larger coil designed for nugget hunting

[kaiser]

Hi guys,

My name is Krister and I’m new to this forum and I gotta say I love it with all it’s wealth on the technical aspect of metal detecting.

I have a couple of questions regarding limitations on a larger coil for detecting gold nuggets at depth which I hope some of you experts could help me with.

I have constructed a type of coil sled that you can push in front of you with ease over pretty uneven terrain without tilting the coil considerably. It also gives the coil a smooth ride with no sudden bumps or jerks when the sled moves over obstacles like tree roots and rocks.

I would like to know if it’s feasible to construct a metal detector designed for a large coil and thereby considerably increasing the depth at which it could detect nuggets compared to existing gold metal detectors.

I would basically like to increase the current and/or the numbers of turns in a larger coil so that the initial magnetic field at the center of the larger coil would equal that of the standard coil size for that P.I gold metal detector (e.g. the standard coil of the Minelabs gpx or White’s TDI).
For instance by using the formula for a magnetic field generated by a coil with n turns; I see that by increasing the radius by let’s say 3 times that of a standard coil (11 inch) I’ll have to increase the product, numbers of coil turns*coil current, with 3 in order to keep the initial magnetic field constant.

My question mark now is how this could be done with out increasing the time delay considerably ( I wanna be able to find 10 gram nuggets or at least half an ounce nuggets, I have no idea about their time constants 20 microsec?)

Which strategy is the best?

Increasing the power (current) only and not change the numbers of turns. This would not change the inductance but would result in a longer time for the current to decay to zero before sampling can take place. Could this longer time be reduced by using thicker wire?

Or is it better to only change the number of turns of the coil so that the inductance goes up or is a combination the best strategy? Lets say I wanna use a coil with three times the radius of a typical standard coil( i.e 33 inches).

Could a double D coil be of any help here? I mean it’s inductance balanced so the Rx winding should not “see” the large fly-back current.

Any thoughts or comment is welcomed.

Cheers /Krister

[grungymike]

power, depth, decay curve

Hi, I'm new here myself and going thru a similar learning curve. I would suggest getting ahold of Carl Moreland's article on the Hammerhead I detector, bbsailor's article on fast coil design, and Reg Sniff's article on PI detector design. These guys cut years off of my learning curve. These article's are available in PDF format over in the project section.

I'm getting ready to build a Hammerhead I and my thinking kinda parallels yours. A lot of the focus is on reducing the initial sample delay to find smaller targets, but I'm leaning toward a bit more power (longer xmt pulse), with a first delay around 12-14 uS, and a strong focus on a good clean signal, which means getting as much noise as possible outa the circuit. My thinking is that while smaller target reception is a good marketing strategy, past a certain point it just isn't worth the time and effort digging tagets like the 4 grain flake I found with my GMT. I consider the ability to ignore really small targets an advantage, I'm looking for the moneymakers.

By the way, one reason I chose the Hammerhead circuit is it has an adjustable xmt pulse width, allowing you to chg the xmt pulse strength
as the situation calls for. In fact just about everything on that circuit is adjustable. I'ts perfect for old knob spinners like me. Course, I don't like automatic transmissions either.

Good hunting...

gm

[Geo]

Hi guys,

My name is Krister and I’m new to this forum and I gotta say I love it with all it’s wealth on the technical aspect of metal detecting.

I have a couple of questions regarding limitations on a larger coil for detecting gold nuggets at depth which I hope some of you experts could help me with.

I have constructed a type of coil sled that you can push in front of you with ease over pretty uneven terrain without tilting the coil considerably. It also gives the coil a smooth ride with no sudden bumps or jerks when the sled moves over obstacles like tree roots and rocks.

I would like to know if it’s feasible to construct a metal detector designed for a large coil and thereby considerably increasing the depth at which it could detect nuggets compared to existing gold metal detectors.

I would basically like to increase the current and/or the numbers of turns in a larger coil so that the initial magnetic field at the center of the larger coil would equal that of the standard coil size for that P.I gold metal detector (e.g. the standard coil of the Minelabs gpx or White’s TDI).
For instance by using the formula for a magnetic field generated by a coil with n turns; I see that by increasing the radius by let’s say 3 times that of a standard coil (11 inch) I’ll have to increase the product, numbers of coil turns*coil current, with 3 in order to keep the initial magnetic field constant.

My question mark now is how this could be done with out increasing the time delay considerably ( I wanna be able to find 10 gram nuggets or at least half an ounce nuggets, I have no idea about their time constants 20 microsec?)

Which strategy is the best?

Increasing the power (current) only and not change the numbers of turns. This would not change the inductance but would result in a longer time for the current to decay to zero before sampling can take place. Could this longer time be reduced by using thicker wire?

Or is it better to only change the number of turns of the coil so that the inductance goes up or is a combination the best strategy? Lets say I wanna use a coil with three times the radius of a typical standard coil( i.e 33 inches).

Could a double D coil be of any help here? I mean it’s inductance balanced so the Rx winding should not “see” the large fly-back current.

Any thoughts or comment is welcomed.

Cheers /Krister

Hi.
If you want to make a detector for large coil, make the Delta Pulse with 1x1 m mono coil. You will find here a lot of threads about it

Regards