Capabilities of a Small Coil

By Reg Sniff
From page 56 of the September, 1999 issue of Lost Treasure
Copyright © September, 1999 Lost Treasure, Inc. all rights reserved

The other day I was talking with a good friend and fellow treasure hunter who lives on the East Coast. That conversation gave me a good idea for this column. It is an issue I have mentioned before, but, after my conversation with my friend, George, I feel it is worth mentioning again.

George was telling me about a trip he had taken to a beach near his home. While at the beach, he met a person who was hunting the beach using a 3-inch coil. George left me with the impression that he wondered about the capabilities of such a small coil.

George went on to relate that he talked to the person using the small coil and was amazed at what this person had found. The man produced several silver coins and mentioned he had found many more with the small search head. My response to George was, "Try one."

I have advocated for years that when it comes to search coils, smaller can be better. I should say that not all small coils are the same. It depends upon the manufacturer. However, as a general rule, I have found coils in the 3 to 6-inch range to have almost the same depth capability as those 8-inch and larger on coin size objects. In some cases they can go deeper.

Why can a smaller coil go deeper than a large coil? The answer lies in two areas, the type of ground and the amount of trash that is in the area. In really mineralized ground, the smaller coil seems to react less to the mineralization.

Although treasure hunters do not really hear the strong responses coming from the ground, it can have a very serious impact on depth. I recently read in a patent about one of the state-of-the-art metal detectors, that the mineralization of the ground produced a signal response as much as 30 times greater than a typical target. Can you imagine, today's detectors are able to ignore a signal 30 times greater than the one you want to find?

Getting back to the smaller coils, I can relate a particular experience about the benefits of a smaller coil. I remember years ago I was chomping at the bit waiting for a new 10-inch coil to arrive. Boy, I thought I was going to clean up in areas I had hunted for years. Was I in for a disappointment. Everything I had read indicated if I wanted to go deeper, I was going to have to use a larger coil. Once I received the coil and tried it, I was truly disappointed in the results. In fact, controlled testing proved the larger coil did not go as deep as the standard coil, which was about 7-1/2 inches in diameter.

For some time I was baffled at the results. It was only after serious studying of the effects of the ground signal and other factors that it made sense. My further testing proved the condition that the smaller coils were better on several different brands, at least where I was hunting.

The mineralization in and around my home is severe by any standards, and it is because of this ground mineralization that larger coils do not fair as well. The larger coils simply react more to the ground signals and, as such, miss the coin size objects.

A further example of smaller is better was shown by a friend of mine named Tony. Like me, he belonged to a particular gold prospecting club in Arizona. That club used to produce a monthly paper noting nugget finds by members. One name that appeared consistently was Tony's. He would usually find as many nuggets as all others combined.

His secret wasn't a super-duper metal detector. It was the fact he was using a 7-inch elliptical coil while others were using 10-inch or greater. The smaller coil would allow Tony to get closer to the ground in many places, and was more sensitive to smaller targets.

For coin hunters searching trashy areas, the concept is the same. Although the search coil may cover a smaller sweep area, it is also more likely to maneuver between trash items better. What this means is the coil has a better chance of being over a coin only and not a coin and trash at the same time.

Before going to a couple of questions, I would like to point out a couple of other unique issues concerning search coils. First is the search pattern. On typical coin hunting detectors, manufacturers use what is called a concentric designed coil.

In the simplest of terms, this type of coil can be described as a large, outer circular transmit coil and an inner, circular coil, the receive coil. Open-faced designs clearly display the layout of such coils.

The concentric-type coil has two distinct patterns of detection, one for the all metal mode and another for the discriminating mode. The all metal mode is the one most depicted, which is a pattern of a swelled V, indicating detection over the entire search head.

In the case of the discriminating mode, things change, especially as the target gets deeper. What happens on deeper targets is the area of detection becomes a swelled V underneath the smaller (inner) coil only, and not over the entire search coil. In other words, to be safe, the true scan area is approximately the size of the inner coil, or about one-half the diameter of the search coil itself.

The exception to this concept is the second design of search coils, the DD, or otherwise called the widescan. On this type of search coil, the transmit and receive coils are about the same size. In simple terms, they are overlapping oval shaped coils. The area of detection in either mode on deeper objects is primarily under the area where the two coils overlap.

In other words, on DD type coils, the detection area is a narrow strip from front to back covering most of the search coil head. If DD designed coils cover more ground and give a narrow pattern, why aren't they more popular and provided with more machines?

The answer is two-part. First of all, DD coils are harder to keep tuned. They will have a tendency to drift because tuning is so critical.

The second reason concentric coils are used is the fact that many believe the concentric-type coil does a better job of discriminating. Most detectors are sold for the purpose of coin hunting. The quality of discrimination of a metal detector becomes a very important selling point.

DD designed coils are primarily used on gold hunting detectors because of their wide scan and they are less responsive to the ground signals. They are, however, somewhat less sensitive to very small nuggets (pinhead size).

With this information out of the way, let's go to the questions of the month.

"I am looking for technical information about metal detector design, schematics, plans, and other information. Can you help me?"

If the plan is to build a metal detector for serious use, my answer is, Don't. It is far more complex than people realize. It has taken many companies years to learn and compensate for the little things that can cause a metal detector to lack sensitivity as well as ignore the external electrical noise that can interfere.

You can find a lot of technical information by reviewing patents on metal detectors. The fastest way to find the specifics you are looking for is to use the Internet. Go to the IBM patent site. Use a search engine and search for patents. Once there, search for metal detectors. This location has most of the patents that would be of interest. Although the information is generic as to the specific design of each stage of a metal detector, it does give general technical information about the concept.

Another source is to contact some of the manufacturers, such as White's and Fisher. They were selling service manuals on different models. I have referred to both for particular information on techniques of design of each and every stage. I hope this helps.

"Have you ever heard of the Brinkman Treasure Sensor 3000? I bought one at a garage sale last summer, but it had no papers with it. Can you tell me anything about it." Thank you, S.L. Beaverton, Michigan.

I am sorry, but I have not seen this particular metal detector so I cannot give you any specifics. I have kept your address in hopes someone will pass along information about this particular model.

I hope somebody who has a copy of the manual or at least an address of the company will provide such information.

There are a lot of metal detectors purchased at garage sales that come with no information. Unfortunately, in some cases some of the manufacturers of such detectors are no longer in business, thus making it difficult to obtain information.

I will say that most companies still in business will be very cooperative in trying to accommodate an owner of a used detector. However, at the present time, I can't provide an address for Brinkman.