I think I understand the basic principles of a collinear antenna. How the Coaxial Collinear Antenna Works? seems to describe it well.
However, I've just repaired a 1090MHz (ADS-B receive only) antenna I built some time ago. Or at least I thought I'd repaired it, turns out I've left the final 1/4 wave still blu-tacked to the window! What purpose does this half-length section serve?
I'm not sure exactly what design I used when I first built it, but they do seem to vary a little: