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



--
Stand up against TTIP and ISDS !

On Tue, 15 Dec 2015 14:40:40 +0000 (UTC), 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

Great question. I'd think that each section radiates from the
outside and velocity factor doesn't apply. Thinking more about
it....I don't know.

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

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.



--
Jim Pennino

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.








--
Stand up against TTIP and ISDS !

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.

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




--
Stand up against TTIP and ISDS !

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

wrote:

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.


It's not just the dielectric constant but also the geometry. A much
lower proportion of the field of a an aerial element is within the walls
of a loose-fitting plastic pipe than the 100% of the field of a coax
inner conductor that is in the coax insulation. I have no idea of how
to calculate the effect, but I suspect that a loose outer PVC pipe will
give a velocity factor nearer to one than to 0.58.



--

Roger Hayter

Roger Hayter wrote:
wrote:

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.


It's not just the dielectric constant but also the geometry. A much
lower proportion of the field of a an aerial element is within the walls
of a loose-fitting plastic pipe than the 100% of the field of a coax
inner conductor that is in the coax insulation. I have no idea of how
to calculate the effect, but I suspect that a loose outer PVC pipe will
give a velocity factor nearer to one than to 0.58.

Good point.

I think if I were going to make colinear out of RG8 coax, for example.
I would make a dipole at the frequency of interest out of RG8 and
then know for sure what the actual effect is going to be.

I know the thickness of the insulating material has an effect, but no
idea what the effect of an airgap is.

It would be an intersting experiment to take PVC pipe, a length of
bare wire and a piece of copper or aluminum tubing and measure the
velocity factor for a loose element versus a not loose element.


--
Jim Pennino