|Ham Radio - Antennas|
Here is an inexpensive J-pole antenna that is easy to construct. In about an hours time, and about $10 worth of materials, you can have a great performing omnidirectional j-pole antenna.
This antenna is based on the same ideas as my 2 Meter J-Pole construction plans.
The j-pole antenna is basically an end fed half wave dipole that uses a 1/4 wave shorted matching stub as an impedance transformer. The j-pole antenna will yield slightly less than 3 DB of gain omnidirectionally.
The material I chose to build the j-pole antenna was 1/2 inch copper pipe used for plumbing. Here are the plans to build a 70 CM J-pole antenna:
The above dimensions for the J-pole are in inches, and aren't typical for a 440 mHz. J-pole antenna. This is what it took for me to get the SWR low. 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 0.75".
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. To Achieve this dimension, I use a 1/2" pipe Tee, and a "Steet Elbow". Before assembling them together, I cut off the exess pipe at the joint before assembly.
|Then 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.|
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 of the J-pole. 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. Its a good idea to provide a ground here. 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.
Here is a photo of a 70 cm j-pole antenna that I use:
This one has been up for about 7 years. You can see how the pipe turns black from the weather. This is normal, and doesn't hurt the performance of the antenna in any way.
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