> 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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