AI4JI's Off-Center Fed Dipole Antenna System  

  This antenna has become quite popular, as the performance is exceptional across the Amateur HF bands.  It's a dipole, fed 14% off center with a 4:1 voltage balun.  It will work 80-6 meters with the built in tuners found in most radios today.  (External tuners work also.)

  The formula:

  for a dipole antenna cut to 3.6 MHz, take 468 / 3.5MHz and you get 130'.  That is the overall length of a standard dipole.  Normally you divide that in half to find the center feed point; however, we are feeding this 14% off-center, so add 14% to 50% and you get 64% for the longest leg which leaves 36% for the shortest leg.    So, 130' multiplied by .64 (64%) gives the longest leg length of 83.2 feet.  the shortest leg length is the remainder, or 46.8 feet.  You can double check this by reversing the math, 130' multiplied by .36 (36%) for the shortest leg is still 46.8'.

  If you wish to build a working version of this Off-Center Fed Dipole, buy or build a 4:1 voltage balun, buy or build a couple of end insulators and get yourself 132' of 14AWG stranded copper wire.  I like to get the white jacketed wire, it disappears against the sky better. 

  Cut your 132' of wire to 84' 2 1/2" and strip back about 6" off of one end, make your balun connection and solder it.  The remaining piece of wire, same deal, strip back 6" make the balun connection.  Measure from the far end of the wire back 6" mark that spot.  This is the end of the wire where it passes through the insulator.  You have a 6" tail to fold back, strip and solder to form the end of your antenna element.
This has been calculated using 14 AWG stranded copper wire, with the jacket on.  Do not use bare copper wire, or solid copper wire or you will not obtain the same results.

I prefer voltage baluns wound on a toroid, they seem to perform better than the current baluns with this antenna. Voltage baluns allow a greater bandwidth; whereas current baluns seem to become less efficient with longer runs of coax.

Please Note: The antennas pictured below are various versions I have been improving on over the past 12 years. I am now building revision D models. These include top mounting tabs for the box, drip rings to protect the coax connector, tabs below for strain relief for the coax and swaging thimbles to protect the antenna elements in the eyelets. My antenna concept and design has been copied and modified many times; however, as of May 2015, no one else has improved theirs (stolen my upgrades).

A completed AI4JI OCF (Revision D) Dipole, ready to hang and work the bands.

Below are some fabrication pictures for the end insulators:

Place heat-shrink tubing on the wire, put on an insulator and strip the end.                 Strip a section off the element side close to your insulator for the splice.

Wrap the bare end over the section your stripped and solder.                                          Slide the heat shrink tubing over the solder joint and heat until shrunk.

Here is how I make my antenna connections to the balun or feed point:

Place antenna wire through eyelet at the proper measured length.                                 Strip a piece of insulation from both sides of eyelet, wrap and solder.

Connect a terminal lug to the free end crimp and solder and attach to the feed point.
The above connection places the strain on the eyelets, without putting any real
strain on the actual antenna connection.  This also allows for movement of the
wires during windy conditions, preventing mechanical wire or connector failures.

Here's how I make my voltage baluns:

I get a heavy duty electrical box, stainless hardware and a toroid I wound.                  I drill and mount the items in the box.

I make the proper connections, seal everything and close the box.                                 Newer versions I build have a drip-ring around the connector.

 Ask about different cores for different power handling capabilities.                             Top tabs for supporting the balun.
I do build custom baluns and ununs. I build current baluns as well.

Bottom Drip Ring and Tabs for Coax strain relief.                                                             Side showing swaging thimble for element strain relief.

The first contact I ever made with the prototype of this antenna was the Calahari Desert of South Africa on 17m with 75 watts. (8400 miles with a 59 report)

The first contact made from the final design, actually pictured above:  KH7U, Honolulu, HI. On 20m with 75 watts. (4500 miles with a 59 report)

        **Please don't email asking about the cores, windings, or balun formulae, I consider those proprietary as I do produce these for sale.**

As of April 2017, I will be building and selling 4:1 Current baluns rated at 1KW. (I can build bigger upon request.)

I will also be building 9:1 UnUn's rated at 700 watts for end-fed long wire antennas.

The optimal wire length is 124.5' which allows the antenna to cover 160 and up.

You can buy one of these antenna systems from me, completely fabricated and ready to hang:

Covers                   Approximate Length     Price (Includes USPS Flat Rate Shipping in the US)

160-80-20-15-10-6            254'                    $149

80-40-20-15-10-6              127'                    $102

60-30-15-6                         87'                      $82

40-20-15-10-6                    65'                      $82

If you just want to buy the completed Baluns or UnUns and add your own wire, here are the prices: (Domestic Shipping Included)

4:1 Voltage Balun rated at 700 Watts        $58  (As pictured above)

4:1 Voltage Balun rated at 1400 Watts      $72

4:1 Current Balun rated at 1000 Watts      $69

4:1 Current Balun rated at 2000 Watts      $99

9:1 UnUn rated at 400 Watts                     $58

You can buy a wound balun from me and put it in your own electrical box for $44. (domestic shipping included)

I accept Cashier's Checks, Money Orders and PayPal.  CLICK HERE TO ORDER

You can email me HERE if you have any questions.



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