John,
You are right, isotropic was what I meant.
To be computable, an antenna needs to be
infinitely small because of the following reason:
- a distance between an EUT and an antenna can be defined
only accurate when both sizes can be neglected compared
to the measuring distance. Both antenna's and EUT's in real
life don't have a single point of reception resp. emission,
so waves of multiple distances and multiple angles of incident
will add together in creating a mesurement result.
In addition, the measurement distance is in the same order
of magnitude as both size of antenna and EUT.
For an antenna to have any other characteristic then omnidirectional,
it must have substantial size compared to wavelength. As the
size must be small in my explanation it
implicates the omnidirectional requirement.
As an EUT is basically a transmit antenna of unknown configuration
the same demands apply as for the receive antenna for a
computable system to deliver accurate results.
Of course, compared to the allover accuracy of EMI measurements
a reasonable accurate result may be estimated using maths, but
then surprises may be expected in single measurements.
I showed in the explanation below that the blindness of an OATS
is a math problem that exists for point transmitters and receivers
only, and that the effect is less prone for real life measurements.
However, in some test setups, this theoretical set up may be very well
approximated, thereby reproducing this effect and creating
surpises in correlations.
Regards,
Gert Gremmen
Approvals manager
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From: [email protected]
[mailto:[email protected]]On Behalf Of John Woodgate
Sent: Monday, March 22, 2004 10:59 AM
To: [email protected]
Subject: Re: What is wrong in my 3 meters FAC measurement
I read in !emc-pstc that Gert Gremmen <[email protected]> wrote (in
<[email protected]>) about 'What is wrong in
my 3 meters FAC measurement' on Sun, 21 Mar 2004:
>I hope I made clear that FAR and OATS measurements cannot be compared at
>all, but for strictly theoretical isotope receivers and transmitters
>that behave as single points.
I agree with the substance of your explanation, but I am puzzled by the
word 'isotope', which is undoubtedly not the intended word. I thought
about 'isotropic', but that is not usually applied to receivers
('omnidirectional'), and it isn't strictly necessary for the receiver to
be omnidirectional for the comparison to be computable: it only needs to
have a beamwidth great enough to accommodate the rather simple pattern
of emissions from the point-source transmitter and reflections from the
ground-plane. A cardioid directional response, for example, appears
satisfactory.
--
Regards, John Woodgate, OOO - Own Opinions Only.
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