I have been trying to get an acceptable 2 meter antenna installed in
my attic. A J-Pole seemed to me the best solution for my attic space.
You may recall an earlier post where I discussed SWR on one of the
ladder line J-Poles I was working with. Well, I gave up on ladder line
and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my
basement work shop to give the best SWR and then took it up to the
attic. The following table shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing.
But, I didn't like the fact that SWR was above 2.0 in my attic and
that things changed between my basement shop and the attic. I guess
this design is influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an
end fed, open stub commercial design that promises less that 1.5 SWR
across the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about
this design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my
basement shop, it should give the same results in the attic.

Anyway, just my subjective observations.

jimbo - AJ7IM

jimbo wrote:
I have been trying to get an acceptable 2 meter antenna installed in
my attic. A J-Pole seemed to me the best solution for my attic space.
You may recall an earlier post where I discussed SWR on one of the
ladder line J-Poles I was working with. Well, I gave up on ladder line
and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my
basement work shop to give the best SWR and then took it up to the
attic. The following table shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing.
But, I didn't like the fact that SWR was above 2.0 in my attic and
that things changed between my basement shop and the attic. I guess
this design is influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an
end fed, open stub commercial design that promises less that 1.5 SWR
across the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about
this design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my
basement shop, it should give the same results in the attic.

Anyway, just my subjective observations.

jimbo - AJ7IM

Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST

On 25 Apr 2006 16:43:11 -0700, wrote:


jimbo wrote:
SNIP
jimbo - AJ7IM

Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST


I started building dipole antennas. they can be mounted horizontal or
vertical and easily moved around. I use cpvc with a dipole wire
inside and a coax-choke balun. They work quite well.

Just another option.
--
73 for now
Buck
N4PGW

"jimbo" wrote in message
. ..
I have been trying to get an acceptable 2 meter antenna installed in
my attic. A J-Pole seemed to me the best solution for my attic space.
You may recall an earlier post where I discussed SWR on one of the
ladder line J-Poles I was working with. Well, I gave up on ladder line
and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my
basement work shop to give the best SWR and then took it up to the
attic. The following table shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing.
But, I didn't like the fact that SWR was above 2.0 in my attic and
that things changed between my basement shop and the attic. I guess
this design is influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an
end fed, open stub commercial design that promises less that 1.5 SWR
across the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about
this design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my
basement shop, it should give the same results in the attic.

Anyway, just my subjective observations.



Not to detract from the other posters ... IMO they all made valid comments,
but I offer this:

As the builder of about 20 copper pipe j-poles, I discovered that I can get
a better final VSWR if I include a 100 pF (or so) cap in the side fed by the
coax center.

I had been attaching an SO-239-type bulkhead connector to the short side of
the J and running a piece of solid wire from the center pin over to the long
side of the J. I could get the VSWR to a dip near the middle of the band by
monkeying with the feed point and/or trimming the length but the lowest I
usually got was around 1.5 : 1. As soon as I used the cap in place of the
straight wire, I could get 1:1. It works for me ... your mileage may vary.

I built more j-poles for 2M than all the others combined and 20 is not a lot
of antennas. For some reason, the cap seems to help the 2M size more than
it helps the others, but these antennas are too few in number for scientific
judgements by this amateur plumber.

73,
John

jimbo wrote:
I have been trying to get an acceptable 2 meter antenna installed in my
attic. A J-Pole seemed to me the best solution for my attic space. You
may recall an earlier post where I discussed SWR on one of the ladder
line J-Poles I was working with. Well, I gave up on ladder line and
constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my
basement work shop to give the best SWR and then took it up to the
attic. The following table shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing.
But, I didn't like the fact that SWR was above 2.0 in my attic and that
things changed between my basement shop and the attic. I guess this
design is influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an
end fed, open stub commercial design that promises less that 1.5 SWR
across the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about this
design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my basement
shop, it should give the same results in the attic.

