by Michael Bernzweig and Jim Caviness
If you are new to metal detecting, pipe and cable locating or ground penetrating radar, you might wonder what people mean by operating frequency. This article explains in great detail what operating frequency is and what it has to do with locating underground metal and structures. Understanding the difference between a low frequency, high frequency, multi frequency and a pulse induction metal detector can seem challenging at first glance. We will demystify frequency for you. Simply stated, you can improve your searches when you use the different operating frequencies. This article will help you understand how operating frequency signals work in greater depth. Understanding frequencies will help you in your search for underground utilities, coins, jewelry, gold, or other metals.
Frequency is the number of waves per unit of time measured in kilohertz or kHz. This is the number of electronic waves passed through a metal detector into the ground when you are detecting metal objects. An example of 12 kHz frequency is when a metal detector transmits and receives 12,000 times every second. If a signal repeats itself 15 times every second, its frequency is 15 Hz.
A typical metal detector's best frequency for coins jewelry and relics is between 5 kHz to 15 kHz. This is the range where you will find most metal detectors for general-purpose are tuned. Many models will include preset search modes that alter the frequency to find specific types of targets, The range is also the easiest to manage for metal detecting newbies. More specialized metal detectors can be purchased once you understand the basics and begin mastering metal detector searches. If you are prospecting for gold nuggets operating frequencies range significantly higher, typically in the 17 to 70 + kHz range is ideal.
The lower frequency the more easily your detector will locate iron and other lower conductivity metals typically found in underground pipes and cables, and relics. The higher the operating frequency, the more easily your detector will locate high conductivity metals like gold. While wearing wireless headphones or high quality metal detecting headphones you will most easily hear the signals your device makes as you search for treasures. Soft or marginal signals produced by your device are typically difficult to hear with out headphones.
Frequency and Conductivity
All metal objects located in the ground conduct electricity in some way with different types of metal detectors. A VLF style industrial metal detectors or even a hobby metal detector device will tell you the type of metal it has found by how well its electronics conduct electricity and the frequency pulses it passes into the ground. These are determined by the speed that the signal sent back to the device decays over time. Modern metal detectors can discriminate between metals to identify typically on an lcd screen the type of metal it is. They can allow you to decipher the difference between ferrous and non ferrous by using discrimination to stop the device from picking upon them. Pulse induction and beat frequency oscillator metal detectors make use of the specific frequency of electromagnetic field or pulses passed through the ground to locate the buried target. There are different advantages and disadvantages of various frequencies used for metal detection.
Low vs. High Frequency
Lower frequencies possess longer wavelengths. They also have greater depth as low frequencies have long waves that penetrate the ground more easily than high frequencies. Low frequency is the best way for high conductivity target detection. Items such as silver are easier to locate using lower frequencies. Two of the main disadvantages of using low frequencies is that they are not good at locating small targets. In addition, it is difficult for finding gold but best for finding iron.
High frequencies metal detectors work with shorter wavelengths than lower frequencies. These are excellent for locating small targets. Items such as small gold nuggets are ideal targets for high frequencies. Iron and ferrous metals are low conductivity targets and low frequencies are better of locating them than higher frequencies. The wavelengths achieve less depth when searching. You have a high accuracy in locating targets that are near the surface. Finally, high frequencies are more sensitive to ground mineralization which can cause interference when searching. For this reason it is important to ground balance to filter out these minerals.
Metal Detector Frequency Chart
In each metal detector frequency chart diagram below, we have compared the frequency and frequency type by technology and by brand. We have broken each chart down by Locator type, frequency detection capability, sonde capability and sonde frequency.
Schonstedt Locators Operating Frequency Comparison Chart
This frequency chart above provides a detailed comparison of the Schonstedt models. By referencing this metal frequency chart you can easily select the best pipe and cable locator for your project.
Leica Locators Operating Frequency Comparison Chart
This chart above provides an in depth comparison of the Leica Geosystems Utility Detection Radar, DD Utiltility Locator and Ultra Utility range of devices.
Goldak Locators Operating Frequency Comparison Chart
For this comparison above, we compare the options for metal detector coil frequency and sonde frequency for the Goldak products range. You can easily see their single frequency and multi frequency options.
