IB coils - what's more important?

[Jeff Kinzli]

To get them balanced and working halfway well, what should I consider most important?

- # of turns in transmit versus receive coil
- inductance of coils
- resistance of coils

I assume that if I want to build a larger or smaller coil than "stock", I would start with the inductance/resistance values of my "stock", working coil, and then build from there, right? Do I need to even worry about the # of turns in either coil if the inductance/resistance is right? Seems like the more of a ratio of turns I have the more sensitive it would be, is that true?

I'm a novice coil builder here struggling to get the first one built, so any pointers would be much appreciated! For what it's worth, my first project is a simple 2-coil (transmit/receive) IB coil for a Whites IDX.

[Robert Hoolko]

Re: IB coils - what's more important?

Jeff

I have never built a coil so don't put too much faith in what I say.

I would think the most important consideration in balancing the coil is geometry. That is, arranging the physical relationship between the various parts of the transmit and receive windings so that the receive signal balances out when the coil is held in the air. I would think this would be the most difficult part of winding your own concentric coil. The geometry also effects the capacitive coupling between the transmit and receive windings which you would also like to be zero. I don't know of any simple guidelines for setting the geometry of the coil. For a start I would just copy the geometry of an existing coil.

The turns ratio between the transmit and receive windings will affect the amplitude of the target signals, but I think this is less important than it sounds. I would not worry about being off by a factor of 2 or so, the detector can handle a wide range of signal amplitudes.

The inductance of the transmit winding is not critical, but LC is. You can adjust the size of the capacitor to get LC right. I would try to get your Tx inductance within a factor of two of an existing coil.

The resistance of the transmit winding affects the Q of the coil. I don't have any equipment for directly measuring Q, but I looked at the BM950 once and guesstimated that the Q was around 15. A high resistance would lower the Q and significantly reduce the transmitted magnetic field. I would be careful to keep the Tx resistance low.

I think the receive inductance and resistance are of less importance. They are not part of a tuned circuit, and you can be off by quite a bit without much of an adverse effect.

When scaling a coil I would try to keep the Tx inductance and resistance approximately the same as the original and the turns ratio approximately the same. I would let everything else follow from that.

A larger Rx to Tx turns ratio will give a larger target signal, but it will also give a larger ground signal and more noise. Increasing the turns ratio does not improve the signal to noise ratio. I doubt that increasing the turns ratio beyond what White's uses would give any significant improvement under typical hunting conditions.

Robert