Power vs detection depth

[Nupi]

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.

You said:

When you use a PI detector with a mono coil, the target is detected purely by eddy currents generated
in its surface.[but through the Eddy current is a Magnetic Field generated] Depending on the size of
the target, its orientation and conductivity; these eddy currents [and the Magnetic Field] the continue
to flow for a given time. [at this time the Magnetic Field of the target, works against the Magnetic Field
of the coil, through this the Magnetic Field of the coil takes longer to go away on the end of the puls.
The flyback will take longer] work If they have not died away by the time the main sample is taken,
then the target can be detected.[If I understand correctly, the PI measures the duration of the
obstruction time of the Magnetic Field of the coil through the induction of the target.
The operation of the PI detector is on the same basis, just another detection method but the coil
receive not the target signal.
It works on the change of the Magnetic Field of the TX through the induction of the target who opposes, is that right?]


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,
[This, I do not understand, how can the RX receive while the RX is nullit. He even receives
almost no signal from the strong radiant TX, much less a small signal that first must penetrate through the soil to the RX coil]
it's not immediately obvious that detection is still the result of eddy currents flowing in the target.
[when I look at my theory that's for me clear why the RX can not receive and it is not necessary that
the RX recieves a signal from the target to detect a target].
nupi

[Nupi]

[QUOTE=baum7154;193793The 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[/QUOTE]


In order not to repeat, you have maybe enough my explanationin in post #31 ?

regards,
nupi

[Qiaozhi]

Nupi - I think you're actually agreeing with everything that others are posting, but your understanding of the English language is causing confusion.

Strictly speaking there is no signal being transmitted by the target that says "Hello, I'm a metal target. Can you hear me?", which is then subsequently heard at the receive coil. In a PI the target is detected by a change in the decay time of the TX pulse. Whether you consider this to be "receiving a target signal", or a distortion of the TX decay by a magnetic field (Lenz's Law) generated by the target, is basically a matter of interpretation. At the end of the day you've detected a target. The same goes for a VLF. A signal is generated in the RX coil that indicates a target, even if this is the result of the magnetic field generated by the eddy currents causing a distortion in the IB field coupling. I think that Carl explained it all very succinctly in his post (#30).

[Nupi]

What about BFO and Off Resonance method?

I agree with what Qiaozhi said. In a BFO the Magnetic Field is also influenced
by the inductian of the target. but because that the bfo changes frequency of the oscillator which is
with is mixed with a reference oscillator and detected.
The bfo coil also not receive any signal from the target.

[Qiaozhi]

The bfo coil also not receive any signal from the target.

To make things clearer, consider a similar situation:

Recently we've seen news headlines saying, "Scientists detect an Earth-like planet around a distant star".
In reality, the planet itself was not detected directly, but its existence was implied by a reduction in light intensity as the planet crossed between us and the star. In the same way, a target is detected by measuring a change in the decay curve of a PI, or the phase-shift (and/or amplitude change) produced in a VLF. Therefore, in our case, the headline might read "Treasure hunters detect a metal object located deep in the ground". The metal object was not detected directly, but by the effect it has within the detector.

You could take this to an extreme by considering that what we interpret as "sight" is simply photons hitting receptors in the eye. In that case, you could state that we never actually "see" anything.

[Funfinder]

thx 2 all for the interesting topic.


> In that case, you could state that we never actually "see" anything.

Absolutly right - it's just a signal-interpreting-process our brain has learned to deal with and to see the "reality" with and in it.
And if our self-recognition-system is turned off - while sleeping and dreaming - we interpret it's real what we see, but it's far away.

The interesting point is how to proof "detection results" - this also is possible pure mathematically by extrapolation or other methods.
To confirm or "see" that it's reality not the whole picture has to be present - solving Sudoku per instance needs justs a part of it.



That's why also the absence of "energy" or substance works - to detect cavity. But the correct amount is needed.
Depending how light or dark is a foto-negative or positive, the brightness of the dia-scanner has to be adjusted to get the highest
information (contrast-factor and best color) out of it.

So the best would be if the metal detector contains adjustable TX power and a coil switchable from narrow to wide.

And of course it is very comfortable if the detector is very lightweight and is able to work with just 5v and a low amount of milli-amps,
but often the therefore needed high-sensitive detection-circuits are far too disturbable by ground- and electro-smog influences.

The title above should be: "power = detection depth"
Its comparable with a small lamp versus a ultrabright lighthouse through coastal fog.

But the MDs power also must be good concentrated and not spread over a huge area by very large round coils,
because in this case the ground-disturbance can blurres out the little or deeper metal target.
The eliptical DD coils are already the best solution but even better would be a row or cluster of little, very powerful and balanced coils.
At least 3 - 5, all hang on their separate circuits.

The main problem most companies have to create real powerful detectors are:
- many people don't like too heavy detectors, they like it small, simple and easy
- The are used to work with usual electronic technology - not high voltage stuff


Of course for professional and really ultra-deep detectors there is no other way than using really high power circuits that are still very sensitive
while using high-voltage (around 50v - not so really high but much higher than usual electronic stuff uses) environment and "concentration coils".

Compare it with a scanner or laser - the used light-beam there also is very concentrated. Or with a directional antenna.


The less voltage or power is used, the higher sensitive has to be the "contrast-factor detecting"-circuit and the easier this one is disturbable!
Which means per instance if the ground is mineralized or there is some e-smog nearby, this stuff is no longer really useful!

