I tried to make the same antenna with 1/2" hardline & had several RF
analyzers to help me, it never worked out. This article helps
explain why coax wont work, I used wire and would my on coils for
phasing, i also took apart a Madrax antenna to get the measurements,
they used brass tubes inbetween the coils, not because its brass but so
the 1/2 wave length can be easily adjusted.
http://www.centurion.com/home/pdf/wp_omni_wireless.pdf