The study of modern metal detectors and their theory of operation can at times be completely overwhelming and a very complex subject to digest and understand. We have attempted in this article to explain; in a straightforward and easy manner, some of the fundamentals of the basic theory behind a detectors operation. It is our hope that the information presented here will clarify some of the questions you may have about detectors, and perhaps allow you to enjoy another aspect of our hobby.

    In its most basic and simple form, a metal detector can be represented by a transmitter, receiver, and an antenna. In Diagram # 1, we see in block form what represents the transmitter, receiver, and all of the electronic stages and components necessary to transmit and receive an electromagnetic wave or signal. You will also note the antenna or search coil which is necessary to transfer the information ( electromagnetic field ) to and from the transmitter/ receiver into the soil and back again; providing there is a large enough target to do so.

    The transmitter makes use of an RF oscillator to generate a specific frequency; our transmit or operating frequency. For our discussion we will use the frequency of 20,000 hertz or cps ( cycles per second ) as an example. This signal is delivered to the search coil by means of a shielded cable ( coaxial, etc..). The signal which is comprised of current and voltage components creates an electromagnetic field ( emf ) around the coil as it begins to flow through the windings inside the coil. This field will be called the primary field. It is necessary at this point to take a quick look at this signal that is creating the electromagnetic field within the coil, ( see Diagram # 2- Signal Analysis ).

    You will notice that the current ( and or voltage ) starts at 0 degrees and ends at 360; this is one complete cycle or hertz. Notice that the current is positive from 0 to 180 and then changes to negative from 180 to 360. This process will be repeated over and over again until we have reached 20,000 of these cycles. When we are at the end of our 20,000 cycles, one (1) second of time will have elapsed and the process starts all over. This means that the polarity of the current ( and or voltage ) will also change from positive to negative ( or vice-versa ) 20,000 times every second.

    It is important to realize that anytime a current/ voltage flows through a wire or any conductor, an electromagnetic field will be generated around that wire or conductor. Since the polarity of the current/ voltage is changing 20,000 times per second, the polarity ( or poles ) of the electromagnetic field will also change at the same rate. The electromagnetic field will point into the ground during one half of the cycle and then away from the ground during the other half of the cycle.

    We now have an electromagnetic field that is being produced inside the search coil ( or loop antenna ) that we will use to help locate our targets. If you would like to prove this to yourself and see the effects of the magnetic field, simply hold an inexpensive compass near the search coil ( with the detector power on ) and watch the needle move as you slide the compass through the magnetic field. You will obviously hear a noise in your headphones ( or speaker ) because of the metal within the compass.

    This changing ( or motion ) of the electromagnetic field is the tool we use to induce current to flow on the stationary target. The concept to remember here is that we can generate, ( induce a current to flow ) by either of two methods; A) by moving a wire or any conductor back and fourth through a stationary magnetic field (elementary generator) or B) by moving a magnetic field back and forth past a stationary wire. Since we can not move our target when out metal detecting, we will have to settle for moving                     ( changing direction- polarity ) of our magnetic field ( see Diagram # 3- Elementary Generator ).

    With our stationary target ( i.e., gold nugget ) and our moving ( polarity changing ) magnetic field we will cause a current to flow; or be induced, within the gold itself. Remember, that if current flows in a conductor a magnetic field will ( and must ) be created. And now we see that it is because of these currents ( eddy currents ) the gold itself will now have its very own magnetic field! It is this secondary field that we are trying to establish and find.

    Once we have established ( or caused to happen ) this secondary field, regardless of what or where it came from ( i.e., gold, junk, etc..) our primary field will cross or intersect with it and “ feel ” or detect it ( provided it’s amplitude is large enough to be detected ) and alert the operator. The target will rob ( or borrow if you like ) some of the energy from the primary field to establish it’s own field ( the secondary field ). When this happens, the electronic circuits within the detector sense this and create an audio tone that will also alert the operator. Other conditions such as soil mineralization for example can also cause the detector to “ sound off ” by distorting the primary field creating an induction imbalance in the windings within the search coil.

