Jim Richardson wrote:
"Could you build a colinear of a single long line bending it back at 90
deg. every half wave---?"

See the "Super J" in the "ARRL Antenna Book", page 16-25 in my 19th
edition. That is more or less how it is made. The two 1/2-waves,
in-phase, are end-driven with phase inversion provided by a
short-circuited 1/4-wave stub between them.

At frequencies higher than 2-meters, this would work for more than (2)
1/2-wave sections. The practical limitation is mechanical.

Best regards, Richard Harrison, KB5WZI