hoosing the Right Coil for the Job: Info on Search Coil Types, Designs & Sizes

Both the type and size of a search coil you use make a big difference in the process. That is why it is important to choose a right search coil for your metal detecting task or your particular style of hunting. The search coil types in use today are represented by the following coil configurations:

1) Concentric/Coplanar

This search coil's design incorporates two transmit coils (windings) and one receive coil of unequal diameters aligned on the same center. This allows all three windings to occupy the same plane or level (as shown by illustration on the right).

The concentric/coplanar style is the most common and is used on most detectors designed for coin, jewelry, and relic hunting. Concentric coil's electromagnetic field detection pattern is conical in shape (extending above and below the coil windings) with the greatest density directly under the coil's center.

Concentric search coils can be thin, thick, solid or doughnut-shaped. Some concentric coils give the appearance of a spider's web. These are called spider coils or open web designs. Some metal detector users find the spider coils to be a little sturdier and seem to present fewer false signals than the standard concentric coils.

Concentric coils are also made in elliptical and semi-elliptical designs. The main advantage of elliptical design is that it can cover more ground for each sweep of the coil when compared to a round coil with the same coil area.

Elliptical coils can also be maneuvered more easily in dense vegetation and between rocks on rough terrain than the round concentric coils. However, it can be more difficult to achieve precise pinpointing of small targets with the elliptical design.

Concentric Coil Disadvantages: • Because the concentric coil's electromagnetic field detection pattern is conical, it imposes a disciplined overlapping of sweeps to ensure thorough ground coverage; otherwise, many targets will be missed (as shown by illustration below).

Concentric/Coplanar Search Coils and Their Cone-Shaped Detection Pattern

• Concentric coils are affected by heavy ground mineralization, "hot rocks" and salt water: they are less stable in these extreme environments.

Concentric Coil Advantages: • Pinpointing with concentric coils is easy and precise because the strongest signal is always in the center of the coil. • Concentric coils are the most compatible with the Discrimination function and discriminate very well.

2) Double D (DD) or 2D Wide Scan

The Double-D coil is characterized by two "D" shaped (transmit and receive) windings aligned back to back (like two opposing letters D) and overlapping. This configuration results in an elliptical shaped (from the toe to the heel of the coil) sensing area which provides a chisel-shaped signal that covers the ground uniformly.

Double-D Coil Disadvantages: • Pinpointing with DD coils can be challenging and would require a skillful approach (described on page 14) unless you use these coils with a metal detector that has the enhanced or improved Pinpoint feature. Also, the larger a DD coil, the harder it is to pinpoint targets. • DD coil's inability to detect targets situated close to trees and stone fences if sensing area is inboard by design. This problem was solved with designing an elliptical shaped 2D housing which allows the sensing area be closer to the housing edge. Detection with an elliptical DD coil next to trees and fences became possible, provided the search path is parallel to the obstacle.

Double-D Coil Advantages: • Double-D Coil is not as affected by heavy ground mineralization as concentric and monoloop coils because the Double-D design "sees" 70% less of the ground mineral effect while covering more ground in one sweep. DD coils are more stable in heavily mineralized and saltwater areas and give you smoother operation in areas where the ground matrix tends to shift. • DD coil does not require disciplined overlapping and provides a wider scanning of the ground than other coils (as shown by illustration below).

Double-D Search Coils and Their Electromagnetic Field Detection Pattern

Even though the Double-D search coil seems to have more advantages over the concentric coil, its performance, as performance of a search coil of any type, depends on the operating frequency of a metal detector. For example, the gold nugget detectors operate on high frequencies up to 70 kHz to detect the smallest grains of gold. In other words, their operating frequencies cover only one end of the "target spectrum" so to speak. Also the size of the coil plays an important role in this process (see more details on page 5).

Metal detectors that operate on lower frequencies cover another end of the spectrum. Most of the regular All-Purpose Single-Frequency detectors operate on a frequency that covers the "golden medium" of the "spectrum" but not all of it. The problem with one frequency is that it may work well in one area and often offer only marginal performance when used in another location.

Ground mineralization, trash content and target size all have an effect on how well a detector transmitting a single frequency would operate. The Dual-Frequency metal detectors perform slightly better in terms of target recofnition and have a little more detection depth than single-Frequency units.

And only the machines of the third type - the Multi-Frequency detectors, can provide a full coverage of the "target spectrum," greatest detection depth, maximum sensitivity over a wide range of targets and their more accurate identification. Such detectors have either 17 frequencies ranging from 1.5 kHz to 25.5 kHz, or 28 frequencies ranging from 1.5 kHz to 100 kHz. Under extreme conditions, even if a few frequencies of a Multi-Frequency detector can not penetrate the highly mineralized ground, most of the frequencies will complete the job.

Detection Patterns of Concentric and 2D Search Coils Operating on Single-, Dual- and Multiple-Frequency

This should be considered when deciding which detector and what type of search coils to use for your metal detecting projects. If the Multi-Frequency detectors are too expensive for you, you can still use detectors of the latest VFLEX Single-Frequency technology which now offers an easy solution. You can buy a Single-Frequency detector with three search coils and set the frequency (low, medium or high) of your detector by simply utilizing one of these coils. Thus, by interchanging three coils, you can have three metal detectors in one!

