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WWVB Ferrite Antenna Revisited

Discussion in 'Electronic Design' started by Bob Agnew, Oct 21, 2005.

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  1. Bob Agnew

    Bob Agnew Guest

    I just dug up an old design of mine where I was planning to use 7 Amidon
    R33-050-750 0.5 inch by 7.5 inch rods in a bundle. i.e. one rod in the
    center and six wrapped around it yeilding a "bumpy" 1.5 inch diameter
    bundle.

    Somehow in my madness, I had calculated that this design would increase the
    effective antenna cross sectional area and improve the S/N.

    I had calculated that I needed about 1000 turns to get 70 mhy. I found
    another of my old calculations that predicted 883 turns, AWG 38 spaced one
    wire diameter apart at 126. turns per inch. I had planned to wrap two
    strands at once, then coat with Q dope. When it was dry enough, I would
    remove one of the winding, then wrap the layer and start the next one.

    Any comments, even those questioning my sanity, will be greatly appreciated.

    Now that I am semi-retired, I thought I'd give this another try, but I
    wanted to run it by the experts here.
     
  2. Bob Agnew

    Bob Agnew Guest

    Forgot to mention that I was considering using a PVC 1.5 inch ID pipe as a
    coil form.
     
  3. Bob Agnew

    Bob Agnew Guest

    Whoops -- Major correction. Only one layer required.

    It's been a long time and memory is poor.
     
  4. Bob Agnew

    Bob Agnew Guest

    I just found a served Litz magnet wire at
    http://www.mwswire.com/slitz6.htm

    This wire is insulated with Nylon/Poly and is AWG 38 with a mean OD of
    0.0075 in. This would yeild about 133 TPI close spaced. This is very close
    to the ideal spacing of 0.00793 in.

    I'm considering winding this Litz wire close spaced.

    Any comments?
     
  5. Mark

    Mark Guest

    why do you think a larger antenna will improve the S/N. At low
    frequencies the noise floor is typically set by atmospherics. with a
    bigger antenna you pick up more signal and more noise.

    Mark
     
  6. Bob Agnew

    Bob Agnew Guest

    I did the calculation for S/N as a function of effective aperature. It was
    quite complex.
     
  7. Jim Yanik

    Jim Yanik Guest

    At Tektronix-Orlando FO,the WWVB antenna we had for our Spectracom 8161
    60Khz WWV receiver/comparator was only a ~6" ferrite rod with a preamp on a
    flexible PCB,all slid into a 2" PVC pipe.

    We got a fairly decent reception,with the antenna indoors,as our building
    owner would not allow us a rooftop mounting.The hard part was keeping the
    antenna away from other instruments that generated noise.
     
  8. Eric

    Eric Guest

    I think you're right, Bob. It's quite likely this will yield great
    results.

    You might also consider a loop antenna like the Palomar commercial
    units. Those are sweet - and they have a simple tuner. At those
    frequencies a simple tuner can help a lot to reduce off-frequency
    interference.
     
  9. I haven't done a finite element analysis on such things, but I think
    you can approximate the same effect as a fat bundle by adding a big
    ferrite bead on each end of one rod, (pick one that just slips over
    the end) including the same inductance per turn. Inserting the rod
    about a diameter into the bead should be enough.

    This gives you a lower resistance and lower stray capacitance coil
    than wrapping around a big bundle. I would also use a very thin coil
    form, like one made of a few layers of epoxy and paper wrapped over a
    few layers of Saran Wrap over the rod. The PVC pipe adds turn length
    without doing anything useful for it. The ferrite rods are pretty
    good insulators, all by themselves. If you can measure resistance
    between two points on one with an ohm meter, it is not RF ferrite.
     
  10. Jim Yanik

    Jim Yanik Guest

    BTW,Our office was on the ground floor,and the outside "wall" was all
    glass,and 'looking' in the proper general direction.
     
  11. Bob Agnew

    Bob Agnew Guest

    Guess there's only one way to find out. Looks like its back to winding.
    Now if can only find those 7 ferrite rods ;=}} I may have to reorder. I
    was using the Amidon R33-050-750 rods.
     
  12. Bob Agnew

    Bob Agnew Guest

    OK -- I dug up the Amidon application notes. The reasoning that I used to
    justify a large bundle is as follows:

    Loop induced voltage = 2*Pi*N*A*Mu_sub_epsilon*F / Lambda

    where :

    Mu_sub_epsilon = effective permeability of the rod
    F = Field strength in microvolts per meter
    N = number of turns
    A = cross sectional area of loop in square meters
    Lambda = wavelength in meters

    The idea is to maximize A. The usual assumption is that the dominant noise
    is the noise in the receiver bandwidth and not atmospheric noise.
    When atmospheric noise dominates, the strategy fails as someone has pointed
    out.
     
