What is the relationship of uH and sensitivity?

[WM6]

I had same machine with same coil data, named "Black Knight".

The same model currently selling in EU as "Klarstein" (seller from Berlin) Silver Edition 71.90EUR.

http://cgi.ebay.fr/KLARSTEIN-DETECTE...item3cbbf72b80

[Carl-NC]

"His detector was not that much better then mine, yet he was finding things that I was missing."

If you are using a Sharper Image metal detector, then you are close to the bottom of the barrel, and it's likely his detector really was much better than yours. You will come out way ahead if you keep the coil and replace the detector, than if you keep the detector and replace the coil. Since your detector already paid for itself, I recommend you get a new detector paying for itself.

For a sinusoidal voltage driven TX coil, the magnetic field is inversely proportional to the number of turns, or the square-root of inductance. Pulsed coils can be different depending on the drive circuitry.

- Carl

[golfnut]

My take - and I could be way off here...

1)The actual inductance value is not directly linked to sens.

(A bigger inductance value or L value will shift the frequency of parallel resonance down)


2)A lossy (higher resistance) coil in (same L value) in a tuned circuit broadens the resonance.

(The 'bandwidth' of the tuned circuit is wider and looses more signal at the centre frequency)


To get a larger phase change in a resonant circuit - it must have a narrow responce.

If a small pull hits a narrowband system the phase sift is relatively higher than the sampl 'pull', shift or partial detune on a lossy broadband circuit.


One way to get more of a ohase pull from a met dec then is to use a coill which has lower resistance (loss) to sharpen / narrow its bandwidth.

Q or quality factor is a key design consideration for coils.

It is inversely proportiona to R

Q=2 pi freq L/R

If R goes down , for the same L value, the loss goes down and broadband responce is pulled in to a narrower range - more likely to give a greater phase pull from a disturbance.

This is a way to get more sens from a coil - less lossy is more 'touchy'
Steve

[Beach]

"His detector was not that much better then mine, yet he was finding things that I was missing."

If you are using a Sharper Image metal detector, then you are close to the bottom of the barrel, and it's likely his detector really was much better than yours.

Mine is cheep alright.

On the other hand his was build around 1975 and he has been using it ever since. I thought electronics had improve since then....He sure was finding stuff that I was missing.

You will come out way ahead if you keep the coil and replace the detector, than if you keep the detector and replace the coil. Since your detector already paid for itself, I recommend you get a new detector paying for itself.

I was thinking about getting a better detector myself, only everybody that I speak to, who has a good one, has told me that theirs has yet to pay for itself.

For a sinusoidal voltage driven TX coil, the magnetic field is inversely proportional to the number of turns, or the square-root of inductance. Pulsed coils can be different depending on the drive circuitry.
- Carl

Inversely proportional......mmmmm....now I think I am getting it. The time it takes to dig a hole is halfed if your buddy helps you dig it. The time is inversely proportional to number of diggers.

More turns equals more power at the expense of power consumption.

I am sure it is not that simple...am I getting closer?

Thanks again!

[Beach]

My take - and I could be way off here...

1)The actual inductance value is not directly linked to sens.

(A bigger inductance value or L value will shift the frequency of parallel resonance down)


2)A lossy (higher resistance) coil in (same L value) in a tuned circuit broadens the resonance.

(The 'bandwidth' of the tuned circuit is wider and looses more signal at the centre frequency)


To get a larger phase change in a resonant circuit - it must have a narrow responce.

If a small pull hits a narrowband system the phase sift is relatively higher than the sampl 'pull', shift or partial detune on a lossy broadband circuit.


One way to get more of a ohase pull from a met dec then is to use a coill which has lower resistance (loss) to sharpen / narrow its bandwidth.

Q or quality factor is a key design consideration for coils.

It is inversely proportiona to R

Q=2 pi freq L/R

If R goes down , for the same L value, the loss goes down and broadband responce is pulled in to a narrower range - more likely to give a greater phase pull from a disturbance.

This is a way to get more sens from a coil - less lossy is more 'touchy'
Steve

Gunna have to chew on this stuff for a while...

Thanks!

[wam]

Hi, the coil is 2 mH TX and 96 mH RX ? - if so it will be very good coil.
If You want more depth with the same battery pawer it will be possible if You will get the same coil but larger size. It will be hard to find coil like this among "western" detectors.
Detectors are about TX 1mH, RX up to 40mH. Check VLF frequency for Your detector, China makes usualy 6.5 kHz detectors. Asian detectors with 30 cm coil or 40 cm coil are expensive and not available in the west. If You want to make coil by Yourself it will be a lot of work and time. I believe that You are not electronics man but for some one who now how, it would be possible to change circuit so depth may be 5 - 10 cm more for coins the most. If You want to go above this it will need a total rework for detector and a lot time - if unhappy - sell it, top up with some $ and have fun with next detector.
Regards,

[porkluvr]

For a given coil diameter, supply voltage, and transmit frequency (assumes sinusoid), a lower inductance increases the transmit field strength.

http://hyperphysics.phy-astr.gsu.edu.../solenoid.html

Carl, have you lost your marbles? Now, it bothers me that Beach would ask such a broad question as he did, not so much as hinting whether he was talking PI or IB detector - or maybe it does not matter so much... but your answer has me absolutely flummoxed.

What you say seems so obviously wrong; it goes against intuition and every thing I have ever read on the subject.

It even contradicts what you wrote in the Hammerhead article (although I do think you got that wrong, there being no square factor):

"...field strength is proportional to the square of the number of turns, so doubling the number of turns results is four times the field strength".

I believe that should read PROPORTIONAL, and not proportional to the square... but now you say it is proportional to the inverse?

