[RFI] wireless power at 6.4 MHz?
jvpoll at dallas.net
Sun Nov 19 20:35:24 EST 2006
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
A PICKUP LOOP WHICH IS SENSITIVE ONLY TO RF
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.
> 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
> 73 Tom
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