    There are many factors that will inhibit us from finding or creating this secondary field.   If the target is buried to deep, our primary field will not penetrate the soil matrix with enough energy to set-up a secondary field that is strong enough for us to detect. All things being equal, the larger the coil ( search head ) diameter the deeper the electromagnetic field will penetrate into the soil. Theoretically this field will travel forever, but the field strength would be so weak that it would be impossible to detect at any appreciable distance. As a side note, the wavelength that you are operating at with a frequency of 20,000 hertz is 15,000 meters. This means that your original ( or first ) wave ( field ) will have moved 15,000 meters ( approx. 9.32 statute miles ) from the source; your detector coil, before the next wave has moved. Remember all this takes place in one second of time ( it is traveling at the speed of light )! To find the wavelength of any frequency you can use this simple formula: wavelength in meters = 300,000,000 ( meters per second ), divided by your frequency in hertz ( or cps ).

    This electromagnetic field we have created is just a very small part of the total electromagnetic spectrum. Within this spectrum you will find radio waves, microwaves, X-rays, and visible light. We can actually calculate the speed of light ( designated by the letter “ C ” ) by simply knowing the frequency and the wavelength of the signal. C ( speed of light ) = wavelength in meters ( times ) the frequency in hertz. To prove that our signal is traveling at the speed of light, simply multiply 15,000 meters ( our wavelength ) times 20,000 hertz ( our operating frequency ) and you will arrive at 300,000,000 meters per second ( approx. 186,000 miles per second ); which is the speed of light. From the above two formulas you can calculate wavelength and the speed of light. It is now very easy to calculate your frequency. Frequency ( in hertz ) = C ( 300,000,000 meters per second- speed of light ) divided by wavelength ( in meters ).       

    Now back to the large coil. As with most anything in life, there are both good and bad points. The bad news with deeper penetration is that you have less sensitivity to smaller targets because the field’s total energy is spread out over a much larger area. This means you will have less magnetic flux density ( tesla ) per square meter. The opposite of this is true ( all things being equal ) with a smaller coil. There is less penetration, but more sensitivity per square meter. If the target is too small we might not hit it with our signal; or if we do the current induced within it might not be sufficient for us to detect.

    Generally speaking, the larger the area of the target itself ( not volume ), the more current we can induce within it; hence better detection. With a larger target we will have more lines of force ( from our emf ) cutting across it and inducing more current. A way to improve one’s chances of finding smaller targets is to increase the signal frequency ( transmit or operating frequency ) of the detector; if it has the capability to do so. The higher the frequency ( 30,000 vs. 20,000 cps ) the more times per second your signal will change from (+) positive to (-) negative; as will the direction ( polarity ) of the electromagnetic field.

    In an elementary generator ( see Diagram # 3 ), the faster something ( conductor, magnetic force, etc..) moves or changes direction, the greater the induced current flow. By making the electromagnetic field direction change faster ( 30,000 vs. 20,000 hertz ) the more induced current you can make flow on your target; thereby providing a better chance of detecting smaller targets. The following is a brief summary of Michael Faraday’s historic discovery made in 1831:

“ Whenever a magnetic force increases or decreases it produces electricity, the faster it increases or decreases, the more electricity it produces. ”

Again the good with the bad. Because of the energy spent increasing the signal frequency  ( don’t forget that you are mobile using a battery with a limited amount of energy ) you will have less energy per cycle and hence less depth penetration.

    It must be kept in mind that there are many other factors not mentioned in the discussion above that will influence the detector’s ability to find a target. A few examples of this would be the following:

a)  target material composition which can help or hinder the flow of current ( good or bad electrical conductor )

b)  target orientation with respect to the electromagnetic field

c)  operator experience 

d)  coil sweep speed

e)  soil composition or matrix ( highly mineralized vs. low mineral content)

    We must understand that the primary field will not only energize ( create a secondary field ) the target in question, but will also cause a secondary field to be created in anything that will conduct electricity; and even some that don’t conduct so well. Within this statement we find the devil in the details. In many gold bearing areas throughout the world; especially Australia and the southwestern United States, we find the soil matrix to be highly mineralized. Mineralized ground in our context is not implying that a particular area is rich in gold, silver, platinum, etc. We label them as mineralized based on their relative composition and or content of certain minerals, not necessarily because they contain precious metals.

    Detectorists often think of highly mineralized ground as being “ hot.” When we speak of the ground in this manner we are simply referring to the various conductive and magnetic properties they exhibit, not temperature. The soil itself is not actually hot to the touch, but it can cause serious problems for a metal detector.

