Dear all,
The original poster ("KC CHAN [PDD]" <[email protected]>) have not mention
about the frequency range of the intended measurements, nor even the
ball-park figure about the dimension of the H-field loop 'sensor' or
pick-up. So all this is just a lot of conjecture. :-)
Note that the situation already exist if there is a electrostatic shield
around the loop. i.e. I assume that the loop is fabricated from a solid
copper-tube (type of shield) co-axial 'cable'. This would amount to
multiple shielding the loop to selective frequencies using Lenz's Law. It
works out to be a case of trying to make a pickup from the primary turn of
a (loosely coupled?) air core-transformer with two or more 'single-turn
secondary windings of different styles, all shorted'.
I must say that would create some kind of challenge with any attempts to
calibrate this combination especially with the cross sensitivity (axial and
transversal) of the modified loop.
I suppose all one gets from doing this is some idea of the presence of
other frequencies but what kind of certainty about their amplitude or
strength? Consider the secondary (scattering) effect of all those induced
currents flowing in the 'shorted secondary' especially at resonance.
Just another of my 2 cents worth
:-)
Tim Foo
Cortland Richmond
<[email protected] To: "KC CHAN [PDD]"
<[email protected]>, ieee pstc list <[email protected]>
om> cc: (bcc: Wan Juang
Foo/ece/staff/npnet)
Sent by: Subject: Re: Active loop
antenna overload
owner-emc-pstc@majordom
o.ieee.org
09/26/01 01:09 PM
Please respond to
Cortland Richmond
I think we may be assuming this overload is caused by the EUT. But this is
just as likely to be caused by something else. Medium Wave and Long Wave
broadcasting produces powerful fields at some distance from an antenna.
This has a fix.
If you place a narrowly resonant loop antenna, with feed point shorted, in
close proximity to the wideband loop used for measurements, it will reduce
pickup at the resonant frequency to a small fraction of the normal amount,
and you will be able to at least SEE signals front-end overload may have
prevented you from being able to notice. At frequencies far from the
secondary loop's resonance, the measuring loop antenna factor will not be
much affected, and you will even be able to get a useful idea of the fields
you are seeing from the EUT. If you wish, you can have your loop
recalibrated with the resonant loop permanently attached, which will
restore the ability to make acceptable measurements.
Cortland
<snip>
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