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Thread: A bit baffeled by OCFD antenna

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  1. #11
    Join Date
    Apr 2001
    Location
    Shropshire. England.
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    17,152

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    I have often wondered why one would need an off centre fed dipole when moving the feed point a few feet towards the centre would nulify the problems or ishooz, if its preferred, associated with this antenna.

  2. #12

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    Quote Originally Posted by N3JBH View Post
    My take on this 1- the OCFD is idea to begin with as it is laden with issues. 2- while the balun is used to step up the impedance it is more needed to attempt to circumvent the common mode issues 3- being it is particularly the same length as a all band dipole 80 meters through 10 meters it really is not all that wonderful being you still need a tuner and a balun. My take on all this is what i have is probably much better. A 136 foot dipole that is fed with what is known as 450 ohm ladder line and a 1:1 very good current balun. As i can tune any frequency i can legally use from 80 through 10 meters. And have lower line loss as a bonus. I assume there be those that would naturally disagree with this and i welcome any advice to contradict it as i am still willing to listen. Jeff
    The feature of an OCFD which makes it attractive to some is that by feeding off centre you can find a position where the impedance is broadly similar on several (many) bands. That impedance is of the order of a few hundred Ohms, so it will present a reasonable match to window line or, with an appropriate impedance transformation, to coax. So, on many frequencies you will not need a tuner at all, or at most the internal auto tuner which some radios have.

    That aside, it radiates similarly to a centre-fed wire of the same length.

    Steve G3TXQ

  3. #13
    Join Date
    Oct 2008
    Location
    Louisiana Gulf Coast
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    1,014

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    Quote Originally Posted by W5DXP View Post
    Steve, since the impedances looking into each leg of the OCF dipole are quite different, doesn't balancing the feedpoint currents automatically unbalance the feedpoint voltages? Would the radiation pattern be different between balanced currents and balanced voltages?
    No, it is not two separate antennas, it is ONE antenna that you are driving at various frequencies. It does not have two different impedances on the two legs. It has one impedance at any particular frequency.

    I have often wondered why one would need an off centre fed dipole when moving the feed point a few feet towards the centre would nulify the problems or ishooz, if its preferred, associated with this antenna.
    Because if you move the feedpoint to the center it will not work the higher even numbered bands. For an 80 meter center fed dipole, for example, at 40 meters, 20 meters, 10 meters, there are high impedance nodes in the very center where the feedpoint is located. By moving the feedpoint off the center, it increases impedance for the 80 meter band, but that is taken care of with the 4:1 balun. But moving the feedpoint off center also moves it closer to the antinodes (points of low impedance, maximum current flow) of the higher bands. And while it is not directly on those higher band antinodes, it is in the same 200-300 ohm range, and the 4:1 balun matches that to 50 ohms pretty well there, too. Since it is unbalanced, using a current balun keeps RF off the shield and in the antenna.

    That is the whole point of the OCFD, to work those higher even numbered bands.

  4. #14

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    Quote Originally Posted by AE5JU View Post
    No, it is not two separate antennas, it is ONE antenna that you are driving at various frequencies. It does not have two different impedances on the two legs. It has one impedance at any particular frequency.
    If that were true, the V/I ratios at the feedpoint would be equal and we wouldn't need to worry about common-mode problems. But we know if we balance the voltages, the currents are automatically unbalanced and vice versa rendering the V/I ratios not equal. Here's an EZNEC example of what we are up against.
    Attached Images Attached Images
    73, Cecil, www.w5dxp.com
    Can CO2 emissions save us from the coming ice age?

  5. #15

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    OK, all very good information. I have a couple of questions, as I recently put up @ about 35 feet
    the MFJ-2010, 67 foot long dipole. MFJ sells them for $59, and I have been very happy with mine.
    Q1- If I were to build a second, longer ocfd, should I but a 4-1 CURRENT balun or a VOLTAGE
    balun?
    Q2- I bought some ferrites from a cool guy on here. If I snap them onto the feedline, would that
    be beneficial, and why.
    Q3- will a coiled part of coax replace the balun, and if so, how many turns and what diameter?

