Have you ever tried to get through the concept of metal detector operating frequency? When I took up this hobby, I tried to do some research. But I didn’t succeed and bought a metal detector that seemed good because of its price. After some time passed, I understood that such an approach doesn’t work. You should understand the operating principles, advantages, and disadvantages of this or that technology and use them to the maximum.
This article contains the results of my research. It has the answers to the following questions:
What is the operating frequency for? What does it influence on?
Which one is better? How metal detectors with high operating frequency differ from the ones with the low frequency?
What device is better – single or multi-frequency one? What is multi-frequency for, and what benefits does it give you?
What is the frequency?
Let’s open a book on Physics. Frequency is the number of occurrences of a repeating event per unit of time, and it is measured in kHz (kilohertz).
The operation principle of a metal detector using any of these technologies – VLF (Very Low Frequency), PI (Pulse Induction), or RF (Radio Frequency) is the same.
The device operation is using electromagnetism principles. A metal detector emits an electromagnetic wave and immediately receives disturbance signals that appear when the waves emitted by a search coil hit some obstacles. Disturbances appear when in a nominally homogeneous environment, an object conducting electricity is encountered.
Objects with high electrical conductivity and those with low one create different disturbances, and this is what allows a metal detector to tell one from another. Discrimination is a process of ignoring some disturbances with specific characteristics – this is the main principle of how it works.
Frequency in a metal detector is a number of waves that are emitted into the ground to detect some metal objects in it. For example, the operating frequency of 10 kHz means that your metal detector emits and receives 10000 waves per second. A standard frequency range used in most metal detector models varies from 3 to 100 kHz.
Each frequency is responsible for detecting some specific target size, which it can find and for how deep it can penetrate into the ground. Thus, one and the same frequency can function in completely different ways at various conditions. In general, everything is clear, but it’s not clear how it will help you in metal detecting. Let’s take a closer look.
What does metal detector frequency affect?
Let’s take VLF metal detectors as an example, since they are the most popular ones.
There are three categories of them according to the operating frequency:
- Low frequency — up to 8 kHz.
- Medium frequency — 8-20 kHz.
- High frequency — higher than 20 kHz.
Pay attention that such a division is quite conditional. This classification may be different in other resources, and there is no standard frequency table. Moreover, if not so long ago, metal detectors for beginners used 6-8 kHz frequency, and it was considered to be a medium frequency, now they have 12-15 kHz operating frequency.
Such a situation occurred due to rivalry in the market and its development.
For these metal detectors the classification is the following:
- 2-6 kHz — low frequency
- 6-12 kHz — medium frequency
- 15-22 kHz — high frequency
- from 30 kHz and higher — very high frequency
The higher operating frequency is, the better the device detects small targets, and less it is affected by electromagnetic noise (electricity transmission line, mobile phones, etc.). However, the detection depth and the device’s ability to function on highly mineralized soils is lower in this case.
When using low frequencies, the detection depth increases, while the device sensitivity to small targets, especially close to the ground surface, decreases. This is the frequency used for deep search metal detectors such as Whites TM 808.
When using high frequencies, things are completely the opposite. The detection depth decreases, sensitivity to small targets near the ground surface, as well as to gold and nickel targets increases.
As for the metal type, low operating frequency increases the metal detector sensitivity to silver and copper, but it becomes less sensitive to gold and nickel.
If you aim at gold prospecting, most of the metal detectors for this purpose use high operating frequency. Firstly, gold has low conductivity, and secondly, gold in nuggets quite often consists of small particles.
The minimal frequency for gold nuggets prospecting metal detector is considered to be 14 kHz. When using lower frequencies, the possibility of detecting small targets significantly decreases.
Low operating frequency
- Long wavelength;
- Detection depth increases since a wave with longer wavelength easily penetrated the ground;
- It does well when looking for high conductivity targets, for example, silver;
- It doesn’t suit much for detecting small targets;
- It doesn’t work for low conductivity targets detecting, for example, gold.
