Laszlo Lebrun wrote:
On Tue, 15 Dec 2015 19:23:21 +0000, jimp wrote:

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...

Crossposting is generally not appropriate.

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

Anything that radiates and is covered with just about anything but air
must have the length adjusted to account for the covering.

The length of a dipole made from insulated #12 wire will be different
than the length of the same dipole made from bare #12 wire.

The difference in length will depend on the characteristics of the
insulating material.

The velocity factor of the coax will determine the length when it is
used as a phasing section.

The outer insulating material will determine the length when the shield
is a radiating element.

That is new to me. Are you sure? That would mean you have to adjust again
when you put the antenna in a PVC pipe to protect it from the weather...
8-0

Yes, I am sure, but again the characteristics of the material, specifically
the dielectric constant, will determine the length adjustment.

There are various types of PVC and I can only find dielectric constants
for low frequencies in the range of 3.1 to 3.9, which is a velocity
factor of 58% to 51%.

You can determine the velocity factor at the frequency of interest,
which I assume is somewhere in the VHF region, experimentally.

Make a dipole out of bare wire and find the resonant point.

Put your PVC pipe over the dipole and find the resonant point.

Calculate the velocity factor.



--
Jim Pennino