This one is very interesting design. I was used something VERY similar before time and all that I remember is, that it defiantly WORK fine, but it is pain to tune this.
All pictures are from this page : http://www.guw.cl/sitio/index.php?option=com_content&task=view&id=78&Itemid=36
This antenna is combination from usual J Pole antenna and Franklin array antenna /AMOS antenna only onmidirectional /.
Gain of this omni antenna is around 6dbi, but you may add more by adding more sections. Double the sections to add 3dbis.
Dimensions :
And feeding point and tune stub :
Some suggestions :
First CHECK dimensions of Your antenna.
Second : Matching to the feed-line is archived by sliding the connection of the feedline back and forth along the stub until an SWR as close as possible to 1:1 is obtained.
Third : Try to add quarter-wave ground plane.
Four : You can bend the half wave section to a half circle to be able to use small diameter of plastic tube :
http://en.wikipedia.org/wiki/Slim_Jim_%28antenna%29
Some more pictures :
