Editing Direction finding (section)

=== Brief history of microwave development ===
Earlier in the century, [[vacuum tubes]] (thermionic valves) were used extensively in transmitters and receivers, but their high frequency performance was limited by transit time effects.<ref name = Gilmour>Gilmour  jnr. A.S., "Microwave Tubes", Artech House, 1986</ref>{{rp|192}}<ref name = Beck>Beck, A. H. W., "Thermionic Valves", Cambridge University Press, 1953</ref>{{rp|394}}<ref>Baden Fuller A. J., "Microwaves"Pergamon Press, 1979</ref>{{rp|206}}  Even with special processes to reduce lead lengths,<ref>Hooijmans P., "Philip's tuner history". Find at http://maximus-randd.com/tv-tuner-history-pt1.html</ref> such as frame grid construction, as used in the [[EF50]], and planar construction,<ref name = Gilmour />{{rp|192}} very few tubes could operate above [[UHF]].

Intensive research work was carried out in the 1930s in order to develop transmitting tubes specifically for the microwave band which included, in particular, the [[klystron]]<ref name = Gupta>Gupta K.C., "Microwaves", New Age Intnl. Pub., 2012</ref><ref name = Gilmour />{{rp|201}}  the [[cavity magnetron]]<ref name = Gilmour />{{rp|347}} <ref name = Gupta />{{rp|45}}  and the [[travelling wave tube]] (TWT).<ref name = Gilmour />{{rp|241}}<ref name = Gupta />{{rp|48}} Following  the successful development of these tubes, large scale production occurred in the following decade.