BarnacleBill
New member
Over the past few years Tom(Jackpine Savage) and I have assumed different sides on the topic of coins producing halos that have spent some time in the ground. Recently Tom was involved in yet another thread concerning halos on another forum that I happened to take notice of. A comment by one of the other posters in that thread sparked some thought on my part concerning halos, and for some reason I believe I came to some clarity on the matter. What I am proposing is a theory, but it is a theory I have some put some effort into proving, and I am pretty comfortable with where I stand. Of course part of the reason for this post is to open my theory up to challenge and see if it can withstand some learned observations.
The conventional wisdom on halos usually follow along with certain beliefs. One is that the coin must be of a metal capable of, or prone to leeching into the surrounding soil, a commonly mentioned one is the USA penny made of copper. Another belief is that as the halo increases in size, a penny for example, would begin to appear to a metal detector as a larger coin. i.e. The penny pictured below with the quarter, has a portrayed halo that should make the penny detectable as deep as the quarter. But the one challenge to this is that after a coin with a halo has been recovered, the halo is not detectable. That observation has been challenged though, by the thought that once the soil matrix with the leeched metal is disturbed, it is now so diffuse it is undetectable. And if you were able to magically remove the coin without disturbing the surrounding soil, then the halo would remain detectable.
[attachment 76598 QPhalo.jpg]
One of the widely accepted principles in electricity is that current will follow the path of least resistance, and this is the basis of what I am proposing is occurring with the halo effect. It's well known that copper is very conductive, and leeched copper much less so, but the leeched copper is usually more conductive than the surrounding soil. Therefore there is little reason for a current to be induced in the copper coin and then travel towards a higher resistance(leeched copper). Quite the opposite, the current induced in the leeched copper will flow towards the lower resistance(copper coin).
So in a nutshell, I believe that the current induced in the halo travels to the coin and increases the amount of current flowing in the domains in the coin. The result is that the coin is made more easily detectable. An analogy is that the halo is acting like the radials on an antenna by collecting signal and delivering current to the coin. Now, how to prove this behavior?
One of the pieces of software I own is based on the "Numerical Electromagnetic Code" which is used to create computer models of antennas. I have designed countless antenna systems with this software and measured using RF ammeters, the current dispersion along the antenna systems. If I constructed the antenna correctly, matching the computer model, I have never seen the software be wrong. With that said I hope what follows doesn't get too technical for most to grasp.
Using the software I created a 5 KHz square loop antenna that acts as the transmit coil on a metal detector. I created another equally sized loop antenna spaced 1/2 wavelength away that acts as the target or coin. The models were created in "free space", it's as if the coil and target are in deep outer space with nothing nearby to affect the readings. The wires used were created with zero loss conductors, to once again hold all other factors as inert as possible.
The green numbered wires have a graphical current magnitude shown by a purple line. There are also current tables that show precise figures for each segment of all the wires. A source, the red circle on wire #1 injects 1V @ 1A & 5KHz into the transmit coil at segment #8. The wires in this software are broken into segments, and segment 8 on wire #1 & #5 are the reference points chosen to compare the current magnitudes and phase.
[attachment 76599 Targetraw.gif]
First the measurements were taken with no radials on the target loop to establish a reference point as a coin with no halo. Then four 1/4 wavelength radials were added to the target loop, but with some added resistance. These radials serve as the halo, or leached copper and I chose to add 20 MegaOhms to the junction points where the radials attach to the main target loop. This is an arbitrary figure, but there is no such thing as a standard since every coin halo & soil combination would be different, and 20 MegaOhms sounded reasonable to me.
[attachment 76600 TargetwRadials.gif]
After comparing the before and after halo modification, segment 8 on wire #5 of the target loop showed a current increase of 12%. Therefore I consider the computer model supports the theory that a halo when present can have an effect on depth of detection, and I believe I have identified the mechanism by which this is occuring. To re-state, the halo is gathering current, and that current is flowing to the coin and increasing it's detectability by a higher current in the coin.