3) Monoloop

Monoloop is the type of search coils in which the multiple strands of wire are wound in a single loop around the circumference of the coil. The electromagnetic field detection pattern of a Monoloop coil is cone-shaped (extending above and below the coil windings), which requires overlapping of sweeps to ensure thorough ground coverage. Monoloop coils can be of either round or Semi-Elliptical designs.

Monoloop Coil Drawbacks: • In highly mineralized soils, a Monoloop coil can be more difficult to Ground Balance and, therefore, possibly more noisy, and will be outperformed by the Double D coil. • If overlapping of sweeps is not maintained, many targets will be missed due to the shape of the Monoloop coil's electromagnetic field detection pattern which is conical (same as the pattern of concentric coils).

Monoloop Coil Advantages: • Monoloop coil provides greater depth and sensitivity compared to a Double D coil of equivalent size in low-medium mineralization.

Monoloop Search Coils and Their Electromagnetic Field Detection Pattern

4) Coaxial

This search coil design has identical diameter transmit and receive windings stacked and aligned on the same axis: the transmit winding is sandwiched by two receive windings in near perfect alignment on center. This creates an electromagnetic detection pattern of more uniform density and performance, and offers the best resistance to interference from high-voltage power lines. The electromagnetic field detection pattern of a Coaxial coil is also cone-shaped.

Because of its specific design, this type is seeing renewed usage only in smallest coils or "mini coils" of 4" in diameter. It would be impractical to use the coaxial coils of larger sizes due to unavoidable increase in coil's thickness and weight.

This type of search coil produces an opposite reaction topside compared to the bottom when sensing a target. If you have the habit of passing a handful of dirt over the top of search coil during recovery to verify whether the target is still in the hole or not (as described in "Plug-Splitting Technique" on "How To Metal Detect" page), you should not do so when using a coaxial search coil.

If a target is accepted on the underside, passing the same target topside will make no sound. The target hiding in the dirt in your hand will seem to "disappear", further confusing your recovery.

Coaxial Search Coil and Its Electromagnetic Field Detection Pattern

Coaxial Coil Drawbacks: • Small detection depth - disadvantage of all "mini coils" • By virtue of their smallest coverage area and conical electromagnetic field detection pattern, coaxial coils impose a very disciplined scan rate and overlap which, if not maintained, may result in missing many targets.

Coaxial Coil Advantages: • Uniform electromagnetic field detection pattern • Better separation of trash and desirable targets in close proximity • Ability to detect closer to metal poles without detecting them compared to other designs • Resistance to 60 Hz AC electrical interference

Successful Metal Detecting Also Depends on Search Coil Size!

As it happens quite often to beginners, they can not find a single good target at the hunt site that has been proven to be "fruitful" and still contains many valuables. In most cases, the reason is simple: they use a search coil of a wrong size. Depending on your "scope of action," you always should determine which coil size is the most optimal for each task before detecting. Sometimes it is necessary to employ two or even three different search coils at the same hunt site. Here is a general information on search coil sizes and their applications:

• Smallest Search Coils (up to 5" in diameter) - "Mini Coils" or "Snipers," are great for extremely trashed areas, really allowing you to "catch" coins in between numerous iron nails and other junk. "Mini coils" provide better separation of trash and desirable targets in close proximity and have an ability to detect close to metal poles and wire fences without detecting them.

These coils are also the best for detecting in tighter spots and around natural obstacles such as dense vegetation, tree roots and crevices. Mini coils are very sensitive to the smallest and shallow targets to which their reactivity is increased. These coils are maneuverable and weightless. Their drawbacks are their shallow detection depth and smallest coverage area.

• Small Search Coils (5" - 8" in diameter) are used for "trashy" areas and highly mineralized ground. These coils are maneuverable in heavy vegetation or on rough terrains. They have more detection depth than "snipers" but less than medium-sized coils.

These coils, both concentric and 2D, have proved to be the most effective in searching around cellar holes and at the sites of old homesteads. If the mineralization intensity is low and the trash content is moderate, the 8" search coil will give you the best depth, target separation and ease of pinpointing.

The 7.25" Double D search coils can easily compete with 9.5" and 10.5" coils under certain conditions and program settings. By virtue of their small coverage area, the small coils impose and promote a more disciplined scan rate and overlap (see more info in "Search Coil Discipline In Details" on next page).

• Medium Search Coils (8" - 11.5" in diameter) usually come standard with your metal detector. These coils are the "golden medium" among all coils and designed for general use in finding the wide range of targets, and under average metal detecting conditions. If the mineralization intensity is high and trash content is low, the 9.5"-11.5" Double-D search coil should be used for optimum depth and coverage. Although these coils do a great job, they lose their effectiveness under extreme metal detecting conditions that might occur on either side of the "average" range. For example, if the hunt site is trashed with iron, the 11.5" coil should be replaced by the 7- or even 5-inch search coil to find targets that are partially masked by nails. If metal detecting takes place at a large farm field, the 11.5-inch coil should be replaced by a larger coil (15" or larger) to cover a vast area quicker.

