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Re: [loopantennas] Ferrite rod antenna epiphany

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  • Chris Trask
    You might want to look at: DeVore, R. and P. Bohley, The Electrically Small Magnetically Loaded Multiturn Loop Antenns, IEEE Transactions on Antennas and
    Message 1 of 3 , Mar 7, 2006
      You might want to look at:

      DeVore, R. and P. Bohley, "The Electrically Small Magnetically Loaded Multiturn
      Loop Antenns," IEEE Transactions on Antennas and Propagation, Vol. 25, No. 4,
      pp. 496-505.

      I have that paper as a PDF file.


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      ----- Original Message -----
      From: "John Popelish" <jpopelish@...>
      To: <loopantennas@yahoogroups.com>
      Sent: Tuesday, March 07, 2006 11:45 AM
      Subject: [loopantennas] Ferrite rod antenna epiphany

      > Up till now, I have been pretty blindly trying variations of tuned
      > ferrite rod antennas, hoping to find a clear pattern that leads to
      > some kind of broad optimum performance. Up to now, most of my
      > preconcieved notions about what is good and bad have not been
      > supported by my experimental data. For example:
      > I assumed,that since every rod antenna I have seen, used a solid rod,
      > then I must get as close to a solid rod as possible when I make a rod
      > of segments. Rods with gaps in them out perform rods with no gaps.
      > I assumed that tight coupling between the rod and coil is a good
      > thing. But when I eliminated the plastic pipe I have been testing
      > various rods with, and would the coils directly on the ferrite, the Q
      > plummeted.
      > I assumed that there would be a close correlation between coil Q and
      > signal sensitivity. While experiments show that this is often the
      > case, there are exceptions (low Q versions that produce more voltage
      > than higher Q versions.
      > Obviously, my understanding has been very primitive.
      > Over the last 3 days of work, I have come up with a new way to look
      > at tuned ferrite rod antennas that fits all the experimental evidence
      > much better than my earlier views.
      > I now see the tuned ferrite rod antenna as two distinct functions:
      > 1. There is a signal collection function that funnels flux from a
      > large volume of space as EM waves pass, through a small volume of
      > space, through the center of a pickup coil. If the coil is untuned,
      > this is about all that is going on.
      > 2. There is a narrow band resonance filter function that boosts the
      > signals center band by roughly the Q of the resonant circuit and
      > suppresses others. This function could actually take place somewhere
      > else, other than on the rod, but the middle of the rod is a
      > convenient place to put it. The quality of this resonance has very
      > little to do with the signal collection function, and is easily
      > degraded by its coupling to that mechanism.
      > Picturing how each of these mechanisms works helps me understand the
      > experimental data I have seen, and suggests alterations to the rod
      > antenna design that optimizes each, separately.
      > For instance, having space between the coil and rod allowes a
      > significant fraction of the coil's resonance flux to be outside of
      > the rod, so that part of the flux does not have energy robbed from it
      > by the ferrite.
      > The gaps in the rod tend to isolate the resonance flux to the part of
      > the rod that is inside the coil, reducing the amount of ferrite that
      > is stealing energy from the resonance with its losses. The ferrite
      > inside the coil has Q cycles to rob energy from the resonance (the
      > same energy bounces around in the resonant circuit Q times before
      > half of it is lost), while the ferrite well outside the coil has only
      > one change to rob energy, since each cycle is independent of the
      > others, so the losses in that part of the rod are not nearly so
      > important.
      > The highest possible resonance Q results in isolation of the resonant
      > circuit from the flux collecting process, so signal goes, even though
      > Q is very high. For a given Q, there must be an optimum coupling
      > between the resonance and the flux gathering process. My seat of the
      > pants guess is that the coupling should halve the Q of a totally
      > isolated resonator.
      > I am working, now, on proving this concept of tuned ferrite rod
      > antennas with some rather radical designs that treat the core inside
      > the coil completely differently that the flux collecting sections
      > (different material, different structure, etc.) These designs will
      > provide an isolatable resonatorthat can be optimized, and a flux
      > collecting system that can be coupled to the resonator by an
      > adjustable amount, so I can explore the Q reduction versus signal
      > amplitude relationship as the coupling factor is changed.
      > I have been wrong about lots things about rod antennas, but I think I
      > may finally be on the trail of something useful.
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    • John Popelish
      ... Loaded Multiturn ... Vol. 25, No. 4, ... I would appreciate seeing it.
      Message 2 of 3 , Mar 7, 2006
        --- In loopantennas@yahoogroups.com, "Chris Trask" <christrask@...>
        > You might want to look at:
        > DeVore, R. and P. Bohley, "The Electrically Small Magnetically
        Loaded Multiturn
        > Loop Antenns," IEEE Transactions on Antennas and Propagation,
        Vol. 25, No. 4,
        > pp. 496-505.
        > I have that paper as a PDF file.

        I would appreciate seeing it.
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