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World Famous Australian Professional Prospector and Gold Tour Guide
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The Confusion Over Coils
© 2000 Jack Lange
I am writing this article because of the many inquiries we get at my shop regarding the large variety of search coils now available for the brilliant Minelab SD series of detectors. The range of coils available has recently broadened to several more coils being made available. Is it really necessary to have such a wide range? How many coils should you have? Which is the best for you? Will they help you get more gold? Some customers save hard to purchase a fabulous new SD2200D detector and are reluctant to spend more money for an additional coil or two. Is that a wise decision? Or should they invest another several hundred dollars to make their Super Detector more viable? These are good questions so I will try to explain the advantages and disadvantages of the various coils presently available. It is important to note that all coils currently being sold new are suitable for any of the SD Series range and are interchangeable.
Four Basic Rules
Before I launch out about the pros and cons of the different SD coils, I want to talk about coils generally. There are four basic rules involved in understanding the different types and sizes of coils available.
The bigger the coil, the deeper it will detect the bigger nuggets, but the shallower it will detect tiny nuggets.The Smaller the coil, the shallower it will detect big nuggets but the more sensitive it will be on tiny nuggets. Double D wound coils are far more stable and create minimal ground noise but they are not as sensitive. This gives them a disadvantage in quiet (low mineralisation) soils but an advantage in noisy (high mineralisation) soils. Monoloop wound coils are more sensitive, but they create far more ground noise and are less stable. This gives them an advantage in quiet soils and a disadvantage in noisy soils.
All SD coils have two basic winding designs: either the double D or monoloop. The monoloop design coil has a single winding of special wire on the outside edge of the coil. The double D design has two loops of wiring which overlap in the central area of the coil.
Double D Coils
Produce far less ground noise and require much less tuning with the SD2100 or SD2000
Give considerably less false signals on mineralised patches or "hot rocks."
In Highly mineralised soils, nuggets can be detected at greater depth.
In very highly mineralised ground, they can be used when monoloop coils become too noisy to use.
They have a full width search pattern at depth.
They are less sensitive to electrical interference coming from nearby detectors, power lines and thunderstorm activity
When used with the SD2200D, they can discriminate ferrous junk.
They don't penetrate quite as deep in quiet ground (low mineralisation) as monoloop coils do.
They will not detect nuggets quite as small as the monoloop coil in quiet ground
They are slightly heavier to equivalent size mono coil, due to more wiring.
The audio signal produced is not quite as sharp or loud as a mono coil.
They have an edge in sensitivity over double D coils. They penetrate a bit deeper than the equivalent size DD coil in light to moderately mineralized soils.
They are slightly lighter than the same size DD coil
They are capable of detecting slightly smaller nuggets than the same sized DD coils
In heavily mineralised soils, they create quite a bit of noise and will not detect nuggets as deeply as the equivalent DD coil.
In extreme mineralisation conditions they cannot be successfully used.
With manual ground balancing SD2000 and SD 21000 detectors they need to be tuned far more frequently.
They will create many more false signals in heavily mineralised soils, and be more sensitive to hot rocks.
The search pattern is much narrower at depth, so the ground cannot be scanned as quickly.
As can be seen from the above, one cannot say that either coil configuration is better than the other. It depends on the particular goldfield, the soil types, and even the size of the nuggets. It also depends on whether one is prospecting old detected ground or new ground. Also with the big coils, the condition of your back can come into the decision making process.
Let's examine each coil individually.
The Minelab 11" DD Coil
This coil is standard issue with every Minelab SD detector whether it is the SD2000, the SD2100, the SD2100 E or The SD2200D. It is standard equipment because it is a useful all round coil, which is easy to use, and lightweight. But compared to some other coils, it doesn't excel in depth tests. It is extremely quite to operate and requires minimal ground balancing. When using a SD2100, at times I have only had to tune this coil three times per day as it ignores mineralisation so well. Combined with the SD2000 it would not effectively detect pieces smaller than half a gram, or a two ounce nugget at more than 35 CMS. It performs better with the SD2100 and especially the SD2200D, particularly on tiny nuggets.