• Large Search Coils (11.5" - 24" in diameter) are designed to penetrate the ground at the most depth. Theoretically, the larger the search coil, the deeper it will detect targets. In reality, it is not always true because the amount of detected mineralization has negative effect on detector's performance: the larger the coil, the more mineralization it "catches." This disables a metal detector in terms of deep ground penetration. The maximum practical size of a large search coil was experimentally determined to be 15" (38 cm). Using a search coil of a bigger size (diameter) would not increase detection depth unless you use a Pulse Induction metal detector.

Large coils are very useful for finding coin caches and deeply buried, large relics. Large coils provide a considerable ground coverage. Detecting with a large coil enables a detectorist to cover a vast territory, such as a large farm field, more quicker than with a medium search coil. The large search coil's drawbacks are its lack of sensitivity to small targets, poor pinpointing, simultaneous rejection and masking of more targets, and its weight. A Hip-mount or Body-mount configuration is recommended for metal detecting with a large search coil for long hours.

The "rule of thumb" governing larger search coil use is as follows: "as the size and depth of the target you seek increases, and the concentration of junk targets in the search area decreases, the size of your search coil should increase."

NOTE: When making a change in search coil size during a hunt, always check and readjust Ground Balance (if it is manual) and Discrimination settings. Changes in the search coil diameter and its configuration also may give depth readings (if your detector features a depth meter) over or under actual target's depth.

NOTE: Because search coils come in all shapes and sizes nowadays, changing the search coil on your metal detector may be the best alternative to buying a whole new machine in case you need to update. However, it can not be that easy as it seems... Please read item 3 in my next note.

NOTE: Avoid using the so-called "Hot" and "Excelerator" cheap search coils due to the following reasons: 1) These coils are built from cheap materials that do not last longer than a few months. 2) As these search coils are put together by the unskilled and underpaid workforce located either in Asia or Eastern Europe, the quality of coil assembly is poor. 3) Your metal detector is NOT TUNED specifically to search coils it did not come with! Unless you have a similar search coil, which is native to your metal detector, to compare to the additional one you have purchased, you would not know that your detector is not performing effectively with a new coil. Even after buying a brand new search coil made by the detector's manufacturer, one has to send both metal detector and new coil to the manufacturer's regional repair center for proper tuning and adjustments. When purchasing metal detecting equipment (or any equipment!) and accessories, always remember that the cheapest stuff is the hardest to afford!

1) Proper Coil Angle

Make sure that the angle of the search coil is properly adjusted: the coil is parallel to the ground when touching the surface.

If the coil is tilted upwards or backwards, a considerable amount of detection depth will be lost, and a few targets will be missed during every sweep as shown in illustration below.

Tilted Detection Patterns of 2D (blue) and Concentric (pink) Coils with Wrong Angularity

To make a necessary coil angle adjustment, simply loosen the bolt that connects the lower shaft to the coil, while holding the detector as if detecting, lightly press the coil to the ground until it sits flat/parallel with the ground. The coil should remain parallel with the ground when you stand with the detector at your side, and the coil is lifted to the sweep height, approximately 25mm (1") above the ground. And then tighten the bolt, but do not over-tighten it.

A shaft length should be adjusted correctly. If the coil is too far from your body, it will be difficult to balance and maneuver, and cause a lot of uncomfortable stretching. If the coil is too close to your body, it may detect your digging tools or any other metallic objects such as steel plates in your boots (always leave them at home!) which you are carrying, causing destructive sounds. Plus, you will experience a lot of stooping and may accidentally step on the coil (I did it a few times).

Some metal detectors have inadequate isolator/washer/fastener designs used for attaching search coils to the detector's lower shaft. If the shaft attachment is not located at the coil's center (open-center coils), the inadequate isolator/washer/fastener system does not hold the coil's angularity stable. To correct this problem, an after-market metal detecting accessory - an attachment called 8-Position Loop Support, is manufactured.

It can be attached to your search coil to keep it constantly parallel with the ground to ensure the coil's detection pattern envelops the coins that may happen to be close to the sweep edge. Search coils of the conventional solid housing style usually do not cause this problem.

Search Coil Discipline in Details

2) Keep Search Coil Parallel to the Ground

Keep the search coil parallel to the ground surface at a constant height to eliminate false signals. Sweeping the search coil in a slight semicircular pattern will maintain a more parallel relationship between the ground surface and the search coil.

Sudden upward motions of the search coil can alter the way automated circuitry of your metal detector "sees" ground minerals and rejected targets causing it to react with false audio signals common to some motion metal detectors.

3) Keep Search Coil Close to the Ground

Keep the search coil as close to the ground as possible to maintain maximum detection depth, especially when you are detecting with a small coil.

If you are searching a lawn with no obstacles, keep the coil in light contact with the grass to achieve maximum penetration. When searching uneven ground, keep an even search coil height above the obstacles and increase sensitivity if possible.

4) No Pendulum Swinging!

Do not swing the coil in a pendulum motion so that it arcs upward at the end of a sweep. This will result in false signals, and detection depth will be lost at these points in ground coverage. This will result in missing many valuable targets.

By ending each sweep with the coil's drifting up, a detectorist, to his/her disadvantage, voluntarily narrows the ground coverage that can be scanned effectively. As a result of such uncontrolled swinging, almost 50% of the area is left unsearched.

Arched Electromagnetic Field Detection Pattern Does Not Reach a Few Coins

One can only imagine how many "keeper" targets may be left in the careless detectorist's path that has been only partially searched. This is one of the reasons why we find many good targets at the "searched" sites. Pendulum swinging is used only in situations when a questionable target needs to be identified (more details on page 19).

