Why are fighter jets equipped with dorsal fin?
There are two features that you may be referring to.

A) Is an antenna that is shaped to provide low drag. Not all F16 have this.

B) The true dorsal "fin":


increases structural strength  
decreases drag by providing a fillet for the vertical stabilizer,   
improves stability about the vertical axis,   
and specifically in the case of the f16 (when installed)
     aerodynamically blends the drag chute pack (box at base of Rudder) 
many aircraft use this as a convenient air intake for 
     equipment or auxiliary cooling.



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