A few questions about collinear coaxial antennas

Hello,

I am about to attempt to build a UHF collinear coaxial antenna and am
trying 
to finalize a design. I have done a lot of reading and am a little
confused 
on a few things. First off I have read contradicting statements whether to

use odd or even number of 1/2 wave elements. 1, 3, 5... or 1,2,4... Also I

don't understand what the 1/4 wave whip is doing on the top without a
ground 
plane (found in most designs), is this necessary for a receive antenna?.

Instead of using coaxial cable, I will be building the 1/2 wave and 1/4
wave 
transmission lines out of ridged copper pipe with air as it's dielectric
in 
order to maximize the velocity of propagation and therefore making true
1/2 
wave elements. Does anyone see anything wrong with this approach?

Thomas