5) Sweep Overlapping

Overlap the sweeps by as much as 75% of the coil diameter, especially if Concentric, Monoloop, and Coaxial coil configurations are used.

Because the effective electromagnetic field detection pattern of the concentric, coaxial, and monoloop search coils is cone-shaped, overlapping is required with this type of coils. For comparison, the following illustrations depict side views of the search coil swept without overlapping, and with 25% and 50% overlaps.

'No Overlap' Case Study for Concentric Search Coil

Even though the underground target assortment depicted in illustrations is imaginable, it includes a couple of realistic cases of coins being masked by iron nails (see more details in "Detecting Masked Coins" on page 18). For the sake of this example, all targets are assumed to be aligned. Also it is assumed that the electromagnetic field detection pattern is uniformly effective, though, it is not in reality, and this will be discussed later on page 10.

As shown in the first illustration, sweeping the concentric coil without overlapping leaves most coins undetected, no matter how the coil's sweeps are placed in the picture. Three sweeps would result in detecting one shallow coin "a" (if its signal is not affected by a small iron nail on the left) in sweep 1, a pull-tab in sweep 2, and a large iron nail in sweep 3.

Concentric Search Coil Sweeps without Overlapping

There is a slight possibility that a pull-tab in sweep 1 would be recovered along with coin "a," and if the hole rechecked for more signals, a coin "b" standing on edge would be "caught" - an impossible case for a beginner (this will be covered on page) but not for a pro. An experienced detectorist most likely would scan the walls of the hole and get the responses from coins "c" and "d", or dig a wide hole and discover these coins in a dirt pile.

The pull-tab in sweep 2 will be dug up if not rejected by Discrimination. Everything else would be masked by mid-sized iron nail, would not respond, and, therefore, ignored by the detectorist. He/she would hear the Threshold going "blank" for a second because of the nail (if iron is discriminated), but likely would ignore it anyway.

Even if the Discrimination control is set up high, a large nail in sweep 3 would produce a silver-like response due to its large size and the Halo effect. A small silver coin "i" beneath the nail is completely masked by it and would not be "seen" by a detector, but would be dug up along with the nail. So now this

recovery could have many possible outcomes. But I will describe the simplest one for the sake of the newcomers to this hobby.

One outcome would be like this: after the dirt is taken out of the hole and piled up, the coin happens to be on top of the dirt pile and above nail. As the pile is checked for a signal, the coin is sounding loud and clear, and then picked up by the detectorist. If he/she scans the pile again, the nail would not respond as the halo effect has been destroyed. If iron is discriminated, the Threshold will go "blank" for a second when the coil is passed over the nail. What happens next is very interesting.

A beginner will be satisfied with his/her coin find and will not pick up the nail from the dirt pile, and will not recheck the pile for possibly more signals again. Well, we know there is nothing else in it, but sometimes there are two coins stuck together underground. After the coins get separated and taken out along with the dirt and nail (in this case) during recovery, the second coin is usually ignored and left behind. And less likely would a beginner recheck the hole.

A veteran detectorist would certainly scan the hole and hear a faint response from the deep coin "h", and possibly get another faint signal from the edge of the coin "f". By digging coin "f" up, this detectorist would not be able to miss the shallow coin "g" standing on edge. And if this person is persistent, he/she would scan the walls of the hole and get a response from the coin "e" despite its close proximity to a mid-sized iron nail in sweep 2. A pro would get all eight coins from this small patch of the ground!

Even from just one outcome chosen, I may develop a web of new possible scenarios, and this could continue endlessly until the last nonferrous target is recovered. But what could be learned from my example? A few very simple and effective measures that can be taken to make sure most of the valuable targets are recovered:

1) investigate every questionable sound 2) dig larger holes (if possible) and scan their walls 3) dig deeper holes (as much as the Fiskars shovel's blade deep) 4) pick up the dug targets from a dirt pile and recheck the pile (or the dirt plug) for more signals 5) at least do not discriminate all nonferrous targets (ideally the Discrimination control should be at minimum level just to reject small nails) 6) OVERLAP!!! At least 25%

Search Coils for Metal Detectors, page 9

Search Coil Swinging Discipline in Details

Sweep Overlapping with Concentric Search Coils

(...CONTINUED from previous page)

The second illustration shows that three more coins could be easily detected when 25% overlapping is taking place.

25% Overlapping of Concentric Search Coil Sweeps

The coin "d" in sweep 2 would be a little problematic to ID because of the pull-tab positioned in close proximity - this would produce a combination of two signals of different tones (if Tone ID is used) or an average between them, or two signals of the same tone character (if your detector is operated in a single-tone mode) but different sound.

The problem here is not the tones of audio responses, but the quality of each signal. In any case (Tone ID or not), both signals may be very broken or "chirpy" due to the positioning of both targets - both are standing on edge! The search coil methods for "separation" of targets, detecting "coins on edge," and identification of questionable targets are described on page 14, page 16 and page 19 respectively.

The rest of this scenario could unfold taking same turns as in the first case described above. At least, in this case, an inexperienced detectorist would leave less coins undetected. To get more coins easy way, one should increase the overlap percentage.

