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N8RT and N8RT/ Beacon

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Projects

OCF Dipole

I was looking for a good performing, multi-band HF antenna that I could fit within the confines of my yard. My original antenna was a 40m dipole made from some #14 THHN wire, hanging from some trees in my yard. But I really wanted something that would work well on 80m-10m. I kept hearing all these great things about the G5RV, but I remained unconvinced.

While researching various antenna designs on the web, I ran across a 7-band “off-center fed dipole” made by Buckmaster. They wanted $225.00 for this contraption. Sounded like a great idea. I did a bit more research on this antenna and what I found is that it is an 80m dipole, 135 feet long, but rather than being fed at the center it is fed at the 1/3 point. The beauty of this for me is that it would work great in place of my 40m dipole, since I already trees to support it at one end, and at the feedpoint. The long end presented a bit more of a problem, but as it turns out I had a tree in the front corner of my lot that could be used, if I made a bend in the long leg of the antenna at the 45′ point. And that’s what I did.

The antenna is very simple. As already mentioned it is fed at the 1/3 point. So for 80m and up, you have one leg that is 45 feet long, and another leg that is 90 feet long. Some tweaking may be needed, but mine was built within an inch or two of those measurements. Further research indicates that the total length might need to be closer to 133′.

Various sources gave different information on the feed point. Some say a 9:1 balun, some say a 6:1 balun is required, and yet some say a 4:1 balun. Others claim that it must be a current balun to work. I already happened to have a 4:1 Unidilla voltage balun still in the package hanging in my garage. So it was decided that I would use it.

I ordered new antenna wire to use from TheWireman.com. I chose their 13ga Toughcoat Silky (#531) wire. It’s a copper-clad steel wire, with a protective insulation. I wanted this antenna to last. I spent about $50 on the wire, including shipping, but it should outlast the #14 wire I was using before, and it sure beats $225 for the commercial version. Add some Dacron rope, the Blaun, and 3 insulators, and I have less than $75 total in the antenna.

Actual construction was very straight forward. My son helped me measure out the two legs of wire, as stated earlier we used 45′ and 90′. We attached the legs to the balun, attached insulators to the ends of the wire, and one insulator for the middle of the long leg that can free slide on the wire. We needed this as a support point where that leg makes a bend at the lot line and changes directions approx 40 degrees towards a tree in the front yard. We tied some rope to the end insulators, and to the balun, and up the trees I went.

Ideal height for the feedpoint is around 30-35 feet. I chickened out before getting that high, I ended up about 20 feet up. I used RG8X for the coax, and let it hang down from the balun. I slid 5 ferrite beads over the coax, and used shrink tubing to hold them in place. By the way, Harbor Freight Tools is a great place to pick up an assortment of shrink tubing.

On-air testing seemed to indicate that the antenna works quite well. One of the first stations worked was in Siberia on 20m. I later worked Switzerland on 40m. Both contacts were with 100W on my Yaesu FT-450. Other stations working the DX were running considerably more power than I was. I’ve since worked lots of DX using this antenna.

Using an Anritsu Site Master I swept the antenna from 5 to 30 mhz. The results are shown below (click the image to enlarge).

As you can see, the SWR dips on most of the ham bands. It’s best on 40 and 20 meters, and acceptable on the other bands, especially for a multi-band antenna. The Site Master doesn’t go below 5 mhz, so results for 80m aren’t shown here. Testing with an MFJ antenna analyzer indicated that the SWR was less than 2:1 across the entire 80m band, and was lowest in the CW portion. The antenna may be a bit too long, but I’m certainly NOT going to change anything! My LDG tuner has no trouble tuning the antenna anywhere that I’ve tried it. The antenna is designed to also work on 6m, but I have not yet checked the SWR there. Overall I’m very pleased with the results obtained, and would not hesitate to build another OCF antenna in the future.

Edited April 4 2011: Received this info via email from Joe KQ4BX:

From all that I read on the OCF, the height determines the type of balun required. At 33’, the feed point impendence would be 200 ohms, requiring a 4:1 balun. The higher it goes, the more the impedance rises, so the balun must change with it. There are a few web sites with the exact impedance at each height.

At the 20 feet you have, the impedance might be low enough to use a simple 1:1 to get RF off of the coax.

Thanks W0HC