Anyway, just my subjective observations.

jimbo - AJ7IM

I guess my conclusion is that the conventional J-Pole design seems to
be more susceptible to near by objects than the much less common end
fed, open stub, J-Pole. My conventional copper J-pole changed much
more than the end fed Arrow J-Pole when moved from the basement to the
attic. And I have noticed on other occasions that even very small
changes in location and/or orientation had a large effect on SWR.

The ARRL Antenna Book has a small section on J-Pole antennas. They
show both designs, conventional shorted stub and the uncommon end fed,
open stub designs. They say that the shorted stub design should have a
4/1 balun at the feed point because a direct coax connection results
in extreme sensitivity to near by objects. And they also say the end
fed, open stub design doesn't require a balun and is much less
sensitive to near by objects, but is harder to tune. (I don't
understand that point, there is one less parameter to fiddle with.)

All of this leads me to wonder why almost all J-Pole designs one sees
are of the shorted stub version and almost none use a balun. I suspect
ease of construction and low cost. My "store bought" Arrow end fed,
open stub, J-Pole seems to verify the ARRL Antenna Book's conclusions
about sensitivity to near by objects. However, do-it-yourself
construction does appear to be much more difficult. I may give it a
try, just out of curiosity.

A final note. My attic is a very difficult place to work. There is no
way I can install an antenna and then make numerous adjustments to get
it tuned to the space. I must be able to do any required tuning in my
basement shop or maybe outside on my deck and then take it to the
attic for installation. So for my application, I want something that
is not sensitive to near by objects. The end fed, open stub J-Pole
seems to fit that requirement.

73s jimbo

On Wed, 26 Apr 2006 09:14:24 -0600, jimbo wrote:
[snip]
The ARRL Antenna Book has a small section on J-Pole antennas. They
show both designs, conventional shorted stub and the uncommon end fed,
open stub designs. They say that the shorted stub design should have a
4/1 balun at the feed point because a direct coax connection results
in extreme sensitivity to near by objects. And they also say the end
fed, open stub design doesn't require a balun and is much less
sensitive to near by objects, but is harder to tune. (I don't
understand that point, there is one less parameter to fiddle with.)

All of this leads me to wonder why almost all J-Pole designs one sees
are of the shorted stub version and almost none use a balun. I suspect
ease of construction and low cost. My "store bought" Arrow end fed,
open stub, J-Pole seems to verify the ARRL Antenna Book's conclusions
about sensitivity to near by objects. However, do-it-yourself
construction does appear to be much more difficult. I may give it a
try, just out of curiosity.

[snip]

A j-pole (open or closed stub) will perform better with a common mode
choke at the feed point. Even better, use two chokes. One at the feed
point and the other 1/4-wave down the line.

This will reduce the higher angle radiation that results from the feed
line being part of the antenna.

Using a choke on ANY antenna fed with coax is just good engineering
practice. It can't hurt - only help. A few turns of coax costs very
little.

73,
Danny, K6MHE


In my many years I have come to a conclusion that one
useless man is a shame, two is a law firm, and three
or more is a congress. - John Adams

email: k6mheatarrldotnet
http://www.k6mhe.com/

wrote:

You can effect the match with either open or shorted stubs when the
dimensions are correct. Performance is not significantly different.

See Mr Cebiks site. He has done a lot of work that is more fact than
lore.


A final note. My attic is a very difficult place to work. There is no
way I can install an antenna and then make numerous adjustments to get
it tuned to the space. I must be able to do any required tuning in my
basement shop or maybe outside on my deck and then take it to the
attic for installation. So for my application, I want something that
is not sensitive to near by objects. The end fed, open stub J-Pole
seems to fit that requirement.


An attic is a problem for many antennas and tuning them is part of it.

