Power vs detection depth

[golfnut]

I think of the Tx RX coil combo as an open transformer.

If this transformer is small, a target has a big effect and couples a good signal thru ( often people make 7" coils and are pleasantly suprised at how good they are )


If this open transformer is large, a target doesnt couple much energy across ( large coils are aften a bit of a disappointment and are often up for sale )

To get a good signal out, with large coil. you need a big target, it isnt necassarilly deeper on small targets.


Lots of Deus large coils up for sale, as people are not interested in coke cans at 30"

S

[Nupi]

What really strikes me is that there are so many different opinions.
If you compare the opinions of different people,
I notice that some are contradictory. My impression is that nobody really knows how it really works. Neither do I.
Maybe it's because we accept theories that are not quite right.

Usually, the operation of an MD are explained in this manner.

"""The operation of the metal most commonly used is based on the principle of electromagnetic induction.
The such a detector consists of two coils. In its simplest form, Due to the one coil, the oscillator,
a resulting high-frequency alternating current which generates an alternating magnetic field.
This field will be a metal object that is not too large distance in the soil will induce an electric current,
called eddy currents (Foucault currents or eddy current). This current,
generates an alternating magnetic field that is detected by the other coil,
the Rx coil and thus the presence of the metal object betrays in the form of a light signal or beep sound."""

I think this explanation is not correct and therefore the practice does not match with the theory.
The RX can not receive the signal from the target.
I think some will say now. This guy is really f..k..g crazy.(maybe you are right)
But so I think about it.

[Dave J.]

The "rule of 64" is almost always misunderstood. It has to do with field strength. Power is proportional to the square of field strength. Therefore just looking at transmit power (keeping everything else the same, as though that were possible which it isn't), we're talking a 4,000:1 increase in power.

The "rule of 64" is a fairly accurate representation of the response at distances from the searchcoil greater than its mean diameter. Closer than that, the details of the searchcoil construction matter more.

Since customers neither want to haul around a truck battery on their back, nor to run the searchcoil hot enough to use for a camp stove, most of the increases in air test hots over the last few decades have been through improved circuit efficiencies and signal analysis rather than more transmit horsepower. And, the increase hasn't been all that great. Back in the mid 1980's there were mainstream machines that air tested past 10 inches (25 cm). Nowadays machines that do appreciably better than the best 1980's machines are mostly equipped with oversized searchcoils, exhibit sluggish response, and/or are battery power hogs.

[Nupi]

I think, this subject matter is very complicated and there are
many different opinions and explanations and when you get involve the "quantum mechanics"
then I get lost.
What I write is my personal opinion and does not mean that I want to know better than a anothers.
When we increase the power we increase the detection depth up to a certain point,
and is determined by many factors.


Some of the factors may be are:
-- The consrtuction of the coil.
-- The construction of the circuit.
-- The kind of "RX" input (signal to noise ratio,...).
-- The type of soil (the grade of mineralization,....).
-- Many types of interferences.
-- The operator (sweep speed,.....).
-- etc....


Before I continue with my perspective on this phenomenon, I have to say that:
I do not believe the explanation that the RX coil receives the induced magnetic field from the target,
generated by the Eddy current.


I believe more this:
The RX coil is coupled by the Magnetic Field of TX (inductance) and the RX coil radiates also.
You can say that you have two TX coils. Now, the "RX" coil positioned at the TX coil so that
the magnetiche fields from TX and "RX"coils neutralize at a certain place. (nulling)
The target will change the magnetiche field lines and brings the nulling in imbalance.
The MD-registered the voltage change on the "RX" coil and detects.


In other words,
When electricity flows through the transmitter coil, it creates a magnetic field. When the coil is moved,
the magnetic field moves with it.
If there is no metal present, the magnetic field will be constant relative to the coil but if you sweep
the detector above a metal objects, the magnetic field will induce an electric current to flow
inside the object. This flowing electric current creates another magnetic field around the object.
This magnetic field will act against or for the magnetic field of the coil according to Lenz's Law so
the magnetic field in the coil will be disturbed.(the nulling of the "RX"coil).
This disturbance (on RX) can be detected and made to sound.


Nupi.

[Gwil]

I'll have to think about this.

[baum7154]

There is truth in what you say but not just the target disturbs the balance between coils but also
the earth mineralization does as well. In a non motion PI detector with a monocoil I believe that the coil does receive the target signal.

[Carl-NC]

Before I continue with my perspective on this phenomenon, I have to say that:
I do not believe the explanation that the RX coil receives the induced magnetic field from the target,
generated by the Eddy current.

Oddly enough, that's what happens, even if you chose not to believe it!

I believe more this:
The RX coil is coupled by the Magnetic Field of TX (inductance) and the RX coil radiates also.
You can say that you have two TX coils. Now, the "RX" coil positioned at the TX coil so that
the magnetiche fields from TX and "RX"coils neutralize at a certain place. (nulling)
The target will change the magnetiche field lines and brings the nulling in imbalance.
The MD-registered the voltage change on the "RX" coil and detects.

RX coil doesn't radiate. Only the TX coil.

In other words,
When electricity flows through the transmitter coil, it creates a magnetic field. When the coil is moved,
the magnetic field moves with it.
If there is no metal present, the magnetic field will be constant relative to the coil but if you sweep
the detector above a metal objects, the magnetic field will induce an electric current to flow
inside the object. This flowing electric current creates another magnetic field around the object.
This magnetic field will act against or for the magnetic field of the coil according to Lenz's Law so
the magnetic field in the coil will be disturbed.(the nulling of the "RX"coil).
This disturbance (on RX) can be detected and made to sound.

