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.
    73,
    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
     
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