Rotatable OCF Dipole For 40 - 10 meters ?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KA7NIQ, Sep 25, 2013.

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  1. KA7NIQ

    KA7NIQ Ham Member QRZ Page

    I was looking at the Comet 422 10 - 40 meters rotatable trap dipole. But the Traps scare me, since I got a really powerful amplifier.

    One cool thing about the Comet h 422 is you can put it in a V Configuration, very handy for those of us who have tree limbs and power lines reducing our ability to rotate a long antenna.

    Then, I got to thinking, why not make a 40 - 10 meter OCF Windom, out of aluminum.

    Now, a 40 meter OCF Dipole will of course be 66 feet long, but would it be possible to load or shorten it a bit, and still keep the multi band capability of a windom type antenna, with coax feed ?

    And, since I am dreaming, would it also be possible to place this antenna in a V Configuration, like the Comet 422 ?
  2. KF6ABU

    KF6ABU Ham Member QRZ Page

    why not put up the Hy Gain Explorer 14 with the 40 meter add on kit you have (per your QRZ bio)
  3. KU3X

    KU3X Ham Member QRZ Page

    First some clarification: A Windom antenna is a vertical antenna, fed against a ground radial system with an off center fed top hat. A half wave dipole, fed off center is just that, "an off center fed antenna", it’s not a Windom.

    As far as configuring the OCF antenna as a Vee or inverted Vee, no problem. You treat an OCF the same way you treat a half wave dipole. An OCF 40 is nothing more than a 40 meter half wave dipole, it’s just fed at a different location which allows you to work more bands with the same length of wire.
    Adding a coil to shorten an OCF is possible, I’ve seen it done before. Rather than add a coil, why not just let the ends hang down a little? Use aluminum for the horizontal section and attach some wire on the ends that you are going to let hang down?
    You will probably only need to shorten the longer aluminum leg of the OCF by hanging wire off of the end since the short end is about 23 feet long. You should be able to use strictly aluminum for that end. You will want to make sure your 4:1 Guanella Current balun is located on the mast.
    Sounds like an interesting project and it is do-able !
    Barry, KU3X
  4. KA7NIQ

    KA7NIQ Ham Member QRZ Page

    That's a GREAT Question, and here is the answer. I have a 50 ft Aluma Tower, and when I bought it used, it did not have any guy wires. So, I was under the mistaken assumption that it was a self supporting tower.
    It is NOT! Well, not really. It will only support 8 sq ft of windload, in a 50 mph wind.
    The Explorer 14 is 7.5 sq ft, and the 40 meter add on kit places it at nearly 9 sq ft of windload!
    We have afternoon thunderstorms with 50 to 60 mph wind gusts here sometimes.
    Yes, the tower is a crank up, but because of power lines, and the power line running to my house, it will not be easy to crank down, once the antenna is up.
    Guy wires are absolutely out, so bye bye brand new Explorer 14, and hello Cushcraft D4 10 - 40 meter dipole, or maybe this OCF Dipole made out of aluminum we are talking about.

    If I can't build this OCF Antenna, I was looking at 40 - 10 meter trap dipoles, like the Comet H422, or the Cushcraft D4.

    I lucked into a Commander HF 2500 Amplifier, so i need something that will take legal power.
    That's why this OCF 40 -10 meter aluminum dipole idea appealed to me.
    It will work most bands from 40 -10, and has no traps to burn up, in case I am not careful with the drive power to the Commander 2500 :rolleyes:
  5. KA7NIQ

    KA7NIQ Ham Member QRZ Page

    Well, for mechanical balance, if one end is made out of aluminum, and is approx 23 ft long, then I will make the other end also 23 ft long out of aluminum, and simply allow the extra length, made out of wire, to "hang down".
    One has to wonder IF that wire hanging down will provide any vertical polarization, and what it will to to the SWR, on a windy day.

    LOL, if someone ever ask me "KA7NIQ, what is your SWR on that homebrew rotatable OCF 40 meter dipole" I will send them this link :D

  6. WB3BEL

    WB3BEL Ham Member QRZ Page

    I must be missing something fundamental...

    Why would you want Offset Fed in a rotatable dipole? I sure wouldn't. Offset fed antennas are prone to common mode radiation without extra care in feeding. If you want multi-band antenna , you could use decent traps or coupled sleeve wires with plastic standoffs to the aluminum tubing main section. Or you could use open wire line with some standoffs on the tower legs.

    But If I were in your shoes, I'd put up a hexbeam and get the benefit of some gain and directivity on 5 Bands. I would shunt feed the tower against ground on 40 and 80 meters. It will be a better DXing antenna on the high bands and the low bands and it will have pretty small windload.
  7. KA7NIQ

    KA7NIQ Ham Member QRZ Page

    A Hex Beam still has over 5 sq ft of windload, and does not offer 40 meters!
    Besides, the HF Bands are a train wreck, so good performance on 40 is an absolute must.
    The Rotatable OCF would give me most bands from 40 - 10 w/o any Traps for my Commander 2500 to blow up.
    3CX 800A7 tubes are not cheap!
  8. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    You do understand, an OCF does NOT have a simple broadside pattern like a regular dipole? So if you are thinking of rotating it like a beam you will get some pretty squirrelly results!

    Some of those results may be useful but you will certainly be scratching your head trying to figure out what's happening.

    The OCF has lobes and nulls all over the place! This becomes much more pronounced as you go up to the higher bands.

    It's up to you, of course. I predict it will be very interesting...

    Wild Bill VE3EKJ
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