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/

"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

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
snip
jimbo - AJ7IM

Here is a link to one I have had good luck with. Works quite well out
of the box. I use two chaokes, one at the base, and one 19 inches below
that. You may need to trim the length of the half wave section, so cut
it a smidge long.

http://www.hamuniverse.com/2mladjpole.html

tom
K0TAR