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Beads, Ferrities ?..

Discussion in 'CB Antennas' started by Oatmeal, Jan 18, 2015.

  1. Oatmeal

    Oatmeal Active Member

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    Any of you guys use these on your coax ?.....
    If so, how well do they work...

    These things work any better than a choke..


     

  2. The DB

    The DB Sr. Member

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    I, personally, prefer using ferrite beads on coax over an air choke, however, both options can work well.

    An air choke uses reactive impedance, which can be affected by things like the length of coax, and can thus in some unlucky situations they can be made less effective. To compensate such designs create a lot of reactive impedance, bu tthis also causes their coverage bandwidth to be rather narrow, although large enough to cover the CB band easily.

    Chokes made from ferrite beads have some reactive impedance, but are mostly resistive which has its own advantages. They aren't nearly as affected by varying lengths of feedline, for example, and they are also far more widebanded than the air chokes mentioned above. If you are using coax that is about the diameter of RG-8, six FB-31-1020 beads do a very good job, and are what I prefer to use when possible. They go for about $2 a piece, give or take. If you are using RG-58 diameter coax, 8 turns through a FT240-31 works very well as well.


    The DB
     
    ghutch likes this.
  3. M0GVZ

    M0GVZ Sr. Member

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    An air wound choke if not wound properly can either do nothing at all or in fact make matters worse as it is reactive impedance and we want resistive. Air wound chokes have very narrow bandwidth so its easy to end up making one that is nigh on useless where you want it. Certainly the majority of suggestions I see on CB forums would result in one that did very little if anything. An air wound one that would work would be 5 turns of RG213 on a 4.25" diameter core.

    Making a proper choke using the correct mix ferrite (31, 43,52, 61 mix) and the correct number of turns around that mix will give you far more choking than an air wound one will, it'll be resistive and not reactive where we want plus it is more broadbanded doing that over several MHz. 8 turns of RG58 round a FT240-61 mix will give plenty of resistive choking for 26-30MHz. You construct them like this

    [​IMG]

    A chart from G3TXQ:


    More reading here:

    http://www.karinya.net/g3txq/chokes/
     
    ghutch likes this.
  4. 543_Dallas

    543_Dallas Sr. Member

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    Ibe been using ferrite on my mobile and base installs with success. The mix has to be right though. I'm honestly not sure what mix the local store is selling but they work well at 27mhz.
     
  5. ghutch

    ghutch Active Member

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    The Mix is VERY important.....31 is good for HF. Place the choke as close the feed point as possible.
    K0BG.com has a chapter devoted to all things chokes, mixes, etc.. Good reading
     
  6. flea49

    flea49 Member

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    The air choke works better but it causes a lost of some rf power because you have to wound 18 ft.
     
  7. The DB

    The DB Sr. Member

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    18 feet of coax has to be wound? Offhand that sounds way off so lets see...

    An air choke has five windings of coax on a 4.25 inch diameter form.

    C = π * D, where

    C = circumference of the circle
    π = mathmatical constant pi, rounded to 3.14 here
    D = diameter of the form, 4.25 inches plus the diameter of the coax, assuming .5 for that diameter, this is because we are adding the distance from the diameter to the center of the coax on both sides of the form

    So...

    C = 3.14 * 4.25 = 14.915

    So the circumference is 13.345 inches. As has five windings around the form we take that figure and multiply by 5...

    14.915 * 5 = 74.575 inches

    There are 12 inches in a foot, so to convert this figure to feet we divide by 12...

    74.575 ÷ 12 ≈ 6.215 feet

    Taking that data and plugging that into a coax loss calculator, I used this one, we get 3.3% loss using RG-58, which would be slightly shorter than the numbers calculated above as it is thinner. That amount of loss is irrelevant, and will be small compared to other losses inthe system, such as ground losses, or simply the rest of the coax run. Change that to RG-213 and we get 1.4% loss, and if you use LMR-400 you get 1.1% loss...

    Yes there is some loss, but not enough that you or the people you talk to will ever notice it...

    Personally, as I said above, I would use ferrite over an air choke any day, and I do. It is resistive, which in this case is better than being reactive for any nuber of reasons...


