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METAL DETECTOR COILS, HOW TO PICK ONE;
The Confusion Over Coils
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 mineralization) soils but an advantage in noisy (high
- 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
mineralized patches or "hot rocks."
- In Highly mineralized soils, nuggets can be
detected at greater depth.
- In very highly mineralized 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 mineralization) 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
- They are slightly lighter than the same size DD
- They are capable of detecting slightly smaller
nuggets than the same sized DD coils
- In heavily mineralized soils, they create quite
a bit of noise and will not detect nuggets as deeply as the equivalent DD coil. In extreme
mineralization 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 mineralized 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
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 mineralization 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 optimized 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"
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"
This coil has excellent depth penetration in most soils on nuggets over 1 gram. By the
way, you can tell how mineralized 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 mineralization) The 18" coil gives good depth with
any of the SD model detectors. I have detected a 4 ounce nugget at 90 centimeters
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
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
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 Coiltek 18" DD
When first released, this new Coil started a mini gold rush to those known nugget areas
where the soil is heavily mineralized. 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 mineralized. 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 mineralized 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 Coiltek have reduced its
weight recently. In this situation I would prefer the lighter 14" Double D
The Coiltek 14" DD
I make this my basic prospecting coil when prospecting highly mineralized 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 Coiltek 14"
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
This is an excellent coil to use in an area
with varying soil depths and differing nugget sizes, in all but very highly mineralized
The Coiltek 24"
This is a specialized 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 Coiltek 24" DD
Once again, in highly mineralized soils, it will shine the most. And if I was going
to a big nugget gold field, which had extremely mineralized 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 mineralized 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!
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.
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
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
- You could pinpoint with accuracy by using its
- 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
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!