The third illustration shows the coin "c" getting fully encompassed and coin "e" partially being "touched" by the electromagnetic field detection pattern when 50% overlapping is taking place.

50% Overlapping of Concentric Search Coil Sweeps

50% overlapping would yield much better results in reality than in my illustration. It is just that the picture format does not allow me to "paint" a more realistic picture of target positioning and density. In real world, if a hunt site contained as many coins per one square meter of ground coverage as shown in my illustration, this would be a fanatstic hunt site, and every inch of the ground would be investigated, and every target would be dug up!

If to place all sweeps that were shown in previous illustrations into one picture, we get an interesting situation: three spots that may or may not contain coins are missed by the detection patterns of all sweeps - "no overlap," "25% overlap" and "50% overlap," combined. On picture below, those spots are 1) location of coin "b", 2) location of coin "e", and 3) location of coin "h."

Electromagnetic Field Detection Patterns of All Sweeps Combined

Based on this observation, I can conclude that even if you metal detect the same area three times following the same direction of search, each time increasing the sweep overlaps by 25%, you would still leave roughly 25% of the underground soil unscanned due to the physical shape of the electromagnetic detection field of your concentric coil. This is another reason why we find deep-buried coins at "hunted out" sites.

To decrease the percentage of undetected underground that falls within the reach of your coil's electromagnetic detection field, you can do the following: 1) increase the sweep overlap up to 75% 2) rework a hunt site at angles perpendicular and oblique to the original search patterns or gridding patterns 3) use search coils of different sizes and switch them during each hunt to get most of valuable targets out of the hunt site (see details on coil sizes on page 5) or 4) use Double-D (or Wide Scan) search coils (see details on page 3).

Now let us take a closer look at more realistic stuff in relation to the electromagnetic detection field pattern of the concentric search coil. This cone-shaped pattern contains regions of decreasing effectiveness with respect to target size and depth as shown on illustration below.

Concentric Coil's Electromagnetic Field Detection Pattern

Detection pattern effectiveness is directly proportional to the surface area of the target. In other words, you might be able to metal detect an iron bucket buried at two feet, but you will not detect a coin at the same depth.

If a dime is buried 7-8 inches deep, it would fit into the effective detection area of the concentric 8" coil, which would be approximately the size of a half-dollar as shown on illustration below (the dime is not up to scale and should be half the size in the picture).

Concentric Coil's Effective Detection Area for Coin-Sized Targets

With 50% overlap, the possibility exists that you can miss the coin during the next sweep. 75% overlapping of the previous sweep will greatly increase your chances that the most effective detection pattern area will not miss the dime.

Search Coils for Metal Detectors, page 11

Search Coil Swinging Discipline in Details

(...CONTINUED from previous page)

Sweep Overlapping with Double-D Wide Scan Search Coils

Due to the 2D coil's advanced design and characteristic shape of detection pattern (wider, narrower and deeper penetrating - see details on page 3), overlapping with Double-D search coils is not as vital for the process as with concentric coils. As shown in illustration below, even if you do not overlap each coil sweep, you can still cover more ground in one sweep with the 2D search coil than with any concentric coil.

Sweeping Double-D Search Coil without Overlapping

If a dime is buried 7-8 inches deep, it would not be missed by the effective detection area of the Double-D 8" coil even with only 25% overlap. The 2D coil's effective detection area would be elongated and narrow as shown on illustration below (the dime is not up to scale and should be half the size in the picture).

Double-D Coil's Effective Detection Area for Coin-Sized Targets

Search Coils for Metal Detectors, page 12

Search Coil Swinging Discipline in Details

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6) Slow & Methodical Coil Swinging

Sweep the search coil slowly and methodically. An average sweep speed is 4 seconds from left to right to left. Do not try to cover an acre in 10 minutes!

The worst possible thing you can do to yourself is to impose a time limit in which to cover an entire hunt site. Unless you are participating in a competition hunt where the detecting speed is the desired objective, you should not swing the search coil 100 mph to cover 50 yards in 15 minutes!

NOTE: Some metal detectors require fast movements of a search coil for achieving maximum detection depth. Please consult the instruction manual for your metal detector.

7) Careful Usage & Treatment of Search Coil

Do not bang the search coil against solid objects such as tree roots, large rocks, walls, etc. Although the search coil is constructed from durable materials and designed to withstand shocks and bangs, it could be prone to serious damage if not treated with due care. The construction of transmit and receive windings inside the coil is very delicate, and if it is not solid epoxy-filled, sudden jolts will skew the windings; thus, altering the electronic balance of the windings. Also, banging the search coil against solid objects can cause random false signals and inaccurate target IDs. Careful sweeping will keep the coil's performance at an optimum level at all times. Excessive brushing of the coil against the ground should be avoided as well. When attaching the search coil to the lower shaft of a metal detector, ensure the coil cable is not subject to undue stress. But do not allow the search coil cable to become loose and prone to creating false signals.

To solve this problem, use Velcro straps (they usually are provided with some metal detectors) to secure the cable being tightly wrapped around the shaft. Make sure you leave enough cable slack between the shaft and search coil to allow for the coil's movements occurring, for example, during packing your metal detector into the bag or rucksack. Do not use your search coils without coil covers (skidplates). Protective covers are inexpensive (usually $15) compared to the price of a new search coil. Make sure you remove the coil cover and clean the inside occasionally.

