I had a go at building an OCF dipole antenna.These types of antennae are very versatile as they will resonate on even harmonics, so an 80m OCF will work on 40m, 20m, 10m, and may be 6m.
There are plenty of technical docs on the internet to find when you search ‘OCF dipole’. The best article I found was by Ron Bertrand, you can find his PDF on www.res.net.au
I was planning on making an 80m but I did not have enough wire, so I had to settle for a 40m to start with.
I measured out 20m of insulated wire. From what I have read, the velocity factor is a bit lower for insulated wire so it should be somewhat less than 19m. I left the ends with about 750mm flopping down in case I had to clip the ends.
The feedpoint for an OCF is about 1/3 the distance from one end. I measured about 6 meters from the end that was closest to my shack. The impedance at the feedpoint will be around 300 ohms, it does not have to be spot on.
A 4:1 balun is used to transform the 50Ohm impedance to something close to the feedpoint. I made a simple current balun with bits from Jaycar electronics. The largest torroid they have is 35mm diameter, which is ideal. $4 for a pack of two.
I used enamelled wire 1mm thick, a small roll of 14m is $10, the balun uses about 3-4m, so costs about $2.50.
The balun completed in the box is screwed onto a piece of plastic from a kitchen cutting board. With a few holes drilled in the base for the rope and wire support it is ready for testing.
One end of the antenna I climbed up a ladder about 4m against a large tree. This is a bit dangerous and I don’t recommend it. Tie a weight to your cord and throw it through the fork you have chosen. Or I have heard of people using a fishing rod to cast a line up over a limb.
The SWR was about 1.7 which is not too bad, I could clip the ends and get it lower also. Once the antenna tuner kicked in SWR was sitting on 1.
The antenna worked well on 20m, and 40m. I made a coupled of contacts on both bands and got good signal reports.