Can anyone explain what is the teardrop-shaped item behind the cockpit?
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Clearly it's not on all models. this one has it but this one does not and this one has something different but in a similar position – Jamiec Oct 21 '15 at 11:10
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It is the RDF antenna 'bullet' (or teardrop, actually) fairing. This image shows the (loop) antenna itself.
Source: mission4today.com
Initailly, RDF antennas were circular loops mounted above or below the fuselage.
"Earhart-electra 10" by Originally uploaded by w:User:Logawi - Transferred from en.wikipedia, description page is/was here.. Licensed under Public Domain via Commons.
Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing.
Source: airpages.ru
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2The change, IIRC, is that the RDFs evolved into ADFs, which rotated to find the relative angle from the nose at which the beacon signal was strongest, allowing the pilot to get a fix from the beacon without having to change course or manually manipulate the antenna. The motorized rotating antenna, however, was much less aerodynamic and prone to damage at high airspeed, so it needed a housing to contain it. – KeithS Oct 21 '15 at 19:54