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.
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.
[...] impedance it is matching current for multi band use if my memory does not fail me, which it might. OCF Dipole Whoa! Holy Cow! nice article on the OCF Airplane build it and have fun, experimenting is the best part of the [...]
Nice article and website. I’m planning a 40m ocf and
looking forward to see how it works.
73 Randy
i built one for 160M using 180/90 ft. and it works great, i used a 4:1 balun and feed it at the 66 ft level. i used to have a full wave 80 meter vertical diamond shaped wire fed at 100ft that didnt work as well. worked alot of dx from africa including libya, jordan etc with 100 w on 17M. wish i build it alot earlier.
KF7NN… did you use a voltage or current balun for your 160m version? I'm getting ready to build a 160m OCF with a Unidilla 4:1 voltage balun. Plan to hang it 30-35' off my tower.
Kevin – N0KDZ
Excellent article. Truly inspiration to those of use with obstacle laden yards who want to work some decent DX. And you should have kept your old callsign; I have some rather creative phonetics for your old call. LOL
No matter how many times I said it phonetically on HF, they always got it wrong. Hotel Charlie seems to get through every time.
Thanks for the article. I was thinking of installing an 80M inverted V in my limited back yard, can probably just fit. I was planning 450 Ohm ladder line to a tuner. While planning it, a local ham suggested I think about an OCF instead. I have a tree limb at about 55 feet that already has rope around it , waiting for an antenna. It is a single tree, no second tree for traditional dipole. Any thoughts on a home brewed OCF with a 55′ apex ? Yard space is limited I probably have 40-50 feet on either side of the tree that provides the apex.
I am just ASSuming here, the long end of the dipole wire is the center or positive side of the balun?
Dont worry about what is the hot/center. The balun does the heavy lifting. You need a 4:1 current balun at the feed point to bring it down from 200 ohms to 50 ohms. Google 4:1 guanella balun. Simple to make and better than the voltage balun. If you have access to an antenna analyzer you put a 200 ohm carbon resistor on the output and you should have 50 ohms across the spectrum.
I am a huge fan of build-it-yourself antennas. These are easy to build, inexpensive and work great! I'm getting ready to put one up using my MFJ 43' vertical for support. I'm going to take the stinger off the top and use my ground radial wires for OCF elements. It won't be built like a tank but it should last long enough to be worth the price. I already have a 4:1 current balun so this will be fun!
Thanks for the great article!
Kevin WWØJD
I built to 80 meter version. Feedpoint at about 40 feet. Used a 4 to 1 current balun. Great match on 10, 12, 17, 20, 40 and 80. I like the fact that I can use an amp WITHOUT a high power tuner. Just use a high power(3KW) balun. Unbelievable antenna!
Jim
AB4XO
I have built plenty of wire antennas over the years with great success. For once, I decided to buy the commercial buckmaster high powered version. No compromise, I love it. Yes, it is expensive as all get out, but I saw immediately upon arrival I could not duplicate what they did in the construction, wire etc. in building the antenna. It also performed exactly as the charts show. In fact, I believe it is the cheapest antenna I will buy due to how long it will last. Performance is outstanding, no tuner required and it will handle all the power I will ever need. What more could you ask for? You just set it and forget it.
I also made my own 40m OCF. I purchased a spool of .14 gauge THHN insulated wire from Lowe's. It's 500 feet, but does multiple projects. I bought a 4:1 balun from Balun Designs. Bob Rumsey is very helpful, and the plans are on his web site. It's a simple 36/64% combination. His balun comes in a NEMA case, with a silicone gasket and SS hardware.
A synthetic rope holds it to a pulley. Coax putty is on the connection, and 100 feet of RG-8X goes under the house to the radio.. The ends are probably 15 feet off of the ground. I've been using it on 40/20/15/10m. This afternoon, I put my Yaesu FT-450AT on 17m and 12m, and hit the tune button. It tuned it. As the internal tuner only goes to 3:1, I might try these bands. The SWR can't be too far off. I had been told it only worked on even multiples (and 15m), but if it tunes…
I've had some good luck with it. I commonly work all over the USA, and up into Canada, as well as Europe. It has gotten me the Canary Islands, Alaska and Hawaii, and Australia, as well as a bit of South America.
Great article! After being out of ham radio for the past 17 years, I just got a new rig, bought a Comet vertical (which is great dummy load, I might add) and based on my poor results, I've been on the search for a good, dependable, long term wire antenna. Thank you for taking the time to write this. My search just ended. Putting one up!
Enjoyed the article. I've looked at it several times as I have studied to plan and build my own 80 meter OCF dipole. I got the antenna built and up today about 35 feet. With a little adjusting of the ends I have better results than I expected. I have less tha 1.5 on 80, most of 75, 40, 20, 17, 10 and 6. 12 meters is like 1.7 which is ok and 15 meters is off the charts; so, no good without a tuner. But 8 bands with one antenna and no tuner is pretty impressive and reports are impressive as well. Comparing my dedicated 75, 40 and 20, I see them very much the same. Mine ended up 88' on one side and 44' on the other; maybe a few inches longer on each end but not much more.