    The DB
     
  8. M0GVZ

    M0GVZ Sr. Member

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    It only works better if you use a vector impedance meter or RF bridge and parallel resistor to verify that you've wound it correctly and it is choking at the maximum its supposed to at the frequency you want. 18ft is not the correct answer, neither is 15ft or 23 ft or 17ft or 27 ft or any fixed figure. It all depends on the coax you're using and the diameter of the coil. And that is why using lengths people suggest on the internet is usually going to result in a coil that is at best ineffective on the frequencies you want and at worst, actually contributing to common mode RFI.

    Using ferrites you can get the manufacturers data sheet so you know exactly what choking they offer at a specific frequency.

    Because chokes constructed from coax wound round ferrites are quite broadbanded in comparison to an air wound choke, offering the highest resistive choking over 20+MHz compared to around 2MHz of peak reactive choking of an air wound choke, it isn't as critical you get the construction correct so can go on the number of turns given on the internet as a turn or two difference isn't going to put you massively off target.
     
    #8 M0GVZ, Jan 25, 2015
    Last edited: Jan 25, 2015
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Source of this incorrect info please so we know who to avoid for other imcorrect info in the future.
     
  10. The DB

    The DB Sr. Member

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    I've found many sites and .pdf's that all seem to include this picture...

    [​IMG]

    And most every site/pdf that does also has either the same or very similar text. It is like one person wrote something up, and many others simply copied it. To bad it isn't true, and for that matter, why waste all that coax on the CB band when you can make a very effective choke (as I have repeatedly said, this is not a balun) with far less coax.

    Apparently many believe that all you need to have a "balun" is a given amount of coax coiled around a form, and the width of th

    For those who want a source for this picture, there is a callsign in the picture...


    The DB
     
  11. tecnicoloco

    tecnicoloco Tweaknician in Training

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    [​IMG]
     
  12. tecnicoloco

    tecnicoloco Tweaknician in Training

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  13. jazzsinger

    jazzsinger Bullshit Buster

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    [

    I get slightly different figures.




    for diameter your measuring to centre point of coaxial on either side of core. In effect measuring the diameter of the cable itself.




    Using 5mm/.19" rg58 I get rounded up.




    3.14 x 4.25" + .19" = 4.44" x 5 turns.




    3.14 x 4.44" = 13.94" x 5 turns = 69.70" / 12 = 5.80 feet.




    For 10.3mm/.405" Rg213/u I get,




    3.14 x 4.25 + .405 = 4.655" x 5 turns.




    3.14 x 4.655" = 14.62" x 5 = 73.08" /12 = 6.09 feet.




    Allowing for both cables having same velocity factor both are very close to a 1/4 wave electrically in the 27mhz range.




    Can't help thinking using an exact 1/4 wave at centre frequency would optomise them for cb band




    I know its minor differences, but



    one of us is out.

    I first saw this 18-21 foot fantasy on hamuniverse years back and like the A99's bullshit gain has propagated everywhere except where it should have been propagated. Into the trash can or my infamous a99 mod as its known.

    All the best Jazz 73
     
    9C1Driver likes this.
  14. The DB

    The DB Sr. Member

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    Hey Jazz.

    The velocity factor is irrelevant in this case, as is the length of coax used. The velocity factor is for the signal between the center conductor and the shield, and that is determined by the dielectric, which is generally some form of polyethylene or some other like material, with the better ones being a foam version of such material. The common mode currents we are choking with such a device travel on the outside of the shielding. On the outside of the shielding you have a different material, generally some form of PVC, but mostly air for the dielectric. Because of this the velocity factor for common mode currents is different, and generally much higher, than the number written on the coax itself.

    Further, how the device actually works is the common mode currents are put through an LC filter. The coils of the choke are the L, or inductive, component of the filter, and the space between the coils, generally determined by the thickness of the insulator material, determines the C, or capacitance, component of the filter. So long as the L and C components combine to create a resonant frequency near the frequency you are using the air choke will be effective.

    Long story short, the length and velocity factor of the coax doesn't matter. RG-213 and LMR-400, with their different velocity factors, use the same form and the same amount of coax for the same effectiveness.


    The DB
     
    543_Dallas likes this.
  15. jazzsinger

    jazzsinger Bullshit Buster

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    I'm well aware the velocity factor doesn't affect the shield outer was merely an observation that both chokes contain an electrical 1/4 wave of coax or thereabouts.

    I'm also aware of what the turns do.

    I notice you omitted the bit that your figures are wrong ;) lol :)
     

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