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ZS6BKW Antenna Review: 1000 QSOs later...

Discussion in 'Amateur Radio News' started by K0LWC, Dec 20, 2020.

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  1. K9UR

    K9UR Premium Subscriber QRZ Page

    At HF frequencies these measurements are not at all super critical and lots depends on other factors like the earth under which the antenna sits and the surrounding objects and height abound ground and feedline length to your shack etc.

    Just follow the basics. 46 feet of wire each side of the center insulator. 39.5 feet of ladder line. An RF Choke and coax to your shack. Don’t over analyze it ! You can easily trim the ladder line to resonance where ever you like to operate in the band.

    These things are simple to make and modify. You don’t gain any measurable advantage by being perfectly resonant either.

    Experiment.
     
  2. VK2XTC

    VK2XTC XML Subscriber QRZ Page

    An update:
    Over the weekend I was able to test the antenna in a very quite radio location about 200km (125M) West of Sydney.(Grid = QF46IH)
    I was able to get it up to almost 20 meters (65 feet) [about 1/2 the height of the gum tree] with the ends at approx 15 meters (50feet).

    My measurements are 28.4 meters (97.13feet) with the ladderline of 12.2 meter (40 feet) and I have a 1:1 Balun.

    The results (best dip) are:

    Frequency 2.91Mhz 3.060Mhz 3.240Mhz
    VSWR 1.98 1.37 2.00

    6.3Mhz 6.610Mhz 7.070Mhz
    2.02 1.50 1.99

    12.680Mhz 12.910Mhz 13.300Mhz
    1.99 1.8 2.00

    26.920Mhz 28.00Mhz 28.820Mhz
    2.00 1.71 2.01

    49.060Mhz 50.420Mhz 51.400Mhz
    2.00 1.25 1.30

    Based on the numbers above I believe the dipole section is long for 80/40/20.

    Still looking to get an answer as to what frequency to "Tune" the antenna as the fundamental Freq ?
    And how to measure the VF of the Ladderline?

    Please note that I'm using PVC coated Steel Wire Washing line. (60 meter roll)
    If anyone can provide a VF for Steel Wire rather than copper , would be appreciated.


    Thanks

    Brian (VK2XTC)
     
  3. VK2XTC

    VK2XTC XML Subscriber QRZ Page

    Thanks for responding Joe,

    Can you assist me with some of the Basics ?
    Such as do you adjust L1 (Dipole Section) to get it to the correct Frequency (Lowest SWR on the section of the band you want),
    and then adjust the ladder line for a deeper dip ( resonance) , or do you do the reverse.

    Is tuning performed at the Primary frequency, and then ladder line length to bring the rest of the bands into resonance ?

    What is the process by which you tune one of the these antennas ?

    thanks

    Brian VK2XTC
     
  4. K9UR

    K9UR Premium Subscriber QRZ Page

    The dipole wires and the feedline are combined into an antenna "system". Let us say that the antenna "system" can be tuned a number of ways. There is not a "right or wrong" way to tune them. You can either tune the antenna by adjusting the lengths of the L1, or you can adjust the length of the ladder line L2. You could even do both. The end result is quite similar.

    Practically speaking, it is far easier to trim the ladder line a bit - which is usually done safely at ground level and without the need to erect and dismantle, erect and dismantle the antenna multiple times - to adjust to the portion of the band that you want to cover. Now the reality is that the ZS6BKW by design typically will cover the entire ham bands that it was designed for (ergo 40/20/17/12 and the higher end of 10M) with reasonable VSWR with no need for lots of pruning or tuning. At most you would add perhaps 30 cm of ladder line or subtract 30 cm of ladder line to dial in the VSWR to the specific frequency that you operate. That's the magic of the 46 feet length and the 39.5 feet ladder line. It so happens to be 50 ohms or close to it on ALL those bands and so there is minimal pruning needed.