73,
Mike Baggett, ki5so
Great article. A few notes from my own research and experimentation:
OCFD is typically fed at 1/3 of the antenna ie. (44/88 ft) as in the article. These numbers relate only to OCFD and Caroline Windom antennea. The G5RV is a different beast.
Though the Original Windom antenna was a single wire feed, the Carolina Windom is coax fed and an improvement on the OCFD design with slightly different feed point, and a "recommended" current balun to assist in better SWR across the bands than a Voltage balun. The antenna also uses an RF choke 11 or 22 ft., depending on a 40m or an 80m/160m Carolina Windom, to handle 15m better and provide a better takeoff angle on some of other the bands, plus feed line isolation… But if you use a voltage balun remember to have the longest leg on the centre conductor side of the coax, on a current balun it doesn't matter.
Now for the tough stuff:
The big issue with what ratio balun to use relates to how far above ground the feed point is and what is the resistance of the feedline solution you are using in relation to the average resistance at the antenna feed point. (Wow, that a mouthfull and there is still reactance to consider at another time). A 9:1 balun would work best with 450ohm feedline to 50ohm Transeiver interface, but I have never experienced
Since new transevers use 50ohm interface to 50ohm coax, we will use this as the common point of reference for our examples. 50 ohm coax at 35ft feed point to the antenna will give about 200 ohms resistance so a 4:1 balun works best to match 50ohm feed line. At 40-50ft feed point above ground you get about 250 ohms at the feed point so a 5:1 balun would render an average of 50ohms, and at 60 ft it would be about 300 ohms at the feed point so a 6:1 balun would render 50ohms at the feed point and be a better choice. There are dozens of other ways to feed the OCFD.
Though the RadioWorks CW 80 or CW160 are built well, but if you can measure and solder, both the Carolina Windom and an OCFD are almost impossible to build wrong. I built my 66ft 40m Mini-Carolina Windom without an RF Choke so it's more truly an OCFD fed at 35 ft above ground… IT KICKS BUTT! and often outperforms my CHA250BX at 1/8th the price.
I cannot say enough good about the Carolina Windom version of an OCFD except: RECOMMEND, RECOMMEND, RECOMMEND…
De Ve3QTH a.k.a. Robert Critch
Thanks for the feedback, and the additional information, Robert! Our ham radio club is getting ready to build a few of these so the info on height vs balun ratio is timely and helpful.
Glad to hear you have had good results with the OCF! I am in the process of putting one up myself- hope to be on the air by tomorrow evening. I got licensed waaay back (late 80's) when I was in high school and never had the money to get an HF rig. Then I got married, kids, etc, etc, etc. I am now finally getting back into ham radio and studying for my General license.
My OCF is about the same specs as yours except I opted for the addition of a 20' vertical radiator coming down from the 4:1 balun. The antenna should be about 30' up once all is said and done, so I am hoping for some good signals.
I just need to get over my severe anxiety abut making that first CW contact on HF!
Mike
KC4APP
I just put up an OCF dipole using an excellent 4:1 balun the OCF Dipoles 1.5 – 54MHz 5kW #4115ocf from Balun Designs. I have put up many dipoles, windoms, and skyloops over the years in different locations all home brew. But this time was a fiasco. I have horrific RF feedback in the shack on 40 meters.
Does anyone have any ideas?
Please email me at W8LWB@arrl.net.
THANKS!
I'd try some ferrite beads on the coax to choke off any common mode RF.
I discovered the Buckmaster OCF a couple years ago, and mine has been up at 35' in the trees since then, even withstood Hurricanes Irene and Sandy. It is built tough, and I wish I would have known about this concept years ago, it could have saved me much work over the years.
It works on harmonically related bands, so I'm thinking of modifying an 80 M version to cover 60/30/15 meters. That would give me 10 bands coverage with 2 antennas.
73, de W3DL
Last year I put up an OCF, cut to the length of an 80-meter dipole. It is fed with 52-ohm cable and a 4:1 balun at the feed point. Here are my SWR results:
10 meters: lowest at 28.40
12 meters: less than 1.3 entire band
15 meters: 21.400 – 1.25, rising gradually to 2.4 at 21.000
17 meters: less than 1:5 entire band
20 meters: less than 1.4 entire band
30 meters: unusable on 30 meters
40 meters: 1.37 at top end rising gradually to 2.4 at 7.000
80 meters: lowest SWR at 3.760, 1.04 Less than 1.6 across entire band
On 6 meters, SWR will be less than 3.0 below 50.100
One more thing, I was wondering what the SWR results would be feeding the OCF with 450-ohm twinlead? I was thinking about building one fed with 450-ohm down to a balun, and then coax to the station. One of the reasons for doing this concerns the weight of the coax. Presently the feed point is about 75 feet up and coax down to the remote switchbox is quite heavy. Any ideas?