If the dirt and soil minerals along with it accumulate inside the coil cover in large amounts, they will be capable of altering the ground balance tuning and detector's performance. To solve this problem, I hermetically sealed the coil covers attached to the coils when I got my detector new and have never faced this problem again since then (you can read all details on page 20 - "Sealing Coil Covers Against Dirt & Moisture").

General Information & Basic Pinpoint Modes

Pinpointing a target refers to aligning the center of target's response width to the designated search coil center for accurate location and careful recovery of the target. Normally precise pinpointing is required on manicured lawns not to damage the grass and soil. In the wooded areas and plowed farm fields, precise pinpointing may be required only at spots with high concentration of junk.

Depending on the type and class of a metal detector, various pinpointing features may be incorporated into the non-motion all-metal mode in which Pinpointing is usually conducted (Discriminate mode is automatically disabled during pinpointing). Each detector's instruction manual includes a how-to description of pinpointing technique and explains the use of pinpointing modes.

For example, Pinpoint mode of one kind will provide a wide audio target response at the first coil sweep over a target.

Wide Audio Response

With each following sweep, it then automatically keeps the audio response narrow, but will allow targets of different depths to be pinpointed with equal precision.

Narrow Audio Response

Pinpoint mode of a different type can provide a linear audio tone: the tone pitch is proportional to the strength of the target response, and the tone volume is proportional to the depth of the target positioned under the coil. Activating Pinpoint while the coil is away from the target will allow the size of the target to be determined.

Full Audio Response

Activating Pinpoint while the coil is near the target will allow portions of the target response to be masked.

Masked Audio Response

Back in the day, to narrow the audio width of target response for defining a more accurate location of center, the technique called detuning was used. Detuning forced the electromagnetic field to become less sensitive to the target, thus reduced signal response width.

Many of today's metal detectors incorporate VCO (voltage controlled oscillator) into the all-metal pinpoint mode. VCO is driven by target voltage to produce varying audio pitch responses for target pinpointing and identification. Although VCO may be difficult and unpleasant to use on large shallow targets, it can be more of an advantage when multiple targets occupy the same pinpointing zone compared to monotone audio. With VCO, by narrowing the target area significantly, it is possible to hear the pitch peaks of two targets in close proximity more clearly.

If your metal detector has a continuous auto-retuning feature incorporated into the all-metal mode, you must be careful when hovering the search coil over a target. If you initiate Pinpoint while over the center of the target, the whole target may be masked as its response will be tuned out. This is why many detectorists experience "vanishing signals" in the all metal mode when attempting to pinpoint a target too quickly.

Pinpointing Metal Detected Targets

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Target Pinpointing Techniques for Concentric & Double-D Search Coils

The pinpointing technique depends on the type of a search coil you use. With the concentric search coil, you scan the target in perpendicular directions as shown in illustration on the right. Move the coil in the east to west direction first, slow the sweep rate down and gradually shorten the distance the coil travels over the target area.

Watch the center of the coil as you move the coil and stop it when the coil center coincides with the highest volume and pitch of audio tone. At this stopping point, move the coil in the north to south ("toe to heel") direction repeating the above procedure. Do so without losing the first center obtained in the east to west direction.

When center is achieved, and the search coil is stopped again, train your eyes on this point. Without moving your eye contact with this center reference, slide the search coil away from the target area.

Your eyes should then be focused on a spot on the ground around which you will cut the ground to form a round or square dirt plug and then pull it out.

I should mention that the concentric search coil's electronic center - the region of the most concentrated electromagnetic field (described on page 10), may not correspond to the actual physical center of the search coil. This may depend on either the design or/and construction accuracy of the search coil. However, today's solid style concentric search coils are wound and aligned quite accurately, and their true electronic center is close to physical center.

If you wish to locate the electronic center of your detector's solid style search coil, remove all personal jewelry, and bench test a small coin in the all-metal mode. Your first objective is to get the smallest sound from the coin as if you are detuning the coin signal. Then you move the coin using the pinpointing procedure outlined above until you locate the center in two directions.

Using a waterproof marker, trace the coin's perimeter on the search coil bottom. Measure and transfer the dimensions of the outlined center to the top of the search coil. Make a mark on the transferred center large enough to be seen from a distance.

Obviously the above information is not applicable to search coils that have open centers. The open-center style search coil gives the detectorist added advantage with respect to immediate ground reference during pinpointing. However, the detectorist must judge target audio center to a non-physical search coil center.

Pinpointing with Double-D search coil requires an altered method because of its different detection pattern (described on pages 3 and 11). Because this type of coil has an elongated detection pattern in the "toe to heel" direction, target centering is not as easily accomplished as it is from from side to side for some detectorists.

When you have centered in the east to west direction, rotate your stance position 90 degrees around that center. Repeat the east to west or side to side pinpointing procedure. The target should then be more accurately aligned with the search coil's center marking.

Perpendicular Double-D Search Coil Position during Pinpointing

Pinpointing Angled Coins

A coin that rests on an angle presents one of the most difficult obstacles to accurate pinpointing because the orientation of a buried coin affects your ability to accurately pinpoint its location with respect to the search coil's center point. For example, a coin on a 45 degree angle will generate a secondary electromagnetic field on angle rather than directly upward like a coin lying flat or parallel to the search coil bottom.

