ad: elecraft

W1JR choke/ balun on a ZS6BKW doublet?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 4X1SO, Jan 8, 2014.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-innov
ad: l-gcopper
ad: L-rfparts
ad: l-Waters
ad: l-rl
  1. 4X1SO

    4X1SO Ham Member QRZ Page

    Hello All,
    I am thinking about installing a ZS6BKW design doublet at my QTH. Each leg is 45' with 39' of 450 ohm window line feeder. Can I put a W1JR choke balun at the window line /coax transition and feed the antenna with a random length on 50 ohm coax? Some published versions of this antenna show no balun and 60' of coax to "smooth out" the matching for use without a tuner on the bands the antenna was designed for. My understanding is that the 60' coax is to help prevent common mode currents on the feedline. Should the W1JR choke/balun eliminate the need for the extra coax? I could get away with using less than 20' of RG213. Am I on the right track in my thinking?
    Kal, 4X1SO
  2. W5DXP

    W5DXP Ham Member QRZ Page

    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.

    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.
    Last edited: Jan 8, 2014
  3. 4X1SO

    4X1SO Ham Member QRZ Page

    Hi Cecil,
    Thanks for your suggestions and input. I can build the balun using an FT240 size core, I have 31 mix and 43 mix available. Which would be more appropriate assuming usage down to 75 meters?
  4. W5DXP

    W5DXP Ham Member QRZ Page

    Looks like 12T on the 43 mix would be a reasonable choice. I think a double core 43+61 choke would be interesting. Maybe we can get Steve to add that combination of mixes to his graphs.
  5. 4X1SO

    4X1SO Ham Member QRZ Page

    Thanks for the link to Steve G3TXQ's site for chokes. The other part of my question concerned the length of the coax feed to the balun. I see on your website how you match antennas using varying lengths of window line. Is the 60' length of coax critical for "no tuner" operation of the ZS6BKW?
  6. W5DXP

    W5DXP Ham Member QRZ Page

    My ZS6BKW is basically a 92 ft dipole fed with 40 ft of ladder-line. Here are the SWR(50) values:

    7.1 MHz is 1.3:1, 14.2 MHz is 1.3:1, 18.14 MHz is 2.9:1, 21.3 MHz is too high, 24.95 is 2.2:1, 28.4 is 5:1, and 29.2 is 1.2:1. These are the SWR(50) values looking into the ladder-line. I need to use my IC-756PRO autotuner on 17m, 12m, and 10m and it just won't tune for a match on 15m.

    Losses in 60 ft of coax will reduce the SWR somewhat but will still probably require a tuner on most bands. A ZS6BKW is NOT a no-tuner antenna. The best one can do is trim for no-tuner operation on a couple of bands, e.g. 40m and 20m for mine. I have to vary the length of the ladder-line to keep from having to use a tuner on the other bands. As you can see from my previous graphs, one doesn't have to vary the ladder-length by much to achieve no-tuner operation but varying the ladder-line length is problematic if the coax/twinlead junction is 60 ft. away. My ZS6BKW is at 40 ft directly overhead so my ladder-line drops straight down to my shack window, i.e. no coax required.

    If you are expecting the standard ZS6BKW to work without a tuner on more than a couple of bands, I predict you will be disappointed.
  7. 4X1SO

    4X1SO Ham Member QRZ Page

    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.
  8. K8AG

    K8AG Subscriber QRZ Page

    I have been using a ZS6BKW homebrew antenna for about 12 years now. I didn't bother with a choke when I first built it. Lots of RF in the house makes for angry family. The rig also acted really funny. I simply wrapped 12 turns of coax about 6" in diameter at the end of the window line. RF gone from the shack. Happy family.

    The most I run is 100W or so. But a commercial choke or beads should work as well. Coax is cheaper.

    73, JP, K8AG
  9. W5DXP

    W5DXP Ham Member QRZ Page

    The problem with coaxial air-core baluns is that they are relatively narrow-banded compared to a good 1:1 ferrite current balun as can be seen from the following web page:

    The 10T/7" "ugly" balun is mostly non-functional on 10m and probably getting close to series self-resonance where it goes through a zero-crossing from being inductive to being capacitive. I suspect that same thing also applies to your 12T/6" "ugly" balun when used on 10m. But it should do an adequate job 40m-20m.
  10. 4X1SO

    4X1SO Ham Member QRZ Page

    Lots of good information has been shared here. After looking at Steve, G3TXQ's great charts on chokes I have another question. How much choking impedance is actually necessary? As a starting point, am I better off putting up a choke whose resistive impedance is 2k ohm over a broad frequency range? If I put up the antenna and don't suffer any CM issues I would think I am good to go. If a certain frequency shows up as a CM "hot spot" I can tailor the choke to increase impedance in that range. What would I do if problems occur in two bands that can't be covered by a single choke let's say 40m and 10m. Sorry for all the questions. I know a lot of this is going to be cut and try.
  11. W5DXP

    W5DXP Ham Member QRZ Page

    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.
  12. G3TXQ

    G3TXQ Ham Member QRZ Page

    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
  13. W5DXP

    W5DXP Ham Member QRZ Page

    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.
  14. WA7DU

    WA7DU Ham Member QRZ Page


    "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...
  15. W5DXP

    W5DXP Ham Member QRZ Page

    Here's one way to do it:

    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 Files:

  16. WA7DU

    WA7DU Ham Member QRZ Page

    That is slick. Thank you.
  17. KM1H

    KM1H Ham Member QRZ Page

    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.

  18. 4X1SO

    4X1SO Ham Member QRZ Page

    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
  19. GM3SEK

    GM3SEK Ham Member QRZ Page

    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 "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
  20. KM1H

    KM1H Ham Member QRZ Page

    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.

Thread Status:
Not open for further replies.

Share This Page

ad: dxeng