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Broadband Butterfly Terminated Dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K5CBM, Jun 29, 2020.

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  1. K5CBM

    K5CBM Ham Member QRZ Page

    A while back, I was told about the Broadband Terminated Dipole that could be attached to the house. HOA/Wife friendly and gets on all of the bands (160m - 6M) --Win!!

    [​IMG]
    http://hflink.com/antenna/#BBTDROOF invented by Bonnie Crystal (KQ6XA)


    K4SFC put up the video of the same kind of antenna and swept the bands for SWR and everything was below 2:1, as promised. However, the equipment that he showed he used in his video cannot be found on the Internet at all, at least according to my Google-fu. Buxcomm cannot be found at all, nor their products.

    So, I started building. Got some wire and 4 #31 ferrous core beads, built a 16:1 balun (http://hflink.com/antenna/#BALUN), put it in a box, and put a SO-239 on it. We tested it first, put 800-ohms on the antenna-side of the balun and measured out 50-ohms on the radio side. Got a match! Got some more wire and made both sides of the dipole and soldered it to the balun. My house is rectangular, so both sides of the dipole should be pretty close to the same length. Within the 2-meters (6') as described in the description. Balun is mounted at one end of the house, on a rafter beam. A little while later, purchased a 1k-ohm 100W wirewound resistor. Put it in a box, and connected the dipole to it. The dipoles run in opposite directions, come through the soffets of the over hang, run down the length of the house, through another soffet, meeting back at the 1k-ohm 100W resistor, which is mounted to another rafter beam in the attic on the opposite end of the house. Purchased 100' of RG-213 to connect the antenna to the ham shack.

    Tested and passed my General. I'm now ready for Field Day 2020, right? (I did one other Field Day a couple of years ago with the local club)

    I setup my impromptu ham shack on the back porch, planning on operating 1D for my first solo Field Day with my IC-746Pro and Raspberry Pi. But, first I wanted to sweep the bands before we fired up the radio. What did I find? SWR beyond 10:1 on every single band except for 6-meters, which was just barely under 10:1. There went my Field Day. Tried to use the tuner built into the radio. Tried to use an LDG antenna tuner. Nothing would get the SWR down enough to operate on. And I couldn't hear one single soul.

    What the heck? Is there something wrong with my balun? Is there a way to adjust the SWR on this type of wire antenna? Maybe I need to build an 8:1 balun to bring down the reactance, therefore the SWR of the antenna? I got questions and I appreciate any and all constructive criticism that I can get.
     
  2. AK5B

    AK5B Ham Member QRZ Page

    Putting antennas within the confines of any house is seldom a win-win situation; lots can go wrong because of coupling issues, common mode issues, RFI issues, low height issues, etc. etc. Sorry to burst your bubble right out of the box---but thinking outside of the box, attic or house is often much more productive when it comes to antennas.

    Next time, consider a simple thin-gauge wire dipole or wire vertical in the trees. 26-gauge wire is nearly invisible once aloft...
     
    WB5YUZ likes this.
  3. K5CBM

    K5CBM Ham Member QRZ Page

    I think I found my issue.

    Not a good enough soldering job on the SO-239. Will fix and try again this weekend. 20200629_205226.jpg
     
    KA0HCP and 2E0CIT like this.
  4. 2E0CIT

    2E0CIT Ham Member QRZ Page

    I'm keen to see how it all works out when the soldering has been done ! :)
    And interested to see if the wire-wound resistor has an effect ...as all the plans I've seen for terminated antennas seem to say a hard-to-find non-inductive one is needed.
    73 Jeff
     
    AK5B and WB5YUZ like this.
  5. KM3F

    KM3F Ham Member QRZ Page

    Wire wound resistors look to RF as an inductor with a resistor in parallel and in series.
    It does not act as you might think at first thought.
    As a matter of fact a 50 ohm wire wound power resistor can be made essentially noninductive if a series capacitance is used to cancel out the effects of the "wire wound inductance" but is still frequency sensitive but with a lower Q than would be if the resistance was not present since it becomes a tuned series configurations that becomes a low resistance at resonance leaving the resistance active as the value.
    .
    Not often done, but a Dummy load can be made with a 50 ohm wire wound power resistor and a series variable tuning capacitor to make a frequency variable 50 ohm test load that mimics an antenna complete with +/-j values.
    I have one built to use as an instructional to show at meetings and presentations about HF antenna where the people can see the results on an antenna analyzer as they vary the tuning.
    .
    With the arrangement shown by the poster, a number of effects are present besides the resistor.
    The resistor will absorb some power as well as have it's inductive effects on the elements.
    Combining all this with a Balun makes for some unknowns that could fool the user just looking at SWR at the end of a Coax.
    Some thought are, the end of each element for the most part is high impedance and dependent on the frequency vs length for that value.
    What does the 1k wire wound do on each Band?
    In any given installation, what are the couplings that exist that would not be present in a field install?
    There are a lot of unknowns doing this.
     
    K0UO, 2E0CIT and AK5B like this.
  6. K5CBM

    K5CBM Ham Member QRZ Page

    Good questions and more discovery for me to do.
     
    KD0CAC likes this.
  7. KD0CAC

    KD0CAC Ham Member QRZ Page

    Keep trying stuff :)
     
    AK5B and K5CBM like this.
  8. K5CBM

    K5CBM Ham Member QRZ Page

    The other night, I brought everything down from the attic, mainly because its just hot in the attic and all my tools are in the nice A/C.

    We (my Father-in-Law and I) resoldered the SO-239 to the balun, better job this time, drilled some holes and ziptied the balun to the box to take the weight of the balun off of the wire to the SO-239. We also terminated the balun to the box and terminated the resistor in the resistor box. Next job is going to attach loop ends to the dipoles to then attach to the devices.

    I added some of my son's lego men for some comic relief.
     

    Attached Files:

    2E0CIT likes this.
  9. AK5B

    AK5B Ham Member QRZ Page

    When transmitting, I bet the electrons passing back and forth will chuckle at the sight of those guys squirming around inside your balun box---they're going to be closely exposed to a lot of RF in there.

    If you hear any laughter or tiny cries for help coming from up there you'll know why.
     
    2E0CIT likes this.
  10. 2E0CIT

    2E0CIT Ham Member QRZ Page

    It's 2020 !!! Where are the Lego women????? (...........etc!) ;)

    When I see an interesting design for a terminated antenna it always seems difficult to find non-inductive resistors at exactly the required specs.
    Somewhere I read about an ancient way to make a non-inductive dummy load by adding just the right amount of salt to distilled water to get its resistivity just right.
    Maybe one could make non-inductive resistors in the same way. I did try it once*
    73 Jeff

    *without any success at all !:)
     
    Last edited: Jul 2, 2020

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