Thank you John, very interesting.  Some of that I knew, some I didn't.
I do know that the rate at which the field falls off is approximately 1/r^3.

-Shawn

John N. Power wrote:

>>From:         Shawn Wilton[SMTP:shawn@BLACK9.NET]
>>Sent:         Saturday, June 12, 2004 12:08 AM
>>To:   PICLIST@MITVMA.MIT.EDU
>>Subject:      [ee:] Magnetic fields
>
>
>>Has anyone done anything like this app note:
>>http://ww1.microchip.com/downloads/en/AppNotes/00232a.pdf
>
>
>>I'm trying to do something similar to this, but I just want to be able
>>to detect a field from about 3 feet away.  But I can't get the darn
>>circuit to produce a strong enough field in the first place.
>
>
>>What I'm doing is this:
>
>
>>I have a simple "transmitter" that consists of a 160uH loop connected in
>>series with a microchip T4422 fet driver running at 12 volts (7812 and
>>10pF caps) and a .1uf 100 volt capacitor.
>
>
>>I'm trying to detect the field with another loop probably about 50uH but
>>much larger radius and my DMM.  I can pick up a frequency change by
>>getting closer, but I can't pick up any voltage just current in the 1-3
>>mA range.
>
>
>>Any ideas, comments, suggestions, discussion welcome.
>>--
>
>
>
>>Shawn Wilton
>>Junior in CpE
>>MicroBiologist
>
>
>>Phone: (503) 881-2707
>>Email: shawn@black9.net
>
>
>>http://black9.net
>
>
> The coupling between two current carrying coils can be visualized
> as the sum of the coupling between each differential current element
> of the source coil and each differential current element of the
> receiver. This coupling decreases with distance and with angular
> rotation of the receiver with respect to the sender. Maximum coupling
> occurs if the source and receiver are parallel.
>
> Because of the first condition, the differential elements of the receiver
> which contribute the most are those which are closest to the sending
> differential element. This means that the best shape and size for the
> receiver coil is that which matches the sender (same size and shape).
> Making the receiver coil larger than the sender increases the distance
> between each source element and its closest element on the receiver.
> Essentially, the coils couple through their perimeters. Matching size
> and shape decreases the integrated distance for a given coil spacing.
>
> The second condition controls how much of the receiver coil responds
> to a given element on the sender. If the coils are circular, the receiver
> element closest to the sending element will be parallel to the sender.
> As you move farther along the receiver coil, the elements not only
> become farther away, but they are also rotated, which further
> decreases the coupling. If the coils are rectangular, the rotational
> effect does not take place, in principle increasing the output
> somewhat.
>
> The induction field produced by current flowing through a coil
> falls off faster than 1 over r squared. To compensate, you need
> some serious current in the source coil.
>
> John Power
>
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