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F distribution
f.i.w.
F layer
f-number
F/O.
f.o.b.
F.R.G.
F.R.S.
F region
f-stop
f-word
F.Y.R.O.M.
F2F
F2T
FA
fa
FA Cup
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F region

F re·gion


noun 
Definition:
 
highest part of Earth's atmosphere: the highest part of the ionosphere that reflects high-frequency radio waves. It is divided into two layers, the F1, which extends upward from 180 km (112 mi) and is present only during the day, and the F2, extending upward from 300 km (186 mi).

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