[RFI] wireless power at 6.4 MHz?

Jim P jvpoll at dallas.net
Sun Nov 19 20:35:24 EST 2006

Dear Tom,

I have read and re-read W8ji's material both previously and 
again today, and I note that, if interpreted incorrectly I can 
see how you say what you say without regard to other 

I believe that what W8JI states is applicable, but, he also
provides the following 'esacpe clause' which few people 
seem to pick up on:

  This screen will not change signal-to-noise in a small 
  loop antenna ***unless the noise source is coupled 
  primarily by direct capacitive coupling***. This means 
  the noise source would have to be have a very high field 
  impedance and be located very close (within inches) to 
  the antenna (called an applicator in this case).  

W8JI, I will proffer, fails to consider sources further away 
can 'couple in' much smaller what I will call "receive-level"
signals that will/can be picked up/couple capacitively to
an antenna, even a so-called loop shielded loop antenna.

Now, bear with me; we 'hams' are not the only experienced 
uses of RF energy at the level where we want to know what
is taking place at an elementary level. There are fields like 
NMR (Nuclear Magnetic Resonance) that have an interest 
in knowng the precise levels of RF fields in and near their 
apparatus, and, the differences in E and H field in that 
apparatus as well.

To that end, note the following excerpt from researchers 
discussing NMR E and H field strength measurement 
techniques :


  The popularization of NMR of large samples in a large 
  magnet bore space has increased the need for a convenient 
  measurement of the magnitude and direction of the rf 
  magnetic field generated by a coil. This is true, for example, 
  in constructing birdcage coils to see if the correct oscillation 
  mode is being generated. The measurement of the rf magnetic 
  field is best performed by the use of a loop of wire in which a 
  current proportional to the rate of change of the magnetic field 
  is induced. For a sinusoidally varying magnetic field, the in-
  duced current is proportional to the magnetic field intensity 
  (with a phase shift). The major problem with measuring the 
  rf magnetic field with a loop of wire is the sensitivity of the loop 
  to the local rf electric field. In particular, the electric field which 
  is generated by the current in the coil must be accounted for. A 
  crude way to do this is to make two measurements, one with the 
  loop and another with the loop shorted, and subtract the results.

  We have been using a variation of the loop which automatically 
  cancels the electric field signal(l) and report its construction here 
  because it seems not to be well known in the NMR community.

Source URL, with images: http://www.nmr.org/346.htm

So, in closing Tom, I would like to summarize with the 
following bulleted points:

  o So-called shield loops have been shown to be practical:

    A practical, well-developed application (which I think I have 
    posted before but which seems to 'fly by' in the ether):
    "NRSC AM bandwidth measurements with the loop antenna"

 o These so-called 'shielded loops' may not be perfect in their 
    'balance' according to theory but the amount of E-field 
    rejection in field test after has shown to be useful; any 
    given application might show more or less 'shielding'
    effectiveness owing to capacitively-induced "displacement 
    currents" that arise due to direct capacitive coupling
    to elements of the shield; THIS is where baluns 
    and other techniques come in to play for better-
    -than-average loop performance in rejecting E-field
    effects nothwithstanding the fact that Loops inherently 
    have an innate ability to 'reject' E-field components.

   o Recent technological in materials involving optics and
      magentics has resulted in the ULTIMATE H-field probe,
      using optics to "bring back the sample":

    "Magnetic Near-Field Distribution Measurements above a 
     Patch Antenna by Using an Optical Waveguide Probe"

I think that about covers it form my end.

Regards, Jim P  // WB5WPA //

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Jim P" <jvpoll at dallas.net>; <k1ttt at arrl.net>; <rfi at contesting.com>
Sent: Sunday, November 19, 2006 3:55 PM
Subject: Re: [RFI] wireless power at 6.4 MHz?

> > I can build a TOTALLY enclosed 'loop' inside an aluminum
> > enclosure, which completely meets the criteria for 
> > 'shielded'
> > magnetic loop, and I can induce current in the loop 
> > inside.
> >
> > Just let me choose the frequencies
> Of course you can, if you breach the shield. It just can't 
> have a time-varying field that goes directly through the 
> walls of the shield nor can it allow one field and not 
> others to pass once the wall is several skin depths thick.
> See:
> http://www.w8ji.com/skindepth.htm
> The point of this is any hi-Q small loop they use to 
> transfer energy over any distance in a room will radiate 
> unless the walls of the room are metal and shield by having 
> a closed path or dimensioned to act as a waveguide below 
> cutoff.
> 73 Tom

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