Cut length on antenna made from coax cable
 

I'm helping a friend build a very simple portable VHF antenna for
hiking. Ignoring all the ladder line J-pole antennas on the web, I
decided a simple coaxial antenna would suffice. Remove 19.2" of outer
jacket, peel back the braid over the outer jacket, tape, and instant
75 ohm coaxial dipole antenna. However, I'm not sure if the driven
element cut length is the free space 1/4 wavelenth, as it would be
with a metal rod coax antenna, or if I should include the 0.67
velocity factor of the coax dielectric covering the center conductor.
Is it 19.2" or 12.9" for the driven element? My sense of smell says
19.2" but I'm not sure and can't find any specific references that
deal with the problem.

Note: I would normally test this on the bench with a reflection
coefficient bridge and sweep generator but I have another project
going on the bench and don't want to move or touch anything right now.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Cut length on antenna made from coax cable
 

On Jul 22, 11:51*am, Jeff Liebermann wrote:
*However, I'm not sure if the driven
element cut length is the free space 1/4 wavelenth, as it would be
with a metal rod coax antenna, or if I should include the 0.67
velocity factor of the coax dielectric covering the center conductor.
Is it 19.2" or 12.9" for the driven element?

Use the 0.67 VF on transmission line currents. Use VF = 0.95-0.97 for
antenna currents.
--
73, Cecil, w5dxp.com

Cut length on antenna made from coax cable
 

On Fri, 22 Jul 2011 09:51:18 -0700, Jeff Liebermann
wrote:

if I should include the 0.67
velocity factor of the coax dielectric covering the center conductor.
Is it 19.2" or 12.9" for the driven element?

Hi Jeff,

Ah! The sleeve dipole re-emerges.

The exposed wire isn't much different from any other insulated wire
antenna. Air dominates the dielectric constant to the now remote
braid and the plastic ever so slightly shifts things. EZNEC includes
modeling insulated wire.

More's the issue of the dielectric coefficient of the braid pulled
back over the outer jacket of your coax. Coupling issues to the
insulated braid there too shift dynamics - and probably dominates the
DC characteristic.

I usually cut long, and slowly snip the exposed wire until SWR
improves. Choking the line would stabilize this.

73's
Richard Clark, KB7QHC

Cut length on antenna made from coax cable
 

On Fri, 22 Jul 2011 11:35:09 -0700, Richard Clark
wrote:

I usually cut long, and slowly snip the exposed wire until SWR
improves. Choking the line would stabilize this.

Another solution I've tried is to put a choke on the line that I can
slide along its length (and later tape down). I then put a connecter
at the far (antenna) end of this line. I then put a bare/insulated
wire on a female connecter and mate the two. This preserves the line
from stripping and peeling back.

I cannot vouch how effective this is as a "Sleeve Dipole" in all the
standard characteristics. The Choke Z would have to be very, very
high to electrically snip the lower end to the right length to tune
it. Another choke a quarter wave away would be necessary. All
doable, certainly, but ad-hoc.

73's
Richard Clark, KB7QHC

Cut length on antenna made from coax cable
 

Jeff Liebermann wrote in
:

Jeff,

You are treading down a well worn path.

There is a flaw in thinking that the shield of the coax below the bottom
of your nominal dipole does not carry significant current on the outer
surface. Whilst you are trying to account for the current distribution
on the nominal dipole, you are ignoring the common mode current on the
feedline.

The antenna will 'work' and you may be delightedm but if you expect that
it will have very low VSWR based on formula dimensions, I think you will
be dissapointed.

You might obtain better feedline decoupling by some form of common mode
choke.

The failure of the sleeve to effectively decouple the dipole is the
reason why several antennas don't work as thought. King gives some hints
that the relative diameters of the conductors bears on operation.

Owen

Cut length on antenna made from coax cable
 

"Owen Duffy" wrote in message
...

The failure of the sleeve to effectively decouple the dipole is the
reason why several antennas don't work as thought. King gives some hints
that the relative diameters of the conductors bears on operation.

Owen


I made a sleeve dipole cut for 375 MHz for listening to the Blue Angels in
flight at NAS Miramar, c. 1992. It worked, but, heck, we could see them
most of the time. I was in Coat Hanger Territory. By the way, the Blue
Angles say very little in flight. :-(

This thread got me thinking. If three or four tuned radials were added,
would this be a better approach? Yes, I realize it gets toward a lot of the
standard commercial monopole antennas, with their 45-degree-down radials. I
mention this because such radials could be added and still keep the builder
in comfy Coat Hanger Territory -- that is, a very light, casual,
cobbled-together antenna -- but something that works a bit better and helps
the decoupling.

