Balun Vs Choke

Discussion in 'General Technical Questions and Answers' started by M7LRB, Apr 16, 2021.

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  1. M7LRB

    M7LRB QRZ Member

    Hi all

    Very quick Q, and realise it is a huge subject....

    As a starter, as a basic op.... Balun or Coax Choke?

    Thoughts??

    L X
     
  2. 2E0HBX

    2E0HBX Platinum Subscriber Platinum Subscriber QRZ Page

    Just my experience... Choke does the better job for me but very much depends on your situation and setup.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you're using a 50 Ohm (matched) antenna, a current balun and coax choke do the same thing; however the "coax choke balun" is pretty narrow band and might cover just one octave of frequency range, while a ferrite balun might cover 3-4 octaves effectively. And a coaxial choke balun can't be just haphazardly tossed together for it to work on the intended frequency range.
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    By "Coax Choke" do you mean a coil of coax, or coax coiled through the aperture of a Ferrite Torus?

    A coil of coax makes a very poor "Common Mode Choke". It is essentially a low-loss inductor, and only acts as a "choke" very near the frequency where the coil of coax self-resonates. It acts like a high-Q parallel-tuned resonant circuit, so has high choking series resisance at just one specific (its resonant) frequency. Often we want a choke that works over several bands, like 80m to 10m, so a coil of coax choke it totally unsuitable for that purpose. Most folks just coil a few turns of coax cable and think they have built a "choke"; not so unless they actually measure the self-resonant frequency and it lands on the band that they want to choke.

    Coax coiled through a ferrite torus (typically 10 to 20 turns of coax through the aperture of type 31 ferrite) creates a dissipative choke that works over a decade of frequency (3-30MHz, a low Q device). It is in every way equivalent to a 1:1 current-mode balun. A choke (aka 1:1 balun) can be constructed by winding either coax or a short parallel-conductor transmission line through the toroid. See here:

    https://forums.qrz.com/index.php?threads/snap-on-ferrites.744730/#post-5735164
     
    Last edited: Apr 17, 2021
    K2XT likes this.
  5. W1VT

    W1VT Ham Member QRZ Page

    I've used coax chokes tuned up with HP impedance meter but most hams don't have anything like that.
    An antenna analyzer is typically designed for measuring impedance close to 50 ohms, not a high impedance parallel tuned circuit.

    I've swapped them all out for ferrite choke baluns. I bought 50ft of RG-400 Teflon coax to homebrew optimized G3TXQ designs. Jim Brown K9YC has higher power designs but Steve's designs meet my needs.
    Zak W1VT
     
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  6. KE8QEP

    KE8QEP XML Subscriber QRZ Page

    Can a nanoVNA with clip leads be used to test a choke/balun?
     
  7. KM3F

    KM3F Ham Member QRZ Page

    Yes a VNA can be used.
    1. Do a calibration.
    2. Equip the VNA ports with some method of clips to connect directly to the choke outer braid only.
    3. Connect the choke between port 0 and 1.
    Set the sweep frequency range to cover the bands your interested in.
    If the choke is wound on a 240-31, expect the max to occur in a narrower range for a single winding of RG58U.
    I wound one as a single winding that peaked in the 75 m band at nearly 3000 ohms where I wanted but was still reasonable up to 20m at 800 ohms.
    A better one can be made using RG400 double wound on the 240-31 core..
    I would keep the turns spaced equally if possible to keep the capacitance as low as possible between the turns.
    At the VNA, I used two UHF female to SMA male adapters.
    Use two Alligator clips with wire end Barrels reduces to a size that will slide into the UHF adapters.
    What you are doing is supplying a signal from port 0 to the Inductance and measuring the AC Impedance with Port 1 receiver.
    In the VNA they pass all control and signal info between ports for readout on the screen display.
    The bandwidth trace value will show on the screen and in the small readout as values.
    If you use a marker to place anywhere, it will give you the Impedance at that frequency point or you can see it as a reading on the screen graticule
    Be sure to set the screen graticule value. I used 500 per vertical division.
    Good luck.
     
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  8. KM3F

    KM3F Ham Member QRZ Page

    I would not recommend any extra lengths because it adds inaccurate reading to the choke.
    Remember this is RF, lead length matters.
    The VNA is high precision, don't take away from it by using clip leads.
    It even shows you the resistance of the outer shield as a very low value depending on the quality of the Coax used.
    Remember, the VNA needs to be setup from Menu selections and resistance selected, not Reactance in the menu. May not sound correct but it is. You may try both to see what the result is.
    As always, your results may vary depending on build, quality and test setup.
    Good luck.
     
    Last edited: Apr 17, 2021
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  9. WA9SVD

    WA9SVD Ham Member QRZ Page


    Not enough information. What application is it for?
     
  10. K8JD

    K8JD Ham Member QRZ Page

    For A Dipole antenna?

    I used dipoles with noting but an insulator where the coax line connected , for 4 decades with no apparent problems, then rabuilt a few antennas and added a 1:1 choke I got from MFJ, main benifit was a coax connector and hanging hook to hoist center into trees.
     

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