What is this wire going from mid-fuselage to the tail on this 737? In  this question the following image is shown, with the question what the two bulges at the aft fuselage are.

However, I'm wondering what this wire is that starts somewhere midway the fuselage, and goes to the vertical stabilizer.
Is it also radar related?



Image Source
It is the HF antenna. It was mostly used in older aircraft, which used them in trans-oceanic flights or in areas without UHF or VHF communications facilities.Modern 737s (and other aircraft in general) have done away with them. On later versions it was fitted in the leading edge of the Vertical fin of B737-200s, as shown in the figure below.



Image from b737.org.uk

Note that these aircraft were delivered in 1982-83. Newer 737s don't have these.