On Mon, 17 Feb 2014 18:04:16 -0500, Fred McKenzie
wrote:

In article ,
Jeff Liebermann wrote:


I hate to admit it, but I think I've built them backwards over the
years. Maybe that's why J-poles have such a lousy reputation and why
I think they suck?

Please pardon my ignorance. I understood that a J-Pole is a half wave
antenna connected to a quarter wave stub. The transmission line is
connected to a low impedance point (50 Ohms?) on the stub.

Yep, something like that.

In the literature, it is usually fed as an unbalanced antenna, but it is
not. Therefore, it should not matter which side has the center
conductor or shield - they are both wrong!

Fred
K4DII

Well, if it really were a balanced feed, I guess(tm) there should be
no difference in the pattern, gain, VSWR, etc between the two
unbalanced methods of feeding the antenna. Yet the author of the
original article shows large differences in the antenna models. See
Pg 10 thru 13:
http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf

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

On Tue, 18 Feb 2014 18:10:39 -0500, "Ralph Mowery"
wrote:

I don't know which one would be the best but I have seen 3 methods of
feeding the J-pole. If you insulate the bottom then you hook the feedline
to the bottom with the center of the coax to the long side. If you do not
insulate the bottom you tap up the matching segment so that you get a 50 ohm
(if that is the coax used) match with the center of the coax connected to
the long leg. Then there is the balun made out of coax that is hooked up to
the matching segment so that a low swr is obtained.

There's also the American Legion J-Pole or the Silicon Valley
Emergency Communications Systems J-Pole:
https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396072666399154
https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396111364421106
This design does one thing right that none of the other J-Pole
mutations seem to consider. The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency. In other
words, a gamma match. The series capacitor is formed by the insulated
turns of electrical wire wrapped around the driven element.

Another thing this design does right is use the zero current point at
the bottom of the antenna as a ground. The problem is that it also
extends the length of the center wire, which makes using a gamma match
all the more important. I think putting the 50 ohm feed point and the
corresponding ground close to each other were either to reduce the
inductance of the connecting wire, or some manner of mutation from
when it was fed by a balance line.

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

In article ,
(David Platt) wrote:

In article ,
W5DXP wrote:

Don't forget the Arrow Open Stub J-pole.

http://www.arrowantennas.com/osj/j-pole.html

I have one of these dual-band types in my "go-kit" (it's actually a
knock-off, built locally based on the plans that Arrow used to have on
their web site) and it's served me well.

David-

Using the Wayback Machine https://archive.org/index.php, I was able to
find the old Arrow plans listed under instruction sheets.

Then I found that the latest version was a single PDF file that has the
same information!

At the recent Orlando Hamfest, someone was selling dual band J-Pole
copies of the Arrow. Now I wish I had bought one.

Fred

"Jeff Liebermann" wrote in message
...


... The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency.

Yes.

I've been making copper pipe and 2-wire transmission line J-poles for almost
20 years. With pipe, I usually fasten the two feed points with clamps and
slide the connections up and down. I'll get a VSWR low-point in-band but
early-on, I discovered that the best VSWR was often about 1.7:1. I had read
about (but never built) a gamma match, so I'd heard about the series cap to
tune out the inductance. I tried a series cap at the feed and it helped.

70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading
somewhere in the band. Does such a 0.3 dB improvement matter? That's not
my call. When I'm essentially playing with the technology, I can take more
time than if I'm working, like to a deadline or a budget.

"Sal"

On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote:

"Jeff Liebermann" wrote in message
.. .
... The length of the wire between the coax
connector center conductor and the driven element is an inductor. In
order to tune out this inductance, one needs a series capacitor, with
the inductor and capacitor tuned to the operating frequency.

Yes.
I've been making copper pipe and 2-wire transmission line J-poles for almost
20 years. With pipe, I usually fasten the two feed points with clamps and
slide the connections up and down. I'll get a VSWR low-point in-band but
early-on, I discovered that the best VSWR was often about 1.7:1. I had read
about (but never built) a gamma match, so I'd heard about the series cap to
tune out the inductance. I tried a series cap at the feed and it helped.

70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading
somewhere in the band.

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

Does such a 0.3 dB improvement matter? That's not
my call. When I'm essentially playing with the technology, I can take more
time than if I'm working, like to a deadline or a budget.

0.3dB is about 6.7% loss. Probably not important or toss a coin?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

"Jeff Liebermann" wrote in message
...

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

That's enlightening.

I need to admit that no such science was applied. I just tried small caps
from my stash to see what would happen. When something good happened, I
remembered it for next time.

The j-pole that seems to benefit most is the 2m version, which I've built
the most of. Perhaps we're not dealing with just the inductance of that 6
cm of wire. There could be some residual inductance in the rest of the
antenna which is being brought to resonance. Maybe when my element lengths
are not optimal, some inductance would be found there. Not sure why the
residual reactance would always be inductive. The first time I add a cap
and it gets worse, then I'll know it isn't so.

