2meter jpole question
 

I just made a 2m jpole antenna out of 1/2" copper pipes, like this one:

http://www.qsl.net/wrav/2mjpole.htm

On this web site the center conductor of the coax is connected to the
longest pipe and the shield to the shortest.

Now looking at other sites I find the center conductor connected the the
shortest pipe.

So what is the correct way to connect the coax?

Kjell Romma, Norway

My 2M "Copper Cactus" J-Pole has the coax center conductor going to the
short pipe...

Scott
N0EDV


Moskog wrote:
I just made a 2m jpole antenna out of 1/2" copper pipes, like this one:

http://www.qsl.net/wrav/2mjpole.htm

On this web site the center conductor of the coax is connected to the
longest pipe and the shield to the shortest.

Now looking at other sites I find the center conductor connected the the
shortest pipe.

So what is the correct way to connect the coax?

Kjell Romma, Norway

On Sun, 21 Nov 2004 13:12:34 +0100, "Moskog" wrote:

I just made a 2m jpole antenna out of 1/2" copper pipes, like this one:

http://www.qsl.net/wrav/2mjpole.htm

On this web site the center conductor of the coax is connected to the
longest pipe and the shield to the shortest.

Now looking at other sites I find the center conductor connected the the
shortest pipe.

So what is the correct way to connect the coax?

Kjell Romma, Norway


My year 2000 ARRL handbook has instructions for a copper pipe 2-meter
j-pole, and the author says it doesn't matter which pipe you connect
the center conductor to. (He does mention a coax choke: 3 turns of
coax in an 8-inch coil, held together with tape.)

bob
k5qwg

Moskog wrote:
I just made a 2m jpole antenna out of 1/2" copper pipes, like this one:

http://www.qsl.net/wrav/2mjpole.htm

On this web site the center conductor of the coax is connected to the
longest pipe and the shield to the shortest.

Now looking at other sites I find the center conductor connected the the
shortest pipe.

So what is the correct way to connect the coax?

Theoretically, it doesn't matter. The bottom 1/4WL of a J-Pole is
simply 1/4 wavelength of a balanced feedline similar to the original
Zepp antenna. I would install a choke on the feedline, however, to
try to keep the feedline from being part of the antenna.

As a point of interest, what is the feedpoint impedance of a 2m
J-Pole if the short is removed at the bottom and the antenna is
fed at that point, aka the original Zepp antenna? Is that like
the Arrow Antenna's J-Pole design?
--
73, Cecil, http://www.qsl.net/s5dxp

Hi Kjell

Electrically it doesn't matter, because a J-Pole consists of a 1/2
wave antenna sitting on top of a 1/4 wave matching section.
Mechanically, one leg of the matching section is integral with the
antenna to save the cost of a fitting.

However, in real life, connecting the center conductor to the stub
lowers the receive noise level of the antenna slightly without
affecting performance.
Albeit, placing three 1/16th wave wires extending from the top cap,
sorta like a capacity hat, will do the same thing.

TTUL
Gary

Cecil Moore wrote:
As a point of interest, what is the feedpoint impedance of a 2m
J-Pole if the short is removed at the bottom and the antenna is
fed at that point, aka the original Zepp antenna? Is that like
the Arrow Antenna's J-Pole design?

Just measured the Arrow Antenna J-Pole and can report that it
*is* a Zepp design scaled for 2 meters. Here's the schematic:

Center
Conductor---------+
1/4WL + 1/2WL
Braid---------+--------------------

The center conductor and braid are usually reversed for a Zepp
but that really doesn't make a whole lot of difference. The Arrow
Antenna J-pole just uses an open stub instead of a shorted stub
like the average J-pole. The 1/4WL open stub transforms the very
high feedpoint impedance of the 1/2WL section down to a low
feedpoint impedance. The 1/4WL shorted stub of the average J-Pole
transforms the impedance from zero ohms at the short to thousands
of ohms at the base of the 1/2WL section. Somewhere in between
those two values lies the desired value of 50 ohms so the shorted
stub has to be tapped.

Actually, I think I like the Arrow Antenna version better for
homebrewing. The 3/4WL section is bolted to the base plate which
is easy. Then an SO-239 can be mounted on the base plate for attaching
the coax and a 1/4WL #14 solid wire can attached to the center of the
socket and be supported at the top by an insulator attached to the
3/4WL section. The Arrow Antenna version can be mostly aluminum while
the ordinary J-Pole, being 100% copper tubing, is relatively hard to
solder.
--
73, Cecil, http://www.qsl.net/w5dxp

Kjell,
Basically, it just doesn't matter...
'Doc