Bigger is Always Better

[Anonymous]

The larger transmit coil is a very useful feature for it substantially eliminates dynamic range problems. In addition, it increases the depth capability of the detector. A normal coil produces an inverse cubic law with respect to the transmitting signal, which on re-transmission by the metallic object also follows an inverse cubic law. This means that the object also follows an inverse cubic law. This means that the signal sampled by the receiving coil has decreased by an exponential factor of six. In the case of a very large coil, the field is substantially uniform in depth thereby the received signal by the coil has only decreased by an exponential factor of three. This effectively doubles the depth for detectioon of metallic particles as compared with a normal size coil.

 

[Anonymous]

For a given object size there comes a point where detection range will fall as coil size is increased. This is illustrated by the accompanying chart. The coil diameter and detection range can be in any units i.e.inches, feet, centimetres provided both scales are the same. All the solid curves to the left of the 45 degree line show increases in range, while the dotted curves to the right show decreases in range. For example take an 11in coil (marked on Coil Diameter scale) and you find that you can detect a small object at just 5.5 inches (A on the Detection Range scale), the intersection falls directly on the 45 degree line and any increase in coil diameter would be on a decending curve giving less range. If, however, you can detect an object at just over 11.5in (B) with the 11in coil, then you can follow the curve up to get increased range with larger coils. A 20in diameter coil will give 14.2in and a 30in coil 15in range. You can see that as the curve approaches horizontal, a big increase in coil size only gives a small increase in range, the difference between a 20in and a 30in coil is less than an inch in range. Continuing along the curve, a 40in coil only gives the same range as a 20in and beyond that it will fall rapidly. These curves do not take into account the fact that larger coils will pick up more noise so that the full potential may not be realised. However they can give an idea as to what improvement is possible. Say you have an 8in coil and you can detect a coin at 12in. Find that intersection on a curve and follow the curve up. You can see that a 15in coil will gain you another 4in; quite significant. The curve maximum is always when the detection range equals the coil radius e.g. 5.5in for an 11in coil.
Eric.

 

[Anonymous]

Hi Eric,
Forgive me for my wickedness, but the above post is a quote from Candy's patent 5,576,624. column 9, lines 30-45.
In the previous paragraph (lines 26-30) he states: "In preference the transmit coil is substantially larger than the detector coil and is positioned substantially over the target volume and the receive coil is moved within the transmit coil to effect searching for any metallic target object."
I think by this he is covering the case where a large (50 yards by 50 yards say) transmit coil is laid on the ground and then you walk around the middle of it with your 15 inch coil.
Not my point though. If the signal really goes from being to the one sixth power to the one third power, it isn't going to just double, it is going to improve by orders of magnitude. Maybe we could get a phd in math to work on that one.
JC

 

[Anonymous]

In practice sure the depth just doubles with the huge coil laid on the ground, but the way it is stated. Of course larger coils spread the flux over their area, and smaller ones concentrate it in a smaller/shorter area. With the huge coil laid on the ground (even 10 yards by 10 yards) you better plug it in the wall if you want to have any flux density, even if you've picked up a third power.
JC

 

[Anonymous]

To those who are looking for larger objects that may be deep down using PI. This larger coil idea may have some merit. The trick is the flux density has to stay the same (power/square foot area) to pick up the power of three (at the same distance). The extreme is to have the transmit coil huge, but how much bigger would it have to be to get say half of this power of three. This is where the math gets messy. And help is needed.
Also what diameter coil is one turn 1 millihenry? Have to lay it on the ground but could be worth it.
JC

 

[Anonymous]

Hi JC,
Something is getting screwed up here. I

 

[Anonymous]

Thanks once again for your response. I had seen the Lorenz Pulse (I think) that had a large free form coil. Hadn't seen this one. Even for small gold if you had to lay down a large transmit and search with a small receive coil it might be worth it, if you could get signals much much stronger at the same depth as now. Twice depth at same sensitivity.
The thing that is not stated in the Candy patent (part I don't like) is that this improvement only occurs if the same magnetic flux density (field strength/square foot) from the transmit coil is present. As the area of the coil is increased the current through the coil must increase in proportion. This becomes impractical real quick.
Large objects see a larger portion of the weak energy there is, so you have a chance of detecting them.
What I am investigating is a need to detect buried gasoline tanks (petrol) at old gasoline stations. Apparently many years ago, old gasoline stations (sometimes newer ones) would get bulldozered over and highways expanded or exit ramps put in. Hundreds were done this way, and they have lost track of where they were. Back then they left the gasoline tanks in the ground. Now the worry is that these tanks are corroding and leaking whatever gasoline was left in them into the ground water.
The US government has a big bag of money for a detector that can detect buried (huge) steel gasoline tanks. The kicker is they must be able to drive around with this detector at a reasonable speed (not sure exactly how fast this is) in order to cover the area they need to (good chunk of the country).
JC

 

[Anonymous]

Hi JC,
Vehicle mounted PI I developed in the 80's. Originally as a detector for protecting agricultural machinery such as forage harvesters etc. Control box was in the cab and had audible warning plus output for a chart recorder if needed. Coil is a differential noise cancelling type 5ft x 18in. Could be made bigger if needed. Gas storage tanks are very big targets and should be detectable at 10ft or more.
Eric.

 

[Anonymous]

Hi Eric
What effect did the metal from the vehicle have on performance? How did you compensate for it?
You mention that it was a noise cancelling coil. Was this to take care of noise from the ignition system, power lines, radio transmissions etc.?
Does the electronics need a lot of modification to work with this type of coil?
Earl

 

[Anonymous]

Hi Eric,
Really like the picture and the use of differential coils, since most of the highways and many roads have 60 Hz power lines running near them. Also need to reject effects of guard rails and sign posts to some extent. Also will probably need to do something about the cutting of the earth's magnetic field as this vehicle speeds down the road.
Asked about vehicle speed again and was told as fast as possible (???). This will mess up the long integration times I normally like to use. Have always direct coupled the signals all the way, but this time may have to AC couple and look for a blimp out of the integrators. May give the retangular coils a try also.
Thanks again for the neat picture, and information.
JC

 

[Anonymous]

Hi Earl,
The metal from the vehicle had little effect at the distance the coil was mounted. The bottom coil is TX/RX1 and the top coil is a RX2 whose signal is subtracted at the first amplifier from RX1. Any vehicle signal equidistant at the side tends to cancel as does noise from power lines and the coils bouncing up and down in the earth's field. One problem is electrical noise from the vehicle; mainly the alternator. This gives horrendous rectification spikes whose frequency varies according to engine speed. This is radiated by the vehicle wiring. We made heavy duty filters which helped a great deal and also tried a switching the alternator field off when detecting. This was the best but means that the battery isn't charging when detecting. Diesel engines are best too, rather than spark ignition.
The circuit board I sent a while back is the very one that was used in this type of unit.
Eric.

 

[Anonymous]

This setup was once on the cover of a treasure hunting magazine, and I think it included an article. I would have to root through some boxes to find it.