The SD22200D has an edge over the SD2100 with DD coils, and seems to be optimised for this coil configuration for sensitivity and certainly for discrimination abilities. In fact the SD2200D won't discriminate with the Mono coils, except when using the more difficult tone discrimination mode.
When using the SD2000 or SD2100, the 11" Mono coil is significantly more sensitive than the 11"DD coil in all but very noisy soils. But this is not the case when using the SD2200 where even in quiet soils the advantage of the mono coil is not all that great, giving perhaps only 12% more depth, in ideal soil types.
The Minelab 11" MONOLOOP Coil
This coil has been a top seller since their release about 20 months ago. This is because it will find little nuggets at greater depth in spots that others have already searched before. It also gives reasonable depth on big nuggets but generally detects about 20% to 25% less deeply than the 18" mono. It is a good all round coil in thrashed ground where smaller nuggets are likely to be predominant. Like all Monoloop coils, it prefers quieter soils and needs frequent ground balancing, unless you are using the auto ground balancing SD2200D. Ground coverage is not very fast with this coil because of its limited size.
The Minelab 18" MONOLOOP Coil
This coil has excellent depth penetration in most soils on nuggets over 1 gram. By the way, you can tell how mineralised the ground is by the amount of noise the detector makes when waving any monoloop coil over it. (With the Sd2200D, you would need to have the auto ground balance on fix to test for mineralisation) The 18" coil gives good depth with any of the SD model detectors. I have detected a 4 ounce nugget at 90 centimetres and a solid five gram nugget at 40 CMS. But it is not a user friendly coil. When conditions require careful scanning it must be tuned frequently (with manual ground balance) as it's mono wiring and large size cause it to "absorb" a lot of mineralisation.
The sensitivity it has toward gold at depth also makes it more sensitive to changing ground conditions. For that reason and because of its weight, it takes a bit of effort to use it. Beginners are advised to practice with a double D coil for a while before attempting the use the big 18" coil.
But boy, have I found some gold with it! I remember on one occasion I returned to an area where I had found about 5 ounces with an SD2000 and the 11"DD coil. I had detected a house block sized patch with it very thoroughly, not missing even a square foot. Later when the 18" coils became available I detected the deeper ground with the same detector, in the same house sized patch and I got about five surprisingly clear signals for two ounces. All of them were deeper gold nuggets, which the 11" DD couldn't find.
Minelab have continued to improve this coil's stability and quietness with a new, flat bottomed, better finished design which took a flat skid plate. If you are still using the old Dish shaped design and are finding it noisy and giving signals when brushing grass, then consider the new 18" coil. Its quietness could surprise you.
To pin point a target with the bigger coils, you must turn the coil onto its edge for accurate location.
Large coils should be used in deeper ground where gold has either been detected before or it is likely to be found.
The Minelab 8" MONO Coil
This is the smallest of the SD coils and therefore by very nature of its size, will detect tiny pieces of gold. I remember detecting two small nuggets with the 18" mono coil. I scanned the immediate area and could find nothing else. I then thought I would try the little 8" mono on exactly the same patch, which was only room sized. There were signals everywhere! I collected 40 nuggets for just over 14 grams. During the two hours it took to detect the small gold patch, I must have balanced the SD2100 dozens of times in order to keep perfect sensitivity for those tiny pieces. Having the right coil for the job can make a big difference. This handy little coil is good for detecting creek and gully beds among the bedrock crevices and boulders where only small coils can fit. It is also ideal for detecting between tall clumps of long grass where you can't fit the bigger coils. The downside of this coil is that it will detect large nuggets at only 70% the depth of the big 18" coils, and because of its size, takes much longer to detect an area.