Yup, this is pretty much how it works. In the statement above you said you don't believe it works this way, but apparently you do. Progress is made, and in just 3 short paragraphs!

[WM6]

with a monocoil I believe that the coil does receive the target signal.

Or changing in existing (thank circuit) load only.

[Qiaozhi]

Before I continue with my perspective on this phenomenon, I have to say that:
I do not believe the explanation that the RX coil receives the induced magnetic field from the target,
generated by the Eddy current.

When you use a PI detector with a mono coil, the target is detected purely by eddy currents generated in its surface. Depending on the size of the target, its orientation and conductivity; these eddy currents continue to flow for a given time. If they have not died away by the time the main sample is taken, then the target can be detected.

When you use a continuous wave detector (often referred to as a VLF) the small eddy current signal from the target is swamped by the TX signal, so it is necessary to blank out the TX using a different method. In this case, the TX and RX coils are positioned in such a way that they are in what's called "induction balance", and any target signals can readily be seen, even though the transmitter is still running. Because the detection process involves monitoring the phase-shift of the received signal, it's not immediately obvious that detection is still the result of eddy currents flowing in the target.

Remember that an IB coil is essentially a very poorly coupled transformer, and the metal target acts as the core. This is why mains transformers have laminated cores, in order to reduce the flow of eddy currents which generate heat and cause losses.

[WM6]

What about BFO and Off Resonance method?

[Qiaozhi]

What about BFO and Off Resonance method?

With BFO and Off Resonance, the detection process is still the result of eddy current generation.
The TX signal generates eddy currents in the target, and these current loops act like a small coil winding, which in turn couple with the TX coil causing a change in inductance.

[Nupi]

There is truth in what you say but not just the target disturbs the balance between coils but also
the earth mineralization does as well. In a non motion PI detector with a monocoil I believe that the coil does receive the target signal.

For clarity, I want everyone to answer, but you must be patient with me.
It takes me a lot of effort to get English on paper.


Of course, disturbs earth mineralization the ballans. Therefore you detect them too.
Tree roots, cavities in the ground, etc...
As I know, but I could be wrong, every metal detector that I know operates with the principle that
the magnetic field, fux, changing by approach from metals, minerals etc.... Only the detection metod is different
from each other.
I do not know much about PI. I do not know much about many things. But I want to learn every day.
What I know, receive the PI nothing. He compares the flyback change. which is caused by the change
of the magnetic field from the coil which is counteracted or acted by the magnetic field of the target, Eddy curent.

nupi.

[baum7154]

I do not know much about PI. I do not know much about many things. But I want to learn every day.
What I know, receive the PI nothing. He compares the flyback change. which is caused by the change
of the magnetic field from the coil which is counteracted or acted by the magnetic field of the target, Eddy curent.

nupi.[/QUOTE]
------------------------------------------------------

The PI emits a pulse, must wait for the coil to decay, and then listen for the target eddy as it decays. This is why it is necessary for a detector to have fast coil/system quieting in order to detect small and fast decaying materials like gold. I don't think the basic PI principle compares the flyback change so much as it trys to detect the actual target emissions. If all it did was compare the flyback response, the speed of the coil would be less important I think.

regards,

Dan

[Nupi]

RX coil doesn't radiate. Only the TX coil.

That's right, the RX, not transmit at that moment, it's nullit.
As we, the RX, shifted from the nulling line, transmit the RX also by the induction of TX magnetic field that always transmit.
But I thought that was obvious from my story. What I wanted to make clear is that the RX is not a passive receiving antenna.
RX receiving nothing only interference. Shifted from the zero line we transform RX coil into a TX-2 coil.


I do not understand you, you say that I in just 3 short paragraphs I changed my beliefs, but I see you changed in 2.
I say above and below the same.
I want to clarify it for you. (I know my English is bad and sometimes confusing)


"In other words,
When electricity flows through the transmitter coil, [No electricity flows through the RX, RX is nullit]
it creates a magnetic field. When the coil is moved, the magnetic field moves with it.
If there is no metal present, the magnetic field will be constant relative to the coil [RX is constant relative nulling too] but if you sweep the detector above a metal objects, the magnetic field will induce an electric current to flow
inside the object. This flowing electric current creates another magnetic field around the object.
This magnetic field will act against or for the magnetic field of the [TX] coil, according to Lenz's Law so
the magnetic [TX] field in the [TX] coil will be disturbed.[an so] (the nulling of the "RX"coil).
[here, the RX coil receives nothing, no signal from the target that the RX recieves,
the inductance between TX and RX that keep both in balance, is disrupted. RX is not in zero position.
The RX, receives induction of the TX and RX generates a voltage, which is detected]."


nupi

[Carl-NC]

Nupi, here are 2 different ways to think about it:

"The TX coil generates a magnetic field, the magnetic field creates eddy currents in a metal target, the eddy currents produce a counter-magnetic field, and the counter-magnetic field is detected by the RX coil."

"The TX coil generates a magnetic field, the magnetic field creates eddy currents in a metal target, the eddy currents produce a counter-magnetic field which distorts the TX field, and the distorted TX field upsets the null of the RX coil."

Technically, they are both correct. However, it's important to remember that not all detectors use induction balanced coils; PI & BFO can use mono, and even VLF doesn't necessarily have to have IB coils; you can do VLF even if the RX coil in unbalanced. So the second statement really only applies to an IB detector, whereas the first statement applies to all (induction-based) metal detectors.

- Carl