|Ham Radio - Antennas|
If you are looking for an inexpensive, and easy to construct antenna, the J-pole antenna is a great one! In about an hours time, and about $15 worth of materials, you can have a great performing omnidirectional j-pole antenna.
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 3/4 inch copper pipe used for plumbing. Here are the plans to build a two meter J-pole antenna:
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 2 1/4 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 2". 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 for constructing a 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.
The photo below is one of my j-pole antennas that I use. This one is not the 2 meter version described here, but rather the 70 cm version of the J-Pole antenna.
So there you have it. I have built several j-pole antennas, and they work great! 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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