Helical delay line? They used 'em back in the bad old days of NTSC TV (and often, tube sets at that), to account for the group delay of the chroma circuits (the luma was being delayed so it arrives at the CRT drivers correctly).
Anyway, because any given turn has direct transmission line behavior to its neighboring turn, but also magnetic coupling to more distant neighbors and so on, the dispersion of a helical delay line is quite pronounced.
You also have a sliding scale, from a long, thin helix in free space, to a helix that's so heavily shielded (solid shield inside and outside, and shield interleaved between turns, essentially making a coil of coax transmission line). The ones I've seen in ancient tube TV sets had a foil strip inside, not really providing any shielding but providing an electric ground reference, which keeps the velocity factor and bandwidth low, as is suitable for the application (low MHz).
Speaking of Tek, an example they used was a fairly highly twisted twinax, which was just slightly dispersive, enough that you saw some pre-edge wiggle but not bad enough to be a problem. That's a consequence of having the turns just close enough together (and not shielded between), to get some useful advantage in velocity factor.
Tim