Goldak EMS Markers & Locators Operating Frequency Comparison Chart
This final metal detector frequency chart above covers both the single and dual frequency EMS Marker and locator range of devices. This chart compares the unique operating frequency required for each type of utility location such as Power, Telephone and Gas EMS markers etc.
Which Frequency Is Best For Metal Detectors?
There are two types of frequencies: Single Frequency operation and Multi-Frequency operation. Single Frequency is also known as continuous wave. It has one frequency selection and is typically found in mid frequency VLF metal detectors like the Garrett Ace 400. Single Frequency is often found in entry to mid level devices. Higher end devices may include a range of single frequency options.
Multi-Frequency models are typically more advanced metal detector devices. They use more than one frequency at the same time. The first generation of these detectors used full band spectrum (FBS) frequency technology. The latest multi frequency models are fast and provide excellent detection depth. These technologies include Fast Multi Frequency (FMF) and Simultaneous Multi Frequency (SMF). Models like The Legend from Nokta Makro and the XP DEUS 2 incorporate these newer technologies. If you are searching for underground utilities, a complete utility locating system that operates at multiple frequencies may be a good option.
If you are searching for targets at a deep depth, then you want to use a low-frequency device. For example, coins are usually close to the surface and have higher conductivity. The best frequency for searching for coins is 10 kHz and lower. If you are searching for tiny gold nuggets or silver coins, you will benefit from metal detectors with higher frequencies.
Depending on the target orientation in the ground, the larger objects benefit from higher frequency devices. The best frequency to locate large objects is 7 kHz or higher. For smaller objects, the best frequency is 7 kHz or lower. Jewelry is one search item that varies in size.
Type of Metal
The frequency you choose will depend on the type of metal you are searching for. The best place to find ferrous metals and iron is at 10 kHz and higher. Gold can be found at 14 kHz and higher. At frequencies between 3 kHz and 7 kHz, silver, copper, and brass can be found. At frequencies between 4 kHz and 8 kHz, nickel and aluminum are most readily found.
Multi-Frequency vs. Single-Frequency Metal Detectors
The majority of metal detectors are single-frequency devices. They operate at only one frequency. Single-frequency devices typically operate at or under 10 kHz per second or above 30 kHz per second. A number of newer devices can detect at two or more frequencies at the same time. These metal detectors devices are called simultaneous multi-frequency detectors. Multi-frequency detectors are reliable and more capable due to their range of target detection. Think of multi-frequency devices as having more sensitivity than single-frequency metal detectors. Multi Frequency units are also popular for locating pipes and cables, specific frequency sondes and sewer camera frequencies. These models can detect both low conductivity and high conductivity targets at the same time. This offers a major step forward in detection technology
What May Affect Frequency in Metal Detectors?
While on treasure hunts or searching for buried utilities, there may be a number of factors that may affect the frequency of using metal detectors. When a treasure hunter or locating crew is on the lookout for items, it's possible to experience interference with the frequency due to a few things.
Ground mineralization has an effect on the normal use of a metal detector. Pulse Induction metal detectors are not affected by mineralization. However, a lower frequency is most often the best choice for a highly mineralized search area. A higher frequency will more likely be affected by the ground’s mineralization. Temperature fluctuations can also impact the frequency. Pulse induction sends a signal to the ground via the search coil. With pulse induction (PI), a relatively powerful, momentary current magnetizes the ground. It is possible to precisely measure the time to zero volts in the absence of metal. This technology filters out or ignores the effects of soil minerals.
Search Coil Size
For metal detectors, it is a general rule that the larger the search coil, the lower the frequency it produces. Conversely, the smaller the search coil, the higher the frequency it produces. Although large search coils are great for ground penetration, they lack sensitivity due to their size. Larger coils will cover a larger area with each sweep, and they will be able to detect much deeper objects. Smaller search coils are not as good at penetrating the ground as larger coils. The advantage of this is that they will have better sensitivity and will be able to locate individual targets that are close to each other.
Concentric versus Double-D Search Coils
Concentric search coils offer the easiest pinpointing for beginners as they locate the target in the center of the coil. They are not as good at filtering out minerals in the grounds. Double-D or DD search coils are very good at reducing the impacts of minerals in the ground. For this reason they are the choice of most experienced users or for use in areas with lots of minerals and targets in the higher frequency range. The DD coils pinpoint the targets closer to the heal of the coil.
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