80m Off Center Fed Dipole

An Off Center Fed Dipole is an easy way to use one antenna for multiple bands. The 80m version of the OCF Dipole is supposed to be capable of 80, 40, 20, and 10m bands. With a tuner, you are also supposed to be capable of using some of the WARC bands. Construction is very similar to the standard dipole except a 4:1 Balun is required to match the feed point.

This Off Center Fed Dipole is set up for 80m. If you want a different frequency, calculate the length of a standard half wave dipole. Instead of dividing the length in half, multiply it by .64 and .36. Basically, one leg is 64% of the half wave and the other is 36%. At this position, the feedpoint impedance should be about 200 ohms.

Use a good quality 4:1 Balun to match your OCF Dipole. For this project, I decided to build mine. To prevent common mode currents, a 1:1 current choke was also added. The core these baluns are constructed on is capable of 200 watts. This setup will be running no more than 100 watts. It is a good idea to overrate these so you do not run into overheating problems.

Aluminum flat bar and a small piece of aluminum DINN rail is used to carry the load of the 80m Off Center Dipole. All screws and nuts are stainless steel.

After cutting each leg to length, a loop was added to the end to attach to the balun. To make the loop, use a pencil or pen slightly large than the termination post. Strip the wire back and form a loop around the pencil. Solder the loop together so it does not come loose. Use heat shrink at the base of the loop to help keep water out.

After attaching the legs to the balun, use a good rubberized tape to weatherize the connections. To keep the Aluminum DINN rail from chaffing the wire, plastic ties are used to isolate the legs. The coax is also attached to the aluminum flat bar in the same fashion.

The insulator used is the same design I have used in other projects. They have worked very well and are much more ascetically pleasing than PVC couplers.

After getting the antenna in the air, I did not have time to tune it. Just to see where it was at, I ran a couple tests with the antenna analyzer. Most bands were showing 2:1 or more except 20m. Since the SWR wasn’t to bad, I decided to do a little testing with FT8. Several 20m contacts were made. With the assistance of the tuner, contacts were also made on 40 and 80m. Even without proper tuning, the Off Center Fed Dipole is performing well. When tuning is done an update post will be made.

Links

40M Dipole

Hub for Portable Dipole and NVIS Antennas.

http://www.66pacific.com/calculators/dipole-antenna-length-calculator.aspx

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