Tuesday, September 6, 2011

Off-Center Fed Dipole / Windom Antenna

Off-Center Fed Dipole / Windom Antenna

This antenna has become quite popular, as the performance is exceptional. It's a dipole, fed 14% off center with a 4:1 balun. It will work 80-10 with the built in tuners found in most radios today. External tuners work quite well 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 to add 14% to 50% and you get 64% for the longest leg and 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'.

You can calculate in the velocity factor for 12 or 14 AWG stranded copper, jacketed wire. Simply take the lengths and multiply them by .975 and you will get the RF lengths of your wire. With these antennas, and the fact that we are using tuners, I have never found shortening the physical length by the velocity factor to give the RF length, any real performance significance.

So, all you hams out there wanting to build this working version of the Off-Center Fed Dipole, get a 4:1 current balun, feed that with the lowest loss cable you can find, go to Home Depot and buy yourself 132' of 14AWG stranded copper wire. I like to get the white colored stuff, it disappears against the sky better. Buy yourself a small length of 1" PVC pipe, cut two 6" lengths for your end insulators and get some rope to tie the thing up with.

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 balun eyelet out to 83' 2 1/2" and mark that point. That is where the wire passes through the end insulator. Similarly, for the opposite end, measure from the balun eyelet out to 46' 9 1/2" and mark, that is where the wire passes through the other insulator.

Below are some fabrication pictures for the end insulators:

Place heat-shrink tubing on the wire, put on an insulator and strip the end. Pull wire through insulator, find where it meets antenna and strip.

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 a 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, solder and attach to balun's antenna 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 wind storms without them to breaking.

Here's how I make my high power baluns:

I get a project box, stainless hardware and a balun I wound earlier. I drill and mount the items in the box.


I make the proper connections and seal the box. Then I test them. First contact of the box pictured: KH7U, Honolulu, HI. On 20m with 75 watts

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