How to choose a coil for a metal detector?

A coil for a metal detector means as much as tires for a car (winter and snow tires, summer ones) or it can even be compared with shoes we wear depending on the weather outside. When working in the field we wear boots, in summer – sport shoes, flip flops – to the beach and robust boots – to the forest.

No one is going to wear flip flops in winter instead of winter boots. The same is about metal detector coils. They are made for different search conditions, not weather ones.

These conditions can vary the same as weather. These are the things that effect your treasure hunting process:

  1. — amount of metal litter around,
  2. — soil mineralization level,
  3. — coil size and operation frequency,
  4. — the target of your treasure hunting

It is desirable to use different coils for all these conditions since this will increase efficiency of your metal detector and sometimes this appears to be the only way to continue your treasure hunting trip. Keep in mind, that even the best metal detector can’t see the target if it is surrounded with large amount of metal litter.

Condition 1. Amount of metal litter

The coil should be chosen depending on the amount of metal litter around. If while going metal detecting you hear several signals after waving with MD once, this means that it’s time to change the coil for a smaller one. When there are two or more targets under MD coil, it can’t define exactly what is there and therefore the target is misidentified and missed as a result. Use smaller coils in this case (from 6 to 9 inches large).

Condition 2. Soil mineralization

Mineralization is the main parameter that influences the MD search depth and the kind of coil you choose. Since it is inside the ground where we perform treasure hunting, the soil prevents electromagnetic signal from penetrating trough. The smaller is the coil diameter and square, the less soil mineralization level influences your device.

E-Trac and СТХ 3030 MD have a special scale (СТХ has digits) that shows soil mineralization level. If it is below 1/3, it means that the soil is highly mineralized and you should use smaller sized coil or ellipse shaped one. If the level is above 2/3, the soil mineralization isn’t very high and you should install a coil of larger size (15”) to increase MD search depth. Use DoubleD coils since they are less sensitive to soil mineralization level.

River beach most often has low level of soil mineralization and you can use standard MD coils. Large 15” coils can also be used; although, they are less sensitive to small targets and it is almost impossible to find a chain when using such a coil.

Sea beach — salty wet sand. For treasure hunters this is one of the difficult sites for searching due to high mineralization level of the soil caused by the sea salt. DoubleD coils of small diameter and ellipse shaped coils are usually used for beach hunting.

Fields, velds — for small search areas we use 15” and 18” coils. Large coils allow increasing the search speed and depth.

Condition 3. The coil size and operation frequency

This is about coin hunting. If you go metal detecting in a field, then take the largest coil from all available. If you go searching for small targets, like jewelry hunting on the beach, then use a standard or a small sized coil. The larger is your MD coil diameter, the worse becomes detection of small targets.

When working with multi-purpose multi-frequency metal detectors it is important not only to choose a proper sized coil and select its type – DoubleD or Concentric (mono), but its operation frequency as well. Use low frequency coils (3 or 7.5 kHz) to search for large targets. The lower the frequency is, the deeper your MD can “see” into the ground.

However, don’t use large sized coils (12х13” with 18,75 kHz frequency) to detect large targets. High frequency attenuates rather fast in the ground and doesn’t penetrate deep enough. As a result, coils of large diameter won’t help you find a desirable target.

As for small coins and beach hunting, you’ll need a high frequency coil (18.5 kHz). This frequency is much better then low ones, when you want to find small targets. As for the coil diameter, it should be about 9—10”. The smaller MD coil is, the more sensitive it is to small targets. It is inadvisable to use larger coils, since the larger it is the less sensitivity it shows to small targets.

HERE IS THE SUMMARY

— Small coils (6—9 inches) are used to search inside buildings, basements, garrets or sites with lots of metal litter, on highly mineralized soils and to look for small targets

— Medium coils (10—11 inches) are multi-purpose ones for the most of search conditions at low level of soil mineralization and around not so littered sites.

— Large coils (15 inches and more) — they will help you to look for treasures and coins hidden deep in the ground around large areas. However, discrimination function performs worse once the coil gets larger, as well as the ability to see between targets located closely together. Large coils are of no use when going metal detecting on highly littered sites.

Oval shaped coils are good for mineralized soils and when there are some stones around. These are more noise-resistant, than round coils. Though, if the soil isn’t mineralized it’s better to use round ones since their search depth is higher.

— The coil operation frequency. High frequencies like 18,75 kHz are perfect when searching for small targets. Low frequencies like 3 and 7,5 kHz are used for large targets detection. You should keep this in mind when choosing your MD: if you are going for large targets buy a metal detector with 3-7.5 kHz operation frequency. If your targets are somewhere on the beach, choose a device with a high frequency 16—18,75 kHz.

— Don’t just replace a standard coil thoughtlessly. For example, E-Trac has 11” standard coil. Quite often beginners in treasure hunting after reading commercial reviews about some new wonderful coils, replace the standard 11” coil with an oval shaped 12х8” one or with 12” round one being under the illusion that this will increase the device search depth. But there are no miracles here! A coil is just a bunch of wire. A physical law says: if you increase the coil diameter two folds this will give you 50% growth of the search depth. Which means that after replacing 11” coil with 12” one you’ll gain just a few millimeters of the device search depth. Therefore, it sounds more reasonable to spend money on a coil made for some specific search conditions, not on just some commercial trick. For instance, you can buy a small coil to use it around some littered sites or a large 15” one.

Try to buy coils produced by your MD manufacturer or its partner companies. For example, CoilTek is Minelab partner company. This approach will help you to avoid buying some nonstandard and sometimes quite low-quality equipment.