[RFI] [BULK] Re: Ambient Noise Levels

Jim Brown 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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