Coil field direction

[Brian Deese]

On a PI coil when we wind a Tx coil and we are choosing which direction to send current into the coil,how do we choose that? do we want the north on top or the south on top? Speaking of field direction.

[joop]

In a PI system with just one coil field direction does not matter I think.

[Dixix]

In the instructions for the Surf PI I read: "To connect the coil to the detector, connect the inside of the coil leads to the" + "and the outside leads of the coil wire to the" - "on the circuit board . dont worry if one of the wires overlaps the face of the coil. "

But is a spiral coil....

Dirk

[Brian Deese]

Yeah its not hard to find drawings of coils on the internet where is current flows into the wire on the right the magnetic flux rotates clockwise so north is on top,but is you feed the left the flux rotates clockwise which puts the south on top.In the book inside the metal detector it says this is very important to metal detector design but does not say what orientation we want.

[joop]

Direction does not matter for a PI coil. We just want a static field for a short time, that is how the PI detector works. Only the field strength is important. When in air, the strength both above and below the coil is equal, no matter what the direction is. The only purpose of the coil field is to induce eddy currents into metal which is present in the gound. These eddy currents don't care about field directions.

[Qiaozhi]

The direction only matters for an IB coil. For a concentric, the relative phases of the coils is important in order to achieve coil balance. For a DD, the relative phases will affect the polarity at the preamp input. But for a mono coil, either way round will work just fine.

[Brian Deese]

The direction only matters for an IB coil. For a concentric, the relative phases of the coils is important in order to achieve coil balance. For a DD, the relative phases will affect the polarity at the preamp input. But for a mono coil, either way round will work just fine.

Ok then,So I want to build a figure 8 coil. That coil type has seperate Tx and Rx and is considered an induction balanced coil,correct? So which way do we want the current flowing in this case?

[Qiaozhi]

Ok then,So I want to build a figure 8 coil. That coil type has seperate Tx and Rx and is considered an induction balanced coil,correct? So which way do we want the current flowing in this case?

By using the term "figure 8", I assume you're referring to the configuration shown on page 57 of "Inside the METAL DETECTOR"?
I'm asking this, because sometimes people refer to the O-O (Double-O) configuration as a number 8.

The figure 8 coils are only used with VLF machines that can handle opposite phase quadrants. If you attach such a coil to the Surf-PI, only one half of the figure 8 will respond to targets. It might work OK with a bipolar PI detector, but I've never tried it.

[Davor]

Dixix is in dilemma with a reason, because the shield is hard wired to one side of a coil, so reversing this connection would give you grass effect on steroids.