The open-ended feeder is used to inductively load the unused elements, making
them into reflectors.
The difference between old coax & new coax results in different amount of
The difference in inductive loading changes the reasonant frequency of the
elements, which, due to coupling change the reasonant frequency of the driven
So, if the driven element has moved up, the reflectors have moved up because
the new coax produces less inductive reactance & you need to add more.
Seems simple, but I'm still working on my first cup this morning....