Antenna woes

Polystyrene is a thermoplastic, and has a certain amount of "memory." What happens when you stretch it and it cools is that the residual stress from the stretching is uneven, especially since sprue is rarely perfectly round. Over time, the residual stress causes it to warp if the ends are not fastened down.

You can use it "as is," just attach both ends with the piece as straight as you can get it, then warm it gently with an extinguished match or soldering iron. It will shrink straight. Alternatively, dip each piece in hot water for a second, then tape it down straight to something rigid.

I find that stretched sprue will eventually bend and sag on the model as well.  For antennas on 1/35 armor, I use 0.020 styrene rod from Plastruct or Evergreen.  Cut it to 2 3/4" for a standard modern (8 foot) US antenna.  I slightly heat the top end to give it a rounded ball to replicate the antenna safety ball on the end.  The same can be done for any era antenna.  I like how they look better than stretched sprue.

Hope that helps.  

Ditto the previous poster who said that sprue has a "memory." What I have found that seems to make a huge difference in whether the stretched pieces stay straight long term is how they dry - cure might be a better word.

After I stretch out a piece to the desired diameter, I take it and tape one end to the top wall section of the pass-through between my kitchen and dining room/worktable, which leaves the opposite or bottom end hanging free. To that I attach a larger black metal spring clamp (any light weight would do) and then let it just hang there for a day or two. Kind of like prestressed concrete bridge sections, if the sprue cures under tension like that, it has shown much less of a tendency to turn into something resembling a curlycue pig's tail.

And I also make whatever stretched sprue I need fresh for each model, I haven't had much luck storing it long term either.