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Thread: Ugly Balun Theory

  1. #1

    Default Ugly Balun Theory

    Does anyone know how "ugly baluns" function? The most common answer among the google results suggests that the turns give the shield inductance and it is this added inductive reactance that stops the current from flowing

    Brief testing using a signal generator and and spectrum analyzer across the shield of a coax wound on a piece of scrap 4" PVC (20 or so turns of rg-58 i think) suggest this is not the case as the attenuation across the shield appeared to show resonance. There appeared to be a "sweet spot" on the frequency dial where the most rejection happened.

    So now im left with the theory that this inductance combined with the inter-winding capacitance is responsible for the resonance. Yet, after hours of looking, the article that describes ugly chokes in this manner has evaded me. Anyone have insight into the actual tuning of an ugly balun?

  2. #2

    Join Date
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    It sounds like you are using too much "inductance". I've never seen a choke with the number of turns you described.

    Sent from my moto e5 play using Tapatalk

  3. #3

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    If inductance alone is responsible for the choking effect, i would expect more turns to be better. This seems to not be the case. Also, if these "ugly baluns" did follow that theory, their performance would get better as frequency goes up (assuming a known-to-be multiband antenna is used), another trend i did not see when testing. Perhaps my rapidly assembled test fixture and probe cables added parasitics that i didnt account for. Another one for the back burner till life allows me the time.

    Thanks!

  4. #4

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    I thought useful inductance was 'turns/frequency range' specific?

  5. #5

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    Quote Originally Posted by WZ7U View Post
    I thought useful inductance was 'turns/frequency range' specific?
    Ive read so many times on forums etc that its the inductance that chokes the signal. More inductance=more inductive reactance, right? And that would mean a better choke (especially as frequency goes up), right? Well, wrap a few too many turns and suddenly it becomes absolutely worthless. Im in search of a mathematical reason for why that happens and how to get it right the first time. Ive went as far as tightly taping 2 pieces of coax next to each other to get an idea of the amount of capacitance between turns and use that in combination with the inductance to see if the parallel resonant frequency is near where certain designs have been built for, and there seems to be a close relation. However, my experiments have been fairly crude and I thought it would be worth seeing if anyone else has really dug into this before going further.

    Im ordering a 2 port VNA very soon (still researching options). When I do, a test setup for seeing the filter response of the shield wrapped in various configurations is going to demystify it for me. Ive simply had too many unpredictable results winding ugly baluns based on other peoples instructions.

  6. #6

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    I will be looking forward to your results.

  7. #7

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    Quote Originally Posted by WZ7U View Post
    I will be looking forward to your results.
    Just ordered the nanoVNA-F, hope the reviews were not paid although I suspect a 1GHz device should be fairly accurate at HF and VHF.
    I will share the results as soon as I learn the device and get it set up. The shipping info predicts Jan29th to Feb20th.

  8. #8

    Join Date
    Nov 2019
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    You might be interested in this web page on Common-mode chokes by G3TXQ. He seems to have the same interest as you and he could explain it better than I could. At the bottom of the page G3TXQ explains his test setup using a VNA.
    Martin, K7MEM
    http://www.k7mem.com
    Ash Fork, AZ - 60 miles from the Grand Canyon on Rt-66. Elevation 5,300 ft.

  9. #9

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    Thank you!

    I will read that right now. Waiting for my better half at the clinic so thats perfect timing!

    Edit: wow! Thats super cool. It brought a few things to my attention i never once considered. I cant wait to get this on the big screen at home and stare at it for a while. Now the wheels are turning! Great find! Thanks!
    Last edited by brandon lind; Thu 9th Jan 2020 at 15:50.

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