[attachment 76601 Pg1amp2.gif]
[attachment 76602 Pg3.gif]
HH
BarnacleBill
P.S. If this is too technical, don't worry about it, just go out and enjoy detecting.
The conventional wisdom on halos usually follow along with certain beliefs. One is that the coin must be of a metal capable of, or prone to leeching into the surrounding soil, a commonly mentioned one is the USA penny made of copper. Another belief is that as the halo increases in size, a penny for example, would begin to appear to a metal detector as a larger coin. i.e. The penny pictured below with the quarter, has a portrayed halo that should make the penny detectable as deep as the quarter. But the one challenge to this is that after a coin with a halo has been recovered, the halo is not detectable. That observation has been challenged though, by the thought that once the soil matrix with the leeched metal is disturbed, it is now so diffuse it is undetectable. And if you were able to magically remove the coin without disturbing the surrounding soil, then the halo would remain detectable.
[attachment 76598 QPhalo.jpg]
One of the widely accepted principles in electricity is that current will follow the path of least resistance, and this is the basis of what I am proposing is occurring with the halo effect. It's well known that copper is very conductive, and leeched copper much less so, but the leeched copper is usually more conductive than the surrounding soil. Therefore there is little reason for a current to be induced in the copper coin and then travel towards a higher resistance(leeched copper). Quite the opposite, the current induced in the leeched copper will flow towards the lower resistance(copper coin).
So in a nutshell, I believe that the current induced in the halo travels to the coin and increases the amount of current flowing in the domains in the coin. The result is that the coin is made more easily detectable. An analogy is that the halo is acting like the radials on an antenna by collecting signal and delivering current to the coin. Now, how to prove this behavior?
One of the pieces of software I own is based on the "Numerical Electromagnetic Code" which is used to create computer models of antennas. I have designed countless antenna systems with this software and measured using RF ammeters, the current dispersion along the antenna systems. If I constructed the antenna correctly, matching the computer model, I have never seen the software be wrong. With that said I hope what follows doesn't get too technical for most to grasp.
Using the software I created a 5 KHz square loop antenna that acts as the transmit coil on a metal detector. I created another equally sized loop antenna spaced 1/2 wavelength away that acts as the target or coin. The models were created in "free space", it's as if the coil and target are in deep outer space with nothing nearby to affect the readings. The wires used were created with zero loss conductors, to once again hold all other factors as inert as possible.
The green numbered wires have a graphical current magnitude shown by a purple line. There are also current tables that show precise figures for each segment of all the wires. A source, the red circle on wire #1 injects 1V @ 1A & 5KHz into the transmit coil at segment #8. The wires in this software are broken into segments, and segment 8 on wire #1 & #5 are the reference points chosen to compare the current magnitudes and phase.
[attachment 76599 Targetraw.gif]
First the measurements were taken with no radials on the target loop to establish a reference point as a coin with no halo. Then four 1/4 wavelength radials were added to the target loop, but with some added resistance. These radials serve as the halo, or leached copper and I chose to add 20 MegaOhms to the junction points where the radials attach to the main target loop. This is an arbitrary figure, but there is no such thing as a standard since every coin halo & soil combination would be different, and 20 MegaOhms sounded reasonable to me.
[attachment 76600 TargetwRadials.gif]
After comparing the before and after halo modification, segment 8 on wire #5 of the target loop showed a current increase of 12%. Therefore I consider the computer model supports the theory that a halo when present can have an effect on depth of detection, and I believe I have identified the mechanism by which this is occuring. To re-state, the halo is gathering current, and that current is flowing to the coin and increasing it's detectability by a higher current in the coin.
[attachment 76601 Pg1amp2.gif]
[attachment 76602 Pg3.gif]
HH
BarnacleBill
P.S. If this is too technical, don't worry about it, just go out and enjoy detecting.
. Most don't know but he is and audio electronics engineer and has designed some pretty impressive products. I assume he has designed a few transformers in his day and knows plenty about proximity effects. He will likely have me running off like a scalded dog!