Maybe I should experiment with varying the spacing across the stub and see
what that does, with and without a cap.

"Sal"

On Thu, 20 Feb 2014 20:38:22 -0800, "Sal" salmonella@food
poisoning.org wrote:


"Jeff Liebermann" wrote in message
.. .

I'm not sure of the frequency of your J-pole. The inductance of about
6 cm of #12AWG solid wire is about 0.05 uH.
http://www.consultrsr.com/resources/eis/induct5.htm
To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be
about 200 pf.

That's enlightening.

One must suffer before enlightenment.

I need to admit that no such science was applied. I just tried small caps
from my stash to see what would happen. When something good happened, I
remembered it for next time.

Well, I was a bit more scientific when I tried it about 10 years ago
in an effort to improve the "American Legion" J-pole. This is from my
memory, which is notoriously unreliable.

I had the help of a return loss bridge, sweep generator, and
oscilloscope.
https://www.google.com/search?q=return+loss+bridge&tbm=isch
I could tweak the antenna, and see the results to the VSWR
immediately. I was finding that the J-pole has a rather narrow
operating frequency range and I wanted to know if something could be
done about it. Mounting the connector on the base resulted in the
narrowest usable bandwidth (at VSWR = 2:1) but also the lowest VSWR.
Using the more conventional feed points furthur up from the base, the
minimum VSWR was horrid, but the bandwidth was wider. Inserting a
series trimmer or wrapping the insulated wire around the 1/4 wave rod
reduced the minimum VSWR, but narrowed the bandwidth slightly.
Replacing the 1/4" dia aluminum rods, with 3/4" copper water pipe
increased the usable bandwidth somewhat, but made the insulated wire
wrap capacitor very small and critical. This may sound very
scientific, but in reality, it was all done in about an hour, with no
photos or recorded data. I had to return the equipment that
afternoon.

About a year ago, the local radio club had a beginners session on
building "emergency antennas", which means a VHF Slim Jim and J-pole
type antennas made from 300 ohm twinlead. The only test equipment
available was an HT and VSWR bridge. I predicted problems and was
invited not to show up. I did anyway, near the end. The problem was
that the antennas were VERY sensitive to their surroundings. It was
very difficult to cut one to length for minimum VSWR, while people and
metal objects moved around the room.

The j-pole that seems to benefit most is the 2m version, which I've built
the most of. Perhaps we're not dealing with just the inductance of that 6
cm of wire. There could be some residual inductance in the rest of the
antenna which is being brought to resonance. Maybe when my element lengths
are not optimal, some inductance would be found there. Not sure why the
residual reactance would always be inductive. The first time I add a cap
and it gets worse, then I'll know it isn't so.

Dunno. I can throw together a 4NEC2 model of a j-pole and see what
might be the feed point impedance for various styles and locations. No
time for that for a few days.

Maybe I should experiment with varying the spacing across the stub and see
what that does, with and without a cap.

It's more fun to fire up the 4NEC2 optimizer, and see what it comes up
with for the best possible antenna given a collection of restraints.
Yet another project...


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

On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote:
It's more fun to fire up the 4NEC2 optimizer, and see what it comes up
with for the best possible antenna given a collection of restraints.
Yet another project...

For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based
optimizer - a 30 segment free demo version is also available. (I'm a
satisfied EZNEC/AutoEZ user with no monitary connections.)
--
73, Cecil, w5dxp.com

"Jeff Liebermann" wrote in message
..

snip

Dunno. I can throw together a 4NEC2 model of a j-pole and see what
might be the feed point impedance for various styles and locations. No
time for that for a few days.

Maybe I should experiment with varying the spacing across the stub and see
what that does, with and without a cap.

It's more fun to fire up the 4NEC2 optimizer, and see what it comes up
with for the best possible antenna given a collection of restraints.
Yet another project...


Too many projects, not enough me. And I face a big non-hobby project of
cleaning and recoating the patio stones. Yuck!

"Sal"
(KD6VKW)

"W5DXP" wrote in message
...
On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote:
It's more fun to fire up the 4NEC2 optimizer, and see what it comes up
with for the best possible antenna given a collection of restraints.
Yet another project...

For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based
optimizer - a 30 segment free demo version is also available. (I'm a
satisfied EZNEC/AutoEZ user with no monitary connections.)
--
73, Cecil, w5dxp.com

I downloaded and used the demo version of EZNEC and I can say it was pretty
easy to get up and running with simple structures. I used it for a 20 m
dipole and I was quite happy when I raised the antenna at FD for its first
outing at full 30-ft height.

I keep meaning to buy the full version of EZNEC. Maybe this will push me
into it.

"Sal"