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e-field probe

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  • Jason White
    I ve read many references to an e-field probe and I ve attempted to find out what that means exactly in my various antenna books to no avail. Web searches on
    Message 1 of 2 , Jan 2, 2007
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      I've read many references to an "e-field probe" and I've attempted to
      find out what that means exactly in my various antenna books to no
      avail. Web searches on the term haven't been very enlightening.. mostly
      information on spectrum analyzer probes. Can someone fill me in?
      (Wouldn't an e-field probe be just about any antenna?) Why is it so
      often mentioned in conjunction with a loop to provide a cardoid pattern
      (which I always assumed was done with a vertical in a particular phase
      in relation to the loop). I ask here because it almost always seems to
      be in relation to some LF/VLF antenna system, which I'm assuming is
      because of the larger wavelength to practical antenna size ratios.

      Thanks for the information,

      Jason W.
    • Peter Schmalkoke
      Jason: I think you already got the basic idea: The most important point is that at VLF frequencies any practical antenna size is very short com- pared to the
      Message 2 of 2 , Jan 2, 2007
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        Jason:

        I think you already got the basic idea: The most important point is
        that at VLF frequencies any practical antenna size is very short com-
        pared to the wavelength.

        With a small antenna size the predominant mode of interaction between
        an antenna and the surrounding space is via the near field, which con-
        sists of a magnetic component and an electric component. Inductors and
        dipoles are dedicated probes for those field components. And sure, any
        erected ("half" dipole) antenna makes an e-field probe when operated
        against ground. Since at radio frequencies the associated wavelengths
        are short compared to VLF, a large distance between the field source and
        the detector site can easily exceed the reach of the near field. Thus
        signal transmissions over large distances at radio frequencies can only
        take place by means of the far field, which puts emphasis on the radia-
        tion resistance. This situation is significantly different at VLF, since
        the actual distances are often within the near field range. Since the
        radiation resistance is lousy with a (small antenna) e-field probe, mat-
        ching it makes little sense and it is operated with a high impedance
        instead. This is why the term e-field probe does seldom appear with
        regard with common radio applications and why it is still elementary
        at VLF.

        Direction finding at radio frequencies or shaping a directional reception
        pattern by means of combined e-field and b-field probes relies on the
        inherently fixed phase relation and the fixed amplitude ratio associated
        with Hertzian far field waves. But in the near field those components
        can take any phase relation and a fixed ratio of the electric and the
        magnetic field intensities does not exist. So RDF and VLF are very dif-
        ferent fields of application for dedicated e-field or b-field probes.

        Regards, Peter



        Jason White wrote:
        > I've read many references to an "e-field probe" and I've attempted to
        > find out what that means exactly in my various antenna books to no
        > avail. Web searches on the term haven't been very enlightening.. mostly
        > information on spectrum analyzer probes. Can someone fill me in?
        > (Wouldn't an e-field probe be just about any antenna?) Why is it so
        > often mentioned in conjunction with a loop to provide a cardoid pattern
        > (which I always assumed was done with a vertical in a particular phase
        > in relation to the loop). I ask here because it almost always seems to
        > be in relation to some LF/VLF antenna system, which I'm assuming is
        > because of the larger wavelength to practical antenna size ratios.
        >
        > Thanks for the information,
        >
        > Jason W.
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