cut to electromagnetic lengths that maximized re radiated radar energy. The practical rule used at the time was to cut strips roughly half the radar wavelength in length so each element would resonate and produce a strong echo back to the illuminating radar. When dispersed from aircraft these strips formed reflective clouds that looked like aircraft or ship formations on radar scopes. The physics is simple resonance and scattering, but the operational effect was dramatic: radar operators could not easily discriminate between genuine targets and the distributed cloud of reflectors.
Deploying chaff effectively required engineering discipline and timing. Crews learned to create continuous “corridors” and shaped clouds by releasing bundles at fixed intervals and at predictable altitudes so returns would present as coherent formations on enemy Plan Position Indicator displays. In some tactical uses aircraft laid elongated lanes to mask ingress or egress corridors. For maritime deception the Allies combined airborne chaff with small surface units towing reflectors and with carefully controlled radio traffic to simulate approaching invasion fleets. The result was a multisensor deception in which radar returns, visual cues for coastal observers, and communications patterns all reinforced the false picture.
Active jamming ran in parallel. The British developed a suite of airborne jammers with codenames like Mandrel and Carpet that targeted specific German systems. Mandrel was designed to interfere with long range Freya early warning radars while Carpet and similar sets targeted shorter range gun laying radars such as the Wurzburg family. Those jammers were mounted on specialist aircraft that would fly in and around bomber streams or in support of strike packages to introduce noise and false signals. The work of specialized groups such as No. 100 Group RAF institutionalized this capability, pairing technical experimentation with operational employment.
Two operational case studies highlight how these techniques were combined. The first large scale operational use of Window occurred during the Hamburg raids in mid 1943 where chaff was used to degrade German night fighter control and searchlight effectiveness. The second is the naval and coastal deception on the night of the Normandy landings where Operations Taxable and Glimmer used chaff to produce phantom convoys on German coastal radar, supported by small boats towing reflectors and by simulated radio traffic. In both cases the Allies engineered a situation in which enemy decision makers received coherent but false situational awareness, drawing forces away from the true objective.
There was a significant strategic calculus behind the initial use of chaff. British planners initially withheld Window from operations because of the risk that the Luftwaffe would copy the technique and use it against British radar coverage. That restraint shows the interplay between technological advantage and operational security. Once applied, the measure countermeasure cycle accelerated: radars were retuned, hardware variants were introduced, and jamming techniques diversified to keep pace. That dynamic is familiar to any practitioner of modern spectrum warfare.
The tactical lessons are practical. First, electromagnetic deception works best when integrated across sensors and with complementary measures such as false communications and physical decoys. Second, simple physical principles scale into operational effects when executed precisely; an otherwise mundane metal strip, cut to the right length and released on time, can change the tactical picture. Third, introducing an innovation into the battlespace creates an arms race in frequency management, direction finding, and signal processing. These lessons remain relevant for contemporary EW where Doppler discrimination, pulse shaping and digital signal processing are the new battlegrounds, but the core tradeoffs between concealment, deception, and detection are unchanged.
For engineers and hobbyists studying historical EW the WWII examples are valuable because they reduce modern complexity to fundamentals: resonance, timing, sensor fusion, and command decision loops. Understanding how Window and the early jammers were conceived and employed provides a tested playbook for how electromagnetic tools can be integrated into wider operational plans, and why legal and ethical constraints matter as these capabilities diffuse beyond military use.