Frankly, I understand what it is to buy something and what it is to build
something; I'd rather build. It keeps me off the street. :-)

"Sal"
(Kd6VKW)

Cut length on antenna made from coax cable
 

One handy hint is that you can adjust the folded back shield by moving it up
and dowm
along the coax and holding it in place with a clothespin or something
similar.

Irv VE6BP

Cut length on antenna made from coax cable
 

On 7/22/2011 11:51 AM, Jeff Liebermann wrote:
I'm helping a friend build a very simple portable VHF antenna for
hiking. Ignoring all the ladder line J-pole antennas on the web, I
decided a simple coaxial antenna would suffice. Remove 19.2" of outer
jacket, peel back the braid over the outer jacket, tape, and instant
75 ohm coaxial dipole antenna. However, I'm not sure if the driven
element cut length is the free space 1/4 wavelenth, as it would be
with a metal rod coax antenna, or if I should include the 0.67
velocity factor of the coax dielectric covering the center conductor.
Is it 19.2" or 12.9" for the driven element? My sense of smell says
19.2" but I'm not sure and can't find any specific references that
deal with the problem.

Note: I would normally test this on the bench with a reflection
coefficient bridge and sweep generator but I have another project
going on the bench and don't want to move or touch anything right now.


Go with your sense of smell. The velocity factor of coax only applies
when the shield is positioned where it is designed to be. Let us know
how it fares.

Cut length on antenna made from coax cable
 

"Sal" wrote in :

...
This thread got me thinking. If three or four tuned radials were
added, would this be a better approach?

A set of radials helps to decouple the feedline, in this case, you would
place them a quarter wave below the bottom of the sleeve.

Ok, you have noted commercial implemenations of this idea... yes it is
done. The quarter wave between the radials and the bottom of the sleeve
carries significant current, ie it affects the pattern and gain.

And still, decoupling is not perfect, though much better.

Whilst the OP's design looks like it is simple, in my experience it is
not. Whilst I have had a lot of success with coaxial dipoles where the
sleeve was three times or more the diameter of the inner element and
substantially air dielectric, I have had little success the the cheap
and nasty approach of applying braid over a vinyl jacket and trying to
adjust it for low common mode feed line impedance and low VSWR. I am not
saying it is impossible, just not as easy as a ground plane!

Owen

Cut length on antenna made from coax cable
 

On Fri, 22 Jul 2011 23:13:25 +0000 (UTC), Owen Duffy
wrote:

You are treading down a well worn path.

Yeah, I know the feeling when I'm stuck in a rut.

There is a flaw in thinking that the shield of the coax below the bottom
of your nominal dipole does not carry significant current on the outer
surface. Whilst you are trying to account for the current distribution
on the nominal dipole, you are ignoring the common mode current on the
feedline.

Agreed. The feedline should be decoupled, which one can do by
wrapping a few turns into a coil and tied together with tie wraps.
Another method is a 1/4 wave decoupling sleeve further down the coax
cable.

The antenna will 'work' and you may be delightedm but if you expect that
it will have very low VSWR based on formula dimensions, I think you will
be dissapointed.

Well, putting a 75 ohm antenna onto a 50 ohm system results in a
minimum VSWR of 1.5:1 assuming the coax is cut for some multiple of
1/2 wave electrical length. Also, most HT's are designed to tolerate
the fairly miserable VSWR presented by rubber ducky antennas and body
proximity.

In this case, the problem to be solved was rather mundane. A friend
was in the process of packing for a weekend camping trip with the
kids. His 13 year old daughter apparently had lost the rubber ducky
antenna from her Yaesu FT-60r. Rather than borrow an antenna, or
build an adapter kludge, he decides it's time for her to build an
antenna. They found some SMA cables, and proceeded to make a simple
coaxial sleeve dipole. However, there was some debate over the cut
length of the driven element because of the presence of coax
dielectric. I get a hasty email with the question and discover that I
really don't know the answer. So, I posted the question here. We'll
find out how it worked when they return.

Whether the home made antenna is optimized for best performance
doesn't seem to be important in this case. Any reasonable antenna is
better than a rubber ducky, so she should be fine (assuming she
doesn't lose another antenna).

You might obtain better feedline decoupling by some form of common mode
choke.

Yes, that will help (as previously noted).

The failure of the sleeve to effectively decouple the dipole is the
reason why several antennas don't work as thought. King gives some hints
that the relative diameters of the conductors bears on operation.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558