On 12/15/2015 2:06 PM, Laszlo Lebrun wrote:
On Tue, 15 Dec 2015 11:00:23 -0600, John S wrote:

On 12/15/2015 8:40 AM, Laszlo Lebrun wrote:

Is it allowed to post here a question related to antennas and without
crossposts? :-)

Reading the content of the other threads, I'm not quite sure...

OK let's try nevertheless:

A colinear antenna is using alternating sections if coaxial cable
whereas the signal is travelling half inside the coax, half outside.
The length of the sections should be half wave considering the velocity
factor of the used coaxial cable.

I would tend to consider the velocity factor only for the parts of the
antenna where the the signal is travelling *inside* the coax cable.

I know, my reasoning is probably wrong, but why?

Thank you for your advice

Three collinear antenna blogs by a very knowledgeable ham:

http://owenduffy.net/blog/?s=collinear

Oh! I was not even completely wrong!
One must really consider different velocities. The interactions are
however symmetric, so it is not enough to make some sections shorter/
longer. The best way is to take high velocity coax to minimize the
differences.

Yup! That's the way I read it, too. Good luck.