Editing Superheterodyne receiver (section)

=== Radio direction finding ===

There was one role where the regenerative system was not suitable, even for Morse code sources, and that was the task of [[radio direction finding]], RDF.

The regenerative system was highly non-linear, amplifying any signal above a certain threshold by a huge amount, sometimes so large it caused it to turn into a transmitter (which was the entire basis of the original [[Identification friend or foe#IFF_Mark_II_anchor|IFF system]]). In RDF, the strength of the signal is used to determine the location of the transmitter, so one requires [[linear amplification]] to allow the strength of the original signal, often very weak, to be accurately measured.

To address this need, RDF systems of the era used triodes operating below unity. To get a usable signal from such a system, tens or even hundreds of triodes had to be used, connected together anode-to-grid. These amplifiers drew enormous amounts of power and required a team of maintenance engineers to keep them running. Nevertheless, the strategic value of direction finding on weak signals was so high that the [[British Admiralty]] felt the high cost was justified.