440 Mhz. J-Pole Antenna Plans
Here is a 70cm (440 Mhz) J-Pole antenna that is inexpensive, and easy to build. I use 1/2 inch copper pipe, and the associated fittings necessary. The dimensions aren’t typical however, this is what it took to get its SWR low.
The above dimensions for the J-pole are in inches.
Measurements on overall length, and stub length are from the
centerline of the separation pipe (horizontal) to the top of
the antenna.
The Connect at measurement is 1 1/2 inches from the top of
the horizontal member to the point of connection. The distance
between the main element of the j-pole centerline and the
tuning stub centerline is .75". To achieve this dimension,
I use a 1/2" pipe Tee, and a "street elbow". Before
assembling them together, I cut off the excess pipe
at the joint before assembly. I cut a length of RG-8X
foam coax to a length of 67" for the feedline, and coil
up 4 turns (as small as you can get it) just below the
horizontal part of the matching section. This will de-couple
the feedline from the j-pole antenna, and help provide
some lightning protection. Connect the center
conductor of the coax to the main element, and the
shield to the tuning stub of the j-pole.
In all of the above dimensions, they are to be
considered starting points. I temporarily attach the
coax using 1 inch hose clamps, and adjust the coax
connection first to the lowest SWR. From there, I adjust
the length of the main element. Then I start over by
re-adjusting the coax connection.
The point where the tuning stub attaches to the main
element is the j-pole antenna's ground point. That is
why you can make it any length. I soldered another
elbow about 1 1/2 feet below the ground point, and
side mounted it to my tower. This too will help with
lightning protection. (provided your tower is properly grounded!)
Only use rosin-core solder. Don't use "plumbing solder",
acid-core solder, or plumbing paste. The acid in these
materials breaks down the solder joint when electric
current passes through it.
To see a photo of a j-pole antenna that I use, click here.
Additional notes on J-Pole construction: