The accurate volume is about 1/3 by 1/3 of the outside dimensions. At least
that's my "measured" experience.
Just means one has to make a rather large Helmholtz for a large sensing
coil. And my recollection of most sensing coils in that range are that
they're over 1/2 meter in diameter. I've seen almost 1 meter in diameter.
There is another way. That is to use a "calibrating" coil. Whose field is
very well known (in free space) along its axis. Then, at a fixed distance
the sensing coil should give a standard reading. [ Be at least 3 diameters
of the largest coil dimension away from the driving coil, the further, the
more accurate. Also, be a long ways away from magnetic materials like rebar
in concrete under you ]]
There is one more way to make a field for calibrating a loop that
has a much larger accurate internal volume than Helmholtz. It is
approximated with 5 loops spaced appropriately. It is my understanding that
the "useable" volume inside that coil is around 70% Don't know where I put
the information on that coil, but if people are interested, will try to
find.
- Robert -
Robert A. Macy, PE [email protected]
408 286 3985 fx 408 297 9121
AJM International Electronics Consultants
619 North First St, San Jose, CA 95112
-----Original Message-----
From: Ken Javor <[email protected]>
To: Jim Bacher <[email protected]>; Mirko Matejic
<[email protected]>; [email protected] <[email protected]>
Cc: Mothish <[email protected]>
List-Post: [email protected]
Date: Wednesday, January 03, 2001 7:11 AM
Subject: Re: Re[2]: assist me
>
>The easiest to way to calibrate a magnetic field sensor in the audio
>frequency range is with a Helmholtz coil or a solenoidal coil. A Helmholtz
>coil is two coils of radius r with the planes of the loops also separated
by
>distance r. The magnetic field between the coils and at the center of
>either type coil is very uniform and solely a function of coil current and
>geometry (frequency independent). If you monitor coil current with an
>ammeter of sufficient bandwidth (any handheld DVM like a Fluke will likely
>do) that is the only measurement you need to determine the field.
>
>You can find all the equations at:
>
>http://www.geocities.com/CapeCanaveral/Hall/6153/
>
>Have fun!
>
>Ken Javor
>
>----------
>>From: [email protected] (Jim Bacher)
>>To: Mirko Matejic <[email protected]>, [email protected]
>>Cc: Mothish <[email protected]>
>>Subject: Re[2]: assist me
>>Date: Wed, Jan 3, 2001, 9:45 AM
>>
>
>>
>> Forwarding for Mothish, please keep him on your cc list as you respond.
>>
>> ____________________Reply Separator____________________
>>
>> --- Mothish <[email protected]> wrote:
>>>
>>> >
>>> > ----------
>>> > From: Mothish[SMTP:[email protected]]
>>> > Sent: Monday, January 01, 2001 10:24:56 AM
>>> > To: [email protected]
>>> > Subject: assist me
>>> > Auto forwarded by a Rule
>>> >
>>> helo sir,
>>> i got ur email id from iric.com.
>>> iam doing my project work for my engineering
>>> studies.(chennai,tamil
>>> nadu, INDIA)
>>> i have selected magnetic field sensor,for radiated
>>> emmision testing.
>>> i seek ur help for me to know the necessity of loop
>>> calibration(we are
>>> using loop ant. for 20Hz-50KHz range.
>>> plz provide me with the information iam in need of.
>>> or give me some contacts to whom i can clarify my
>>> doubts in future.
>>>
>>> seeking ur help barely.
>>> expecting ur reply.
>>> bye
>>>
>>> SINCERLY,
>>> MOTHISH BABU
>>>
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