  13. Bob Agnew

    Bob Agnew Guest

    ...and of course, atmospheric noise or QRM.
     
  14. Robert Scott

    Robert Scott Guest

    I saw a plan once for a WWVB antenna that was made of a 3 foot
    diameter coil inside of an electrostatic shield. The shield was a
    loop of copper pipe with a small gap so it didn't become a shorted
    turn. The idea was that electrostatic noise could be shielded,
    leaving only the electromagnetic component to get through to the coil.

    Is there any advantage in doing things that way, as opposed to the
    ferrite rod that is being discussed in this thread?


    -Robert Scott
    Ypsilanti, Michigan
     
  15. I have no argument with any of that.

    The rod is intercepting the energy in a volume by steering the flux
    passing through some cross sectional area larger than the area of the
    ends of the rod, through the length of the rod, and thus, through the
    coil. I say an area larger than the area of the ends of the rod,
    because the high permeability of the rods gathers flux that would have
    gone past the rod if it has been air. Larger end areas allow flux
    that is further from the rod's center line to detour through the rod
    without having to crowd in near the more central flux as much. So
    larger ends gather more flux. A simple rod may be easy to
    manufacture, but it is not the optimum shape to gather flux from space
    and transfer energy into a coil.

    My point is that the *effective* area of flux intercepted by the rod
    is related to the areas of its ends, not to the cross sectional area
    of its middle, as long as the permeability of the middle is high
    enough that the flux isn't held back by too small a center area.

    This is from the "Rod Permeability vs. Rod Length divided by Rod
    Diameter" graph on:
    http://www.amidoncorp.com/aai_ferriterods.htm

    Note that for a permeability 10 rod, it poops out when the length
    reaches 10 times the diameter. At that length, flux near the rod has
    little incentive to dip towards the ends and crowd in along with other
    flux to go through the rod, and will just as well run along side the
    rod. I have added a factor of center area to end area to this graph
    which has assumed this factor is always 1.

    So, for 33 material, with a permeability at least 800, as long as the
    overall length is much shorter than 800 times the end diameter times
    the middle area divided by the end area, most of the available nearby
    flux that would have passed through a constant diameter rod will take
    the short cut through the rod (and through your coil), even though it
    is necked down in the middle.

    For example, if you used 1.5 inch beads with half inch holes on each
    end of a half inch rod, the combination would gather as much flux as a
    1.5 inch diameter solid rod as long as the overall length was much
    less than:
    800*1.5*(pi*0.25^2)/(pi*0.75^2)=133 inches, so your 7.5 inch rod
    extended by, perhaps a couple inches, by the beads, is well within
    "much shorter than". Gluing two rods end to end would more than
    double the output because the volume of space that flux is gathered
    from would more than double. This is because the effective area of
    flux gathering is bigger than the end areas, and how much bigger
    depends on the rod length, which is not included in the formula. The
    formula looks pretty suspect to me (like it is for an air core coil)
    because there is no reference to rod length. It gives the same result
    for a zero length rod, unless "effective permeability" does not mean
    rod permeability, but some combination of rod permeability and length.

    Doing it my way should cut your winding resistance and capacitance
    down by a factor of about 4 (with the thin form) while keeping the
    intercepted energy a little higher (assuming the beads have an outside
    diameter 3 times the rod's diameter and extend its length to something
    like 9.5 inches). To me, that equates to higher Q and higher energy
    output.
     
  16. Bob Agnew

    Bob Agnew Guest

    Yes there can be an advantage. I don't remember all the details. Ir gets
    quite complicated. First of all,at 500 Khz., the principal mode of
    propagation is the ground wave. I don't remember what the mode is, TM0 etc.
    The second problem is the ionospheric problems with the D-Layer. The ground
    wave propagates differently that the sky wave.
     
  17. Jim Thompson

    Jim Thompson Guest

    Here in Arizona that's the only style I've found effective against all
    the man-made noise. Mine was 8" diameter of 1/2" copper pipe and used
    a "fiber-tee" junction to avoid the shorted turn.

    See the S.E.D/Schematics page of my website for the circuit I used.
    Unfortunately I don't have a picture of the antenna (this dates to
    1974).

    ...Jim Thompson
     
  18. Bob Agnew

    Bob Agnew Guest

    Sorry -- That was supposed to be 60 Khz. NOT 500 Khz. Everything is
    getting fuzzy now.....
     
  19. Bob Agnew

    Bob Agnew Guest

    Jim -- Did you mean 8 feet instead of 8" as you said?
     
  20. The ferrite rod antenna can be very well shielded by a grounded layer
    of foil that wrapped around the coil, but is slit lengthwise. It also
    helps to wind a coil that is symmetrical with respect to its ends and
    connect it to a fully differential amplifier with matched input
    impedances, so any noise that is common to both ends is rejected.
    This makes shielding much less important.
     
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