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How long the ferrite rod?

Discussion in 'Electronic Basics' started by Richard, May 15, 2005.

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  1. Richard

    Richard Guest


    I'm wanting to exchange "air cored" frame aerials for ferrite rods, for the
    medium waveband (Broadcast band).

    The ferrite rods are 10mm dia, and the MW coil for it is 35mm long, I think
    about 90 turns. Now, how long should the ferrire rods be do you think? The
    thing is, I've got some fairly long rods (200mm long) and I'm thinking
    should I cut the rods so they are say 100mm long or even 66.66mm long. The
    point is, does it matter that much how long the ferite rod is? Could I in
    fact use a 66.66mm rod and it would be no better or worse than a 100mm rod?

    Also, how do you cut a ferrite rod? TIA.
  2. Richard

    Richard Guest

    Actually I bet it's good to have no less than 4" (100mm), so if I do cut
    I'll not cut shorter. No idea whether there is much advantage to rod being
  3. The longer the rod, the bigger volume of space it couples into, and
    the more energy it might intercept from the waves passing through that
    volume. This generality applies to any length much shorter than 1/4
  4. Richard

    Richard Guest

    I suppose there is a formula somewhere showing output volts against length.
  5. Richard

    Richard Guest


    So, I beleive the formula I'm looking for is:

    Loop Induced Voltage (Fhe) = 2πNAµeF
    __________ (in µV)
    Anyway as per:

    And as you point out, for maximum induced volts use the entire rod.
  6. John Fields

    John Fields Guest

  7. Rich Grise

    Rich Grise Guest

    If it's powdered iron, you can cut it with a diamond saw. If it's
    ferrite, you probably can't cut it, but there's a possibility you
    could score it with a triangular file and snap it, like glass

    Good luck!
  8. John Fields

    John Fields Guest

  9. But unless you need to shorten it for reasons other than reception
    quality, there is little reason to shorten it. The only reason radios
    don't come with foot long ferrite rods is cost and overall unit
  10. John Fields

    John Fields Guest

    Except, perhaps, for the central void.
  11. Not really. Transformers are made as small and cheap as possible, but
    making them very much larger than necessary does not add much but
    weight. Making a rod antenna much larger (besides making it more
    costly) also provides lots more signal.
  12. I am also pretty sure that gluing a large ferrite bead (like those
    around power cords for RFI suppression) on each end of the rod, beyond
    the winding, also gets you some extra signal. No need to have the
    bead hole filled with the rod. Just a bit of overlap works.
  13. Rich Grise

    Rich Grise Guest

    Yeah, I read the other responses saying pretty much the same thing
    about an ohnosecond or three after I posted. :)

  14. Rich Grise

    Rich Grise Guest

    Back when I was in the USAF, I stole a 4-400A by checking it out of
    bench stock, and watched a guy build a nominally kilowatt linear out
    of that and a bunch of surplus crap. This was in Okinawa (early 1970's),
    and you couldn't walk down the street without passing a couple of surplus
    dealers. He did it grounded-grid, and for his filament choke, he took
    about a half-dozen tuning slugs from various coils, transformers, etc -
    but these were big fat tuning slugs - about 3/4" diameter and about 1"
    long. He just glued them together. He had found a beautiful silver-plated
    coil for the pi-net output, and of course, it was covered with tarnish.
    I thought he should have just wiped a little Brasso on it, but this idiot
    SANDPAPERED it! AAAAIIIIEEEEeeeee! Later, I came to find out that silver
    tarnish not only doesn't hurt in that situation, but might actually
    be beneficial! But SANDPAPER! Well, it was only HF, and I'm sure that
    wasn't the only ineffeciency with the thing. But he did DX with it.
    I have to admit, it was kinda cool seeing the plate go bright orange
    until he got it tuned properly. :) And it hissed. Somebody told me
    that that was the ka-dink effect. The electrons hit the plate so hard
    and fast, that they knock off secondary electrons, which when they
    hit the glass, go "ka-dink!" ;-)

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