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This article is written for my associates, who, like me, since childhood were fascinated by stories about hidden treasures and sunken, laden with gold galleons.

For those who, holding an ancient coin or an ancient arrowhead, experienced a real storm of feelings and emotions from contact with the spirit of past eras. I know that there are those among you who, passing through mother land in the spring, sheltered by young and fluffy grass feels a heartbeat from the realization of the fact that many necessary and interesting things are hidden by the nurse in their bowels. Oh, how you want after hibernation to plunge into the romance of adventure and exhaustion to revel in the pleasure of SEARCH. Yes, maybe it's a disease, and I know people who have repeatedly thrown this activity, but still BEFORE. Sorry for the bad word play, but this disease is good for health.

Staying outdoors, as well as sparing physical activity associated with digging out finds have a beneficial effect on the body of any age. I will not be mistaken if I assume that to search for metal objects you will need a good metal detector, and to dig them - a convenient shovel. The range of metal detectors offered by manufacturers is quite diverse and before you buy the device sometimes have to choose for a long time. The question "which machine is best?" is difficult to answer. But in order to somehow determine this one must ask yourself a question, and for what purpose it is needed? THE GOAL IS TO DETERMINE THE TOOL! I understand the skepticism of some readers of this article, who have quite a good idea about the real possibilities of metal detectors and have tried several models in the work. And yet, for beginners this article will be useful and will make up for at least part of the lack of information on this topic.

There are five types of metal detectors (MD):

Devices with BFO (dependent generator) Working on the principle of balanced induction (induction balance) Working on the principle of impulse inductive method Working on the principle of dearrangement magnetometers.

The latter devices use the principle of determining small anomalies of the intensity of the earthmagnetic magnetic field, but to search for treasures in general are useless, as only iron objects can detect. Devices with BFO and dislocation work by determining small changes in the inductiveness of the search coil under the influence of iron objects. Both methods are characterized by poor sensitivity.
On the contrary, pulse detectors have many advantages: the most sensitive among all detectors, which in modern times are available, are insensitive to the influence of the earth, have a simple design. Their work uses magnetic pulses that can cause current in all metal objects that fall into the magnetic field. In between pulses, the receiver receives a response that is amplified and processed electronically. With their undeniable advantages, they have drawbacks: to work you need powerful batteries, extremely sensitive to small iron objects. Since the mid-1990s, many pulsed MDs have been supplied with discriminatory devices.

MD with balanced induction has become standard detectors for universal use. In the search head they have two coils, one of which leads a variable magnetic field. The other coil is located so that the field is normally balanced around it, and on its output there is no electrical signal. In fact, the receiving coil has a so-called residual signal, due to the not ideal design. Metal objects that approach the coils, change the configuration of this field, unbalance the system, and as a result - at the exit of the receiving coil there is a signal. This signal can be reinforced and inform the operator about the find. Modern MDs using this principle have a powerful electronics, processing signal and giving the operator a lot of additional information: the relative specific conductivity of the metal, the depth of laying, etc.

Coils of such MDs can have different configurations. The most practical and technological are coils with "wide" grip, (Figure 1a) and coils with "point" capture (Figure 1b, c). The shaded area in the pictures is the area where the search head is captured. Currently, the second version of the "point" capture design is most commonly used. In the process of the receiving coil, part of the transmitting coil is wrapped around the receiving coil, which is turned on by an anti-phase main (large diameter).

The number of turns of such an anti-phase coil is selected in such a way that the field around the receiving coil is balanced, i.e. the magnetic flow of the large transmitting coil A, which enters on one side is balanced by the magnetic flow of the small B coil, which enters the the receiving coil on the other side. It should be noted that there is a certain proportion in the ratio of diameters of transmitting and receiving coils.

This achieves the best setting of this system, as well as the ability to receive a small level of residual signal in the receiving coil. The question may arise: why do you want to reduce the residual signal? The level of the residual signal depends on many factors: the power of the signal in the transmitting coil, the location of the coils, the number of revolutions of the receiving coil, goodness, the quality of manufacture, the shape of coils, their mutual location, etc. and therefore can have a significant spread from units to hundreds of millivolts. Therefore, if the receiving coil is included in the parallel vibrational circuit (which is done quite often), the level of the residual signal can increase tens and hundreds of times due to resonance.

In this case, it is impossible to get a significant pre-amplification of the receiving coil signal at variable voltage. This, in turn, requires significant gain rates of constant voltage after detection of a useful signal, but then the problem of "drift" exacerbates, leading to unstable metal detector operation. The level of the residual signal (O.S.) can to some extent affect the sensitivity of MD, because after detecting a useful signal in the preterm, the ratio of signal/noise at high levels of O.S. decreases.

I would also like to say that in order to obtain high sensitivity, the total gain factor of the entire receiving tract of MD can be tens of dB. Reckless increase in sensitivity leads to a deterioration of the stable operation of the device, turning, thus, MD's useless search tool. On the fact that sensitivity is the main, but not the main advantage of modern MD, as well as the factors that affect it, you can read the article on the same site. Understanding this fact comes with experience and becomes apparent after the search engine gets on a heavily littered metal trash site. There is no longer a high sensitivity of joy! Sometimes the pursuit of supersensitivity in some individuals degenerates into a disease, when a person focuses on every extra inch of depth specified in the advertising prospectuses. Firms-sellers skillfully play on this fact! Lastly, the depth of detection is usually given by air, i.e. for idealized conditions, but in the ground the specified object at the stated depth you most likely will not find!.

