Concentric coils

[thedigger]

With concentric coils,does having the receive coil on the outer give more depth on a large coil?

[Qiaozhi]

With concentric coils,does having the receive coil on the outer give more depth on a large coil?

Commercial coils have been made with the RX as the outer coil, but the overwhelming vote appears to favor the RX as the inner coil. The main advantage of having a small RX coil is superior pinpointing.
I vaguely remember finding that the outer RX provided a wide-scan capability, but pinpointing was rubbish, and depth was reduced.

[Woody.au]

Commercial coils have been made with the RX as the outer coil, but the overwhelming vote appears to favor the RX as the inner coil. The main advantage of having a small RX coil is superior pinpointing.
I vaguely remember finding that the outer RX provided a wide-scan capability, but pinpointing was rubbish, and depth was reduced.

I also wondered why commercial coil put the Rx on the outside, a bizzare way of thinking? I made a 3 coil concentric that was TX RX TX, the 2 TX windings was a single winding split into 2 series windings. I found this to give much better depth and sensitivity and is a good all round coil.

[Qiaozhi]

I also wondered why commercial coil put the Rx on the outside, a bizzare way of thinking? I made a 3 coil concentric that was TX RX TX, the 2 TX windings was a single winding split into 2 series windings. I found this to give much better depth and sensitivity and is a good all round coil.

Could be an interesting approach. I've never tried that.

[Rudy]

There are a couple of factors to consider in regards to placement of the RX coil. You can do a simple "back of the napkin" analysis, purely on geometric principles.

The Tx field from a round coil reaches the maximum depth at a point on the central axis. That is why overlapping swings maximizes the ability to find the deeper targets.

Now, the signal from the target falls in intensity following an inverse square law. So, the farther the distance to the Rx coil windings, the smaller the signal produced at the receiver. This argues for a small coil located close to the same axis (ergo, concentric), which when the coil is swept parallel to the ground, minimizes the distance from the target to the Rx coil. Note that if the Rx coil diameter is too large, the hypothenuse (ie. the distance between the target and the Rx coil winding) starts becoming noticeably larger than the vertical distance between the target and the ground surface thus increasing the amount of attenuation..

Now, a very small Rx coil is not the answer, as it would not "capture" a significant amount of the field created by the Eddy currents flowing in the target. There is an optimal Rx coil diameter, which is a tradeoff between sensitivity to coin sized targets and capturing as much of the field from the target as possible.

This optimum size results in an Rx coil with a diameter smaller than the Tx coil.

[Qiaozhi]

There is an optimal Rx coil diameter, which is a tradeoff between sensitivity to coin sized targets and capturing as much of the field from the target as possible.

This optimum size results in an Rx coil with a diameter smaller than the Tx coil.

I guess that no-one is in agreement over the optimum RX size. For example, recent Tesoro coils have the RX less than half the diameter of the TX, whereas Fishers have an RX greater then half the TX diameter.

Given that the target signal versus distance follows a 6th power, the difference is probably negligible.

[Tinkerer]

I guess that no-one is in agreement over the optimum RX size. For example, recent Tesoro coils have the RX less than half the diameter of the TX, whereas Fishers have an RX greater then half the TX diameter.

Given that the target signal versus distance follows a 6th power, the difference is probably negligible.

I am using 280mm for TX and 150mm for RX at present for the IB-PI.
One of the factors to consider is the k factor between the 2 coils. If the endeavor is to reduce the Flyback in the RX coil, it helps to have maximum distance between the 2 coils.
As the TX and RX coils get closer the mutual induction (k) increases. this means that the Bucking coil needs more inductance to counter act the TX pulse in the RX coil.

During the receive cycle, the RX coil and the Bucking coil, being 180 degrees different in phase tend to null the RX signal. However, the inductance of the Bucking coil is only a fraction of the inductance of the RX coil, so there remains the greater part of the signal in the RX coil.

In other words, we want a bucking coil with the minimum amount of inductance.
The closer the RX coil is to the TX coil, the more inductance the Bucking coil will need.

Tinkerer

[Rudy]

I guess that no-one is in agreement over the optimum RX size. For example, recent Tesoro coils have the RX less than half the diameter of the TX, whereas Fishers have an RX greater then half the TX diameter.

Given that the target signal versus distance follows a 6th power, the difference is probably negligible.

Qiaozhi, I said it followed an inverse square law simply because most people are more familiar with inverse square, however you are correct in saying it is 6th power, when considering the fact that the radiated field is isotropic.

Besides the electrical signal pickup from the target, other factors influence the decision. Things like how the manufacturer intends to induction balance the coil assembly, where to locate capacitors (if used), as well as mechanical consideration, like manufacturing the mold, location of the attaching flanges, open vs. closed coil,...