In article ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

I think the real problem is this antenna for 2 meter operation is 20
feet long!

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

Jeff,

Do you have a sim or model of that situation, showing that there aren't
significant RF currents on every other half-wave section? Somehow I
can't make sense of how that would happen.

My mental model of the alternating-sections design has been that all
of the sections do radiate... the alternating hookup forces them to
radiate in phase with one another, rather than out of phase (and thus
tending to squint the pattern badly upwards and downwards).

Now, I have heard that the alternating-coax collinear doesn't have as
much gain as an array of separate dipoles hooked up with a phasing
harness... but I've always read that as being explained by the fact
that the upper sections are carrying smaller RF currents than the
lower because some power has been radiated away before the signal
reaches the upper part of the antenna.

So, I'd appreciate enlightenment here!