What is the wire running from the CG to the tail? I was looking at airplanes a while back and noticed that there is sometimes a wire running from about the CG near the wind to about halfway up the tail.  I see this more on 737s but I have also seen it on C130s.  What is this wire?  To hold up the tail?  I have seen: What is this wire going from mid-fuselage to the tail on this 737? but I am more interested in all of the other planes that have it too.  Are there planes that actually use guy-wires for support or are all of the wires I see antennas?
Sounds like an HF antenna. It absolutely isn't structural, as neither of those aircraft needs guy-wires for structural stability. (You'd also expect to see it on every example, not just some, if it were required that way.)

This article provides a lengthy discussion of HF antennae, for those interested in going deeper into the subject.

This thread has a related discussion of why not all HF antennae are of the long-wire type. Essentially, newer designs often replace the long wires.