DIY Current Balun and OCF Dipole
I heard some hams talking on the air about a new antenna they were trying out. The antenna in question was an Off Center Fed (also known as OCF) dipole and they claimed their version was tuneable from 40 to 10 meters with the exception of 15 meters.
I did a little research and found out the input impedance was around 200 ohms more or less, therefore a 4:1 balun was required, preferably a current balun. Not having such a critter in my junk box, I started researching and decided to try my hand at building one. My junk box contained a large toroid inductor removed from a very big UPS inverter. The toroid was wound with #12 wire so I carefully unwound the toroid and rewound it with two coils and wired it up as a 4:1 balun. I was able to get eleven turns for each coil so I stopped there. I googled the numbers printed on the toroid and discovered that the permeability was 26. I wired up a couple of 100 ohm resistors where the dipole would connect and checked it out with my MFJ antenna analyzer. Lo and behold the SWR was flat from below 80 meters to well beyond 10 meters.
Not having an enclosure for this contraption, I happened upon an old plastic peanut butter jar that would barely contain the new balun. At this point, I figured this kluge had very little chance of working. Here in all its glory is my new balun:
2/24/2018 update: My Skippy Peanut Butter container finally gave up the ghost due to U.V. and the weather. A short memorial service will be held honoring the long life (at least longer than I thought it would last) of this awesome performer. In lieu of flowers, please send donations to the Georgia retired nerds home. Mr. Skippy has been replaced by a MFJ-913 Balun and the dipole is now 80 meters vs. 40.
One side of the dipole is 84 foot 10-1/2 inches long, the other is 47 foot 10-1/2 inches. I used #12 stranded and insulated wire used for electrical wiring. I was able to string this thing between two trees up about 35 feet. Unbelievably, this thing works even on 15 meters with a maximum SWR of 2.5 on any band.
My first contact was Alaska on 40 meter CW. I started working some SKCC stations and thought I would never get away from all the people calling me. This contraption beats my antique R7 vertical hands down.
So if you're thinking of building an OCF dipole or winding a balun, give it a try! You might just create something wonderful.