Secondary Electromagnetic Field Generated by Coin on 45 Degree Angle

Even though you think the signal is physically aligned with the search coil's center point, the target will be to the side. The greater the angle of the coin with respect to the bottom of the search coil, the greater the error can be expected in pinpointing.

Unfortunately coins on an angle less than 90 degree to the search coil will not usually give any special audio signal to set them apart from coins lying flat. This accounts for the reason many coins are scratched during recovery.

If you are coin shooting on a manicured lawn, and you have to keep the recovery areas small and neat, the best way to determine if the coin is lying on an angle is to use a probe (a screwdriver with dull tip). If you metal detect in the wooded area or at the farm field, cut larger dirt plugs to avoid hitting the off-center coins with your shovel blade.

Pinpointing Metal Detected Targets

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Pinpointing Coins On Edge

Coins standing on edge or perpendicular to the search coil's bottom emit audio responses similar to nails or short pieces of wire. Also, these coins will not be detected as deep as coins lying flat at the same depth.

The coins on edge will produce two audio responses in one direction of sweep and one response in the perpendicular direction. If the orientation of the coin's edge is perpendicular to the "toe to heel" (north to south) direction of sweep, the audio response will be two beeps close together and one beep in the east to west direction.

This effect will be exactly opposite when the coil's orientation is rotated 90 degrees. In one beep direction of coil movement, visual ID metal detectors may produce false readings of lower conductivity on coins.

Pinpointing in the double beep direction will require estimating center between the beeps. If a trash target resides in the same proximity, additional signals may confuse you.

To eliminate the confusing double response, the "edging" technique is used. The search coil itself is placed on edge, and when the target is pinpointed by "X-ing" the area, the audio responses will be singular in each direction.

The target will still be located respective of the search coil center, but you will not have the easy view of this center compared to the conventional coil orientation. Before trying this pinpointing method in the field, it is recommended to practice in a test garden environment first.

Pinpointing Coins Next To Trash

The proximity of rejected junk targets to good targets can alter their audio responses. Good targets next to trash or trash itself can produce audio responses which are "clipped", "choppy", "snap", crackle & popped", "chirpy" and "ragged". Knowing which of these sounds your detector makes involves digging a few of them!

Usually if a broken signal is repeatable in all sweep directions, the target most likely is junk. Targets that sound good in one direction and bad in another can be partially masked. The same holds true for targets that give a good signal in only one direction of sweep.

Situations that involve many targets may be extremely difficult to pinpoint in all-metal mode and may force the detectorist to pinpoint in the motion discriminate mode exclusively. To do this, place the search coil in the area of positive sound.

Pinpointing a Target in Motion Discriminate Mode

As you begin the "X-ing" process, rotate your stance around the target area and sweep slowly. Changing the search coil orientation around the target area can sometimes expose the coin to the best advantage providing a clear audio response. It is at this point that pinpointing can be accomplished with a greater degree of accuracy.

All targets will have a different audio character with respect to depth, oxidation and discrimination settings. Get to know your detector's audio responses to various targets better by experimenting at home and in the field. Those detectorists who are willing to dig the extra questionable signals are often surprised at the results.

Masked Targets Detecting Technique

When a target is detected, the time duration before the circuitry can respond to the next target is called "recovery time." If a detector's recovery time is set on a longer duration, this detector can be prone to ignoring targets lying in close proximity to targets that are rejected in accordance to the Discrimination setting.

Use of discrimination to reject iron nails only compounds the problem because ferrous objects lying in close proximity to a good target tend to cause the strongest masking effect and would not allow the positive audio to be heard. To avoid this problem in areas of concentrated iron junk, an ID metal detector should be used without any discrimination.

For example, if a rusted bottle cap is lying closely beside a coin, and the metal detector is set to reject bottle caps, the larger ferrous target can completely mask the response from the good target. Also a metal detector may produce a broken signal or none at all over a good target when an iron target precedes it in a sweep path.

Coin Masked by Iron Nail

By scanning the target area at 90 degrees or perpendicular to the initial direction of sweep, a good target can be encountered with less interference from rejected ferrous or nonferrous targets.

This is one of the reasons we come back to previously searched areas and continue to find more good targets. This is why it is important to come back to your favorite hunt sites and rework each of them at angles perpendicular and oblique to the original patterns of search.

The best advantage in a concentrated trash situation is to use a smallest 4-5" search coil which is able to "extract" good targets from the "trash blanket" and help you avoid scanning both good and junk targets simultaneously.

Questionable Target Identification Techniques

In the motion discriminate mode, both expensive and inexpensive metal detectors produce seemingly good responses to larger, heavily rusted ferrous targets which contain conductive and nonconductive properties. Sometimes an operator of an inexpensive metal detector can use Discrimination as ID function.

This can be done by simply turning the Discrimination knob (if it is utilized) clockwise to increase the Discrimination level, while scanning an unknown target, and see if the target still responds. However, even when Discrimination is set up at high levels, the motion discriminate mode can not ignore large iron objects sometimes.

In this case, you should switch to the all-metal non-motion mode to identify a ferrous target. Iron bearing objects distort the electromagnetic field, and we can use this effect to our advantage when making a decision whether to dig.