The ** Manufacturing 18" DD
When first released, this new Coil started a mini gold rush to those known nugget areas where the soil is heavily mineralised. One customer told me that in one spot he had originally found about 25 ounces of gold using the SD2200D using the standard 11" DD coil. The ground was extremely mineralised. So much so that it was impossible to detect it with a conventional VLF detector or even the SD mono coils. Fitting his new 18" DD coil he was amazed at how quiet the ground had become. Receiving signal after signal he dug out about 21 ounces of gold including an 8 ounce nugget. The 11" DD had handled the noisy conditions perfectly, but couldn't go deep enough for the larger nuggets.
I would use this 18" DD coil where someone (others or myself) had previously found gold in heavily mineralised ground, or when prospecting deep noisy ground, which was probably never detected previously, but looked as if it had good potential. It wouldn't be my preferred coil for prospecting totally new country where I had no idea as to whether or not I would find gold. This is because the coil is a little too heavy for fast swinging, even though ** Manufacturing have reduced its weight recently. In this situation I would prefer the lighter 14" Double D coil.
The ** Manufacturing 14" DD Coil
I make this my basic prospecting coil when prospecting highly mineralised new country with my SD2200D, where to my knowledge gold hasn't been detected before. I need to cover the ground very quickly with minimal effort and this coil suits the situation for the following reasons: (1) It is not heavy, being only a few ounces heavier than the 11" DD coil. (2) It gives a full 14" coverage at maximum depth penetration, so in reality the 14" DD is scanning roughly the same width of soil as the 18" monoloop which has a search pattern which narrows at depth. .
Also, this 14" DD coil searches very quietly so it takes minimum concentration to hear a signal.
Even with the auto ground balance of the 2200D, fast sweeping of a monoloop coil in noisy ground can create distracting ground noises, which can drown out nugget signals. With a double D coil you can swing quickly and quietly. (3) This coil detects substantially deeper than the 11" double D and covers the ground 40% faster.
The ** Manufacturing 14" MONOLOOP
This coil is a real surprise packet. I first tested it in quiet ground on a tiny one third of a gram piece of gold and found that with the SD2200D in "normal" mode without a signal enhancer, it could easily detect it at 6" deep in quiet ground. I was very impressed. Next I tested this new coil on a nugget the size of a $2 coin and it picked it up at an incredible 18" without straining my ears. This was slightly better than the 18" monoloop could do. I reasoned that surely on a big nugget of several ounces, it would fall behind the 18" mono in performance. But it didn't, in fact it was once again slightly better. I then ran the coil over a noisy clay patch fully expecting that, as it was such a sensitive coil, it would be noisy, yet for a monoloop it was surprisingly quiet. Naturally, not in the same quietness class as a double D coil. In heavily mineralized ground the 14" DD would still be the sensible option to choose. Why does this 14" monoloop coil break the rules and perform at least as well as the 18" mono on bigger nuggets, and the 11" mono on smaller nuggets? It's somewhat of a mystery, but it seems to be an optimum size for the mono configuration.
This is an excellent coil to use in an area with varying soil depths and differing nugget sizes, in all but very highly mineralised soils.
The ** Manufacturing 24" MONOLOOP Coil
This is a specialised coil, which many professionals will appreciate. It is an enormous coil and while it is light considering its size, it is too large and weighty for general prospecting. I would only use it in certain situations. I can think of several spots where in the past, I found larger nuggets in deep, quieter ground. I knew there was more gold there but it was too deep to detect. When I return to those spots I will be using the giant coil to give me that extra edge. I get the impression that it will scan between 15cms deeper than 18" coils, on large nuggets over a few ounces weight.
With careful scanning I had no trouble detecting a one gram nugget near the surface.
