A bit baffeled by OCFD antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N3JBH, Jul 27, 2011.

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  1. N3JBH

    N3JBH Ham Member QRZ Page

    Every where i look i see balun's used in a OCFD antenna. Wouldn't Unun be a better choice? Just asking this for my own wisdom i do not have any plans to really make one.
    As i enjoy my 136 center fed dipole and it does every thing i need.
  2. K4SAV

    K4SAV Ham Member QRZ Page

    An OCF antenna needs a choke on the feedline as well as the impedance transformation. You don't want the current going back down the feedline. A current mode balun can provide that. An un-un provides no choking function.
  3. AE5JU

    AE5JU Ham Member QRZ Page

    If the feedpoint is in the direct center, as it is with your 80m dipole, then for the higher even numbered bands, 40 m, 20 m, 10 m, there is a node (high impedance point) directly at the feedpoint. By moving the feedpoint offcenter, you have moved it off of those nodes, and closer to to the antinodes (low impedance point) for the higher bands. There are various usable feedpoints that improve certain bands, from 31%-69%, 33.3%-66.6%, 39%-61%, and others. But what happens is that you are not right on the 50 ohm impedance antinode, but over where it is in the 200 - 300 ohm range. But it is there for most of the other bands, too. For lower OCFD's in the 30'-50' height range, 4:1 current baluns are used to get that 200 ohms down to 50 ohms. When the OCFD is higher, 50' or more, it is closer to 300 ohms, so a 5:1 or 6:1 current balun is used. Now you have an antenna that will work all of the even numbered higher bands, and a few of the others.

    Our club has an 80 m OCFD (33%-66%) that will work 80, 40, 20, 17, and 10 meters. It won't work 15 meters, but we have another antenna, a 20-15-10 m Tribander that will cover that. Of the bands the OCFD will work 40 and 20 are great, around 1.5:1 SWR. 80 is a little high, about 2-2.5:1, but easily taken care of with the tuner. 17 m is a little high, but again, easily fixed with the tuner, and we've made some good dx contacts with it on that band.

    We've also used a 40 m (44' + 22') OCFD for remote special event stations with good success. And one of the club members has a similar OCFD in his attic, with the 44' leg zig-zagged a bit, but working the world.
  4. N3JBH

    N3JBH Ham Member QRZ Page

    I understand the various feed ratio's needed but where i get confused is if this antenna is fed off centered which of course it is then it can't be truly a balanced antenna then correct?
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    Correct - the antenna is not balanced. That means, if you do nothing to prevent it, the currents in the two legs of the feedline will be out of balance and the feedline will radiate. To reduce feedline radiation you need to force balanced currents, and you do that with a device which presents a high impedance to out of balance currents - a BalUn.

    In this context the "Un" part is the antenna and the "Bal" part is the feedline. Even if the feedline is coax we want to force balanced currents in it, so it is still the "Bal" part even though it is coax.

    Hope that makes sense!

    Steve G3TXQ
  6. N0AZZ

    N0AZZ Ham Member QRZ Page

    I have several OCFD a 135' 6-80m and a 270' 6-160m both are coax fed using a 6-1 balun and most bands can be worked without a tuner 15m has to have external one. All can be tuned with internal tuner in my Quadra amp so they must be <3-1 to tune and that is band edges on 80m on mine. 160m needs a external tuner for all but a very small amount of bandwidth.
  7. N3JBH

    N3JBH Ham Member QRZ Page

    Thanks Steve. that makes sense
  8. W5DXP

    W5DXP Ham Member QRZ Page

    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?

    Seems to me an N:1 choke at the feedpoint just ensures that there is a standing-wave common-mode current minimum at the feedpoint while generating a standing-wave common-mode voltage maximum, i.e. 1/4WL back down the coax, there may exist a standing wave common-mode current maximum???

    If the OCF is fed with ladder-line and a choke installed at the shack, can common-mode signals be avoided on the transmission line for multi-band operation? Since the SWR on the ladder-line is low and tuners handle impedances in the neighborhood of 300-450 ohms very well, why not use a 1:1 choke and a tuner instead of an N:1 balun?
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Yes - but we usually want to balance the currents, not the voltages

    I would expect so.

    Not necessarily - it will depend on how high the choke CM impedance is and the coax CM path impedance, which as you know depends on its length, routing and termination. However, we know it 's tough to kill all feedline CM current on an OCFD and sometimes takes two or more chokes.

    Indeed - why not!

    I was simply trying to help the OP sort out the conundrum of why you would use a BalUn between and Unbalanced antenna and Unbalanced coax feedline.

    Hope you are keeping well, Cecil.
    Steve G3TXQ
  10. N3JBH

    N3JBH Ham Member QRZ Page

    "I was simply trying to help the OP sort out the conundrum of why you would use a BalUn between and Unbalanced antenna and Unbalanced coax feedline."

    Hence my confusion in the first place. 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
  11. G0GQK

    G0GQK Ham Member QRZ Page

    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.
  12. G3TXQ

    G3TXQ Ham Member QRZ Page

    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
  13. AE5JU

    AE5JU Ham Member QRZ Page

    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.

    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.
  14. W5DXP

    W5DXP Ham Member QRZ Page

    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 Files:

  15. K9SRV

    K9SRV Ham Member QRZ Page

    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
    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.

  16. W5DXP

    W5DXP Ham Member QRZ Page

    Ideally, it doesn't matter. Practically, most gurus recommend current baluns.

    One cannot answer that question until one knows the ferrite material. Do you know?

    It would work for 20m-10m, a 2:1 frequency ratio. It becomes somewhat impractical for a 4:1 frequency ratio.


    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.
  17. AE5JU

    AE5JU Ham Member QRZ Page

    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.


    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.
  18. G3TXQ

    G3TXQ Ham Member QRZ Page

    Nope - that would only be true if there was no Common Mode path and the load was fully floating.

    Steve G3TXQ
  19. G3TXQ

    G3TXQ Ham Member QRZ Page

    This chart will give you some idea how an air-cored balun compares to a ferrite-cored one:

    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.

    Steve G3TXQ
  20. K9SRV

    K9SRV Ham Member QRZ Page

    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!

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