    Q4- This one will make me look like a LID, most likely, but here goes. If I were to buy an identicle
    MFJ-2010 67' OCFD and I ran it say 45 degrees to the one that is up now, would {theoretically} it
    be possible to buy a T connector, like truckers use w/ dual antennas, and connect the 2 identicle
    OCFDs at the shack, run 1 coax into the tuner and then have an antenna system that would radiate such that nearly all the nulls are covered by one or the other antenna?

    That last one took some guts to ask, as I assume there are innumerable reasons why it wont work
    and blow my transciever to hell and back, but I wanted to know.

    Flame away, and thanks for any and all input.

    John
    KB9ICO

  6. #16

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    Quote Originally Posted by KB9ICO View Post
    Q1- If I were to build a second, longer ocfd, should I but a 4-1 CURRENT balun or a VOLTAGE
    balun?
    Ideally, it doesn't matter. Practically, most gurus recommend current baluns.

    Q2- I bought some ferrites from a cool guy on here. If I snap them onto the feedline, would that
    be beneficial, and why.
    One cannot answer that question until one knows the ferrite material. Do you know?

    Q3- will a coiled part of coax replace the balun, and if so, how many turns and what diameter?
    It would work for 20m-10m, a 2:1 frequency ratio. It becomes somewhat impractical for a 4:1 frequency ratio.

    http://www.k1ttt.net/technote/airbalun.html

    If I were to buy an identicle
    MFJ-2010 67' OCFD and I ran it say 45 degrees to the one that is up now, would {theoretically} it
    be possible to buy a T connector, like truckers use w/ dual antennas, and connect the 2 identicle
    OCFDs at the shack, run 1 coax into the tuner and then have an antenna system that would radiate such that nearly all the nulls are covered by one or the other antenna?
    The two antenna fields would interfere with each other and the mutual coupling would change the feedpoint impedances. Such a configuration can be modeled and I wouldn't have a clue about the radiation patterns or feedpoint impedances until I modeled it. But I will predict that Murphy's Law will not be kind to you.
    73, Cecil, www.w5dxp.com
    Can CO2 emissions save us from the coming ice age?

  7. #17
    Join Date
    Oct 2008
    Location
    Louisiana Gulf Coast
    Posts
    1,014

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    Quote Originally Posted by KB9ICO View Post
    OK, all very good information. I have a couple of questions, as I recently put up @ about 35 feet
    the MFJ-2010, 67 foot long dipole. MFJ sells them for $59, and I have been very happy with mine.
    Q1- If I were to build a second, longer ocfd, should I but a 4-1 CURRENT balun or a VOLTAGE
    balun?
    Use a current balun to keep RF off the shield. The Buxcomm B15C41 CURRENT Balun 4:1 BALUN is an excellent choice for
    this antenna.

    http://www.buxcomm.com/catalog/index...oducts_id=2245

    Q3- will a coiled part of coax replace the balun, and if so, how many turns and what diameter?
    I've found 10-15 ferrites, such as Fair Rite RCT-2, to be a better choke balun than coiled coax (aka "ugly balun"). I had some "RF in the shack" problems with my fan dipole, with amp, Ten-Tec Jupiter, and a Heil mic. The RF was getting into the mic. Even though I had an "ugly balun" it did not help. Adding the ferrite beads stopped the RF. Replacing the center insulator and the beads with a 1:1 Current balun from Clear Signal also worked well, no RF in the shack.

  8. #18

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    Quote Originally Posted by AE5JU View Post
    No, it is not two separate antennas, it is ONE antenna that you are driving at various frequencies. It does not have two different impedances on the two legs. It has one impedance at any particular frequency.
    Nope - that would only be true if there was no Common Mode path and the load was fully floating.

    Steve G3TXQ

  9. #19

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    Quote Originally Posted by KB9ICO View Post
    Q3- will a coiled part of coax replace the balun, and if so, how many turns and what diameter?
    This chart will give you some idea how an air-cored balun compares to a ferrite-cored one:
    http://www.karinya.net/g3txq/chokes/

    Note that very few of the air-cored designs manages to achieve >1k choking impedance from 20m thru 10m, and none manage >2k. Also read the note explaining how reactive chokes can actually increase Common-Mode problems under certain conditions.

    73,
    Steve G3TXQ

  10. #20

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    Thanks guys,
    I really do appreciate the input, and I will print the link on baluns and put it in my binder.
    My fav. comment, is the reference to Murphys' Law, as it is one thing you can ALWAYS
    count on! Thanks Cecil!

    John

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