The ** Manufacturing 24" DD Coil
Once again, in highly mineralised soils, it will shine the most. And if I was going to a big nugget gold field, which had extremely mineralised deep ground, I would definitely take this garbage can lid with, me heavy though it be. In quiet ground tests on a four ounce nugget, it detected only about 6cms less deeply than the 24" mono coil, and about 7 cm deeper than the 18" mono coil, which makes it versatile for a giant DD coil. This also suggests that in very noisy ground, where you couldn't successfully use a mono coil at all, it would sniff out larger nuggets, which other coils couldn't reach. It would probably detect 50% deeper as the 11" DD, 30 % deeper than the 14" DD and 10% deeper than the 18" DD. There are still thousands of big nuggets to be found in thrashed areas of highly mineralised grounds, which are just out of reach of the smaller DD coils. Monoloop coils would be too noisy to use in these conditions and I would be prepared to carry the extra weight of the 24" DD and have a lot of fun patiently scanning, waiting for the deep sweet signals which can only indicate one thing. So I'll take the big garden pick with m, a pair of leather gloves and band aids for my blisters!
** Manufacturing Elliptical 10" MONO Coil
I use this coil in crevices and among tight places around bushes for a start. It's absolutely deadly on tiny stuff. It was able to detect pieces so small at 2" depth that my sensitive gold scales didn't register any weight and they go down to one tenth gram. It was able to detect a two tenth gram piece at 6". And on a 50 cent sized nugget it detected much better than expected, at about 65% depth of the big coils. It would be a deadly coil in small gold country or for crevicing.
Don't be surprised if your own depth tests give slightly different results to mine. Different detectors and different soils can affect results. Surprisingly, a nugget of a certain weight may be detected at a very different maximum depth to another nugget of the same weight. Even two identical looking coils can give slightly different results when compared.
** Manufacturing 24" Elliptical MONO Coil
I HAVE JUST RETURNED FROM AN EXTENSIVE DETECTING TRIP AND WAS ABLE TO THOROUGHLY TEST THIS NEW COIL OVER A VARIETY OF CONDITIONS. ALL I CAN SAY IS "WOW!"
It was very impressive for a variety of reasons:
I covered an enormous amount of ground each sweep. This meant more gold.
It is very light, being only about the same weight as the 14" coils.
It detected at impressive depth on large nuggets, and was not inferior to 14 and 18" round monos in this regard.
It could easily detect nuggets of half gram at several inches and a 1 gram nugget at about 10"
It was a quiet coil for its size, especially seeing that it is a mono. The only time I changed it with a double D was in very hot red soil.
You could pinpoint with accuracy by using its sharp tip.
Its one downfall being a mono coil, you cant discriminate with it.
If a Minelab SD owner was only able to initially purchase two additional coils, and they already had the traditional 11" DD coil as standard equipment, it would probably be a wise move to get a 24" Elliptical mono plus a 14" DD. If there were plenty of tiny bits around I'd also recommend a 10" elliptical. Remember that The SD2200D primary discriminator doesn't work with any of the mono coils) A larger DD would be a huge plus for very noisy ground where deeper nuggets may be lurking. With this added variety of coils, once you had found a productive patch of ground, you would get most of the gold in that patch.
Personally, as I am a serious gold seeker, I will take at least half of the above coils with me on my next lengthy gold seeking adventure. Why not? Having the right coil for the job can make thousands of dollars worth of gold difference in many situations. I would hate to think that for the sake of saving a thousand dollars in coil money that I had to give up on that 30 ounce patch just where the ground became too deep because I didn't have a big enough coil. Or it would be frustrating to give up on that likely looking rocky ground because I couldn't fit a coil between the big boulders. To some hobbyists who only spend the occasional weekend on the goldfields, buying a heap of coils might be a bit of overkill. But for serious seekers, having the best range of coils will mean considerably more gold, particularly if they visit a variety of goldfields or meet up with very varied situations.
By the way, none of the above coils can be used with any detectors other than the Minelab SD series. Big or advanced coils are not a substitute for a good detector. Thanks to Minelab and Bruce Candy the inventor, for the brilliant Super Detectors, particularly the SD2200D. Without them, the biggest and best variety of coils would be relatively disappointing.
See you out there!