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Thread: OCF

  1. #1
    "Island Bartender" KG4CGC's Avatar
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    OCF

    Old Codger's Forum. Also called an off center fed dipole.

    I was looking online for dimensions to build one or at least an OCF calculator. The basic premise is that instead of feeding it in the center with 1:1, move the feed point over to where 2/3rds are on the long leg and 1/3rd is on the short leg using a 4:1 balun.
    I already have a 4:1 that I bought at a hamfest a few years ago and now ended up making the long leg 66' and the short leg 33'. Boy let me tell you what! All the main ham bands really lit up! Even 80m and the AM BCB. 40m comes in strong. 20m comes in strong. 10 meters even (RX tested on 11 since no one was on 10m) was alive and signals were strong.

    I want to pull it up through the trees about another 20 feet and get the feedpoint further up in the ether. It's probably close to 30' off the ground at that feedpoint. The long leg slopes up at about 45 and the last 18' is starting to curl down in the trees pointing NW. The short leg points due south, almost a perfect 90 from the long leg and slopes down from the feedpoint to about 25' at it's lowest. I will drop it again to add more coax when I get more coax but my question is, what length should I adjust the elements to? It seems pretty bright as it is on the RX. I have not TX tested it yet as I'm changing things around in the operating position. Any advice well be well taken and I wish I had explored OCF dipoles years sooner.

  2. #2
    Lord of the Flies WZ7U's Avatar
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    Charles! Leave it and see if your external tuner can clean it up some. Try it! It may be fine right where it is.

    The one I had up before the storms and tree movement ate it up was 90' one side. 45' the other and it played good for as low as it was. When I put up the vertickle, it wouldn't play nice any more then the weather had its way with it, all on a 1:1 balun. I would like to try a 4:1 sometime with new wire up higher and see how much better it would be.

    Congrats man! HF is a kick.

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  3. #3
    Volcano Tamer PA5COR's Avatar
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    I've been using the original Fritzel FD-4 OcF for 10 years or so, 1500 watt version with 1:6.6 balun and incorporated 1:1 choke.
    Added coil and length of wire on the short end to resonate it on 1850 so it is working on 160 as well, coil was 180 microH and length of wire after that about 7 meters, cut to resonate on 1850.
    Original FD-4 lengths : Fritzel used the shown dimensions of 13.8-m and 27.7-m in the 1960s.
    That way i added a band, you can add more bands by just adding 1/3 and 2/3 lengths for the band(s) you want to add like the fan dipole.
    https://www.eham.net/reviews/detail/1055
    For more details on the OCF look here: https://pa0fri.home.xs4all.nl/Ant/FD4/fd4eng.htm search the FD-4 and read up.
    "If the Republicans will stop telling lies about the Democrats, we will stop
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  4. #4
    Administrator N8YX's Avatar
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    Do you have access to an antenna analyzer, Charles?

    Be interesting to see an impedance/SWR plot across the various HF bands.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  5. #5
    La Rata Del Desierto K7SGJ's Avatar
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    Nice job Charles. That is exactly the same exact antenna I'm using. It has been a very good antenna for all the ham bands. I did check it out with my RixExpert and did a bit of tweaking, and was surprised how well it looked on almost all the ham bands. Of course I run it through a tuner so the rig sees something near 50 ohms. Without a tuner, the newer rigs throttle back as the mismatch gets worse. However, the older rigs, just like me, don't give a shit, and would probably load up to a 53 Volkswagen with no problem. I also use a 6BTV vertical, and between the two, I do okay. I'm not a serious DXer by any means. If I happen to connect with some DX, I will work it, but I prefer to spend some time talking with new contacts. I'm really not in to the I'm eddie, ur 599, 73, bye shit.

    Maybe when I muck out the radio room, we can hook up for a QSO.
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  6. #6
    "Island Bartender" KG4CGC's Avatar
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    I do not have an antenna analyzer but I'll ask around. I may run into one. That's something I should have bought years ago.

    I added 30' of coax at the shack end and got the feedpoint up another 20' and into the branches of the giant black walnut tree. The short leg is no longer at an "L" and the only way to describe it would be to say that if you stood under it and looked up, it would say 5 o'clock.
    Boy ah tell ya hwat. It sure does hear stuff. If I take it down for for any reason it will be to use heavier wire.

    I will experiment with TX over the weekend. Based on how well 80, 40, 20 and 10 come in, I think the tuner will be happy with it. I haven't ever listened to all those bands, including AM BCB, and heard everything without having to retune or switch antennas in the past. The last set up was 33' of window line attached to the same 4:1 balun with 33' leg on each end of the window line and it would ONLY tune up on 80 and reception on AM BCB wasn't nearly so lively. It was good, but not lively. Then, as you tune up the scale in higher and higher frequencies the sensitivity would drop off.

    So, to recap, the long leg faces NW, peaks at about 75' before curling down to terminate at 55'. The short leg is almost SE. Maybe, SSE? One small detail, I had to use a 6" double SO-239 to splice together the coax. That's a 6" section with a lot more shielding and it may affect the performance on certain bands as if I had a ferrite shield on that spot and cause the coax to act as a radiator if I had a tuner good enough to tune 160 on a non resonant antenna. This MFJ-948 will only tune on 160 with an antenna that's reasonably close to being resonant on 160. It won't tune an 80m antenna on 160. I've tried in the past. I'll see what happens over the weekend.

  7. #7
    Lord of the Flies WZ7U's Avatar
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    See if you can hear AM 1060 out of Calgary. All comedy all the time. I get it reasonably well at the coast after dark.

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  8. #8
    "Island Bartender" KG4CGC's Avatar
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    Quote Originally Posted by WZ7U View Post
    See if you can hear AM 1060 out of Calgary. All comedy all the time. I get it reasonably well at the coast after dark.
    860

  9. #9
    "Island Bartender" KG4CGC's Avatar
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    1540 AM ... for the India crowd? CHIN.

    CHIN's lineup consists primarily of Chinese (Cantonese & Mandarin) and Italian programming in the mornings and Tamil programming in the evenings and overnight. They also air Albanian, Bengali, Bosnian, Bulgarian, Croatian, German, Greek, Irish, Jewish, Russian, Somali and Ukrainian programming on Saturdays and Sundays.
    https://en.wikipedia.org/wiki/CHIN_(AM)

  10. #10
    "Island Bartender" KG4CGC's Avatar
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    Quote Originally Posted by WZ7U View Post
    See if you can hear AM 1060 out of Calgary. All comedy all the time. I get it reasonably well at the coast after dark.
    CKMX. I hear something zany in there but it's competing with some other station.

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