One trick I've tried to help others was tune it inside a room and then
note how the SWR changes in location and adjust accordingly.
Tuning in this case was adjusting the length of the longer element
(capped with 1/.4" brass bolt and nut) and stub length without
altering the initial best SWR tap. Usually shortening the longer
element was the needed adjustment.


Allison

Yes, I read Cebik's article on J-Poles several times. I don't doubt
that both designs can be tuned to the same SWR and produce the same
performance. And I tried the brass bolt and nut trick and a coil of
coax at the feed point. But in my basement shop, the shorted stub
copper J-Pole was very, very sensitive to almost any change of
orientation or location. And then everything changed when I installed
it in the attic. But the Arrow open stub J-Pole seemed to produce the
same SWR regardless of orientation or location in my basement or in
the attic.

Anyway, thanks for the insight, jimbo

"jimbo" wrote in message
. ..
I have been trying to get an acceptable 2 meter antenna installed in my
attic. A J-Pole seemed to me the best solution for my attic space. You may
recall an earlier post where I discussed SWR on one of the ladder line
J-Poles I was working with. Well, I gave up on ladder line and constructed
a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop
to give the best SWR and then took it up to the attic. The following table
shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing. But,
I didn't like the fact that SWR was above 2.0 in my attic and that things
changed between my basement shop and the attic. I guess this design is
influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an end
fed, open stub commercial design that promises less that 1.5 SWR across
the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about this
design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my basement
shop, it should give the same results in the attic.

Anyway, just my subjective observations.

jimbo - AJ7IM

"jimbo" wrote in message
. ..
I have been trying to get an acceptable 2 meter antenna installed in my
attic. A J-Pole seemed to me the best solution for my attic space. You may
recall an earlier post where I discussed SWR on one of the ladder line
J-Poles I was working with. Well, I gave up on ladder line and constructed
a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop
to give the best SWR and then took it up to the attic. The following table
shows the results.

Basement Attic

144 1.90 1.80
145 1.65 2.00
146 1.50 2.10
147 1.60 2.25
147.995 1.90 2.25

Actually, probably not that bad for a first attempt at copper tubing. But,
I didn't like the fact that SWR was above 2.0 in my attic and that things
changed between my basement shop and the attic. I guess this design is
influenced by objects close by.

So, somewhere in my surfing I came across the Arrow J-Pole. This is an end
fed, open stub commercial design that promises less that 1.5 SWR across
the band. Here are the results of my experiment with this antenna.

Basement Attic

144 1.10 1.22
145 1.18 1.10
146 1.25 1.20
147 1.30 1.32
147.995 1.40 1.42

Not 1.0 SWR but certainly met the promise. And the best thing about this
design is that near by objects don't seem to have an impact on
performance. If I built one of these designs and tuned it in my basement
shop, it should give the same results in the attic.

Anyway, just my subjective observations.

jimbo - AJ7IM

You should be able to tweak the antenna on the bench until the SWR is more
like 1.15:1 or so. Once you get it right there will probably be a lot less
detuning once it is put in the attic

The J-pole crowd seems to tout fantastic claims about this antenna and I
have never been impressed. Many people have tested the J-pole against
other easy to make antennas and the end fed half wave J-pole usually
does not stand up to even it’s brother, the center fed half wave dipole.
I assume the decrease in performance is due to mistakes in assembling
and tuning the matching section. Here is a link to a group that tested
some J-poles against other antennas in the CA desert and the J-pole lost
to even simple 1/4 wave ground plane types.
http://www.geocities.com/SiliconVall...5/anttest.html
My point is, why not make a simple ground plane for the attic that may
not get bothered by surroundings as much as the J-pole.
Bob


Buck wrote:
On 25 Apr 2006 16:43:11 -0700, wrote:

jimbo wrote:
SNIP
jimbo - AJ7IM
Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST


I started building dipole antennas. they can be mounted horizontal or
vertical and easily moved around. I use cpvc with a dipole wire
inside and a coax-choke balun. They work quite well.

Just another option.