I know you were talking PI in the article and here (it appears) we are talking IB, but I cannot see how to reconcilable these statements and I am baffled. What gives?

Are you possibly mixing up flux density with magnetic field strength, or have I lost my mind?

Please explain this so I can go stand on my head and eat my peanut butter.

[WM6]

Hi, the coil is 2 mH TX and 96 mH RX ? - if so it will be very good coil.
If You want more depth with the same battery pawer it will be possible if You will get the same coil but larger size. It will be hard to find coil like this among "western" detectors.
Detectors are about TX 1mH, RX up to 40mH. Check VLF frequency for Your detector, China makes usualy 6.5 kHz detectors. Asian detectors with 30 cm coil or 40 cm coil are expensive and not available in the west. If You want to make coil by Yourself it will be a lot of work and time. I believe that You are not electronics man but for some one who now how, it would be possible to change circuit so depth may be 5 - 10 cm more for coins the most. If You want to go above this it will need a total rework for detector and a lot time - if unhappy - sell it, top up with some $ and have fun with next detector.
Regards,

Good advice wam.
By building a coil he can lost mostly his time, but learn huge new things about metal detector. So, worth to try.

[Carl-NC]

It even contradicts what you wrote in the Hammerhead article (although I do think you got that wrong, there being no square factor):

"...field strength is proportional to the square of the number of turns, so doubling the number of turns results is four times the field strength".

I believe that should read PROPORTIONAL, and not proportional to the square... but now you say it is proportional to the inverse?

You're right PL, the HH article is incorrect, and should say "proportional." Field strength is proportional to N*I, or ampere-turns. For a constant current, more turns = more field strength = more depth.

But let's consider a sinusoidal VLF detector. For a given frequency the current through a coil is



Field strength is proportional to N*i(t) so it is proportional to N/L, given a fixed frequency, assuming v(t) doesn't change, and ignoring parasitic R's and C's. L is proportional to so therefore field strength is proportional to 1/N.

Remember, this is for the case of a fixed v(t) drive so that i(t) varies with inductance, which is the case for many VLF detectors. If I can hold i(t) constant then field strength would be proportional to N, but I would probably quickly run out of v(t) headroom.

PI detectors can be radically different. If peak current Ip is limited by the parasitic Rs (usually not the case in VLF) then increasing N increases Rs proportionally and current will fall proportional to N. Field strength is N*Ip so it would remain constant. This assumes that coil resistance dominates, which may not be the case.

But peak current could also be limited by the exponential which has an L/Rs time constant. In this case, field strength could go up slightly. But all this ignores parasitic C and the need to adjust the damping R, and how all this affects the switch-off speed. I have not worked out all the math, but obviously a too-high L will result in a flyback that can't settle in time. It's possible that there is a sweet-spot L range that works nicely (like 200-400uH) and above or below that range things get worse. Or it may be that super-low-L PI loops would perform better but put too much stress on the power supply circuitry.

- Carl

[Beach]

Hi, the coil is 2 mH TX and 96 mH RX ? - if so it will be very good coil.
If You want more depth with the same battery pawer it will be possible if You will get the same coil but larger size. It will be hard to find coil like this among "western" detectors.
Detectors are about TX 1mH, RX up to 40mH. Check VLF frequency for Your detector, China makes usualy 6.5 kHz detectors. Asian detectors with 30 cm coil or 40 cm coil are expensive and not available in the west. If You want to make coil by Yourself it will be a lot of work and time. I believe that You are not electronics man but for some one who now how, it would be possible to change circuit so depth may be 5 - 10 cm more for coins the most. If You want to go above this it will need a total rework for detector and a lot time - if unhappy - sell it, top up with some $ and have fun with next detector.
Regards,

Wam, thanks for the input.

If You want more depth with the same battery pawer it will be possible if You will get the same coil but larger size.

Sorry for not be more specific, I don't really need more depth, what I need is more width. My little detector can detect a dime (USA) at 14 inches in the dry sands where I do most of my hunting. But, my coil only has the width of about 6 inches for small targets, maybe less. This means that I would need to take baby steps of no more then 5 inches per sweep to cover all the area in front of me. Walking so slowly would never allow me to cover the area I need to make my time worth while. So your idea about the same coil only bigger is the direction I am going.

Thanks Again!

[Beach]

...Snip... Now, it bothers me that Beach would ask such a broad question as he did, not so much as hinting whether he was talking PI or IB detector - or maybe it does not matter so much...snip.

Sorry about that it is just that I am kind of new to all this.

But thank you in help clarifying some of the mixup.

[Beach]

Snip...

If R goes down , for the same L value, the loss goes down and broadband responce is pulled in to a narrower range - more likely to give a greater phase pull from a disturbance.

This is a way to get more sens from a coil - less lossy is more 'touchy'...snip

OK, so what I am hearing is that if I make a coil that has the same L, but has less r due to say thicker copper gauge wire in the winding, I could get a more percise better reacting coil?

Thanks again!

[porkluvr]

Sorry about that it is just that I am kind of new to all this.

But thank you in help clarifying some of the mixup.

Forgive me for being grumpy - understood you're new here.

Not trying to pin this on you - but one of my pet peeve here, is somebody taking about one minute to pen a broad question that begs a multitude of lengthy replies. And, then the poster will kick back and let everybody else thrash out the details of the question.

Something like this would bother me: "Ahhh, the universe! Any thoughts on the matter?"
(One possible answer: "The Moon! They say it's made of green cheese.")

In all reality it may be fortunate that you did frame a broad question because it looks like any good answer may have to transcend limitations which my feeble mind would like to impose. My math and physics aptitude is not the greatest, and this is a messy subject.

edit: Again, it may have seemed like it but I'm not trying to kick you for your question.

[Qiaozhi]

[satdaveuk]