High operating frequency
- Shorter wavelength (in comparison with low operating frequency);
- It demonstrates perfect results when detecting small-sized targets, for example, gold nuggets;
- The frequency is better for low conductivity targets detection – gold or iron;
- Detection depth is worse (in comparison with low operating frequency);
- Higher precision, especially when detecting targets locates closely to the ground surface;
- It is more sensitive to electromagnetic noise from the highly mineralized ground.
Almost all metal detectors for beginners have a standard medium operating frequency equal to 6-8 kHz. These are Teknetics Eurotek, Garrett Ace 250, Minelab Go-Find, Bounty Hunter Land Ranger Pro, Bounty Hunter TK4 Tracker IV, White’s Coinmaster — the list is rather long in this case.
When using a metal detector with this kind of operating frequency, you can get advantages of both low and high frequencies.
Such as optimal balance of the device sensitivity and detection depth, something like a golden mean. These metal detectors aren’t very noisy in case of functioning on highly mineralized ground or ‘hot rocks.’
Many device models don’t even have a ground balance function. That’s why metal detectors with a frequency lower than 10 kHz are usually used for coin shooting.
Single-frequency, multiple-frequency or multi-frequency metal detector
Most metal detectors use one operating frequency. They don’t have an option of switching frequencies or multi-frequency option. As a rule, these are metal detectors for beginners.
However, there are multiple-frequency metal detectors that are divided into two main types.
Firstly, these are metal detectors with several operating frequencies. These kinds of models allow selecting only one of the available operating frequencies. This means that the metal detector has several frequencies, but it can use just one at a time. You can switch from one to another. After metal detecting using one frequency, you can switch to the next one and so on.
Some metal detectors change their frequency together with replacing their search coil, but most modern device models a coil that can work using several frequencies. Nokta Impact, XP DEUS, Minelab Equinox 600 are one of such metal detectors.
Secondly, these are multi-frequency metal detectors. These metal detectors analyze signals they receive on two or more frequencies simultaneously.
Usually, they have a basic frequency and a couple of secondary harmonic frequencies. These are the ones that are divisible by the basic frequency. For example, 30, 45, 60 are divisible by 15 kHz frequency.
There are few true multi-frequency metal detector models. An important feature of a multi-frequency device is that it simultaneously functions using several operating frequencies, and it can be 2-3-20 frequencies at the same time.
Minelab CTX 3030, Minelab Vanquish, Minelab Equinox 800 are one of these metal detector models. Don’t be surprised that there is just one manufacturer on the list. It has a patent and a license; thus, all attempts of other companies to use this technology are committed for trial.
Experienced treasure hunters advise studying some perspective areas using a high-frequency metal detector and then with a low-frequency one. Multi frecuency metal detector simultaneously emits signals of several frequencies. This feature allows in one wave of a search coil to do three things: studying the ground with the help of low, medium, and high-frequency devices. Correspondingly, the detectorist spends less time with higher efficiency.
How to select a metal detector frequency
What metal detector to choose – multiple-frequency or single-frequency one?
The operating frequency you select highly depends on what you are planning to look for. If you aim at small coins, earrings, other small non-ferrous targets, you should get a device with high operating frequency. In case if your targets are large coins, bullet casings, buckles, etc., you should select a medium frequency. If you are going to look for deep hidden hoards, helmets, or wood choppers, choose a low-frequency metal detector.
A standard approach is the following: during the 1st season to make sure you’ll learn using a metal detector and get the idea about the potential of this specific area where you go metal detecting and what you can find here, it’s better to use a metal detector with a medium operating frequency.
Moreover, such frequency is used in most metal detectors for beginners. At that, taking into account how trashy some areas are – these devices have a mono-coil (concentric one), which is also often used for inexpensive devices of beginner level. Such search coils are multi-purpose ones and perfectly balanced.
When the 2nd season starts, it will be clearer for you in which direction to move when selecting the device coil size and frequency considering this specific area and your existing preferences. By that time, you’ll know what small and large search coils are for, which frequency is more productive regarding some specific targets.
It is more efficient to use metal detectors with several frequencies or multi-frequency ones. Moreover, nowadays, the multi-frequency feature has become available even for simpler device models (see Minelab Vanquish). If you can’t afford getting such a device, get a single-frequency metal detector. Don’t worry, you’ll find your treasures anyway, you won’t miss anything.