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Off Center Fed Dipole

Discussion in 'Amateur Radio Equipment Reviews' started by N5VEG, Nov 16, 2010.

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  1. F1MQJ

    F1MQJ Ham Member QRZ Page

    What OCF do you have

    Hi
    For what i've seen there are many kind of OCF dipole : true conrad-windom with 1 wire set at 1/3 and ground radiants, FD4/FD3= conrad-windom 1/3 with balun, carolina windom=OCF with balun and part of coax between balun and choke balun making vertical part of the antenna, Center load OCf with a serial CL in the midle of the dipole but Off center feed at about 20%
    Could you give me these informations (%feeding, center load, vertical participation of the coaxial, ...) for those who were happy with it
    I'm especially searching answers with half antennas (less than 35m=115feet) the only room i have.
    Thank you in advance
    73's
    F1MQJ - remi
     
  2. N1GPT

    N1GPT Ham Member QRZ Page

    If we applied this logic to its fullest extent, there might not be any hams on the air at all. What about any ham who buys a commercially made radio instead home-brewing one from scratch? Do they need to be shot too?

    I live in an apartment. I don't have a workshop, well-stocked parts bin, or power tools (unless you could my soldering iron). However I do have a Radio Works Carolina Windom 40 on my building's roof. Could I have built this antenna myself instead of buying it? Well, sure, but let's think about that for a minute.

    The antenna has a 6:1 balun and a 1:1 line isolator. I would have to build these. For that, I would need some PVC tubing, end caps, epoxy, wire and some ferrite cores. I don't happen to have PVC tubing lying around so I'd have to locate a supply house and buy some. I also don't have any ferrite cores, and I don't know of any places locally where I can buy exactly the right ones off the shelf, so I'd have to order them. (Realize that this would incur shipping costs too.) I'd need the epoxy and wire too.

    I'd also need some SO-239 connectors, PL-259 connectors, a bit of miscellaneous hardware, end insulators, a 10' section of coax, and of course some suitable antenna wire. I don't have any those handy either, so again I'd need to buy them somewhere.

    Lastly, to assemble the balun and line isolator, I'd need to drill some holes (to properly mount the SO-239 connectors, etc...). For that, I would need a drill, but I don't have one. I could certainly buy one, but after I was done I likely wouldn't end up using it again, and it would just end up taking up valuable closet space.

    On top of material costs, there's the cost of time spent tracking everything down, and the effort of trying not to make a mess in my apartment.

    Now, the fully assembled and tested antenna that I bought costs about $130. Could I build it myself for less money? Actually, if you count the cost of buying a drill kit, no. But let's assume I was able to beg/borrow/steal one. Well, ok: then it might end up being cheaper to build it myself, but not so much cheaper that it would warrant my being taken out and shot. If I cut every possible corner I might reduce the cost even more, but I don't think I would be as satisfied with the results.

    Frankly, I like the Radio Works antenna that I have now, and I don't care how offended anyone might be by that. In my opinion, I think the price is pretty fair compared to other antennas, and considering that the 1:1 line isolator is included in the package. (I've seen other commercially available OCF dipoles with documentation that suggests the use of a line isolator, but they don't include one.)

    If it makes you feel any better, I did implement a home brew solution for mounting the antenna on the roof that met the requirements of: a) minimal cost, b) sturdiness, c) non-conductive materials, d) being easy to for one person to put up or take down, e) avoiding the need for drilling holes and f) achieving sufficient clearance between the antenna and the building to avoid excessive de-tuning.

    As for the performance of the antenna itself, it seems to work pretty well, given the somewhat compromised installation location. I've worked all over the continental U.S, and into Europe, South America, Mexico, Canada, Alaska, Hawaii, Japan, and even once to China (on 10 meters no less), all with 100 watts. The SWR curve is a little off on 20 meters (the resonant point looks to be a bit below 14Mhz), due I suspect to the effects of some nearby metal obstructions on the roof, but it seems to generally meet advertised specs.

    I recently managed to put up a multi-band vertical antenna on the same roof, and I can tell that the noise floor is generally lower with the Windom. The Windom also performs noticeably better than the vertical on 40 meters.

    I ended up choosing the Windom because, sadly, my building isn't big enough to accommodate a typical G5RV antenna, at least in the mounting location that I currently have staked out. Also, a G5RV needs about 30' of ladder line hanging vertically from it, and in my case it would end up dangling outside the windows of several other apartments in the building. I'm sure the tenants wouldn't be to thrilled at that. A beam was also out of the question for various reasons. The 40 meter Windom fits in the available space while still yielding good performance.

    -Bill, N1GPT
     
  3. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I had one of these up in Iowa: http://kh2d.net/windom.cfm

    I had it between two very tall trees, with the feedpoint at 40 feet. The trees were higher, so the elements sloped upward a bit.

    It was a very effective antenna from the middle of the country, and was very easy to match.

    OCF's do have a penchant for feedline radiation, so you must take care to dress the coax away from the antenna at a right angle. A choke balun or line isolator near where the coax enters the building can help. Some windoms use the feedline radiation on purpose to get a vertical component to the radiation pattern.
     
  4. N5VEG

    N5VEG Ham Member QRZ Page

    So far, my homemade OCF dipole has performed just fine! I am using it to call a weekly 10 meter net and everyone seems to hear me OK. I make 5-9 contacts all over the US and Puerto Rico without an amp. No big SWR problems on any band with my Z100+ autotuner. It carries 40 and 20 meters without the tuner. Between the OCFD and my S-9 43footer, I don't need any more antennas.;)
     
  5. WA4OTD

    WA4OTD Ham Member QRZ Page

  6. KK9CQ

    KK9CQ Ham Member QRZ Page

    OCF

    At present I have a 135' and a 270' OCF up. Both are fed with coax and I use a 4:1 balun on both. The smaller one is in a sloping "L" configeration. The larger one is in a tilted "triangle" configeration. During 30 years plus, I have erected and tried (used) many different antennas. The OCF has become my favorite.

    Bob
     
  7. N3UUX

    N3UUX Ham Member QRZ Page

    Bob,

    Can you elaborate on the "tilted triangle configuration"? I've been going through my options for getting on 160 meters. I have a 80 meter OCF dipole up now with a 4:1 balun. To make it a 160 meter, I should be able to just add 135 feet of wire to the 45' side of it, correct?

    Also, what is the height on your antennas?

    Thanks,
    Keith - n3uux
     
  8. KK9CQ

    KK9CQ Ham Member QRZ Page

    OCF

    N3UUX:
    PM has been sent.

    Bob
     
  9. N3UUX

    N3UUX Ham Member QRZ Page

    Bob - I replied to your PM, but I'm not sure it went through. It's not showing up in my sent mail box. If it didn't come in, let me know.

    And thanks so much for the info.

    Keith
     
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