How to choose a metal detector characteristics score comparison
Returning to the theme of the design of the search head, I want to raise a little question, which coil is better - with "wide" or "narrow" capture. In experience with the use of "point" capture is better, as coils with "wide" capture have a small ability of point orientation and have a tendency to receive false signals from iron objects that are on the outskirts of the search zone. But at the same time, "wide-capture" coils allow you to conduct not such a dense scan of the inspected area of land, as coils with "point" capture. This allows you to view large areas for a fixed period of time, and if the site is not too saturated with metal, you get a decent time saving. Working with a coil that has a "point" grip, it is necessary to monitor the density of the scan, each subsequent swing of the coil should not cover the previous trajectory by more than half a diameter.

Otherwise, omissions are unavoidable, especially at the extreme depth of detection. Such coils are good when working on heavily littered areas, they are easier and faster to establish the exact location of the find (targets). According to my observations, very few search engines follow the rules of working with coils that have a "point" capture. The trajectory of their coil resembles a wide zigzag on the traversed areas, as a result of many missed targets. I myself repeatedly repeatedly "called" such areas, finding the items they missed. Be careful, do not rush, remember, just one thing you missed can be more valuable than all the others combined! Information on the design of coils, as well as their optimal use for search, deserves special attention, so this issue will be devoted to a separate article.

Good metal detectors can have many different abilities, such as discrimination (recognition) of targets, i.e. it is possible to ignore various garbage that is of no interest. Almost all modern MDs have a discriminator. Another feature of good MD is the possibility of excluding the influence of land on the search process. This is partly solved with the "Faraday" screen around the coils, and the best is considered to be a screen made of iron oxide.

When self-constructing MD, the screen is made, as a rule, from a puddle of thin copper foil. Special circuit-technical solutions are used to better suppress the influence of the earth. This method in foreign literature was called GEB (Ground Exclusion Balance) - the exclusion of the influence of the earth. It should be noted that the presence of the GEB system in MD does not always allow to be effectively rebuilt from the influence of the ground. Fortunately, in most of our country "heavy" soils are very rare. "Heavy" soils include: raw sea sand, red clay, rocky soils, etc.

The technical literature on metal detectors uses a lot of special terms, so you want to introduce you to some of them. The designation VLF (Very Low Frequency) means a very low working frequency of MD. The ability of MD to discriminate (recognize) the nature of the find depends on the working frequency of the device. At high frequencies, the phenomenon of skin-effect increases, and the quality of discrimination is significantly reduced. Therefore, at first, MD manufacturers used very low frequencies of about 2kHz (70s, early 80s). This led to specific problems, because at these frequencies, despite good sensitivity to copper and silver, sensitivity to gold and nickel was reduced, and in the design of coils there were problems with quality (goodness ).

Modern metal detectors have a wide range of working frequency, this is due to the specifics of their application, as well as for engineering reasons. Most often, the range of frequencies extends from 6 to 20 kHz, but it happens lower. In this frequency range, the devices are good at discriminating against targets and there are no serious problems with the design of coils. Devices to search for gold use elevated frequencies - up to 15-20 kHz and above.

This is also due to the fact that these frequencies improve sensitivity to very small objects, for example - to gold nuggets, often having a meager size and a small weight. In recent years, multi-frequency search has been used to improve the depth and quality of discrimination, which gives advantages under certain conditions. With the advent of cheap microprocessors, this method has received a strong impetus in its development. But I want to make two important observations about multi-frequency search:

1. This method does not significantly increase the depth of discrimination compared to single-frequency devices.

2. The range of targets, which are well discriminated against, is increasing, but in the case of a complex configuration of the latter, they may be cut off.

The designation TR (Transmitter-receiver) means "transmitter-receiver" and also refers to MD, working on the principle of induction balance. This type of metal detector snare appeared in the 1930s. These devices have spread, orthogonally located coils. Devices of this type have working frequencies from 50 to 100 kHz. A serious drawback of them - a big reaction to mineralized soil. Their features should also include immunity to small objects. When looking for large objects, the size of a litre jar and more, in the conditions of low-mineralized and slightly littered soil, these MDs are very convenient. Since the mid-70s almost out of use. In modern times, the typical representative of MD of this type is The Gemini-3 of Fisher. Some MANUFACTURERS of MD have used this designation as an addition to the term VLF apparently to emphasize once again that the device is based on the principle of induction balance.

In the process of working on the ground can change the nature of the soil and, therefore, its mineralization. This will require an adjustment of the land impact compensation system. This is done both manually and automatically if the detector has this mode of operation. It was designated Ground Track. This mode is not recommended when installing high sensitivity.

The term VCO is used to refer to the sound mode of the MD, in which, depending on the size and depth of the object, not only the volume of the sound changes, but also its tone. This sharpens the perception of the hearing of the slightest change in the beep.

Modern high-end metal detectors have a rich service that allows an experienced operator to conduct a fruitful search with the least time. If the search engine additionally has an idea about the principle of the metal detector and its real capabilities, it brings additional dividends.
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