[RFI] [BULK] Re: Ambient Noise Levels
jim at audiosystemsgroup.com
Thu Aug 10 11:51:13 EDT 2006
On Thu, 10 Aug 2006 09:14:25 -0500, Jim P wrote:
> "But for most sources, the transition from
> near to far field occurs at roughly 1/6 wavelength"
>I don't believe sources I cited not a month of two back
>are in agreement with that statement NOR does it
>agree with the empirical results that those using
>shielded magnetic loops experience (which I had
>just alluded to in, I think, my previous post.)
One excellent source is "Noise Reduction Techniques in
Electronic Systems" by Henry Ott. The 2nd edition was
published in 1988, and is in print. You can order it from any
standard source, and also from his website,
hottconsultants.com. See his Fig 6-4 on page 161, which does
a particularly nice job of illustrating the matter.
You will find the same transition point in "Introduction to
Electromagnetic Compatibility" by Clayton Paul, 1992, also
still in print. See page 179. When I presented a workshop at
the IEEE EMC conference in Chicago last summer, it was for
sale in the lobby.
These are both excellent books, and both authors use them as
texts. I haven't met Dr. Paul, who is at the Univ of
Kentucky, but I attended one of Henry's excellent three day
workshops several years ago.
Both Ott and Paul are specifically talking about a small
source like a half wave dipole. When I qualified my statement
with "for most sources," I was thinking of true line sources
or other sources that are large in comparison to the
wavelength of the field they generate. For these sources,
the transition to the far field will be at a greater
Ah, you say, power lines are such a source. They certainly
could be, IF the noise they radiate is coherent -- that is,
if precisely the SAME noise is exciting all elements of the
line. But if the line is being excited by multiple noise
sources along its length, its behavior will be as a simple
point source, and what you will see in the far field is the
uncorrelated sum (RSS) of those far field sources.
Jim Brown K9YC
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