Of course we must see the whole development processs which includes the market situation and the improves of the technology.
For the next future I can imagine already pretty strong detectors using 25v accus who provide enough TX power for 2 search days
and are still lightweight enough.

But the main thing will be creating coils that are much more accurate to focus their power down and read the detection-signals like a ground scanner.
And perhaps DD will be not the best solution for real "concentratic" coils. And it may need fast microchips to calculate the single coil-cells statuses
and combine this to a fiting picture of the ground situation.

For the moment just keep in mind:
Don't built a detector which works already with a 1 volt battery, no matter how sensitive it is, but: The more power, the better and deeper!

[Silver Dollar]

Well more power is not always useful. Just yesterday I boosted the TX on my Bandito by
tweaking a resistor. It went from 8.7V to 9.7V. I thought hmm a little more depth. Wrong!
I lost depth in air test! I'm not sure why? (Saturating preamp?) But have seen it before on
this design. Also I would believe you might have so much power that the soil absorbs the
magnetism and retains it making it impossible to see any targets.

What might gain additional targets is to ramp the power up scan by scan to see different
ones based on the different levels "uncovering" various materials...

Some PI's use this by having several different pulse widths which are equivalent to different
power levels.

[Nupi]

Has/does anyone radiation/fields pattern of different coils?

[Nupi]

Well more power is not always useful. Just yesterday I boosted the TX on my Bandito by
tweaking a resistor. It went from 8.7V to 9.7V. I thought hmm a little more depth. Wrong!
I lost depth in air test! I'm not sure why? (Saturating preamp?) But have seen it before on
this design. Also I would believe you might have so much power that the soil absorbs the
magnetism and retains it making it impossible to see any targets.

What might gain additional targets is to ramp the power up scan by scan to see different
ones based on the different levels "uncovering" various materials...

Some PI's use this by having several different pulse widths which are equivalent to different
power levels.

Try a small piece of ferrite and find a place on the coil where the depth increases with your larger oscillator output.
If you change the power affect the coil-nulling.

[Silver Dollar]

I was just thinking I should have tried retuning the RX cap after increasing the TX power.
I'll try the ferrite too. Thanks!

[Nupi]

I was just thinking I should have tried retuning the RX cap after increasing the TX power.
I'll try the ferrite too. Thanks!

I want to tell you that sometimes lowered the voltage is better for the performs from the MD.
The discrimination can also better work (or not!!!). It's definitely worth it to try it out.
Seek and you shall find (the question is what you will find ) There are many factors that
play a role in the proper functioning of an MD. Even for manufacturers is that a challenge, and especially for us as amateurs.

[golfnut]

Raising the Tx bias/drive on bipolar Tx's does give a little more Tx signal current - the problem is the device will be running more non linear and will be also transmitting more 3rd harmonic and 5th, 7th etc.

It is the inability of the synch dets ( mixers ) to cope with this forest of spurs which makes the overall performance worse.

If the Tx drive was really nonliear then the synch det drive from the phase shifters could also contain multiple frequencies which is bad.

If after altering the Tx drive you added a lossless filter on the preamp input, and one on the preamp output, which only passed the Tx signal frequency, would you realise the tiny gain.

[Chet]

I've read it takes 64 times the power to double the detection depth. The plotted data shows it takes about 10 times the current to get the same signal for each 5 inch increase in depth. If the detection depth is 5 inches it would take 10 times the current to double the distance. If detection depth is 10 inches it would take 100 times the current to double the depth. Is my thinking wrong or are the plots wrong?

Hi Green
I like your test. I assume that the vertical scale is millivolts from the integrator output. I believe that your plots are correct but suggest that another method be used to reinforce your findings.

Another possible way to test this would be to establish a transmit pulse width of say two or three time constants of the coil. Keep the pulse charge waveform from flat topping for comparison with the second part of this test. Measure and record the output level with a target at 10 inches. Next move the target to 5 inches and reduce the pulse width until the recorded output level is achieved. Calculate the peak and average power of each of the two pulse widths used for the final comparison. This hopefully will reinforce your x10 finding.

[green]

Hi Green
I like your test. I assume that the vertical scale is millivolts from the integrator output. I believe that your plots are correct but suggest that another method be used to reinforce your findings.

Another possible way to test this would be to establish a transmit pulse width of say two or three time constants of the coil. Keep the pulse charge waveform from flat topping for comparison with the second part of this test. Measure and record the output level with a target at 10 inches. Next move the target to 5 inches and reduce the pulse width until the recorded output level is achieved. Calculate the peak and average power of each of the two pulse widths used for the final comparison. This hopefully will reinforce your x10 finding.

I wasn't looking at it right at the time, (maybe still not). When the target distance gets out past the diameter of the coil, signal strength decreases at the sixth power of distance change. I was looking at distances less than the diameter. Now I'm thinking it takes more than 64 times the power when out past the diameter of the coil. To double signal need to double peak current. To double peak current, average current would more than double with a PI detector.

[Chet]

Hi Green
I will give that some more thought. If the x64 does apply beyond the diameter limit of the coil your tests are still valuable from the point that smaller targets can be detected by a x10 power increase within the diameter range of the coil. Your three coils used in the test; where they the same coil reshaped into the different dimensions?