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
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.
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.