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W1JR choke/ balun on a ZS6BKW doublet?

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

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  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.

    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
     
    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:

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

    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.
     
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