    The tuning steps may depend if it is a flat top T or an inverted V. If inverted v, changing the angle of the opening can alter the tuning. So can height above earth that the ends are oriented. If the antenna resonates low in the band and you need to go higher (say you are SSB only and the antenna resonates best in the CW sub band) then trim a few cm of ladder line and try it. I usually shoot for best match on the band / frequency that I operate most. But I don't fixate on that - with a ZS6BKW anything below 1.5:1 is good enough and even 2:1 is fine and canuse an ATU in the shack. Adjusting the lowest band, ie 40M will then impact all the higher bands. A small adjustment on 40M will have a 2x impact on 20M and a 4x impact on 10M. So do your trimming in small increments and ladder line is much easier to add to than each of the L1 elements if you do happen to clip too much off.
     
  5. K9UR

    K9UR Premium Subscriber QRZ Page

    Ignore the performance on 75/80M band -- it was never designed to cover that band. Let us start with 40M. Your antenna is too long. You should be at 46 feet per side (total length 92 feet across not 97.13...you're too long) and 39.5 feet of ladder line (not 40 ft, again too long). Both "too long" situations are additive and cause the resonant point to be below the ham bands.

    So as expected this antenna resonates far below 7 Mhz at around 6.5 mhz.

    Dial in the 7 Mhz section first. That will impact the upper bands. Aim for minimum vswr at 7.0 Mhz or 7.1 mhz depending what mode you run in Oz (CW or SSB).

    I suggest you cut the L1 wires to 46 feet per side. Then erect the antenna. Test the VSWR. It should now resonate closer to 7 mhz (higher in frequency than before). Do Not cut the ladder line til you test it out. If it is still below 7 mhz, now cut the ladder line to 39.5 ft. Then test again. IF it still is too low in the band, cut 2 or 3 cms at a time on the ladder line and re-test. You'll dial in 7 mhz first. Then 14/18/28 should fall into place.
     
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  6. VK2XTC

    VK2XTC XML Subscriber QRZ Page

    This ongoing saga continues.

    What I have discovered is that the ladder line section can be adjusted independent to the Horizontal section making it easier to tune.

    My ladder line section is 13.45m long , making it's Calculated 1/2 Wavelength = 150/13.45 = 11.15 Mhz
    Measuring Short / Open circuit for the VSWR dip = 4.430Mhz (open) , 9.040 Mhz (short) , 13.710 Mhz (open) 18.450 Mhz (short)
    Velocity Factor = 9.040/11.15 = 0.810
    150/9.040= 16.593 meters (Calculated Length)
    16.593 * 0.810 = 13.44M (Physical Length) - As measured

    So home made ladder line with PVC Coated Steel Wire Washing Line from the local hardware store has a Velocity Factor of 0.810. It's basically not very good for use as ladder line.

    The L2 section of a ZS6BKW is not , as some have stated, a 1/2 wavelength at 14Mhz (20M). It is actually 0.62 wavelengths.
    0.62 wavelengths at 14Mhz is 13.286meters and if the ladder line VF is 0.90 it would be a physical length of 11.957 meters.
    In my case it's VF is 0.810 so the physical length is 10.761 meters.

    Which means you need to tune the section L2 to 12.55Mhz for VF of 0.9 and 13.94Mhz for VF of 0.810. Not 14Mhz as some have stated.

    This of course aligns with what K7JOE has stated above as 39.5 feet converts to 12.039 meters for a 0.9VF ladder line.

    If it ever stops raining here (nearly a month now) I'll get out and test it with L1 connected at 2 x 14.250 meters long.



    Brian
     
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  7. K9UR

    K9UR Premium Subscriber QRZ Page

    yes, this is why the ZS6BKW is a good antenna on 20 meters. It is what is known as an extended double zepp (or very close) and so it offers some gain, about 3 db, in the broadside in favored directions. As you note, it is not a half wave antenna. As you will find out, the simplicity of it is what makes it stay in the air - light weight and simple. The specific lengths make it work on multiple ham bands meaning it's an inexpensive solution for most hams who want to be active multiple bands - 40/20/17/12/10 and will load up fairly easily on 80.

    Let us know your performance results!
     
    Last edited: Apr 12, 2021

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