|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 balun. It will work 80-6 meters with the built in tuners found in most radios today. (External tuners work also.)
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'.
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 length" of your wire. (This is the real length you require for resonance)
If you wish to build a working version of this Off-Center Fed Dipole, buy or build a 4:1 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.
Below are some fabrication pictures for the end insulators:
Here is how I make my antenna connections to a balun or feed point:
Here's how I make my baluns:
The first contact I ever made with this prototyped 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)
You can buy one of these antenna systems from me, completely fabricated and ready to hang for $89.00 (domestic shipping included).
You can buy a wound balun from me and put it in your own electrical box
for $30 (domestic shipping included)
You can email me HERE if you have any questions.