Feed point chokes for J-poles: how close?
 

In a multi-band J-pole (where higher frequency j-poles are basically
mounted on the long leg of the next lower frequency leg) can the feedpoint
chokes for all the bands be located at the base of the antenna, or do they
need to be kept right by the J's for each band? The "copper cactus" has
the feeds running up the center pole and poking out holes in the tube
for each feedpoint. That would require putting the chokes at the base of
the mounting stub.

--
Ben Jackson

http://www.ben.com/

The purpose of the choke(s) is to reduce/eliminate coupling between the antenna and feedline, thus suppressing feedline
radiation. Running the coax inside the element effectively does this for the upper elements, so the bottom one is the
only one of concern. However, since it will couple to all the feedlines, they all need to be choked, but only for the
frequency of the lowest element. One simple way to do this is an isolated metal tube a quarter wave long just below the
mounting stub.

--
Crazy George
The attglobal.net address is a SPAM trap. Please change that part to: attdotbiz properly formatted.
"Ben Jackson" wrote in message ...
In a multi-band J-pole (where higher frequency j-poles are basically
mounted on the long leg of the next lower frequency leg) can the feedpoint
chokes for all the bands be located at the base of the antenna, or do they
need to be kept right by the J's for each band? The "copper cactus" has
the feeds running up the center pole and poking out holes in the tube
for each feedpoint. That would require putting the chokes at the base of
the mounting stub.

--
Ben Jackson

http://www.ben.com/

OOPS. Forgot-the bottom of the quarter wave sleeve choke needs to be connected to the shield.

--
Crazy George
The attglobal.net address is a SPAM trap. Please change that part to: attdotbiz properly formatted.
"Ben Jackson" wrote in message ...
In a multi-band J-pole (where higher frequency j-poles are basically
mounted on the long leg of the next lower frequency leg) can the feedpoint
chokes for all the bands be located at the base of the antenna, or do they
need to be kept right by the J's for each band? The "copper cactus" has
the feeds running up the center pole and poking out holes in the tube
for each feedpoint. That would require putting the chokes at the base of
the mounting stub.

--
Ben Jackson

http://www.ben.com/