Good sites.

Some may wonder why there are different geometries for the 'same' antenna'.
The reason is that the quadrifilar helix can be designed for practically any
beamwidth up to 300 degrees or so. This results in different lengths and
number of turns. Some designs are open at the bottom and some are closed at
the bottom. It all depends on what you want the antenna to do.

--
Jim
N8EE

to email directly, send to my call sign at arrl dot net