    A vast majority of the goldfields are composed of soil, sand, and clays that contain small grains of iron-bearing minerals. These minerals are predominantly the iron oxides: magnetite, hematite, maghemite, limonite, and lepidocrocite. All of these oxides exhibit a varying degree of ferromagnetism. Ferromagnetic substances can be magnetized by being exposed to another magnetic field; like the one produced by the coil. Any material              ( including the iron oxides, conductive alkali salts, ferromagnesian silicates, aluminosilicates, etc.. ) that will allow current to flow have to set up a secondary magnetic field. It is this natural property that causes the ground in highly mineralized areas to become “ hot. ”

    Now the problem, how do we discern a good field ( target ) from one that is created from the ground? Under certain conditions this mineralized soil will have a signal ( field ) just as strong if not stronger than the tiny target. Fortunately for us detectorists there is an electrical phenomena called phase shift or phase angle.

    In Diagram # 2- Signal Analysis, we showed you one cycle of our signal ( transmit frequency ) with only one of the two components shown; current or voltage. What we need to understand though is that there are really two separate and distinct components; namely current and voltage flowing at the same time ( see Diagram # 4).

    Notice in the top drawing the current and voltage are in phase. By this we mean that their minimum and maximum responses rise and fall in step with each other at the same points along the curve. Again, when this happens we say the current and voltage are in phase and this happens in a purely resistive circuit. There is no capacitance or inductance in the circuit, or if there is they are exactly equal and opposite in value. Now look at the drawing on the bottom of Diagram # 4. In this drawing you will notice that the current and voltage are not in phase. In fact, they are 90 out of phase! The current component of the signal is 90 behind ( lags ) the voltage component. This type of situation arises when the circuit contains more inductance than capacitance. In a circuit containing more capacitance than inductance the voltage component of the signal is 90 behind ( lags ) the current component. The secondary electromagnetic field that is generated by the target has two components that make up this field ( just as the primary field does ), the magnetic component (H) and the electrical component (E).

    The magnetic and electrical lines are always mutually perpendicular and the total energy is divided equally; this is the nature of wave propagation. We are interested in the electrical component of this wave. It is here that we find the same ( although delayed in time and with a different amplitude ) current and voltage that we originally started with. When we compare this new signal with the original signal we will usually find that the phase has changed. If the target is a nice, shiny silver dollar ( which looks inductive ) then typically the current will lag the voltage. They will be out of phase and different from the original signal. Inside the detector a phase demodulator(s) will measure this change in phase ( or phase angle ) and will establish what type of target our search coil is detecting.    The VLF metal detector makes use of this “ phase shift ” to discriminate or identify different types of targets ( i.e., coins, trash, etc. ). Each target will have its own phase shift associated with it.

    As stated earlier, the soil matrix in some locations can be highly mineralized which will also cause a phase change. Under moderate conditions we can compensate for this change by ground balancing our detector and making this our zero reference. Our detector will now ignore this mineralization and detect only real targets; which unfortunately also include the so-called “ hot rocks and ironstones. ” Under extreme or highly mineralized conditions we cannot completely cancel out ( correct for this phase change ) the effects of the soil on the detector.

    Our depth or signal penetration into the soil will be very limited because of the absorption rate and loss of energy. Also, because of all the secondary fields being generated by the mineralized soil, we will experience a distinct increase in the noise level   ( also referred to as ground noise ) making it difficult to distinguish between soil and target. It should be stated at this point that our discussion has been centered around the basic operation of a VLF ( or Induction Balance ) metal detector.

    In order to improve your chances in highly mineralized soil a Pulse Induction detector could be used. This is a different technology that has good and bad points as well, but generally speaking it is more efficient in extreme soil conditions.

    In closing we must remind you that the theory discussed in this article is very general in nature, and is by no means complete. We have selected certain topics that we felt would give the metal detectorist a brief and basic overview of the general workings of the detector. We must apologize in advance for any errors that might have been made in our explanation. Our goal is to expose the detectorist to some basic theory with the hope that it might further stimulate your thirst for knowledge; which will ultimately help enhance the hobby as a collective whole. Good luck hunting!!