For example, as you approach a moderately sized ferrous bearing target, such as a rusted steel bottle cap, the audio signal in the non-motion all-metal mode will begin sounding before the leading edge of the search coil passes over the edge of the target.

Distortion of Electromagnetic Field by Ferrous Object

When encountering a questionable target, remove the search coil away from the target area to a distance at least the diameter of the coil or more. Operating your detector in All-Metal mode, slowly approach the target area again.

As the target audio appears, notice where the leading edge of the coil is in relation to the ground. Mark the spot with your toe if necessary (make sure your boot toes do not contain metallic plates!).

Determining Width of Ferrous Signal

With the coil still traveling toward the target, keep your eyes fixed on the initial sound entry point. If the trailing edge of the search coil goes beyond this point, and the target audio continues and eventually fades, the audio signal width is considered wider than the diameter of the search coil. The target is most likely ferrous in composition.

Large nonferrous targets such as aluminum cans can produce wide audio signals, but the width of those signals will not be wider than the search coil diameter. In other words, the signal will end as the trailing edge of the coil passes the initial sound entry point for nonferrous targets.

Deeply buried coins will not produce much sound width at all. Some thoroughly rusted square nails may escape this technique, but this method will generally save a lot of unnecessary digging on most targets. Using this technique as a way to outline signal boundaries provides for more accurate and nondestructive recoveries at the manicured lawns.

This technique is very helpful for relic hunting. If you are using a motion Discriminate search mode to reject nails, this technique can quickly verify the larger, equally desirable ferrous targets. Such targets could be pistols, gun barrels, cannon balls, battle axes and sword blades.

"Pendulum Swing" is another effective method for identification of questionable targets. When you encounter such a target, keep your detector in a motion discriminate mode and start swinging the search coil as fast as you can in a pendulum motion, with the lowest point over the target area. If the partial signal of high tone disappears, it is likely to be a ferrous target.

This simple technique may work or may not with any metal detector. Find a few singular questionable targets in the field, try this technique, and see if it works with your machine. I have successfully used this method with my Tesoro Cortes which has an elliptical concentric search coil. But I have not used this method with my Minelab E-Trac utilizing a Double-D search coil because this machine offers other effective means for identification of questionable targets.

In addition, there are situations when a positive target response disappears after the recovery process begins. This "fanthom signal" may be caused by either 1) inaccurate pinpointing or 2) a reaction between the discriminate function and the "Halo Effect."

Sealing the Search Coil Cover To Secure Against Dirt & Moisture

Within the first year of using my metal detector, I started noticing short false signals while swinging the search coil in the air. No matter what program settings I was using, the false signals would not disappear.

The puzzle was solved easily when I finally detached the coil cover to clean it inside. As a substantial amount of dirt, sand, and small rock fragments fell out, not only the coil became lighter but also the false signals disappeared. The dirt that got inside, between the search coil and coil cover, through the gap consisted of mineralized soil and caused the false signals' appearance.

Prior to that discovery, I used either duct tape or electrical tape to prevent the coil cover from falling off during metal detecting. Now, to seal the gap and keep the coil cover secured, I used the silicone sealant because, for its water resistance and flexibility, it is the most reliable way to seal the gap against the elements. At the same time, the silicone sealant cushions movements between search coil and coil cover resulting from temperature changes and mechanical shocks.

Silicone Sealant

Your objective is to seal hermetically the gap between the search coil and its coil cover (skid plate). First you have to make sure adhesive surfaces are dry and free of dirt, grease and dust as they would impair the silicone sealant’s adhesion and curing. Put the cover on the search coil tightly and secure it with electrical tape if necessary.

Then you cut the plastic applicator nozzle with scissors at an angle to achieve a 1/8-inch hole in the end. Puncture the seal on the silicon tube with a nail. Screw the nozzle on the tube clockwise until it becomes tight. Place the nozzle on the area for sealant, then squeeze it to apply a small but constant bead of silicone filling the gap all the way around the circumference.

The first mistake one can make is applying too much silicone, this is easily done and can make a complete mess. So apply the silicone sparingly. If you miss a little bit you can always go back afterwards and fill the gap. Use a small spatula (you can cut it from the plastic bottle with scissors), paper or a cloth to remove the excess silicone. If necessary, use the washing up liquid and draw it along the bead of silicone.

While sealing, do not inhale the fumes over a long period or in a high concentration. Make sure there is adequate ventilation. If the eyes or mucous membranes come into contact with uncured silicone rubber, rinse well with water to prevent inflammation. Cured silicone rubber can be handled without any health risks.

Wait until the next day and wipe the coil clean with a damp cloth. If you want the coil to look neat, wipe off the residue with a cloth moistened in a solvent such as white spirit or ethanol, and follow safety instructions. It is best to save the remaining sealant as it always comes in useful. Do this by screwing a tight fitting screw into the nozzle, this stops the air getting to the silicone and curing it.

Clear Silicone Sealant On Standard Minelab 10.5" Search Coil

Brown Color Silicone Sealant On Minelab 7-1/4" Search Coil

Since I sealed the coil covers on all my metal detectors, I have not had any problems so far. Unlike other joint fillers, silicone sealants have the best resistance to weathering and aging. Not even years of open-air weathering will change their physical properties. The outdoors silicone sealant, both clear and in different colors, is sold in any hardware store.