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Thread: W1JR choke/ balun on a ZS6BKW doublet?

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  1. #11
    Join Date
    Mar 2002
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    Tyler, TX
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    Quote Originally Posted by 4X1SO View Post
    Thanks for the clarification on what to expect from the antenna.
    I posted a ZS6BKW mod today on this newsgroup that not only allows for 75m operation but actually improves the SWRs on the other bands at least at my QTH.

  2. #12

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    There are a couple of issues to consider: we want to reduce the CM current on the coax so that we maintain reasonable balance in the ladderline; and we want to ensure that the residual CM current through the choke doesn't cause a heating problem. Both issues depend on the choke's load impedance, and the CM impedance looking back along the coax braid.

    If we assume, worst-case, that the braid impedance is zero, and assume that the VSWR(50) looking into the ladderline never gets above 4:1, a choke with an impedance >1000 Ohms should ensure reasonable current balance.

    If we now assume the choke is wound on a single FT240 size core, we should try to limit its average power dissipation to 10W. To do that, if the average power into the ladderline is 100W, the choke impedance needs to be >500 Ohms.

    So, to meet both criteria a choke >1000 Ohms will do the job.

    Please note that only conducted TX CM current has been considered, and those calculations assume: the choke is resistive; the average power level is not greater than 100W; and the choke load impedance never exceeds 200 Ohms balanced. Change any of those factors and the answer changes

    Steve G3TXQ

  3. #13
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    Quote Originally Posted by 4X1SO View Post
    ... am I better off putting up a choke whose resistive impedance is 2k ohm over a broad frequency range?
    When the RF emerges from the choke, it has a choice of two directions of travel - toward the antenna or down the coax braid back toward the transmitter. The answer to your question depends upon what impedance those two paths presents. Let's take a look at best and worst cases involving conducted common mode signals.

    1. The impedance looking toward the antenna is 50 ohms and the impedance looking back toward the transmitter is 2000 ohms. One might not even need a choke under those conditions.

    2. The impedance looking toward the antenna is 5000 ohms and the impedance looking back toward the transmitter is 200 ohms. One definitely needs a choke and a choke with that much choking impedance is difficult to come by.

    By choosing an appropriate length of ladder-line that ensures a low impedance at the choke, one reduces the choking requirements and stress on the balun. Seems to me, random choices of ladder-line length tends to obey Murphy's law.

  4. #14
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    Quote Originally Posted by 4X1SO View Post
    Thanks for the clarification on what to expect from the antenna. My Kenwood 850 has a fairly good internal tuner that I can try. If that doesn't cut it, I have an old HeathKit SA2040 that I can put on line. I really appreciate your taking the time to discuss this with me.
    Quote Originally Posted by W5DXP View Post
    A ZS6BKW (and G5RV) indeed does need a 1:1 choke balun at the coax/twinlead junction. Assuming the ferrite used in the w1jr balun is an acceptable material, e.g. #61, it will perform the balun function well above 14 MHz. (40m may be problematic.) 90 ft may be too short for the ZS6BKW dipole. If the wire is not insulated, I would suggest you start out with 94 ft and trim for a perfect SWR on your favorite band.

    Other ferrite materials may be superior to #61. Here is a web page that may be helpful.

    http://www.karinya.net/g3txq/chokes/

    Here are the measured SWR curves for my ZS6BKW. It is 92 ft of insulated wire fed with ladder-line whose length can be varied from 39 ft to 42 ft. I do that so I won't have to use a tuner. A 500 pf series capacitor on the coax side of the 1:1 choke allows reasonable operation on 75m but the capacitor should be shorted out for the other bands.

    http://w5dxp.com/ZS6BKsWr.JPG

    DXP:

    "Vary the length of the ladder line."

    That is a new idea to me. Smart, but new. How do you do that? Can you make that adjustment without physically manipulating something at the radio end of the ladder line? Would it still be practical if your ladder line did not drop right into your shack? Thanks for the idea...
    Am le haghaidh Dia. Téigh le Dia.

  5. #15
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    Quote Originally Posted by WA7DU View Post
    DXP: Vary the length of the ladder line."How do you do that?
    Here's one way to do it: http://www.w5dxp.com/notuner.htm

    I'm experimenting with switching lengths using one DPST switch rather than 2 x DPDT switches per loop. Here's another method that I once used for varying the length of the ladder-line. It worked very well and could be automated using a screwdriver motor.
    Attached Images Attached Images

  6. #16
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    Quote Originally Posted by W5DXP View Post
    Here's one way to do it: http://www.w5dxp.com/notuner.htm

    I'm experimenting with switching lengths using one DPST switch rather than 2 x DPDT switches per loop. Here's another method that I once used for varying the length of the ladder-line. It worked very well and could be automated using a screwdriver motor.

    That is slick. Thank you.
    Am le haghaidh Dia. Téigh le Dia.

  7. #17

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    A question not answered is the type of ferrite? A FT-240-43 will work fine 80-10M and wound conventionally with RG-8X or RG-142, depending upon power and VSWR, they are in stock at Mouser under the real Fair-Rite part 2643803802 at far less than Amidon.
    If you intend to build a different antenna to include 160 then the 31 mix is a better choice.

    The JR method adds nothing to the performance.

    Carl

  8. #18

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    Thanks for checking in Carl. I already have the the FT240-43 and the 142 coax. I also have some 31 mix cores. I guess the 43 mix it is. All I need is the appropriate weather resistant container and I'm set. Thanks very much to all who responded and shared their knowledge and experience. VY 73 and Shalom from Jerusalem. Kal, 4X1SO

  9. #19
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    Quote Originally Posted by KM1H View Post
    A question not answered is the type of ferrite? A FT-240-43 will work fine 80-10M and wound conventionally with RG-8X or RG-142, depending upon power and VSWR, they are in stock at Mouser under the real Fair-Rite part 2643803802 at far less than Amidon.
    If you intend to build a different antenna to include 160 then the 31 mix is a better choice.
    Agreed - go to Mouser. After being badly burned by Amidon supplying a large order of 2.4in toroids in completely the wrong material, I won't ever buy from them again.

    The JR method adds nothing to the performance.

    Carl
    The "W1JR" winding with the center crossover has very little affect on the magnitude of the broad resonance peak, but it does move that peak to a higher frequency. With the conventional winding method, the start and finish of the winding can be very close together, so the self-capacitance of the winding is quite large and also quite difficult to control. The crossover winding places the start and finish on opposite sides of the core, so the self-capacitance is not only smaller (which increases the resonant frequency) but also much more predictable.

    Tthe crossover configuration often gives a more convenient layout as well. For many applications I simply secure the opposite ends of the winding with cable ties and let the whole thing hang in the coax. The weight only becomes significant when someone insists on using a box.


    73 from Ian GM3SEK

  10. #20

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    The "W1JR" winding with the center crossover has very little affect on the magnitude of the broad resonance peak, but it does move that peak to a higher frequency.
    Very true Ian but the 10M end is rarely a problem using 43 material. At 80M it becomes a problem to get sufficient choking with a limited number of turns and a single core.
    Putting it in a metal box as JR did is not a good idea either, at least when I tried it at the 1200W level on a G5RV up 70' between 2 pine trees. The RG-213 feed was hot in the shack. I finally ended up with a plastic box plus 12 large 43 mix beads over the RG-213 at the box end. Yes it was heavy but the #12 Copperweld for the antenna and the #12 copper for the balanced line held up well until I built a real antenna system.

    Carl

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