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