There are very good reasons why the J-Pole antenna has become so
popular in the past few years. They are easy to build, cheap,
and when they are built right almost indestructable. They can be
built out of nearly any material the typical amateur can find
around the house or purchase for just a few dollars. In this
issue I am going to discuss a few different variations. The
simplest I found was the TV Twin-lead version I downloaded from
the intenet. Brett, N5SQK brought over the ladder-line version.
He said it will out-perform the one made of twin-lead. It was
from an article by Ed Humphries, N5RCK. The super J-Pole came
from a QST article by Jim McDonald, WB0JQH. He called it an End
Fed Extended Double Zepp. However, call it what you may it is
still a J-Pole. that article along with some interesting hints
was supplied by my good friend Bruce Cameron, WA4UZM. The Copper
Cactus Two-band J-Pole was from a 73 article by John Post KE7AX
and was supplied by Brett N5SQK. There are some ariations in the
dimensions. The dimensions of the twin-lead and ladder line
versions are different because of the different velocity factors.
the dimensions of the Copper Cactus dual band were chosen to make
the antenna usable on both 2 and 440. An interesting variation
on the Copper cactus dual band unit that I did not draw up was to
build two J-Poles. One inverted below the other. Only the top
one was fed. He made the matching secions 1/2 inch longer than
the single one. One short-coming I and others have found with
the J-Pole is that they do not like to be near metal. thus, if
you side mount it on the tower it must be well away from it.
About 2 years ago I tried to stack 4 J-Poles on a 25 foot mast.
I had thme spaced about 7 inches from the mast. It did not work
at all. The double Cactus, since the section normally used to
support the J-Pole is now the lower element, must be spaced well
away from the tower and supported by an insulated support. Post
used a five foot length of 1 1/2 inch PVC. My experience with
PVC antennas tells me that if you do that put a wooden dowell
inside because PVC tends to flow over time and if you don't it
will droop..The usual way of feeding a J-Pole is to solder the coax directly to the elements. I have seen some more ingenious feed methods, but the simplest is the one shown by Jim Post on the Copper Cactus. He attatched an SO239 directly to the main element with a pop-rivet and then soldered it. According to Post you can use a machine screw just as well. A length of #10 wire across to the stub and you have a strong attatchment.
Most designs attatch the shield to the main element and the
center conductor to the stub. But not all, I guesse it doesn't
make a great deal of difference. Actually, since this is a
ballanced feed, a balun would make sense. I tried it and didn't
find much difference. I ran across an article that used an
involved matching-stub-balun system. But it looked too
complicated. I would have to be convinced it is worth all the
trouble,
The Super-J-pole is electrically equivalent to the Ringo Ranger. It is a favorite of Bruce, WA4UZM and he claimed that it out-performs the Ringo. I believe it, but then I don't particularly like the Ringo-Ranger. My experience is that it works well until about the second rain-storm. By then the gama-match is shot. KC4HAZ had good luck with his by mounting it upside-down on the side of his tower. I have had about 4 of them over the years and was happy to be rid of each of them.
Another design I didn't draw up was the J-Pole beam. This was a
simple 4 element beam except that the driven element was a J-pole. It has the advantage that the
beam is
supported by the driven element. If you try to mount a single
vertically polarized 2 meter beam above your low-band beam you
have to use an insulated mast. This obviously overcomes that
problem. If you haven't tried building a J-Pole yet you aren't
really living the Ham life. Run down to that your favorite hardware
store and get youself some 1/2 inch copper pipe and give it a
whirl.