> I have conducted measurements from 1-30MHz in a densely
> populated housing estate, in a semi rural college and at
> a semi rural house that has no objects to reflect the
> field strength back.
The house was built without the use of nails? There is no
plumbing in the home, no contiguous metal piping? No metal
flashing along the roof line?
Even at 1 MHz enough nails like roof, or a wall, make a
"phased array" reflector.
It is my understanding the formula is based upon the
normalization of the spatial Fourier transform - distance
normalized using lambda. The 2 *(D/lambda) is the "width"
of the sync function, or the sin(x)/x.
[opinion]
At low frequencies I trust the magnetic field to be better
behaved. Whatever that means. Measuring HField and then
relating back to free field EFields (seem to me) would more
accurately relate to what you want. Those loop antennas
may have already taken that into account though in their
calbration curves.
- Robert -
On Wed, 2 Aug 2006 13:31:35 +0100
"Iain Summers" <[email protected]> wrote:
> John thanks,
>
> I am aware that the boundary is a smooth transition and
> the main reason for wanting to know is to do with some
> subjective analysis of powerline telecommunications
> measurements that I have completed.
>
> There are various papers that state that the attenuation
> of E with distance is 1/distance(d) in the far field and
> some combination of 1/d, 1/d^2 and 1/d^3 in the near (or
> very near) field.
>
> I have conducted measurements from 1-30MHz in a densely
> populated housing estate, in a semi rural college and at
> a semi rural house that has no objects to reflect the
> field strength back. I have plotted all of the graphs and
> included curves for 1/d^0.5, 1/d, 1/d^2, 1/d^3 and I am
> now in the process of correlating them.
>
> What I'd like to know is how large the measurement errors
> are. eg I have used a loop (which is the recognised
> method) and then converted to E for comparison to others
> results by using E/H=377 which is only true in the far
> field. I though it would be useful to determine what the
> results might be if E/H approached 3000 or 30 say and for
> that thought I'd use the point source formula
> lambda/2*pi and also 2*D^2/lambda to determine how near
> or far the boundary might be.
>
> Sorry that is a bit of a ramble but that's the background
> to it.
>
> You mentioned that D might be an area which was my
> initial thoughts as it is quoted for use as an aperture
> and therefore may not be wholly appropriate to use in the
> instance of the source being a large loop in the
> horizontal plane.
>
> The overall aim of my PhD is not to come up with any
> answers per se but to explain to others who quote EMC
> theory in relation to PLT measurement. There are lots of
> assumptions but no one aver states "the answer is x and
> by the way that is + or - 50%". There are lots of people
> measuring E in dBuV to 2 decimal places with no mention
> of the errors in their measurements. This is fine for
> compliance if everyone does the same thing but some
> people are trying to predict interference based upon
> these results.
>
> Thanks,
> Iain
>
>
>
> ----- Original Message -----
> From: "John Woodgate" <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, August 02, 2006 8:59 AM
> Subject: Re: Near field/Far field what is D.
>
>
> In message <001c01c6b604$c0ed2810$6401a8c0@homepc>, dated
> Wed, 2 Aug
> 2006, Iain Summers <[email protected]> writes
> >There are various references and opinions as to what
> constitutes the
> >near field far field boundary. The common one from
> antenna theory is
> >2D^2/lambda where D is defined as the largest aperture
> of the antenna.
>
> There isn't a sharp boundary. What you take as the
> boundary depends on
> why you want to know where it is, and you don't tell us
> that.
>
> For example, you might decide that you want to know where
> the phase
> angle between E and H is only 1 degree. Or is 45 degrees.
> Or more.
> >
> >Has anyone researched what the limitations of this
> formula are? I am
> >interested in the far field boundary between 1-30MHz for
> large loops of
> >wire.
>
> Large compared with the wavelength at 1 MHz? That's large
> indeed!
>
> >In terms of a loop if this formula is applicable is D
> the circumference
> >or diameter of the loop.
>
> Or maybe the area?
> --
> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and
> www.isce.org.uk
> 2006 is YMMVI- Your mileage may vary immensely.
>
> John Woodgate, J M Woodgate and Associates, Rayleigh,
> Essex UK
>
-
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