Differences in coil Inductance?

[JC1]

Mr. Q

From this point of view it will be safe to state that the sensitivity of any IB metal detector will depend mostly on the electro-
magnetic properties of the search head loops. The most important of those properties is the Q, quality factor.
This article will not get any further in to the academic aspect of the Q ,but wish to tell that the Q is the main of all factors which
could influence the performance of any search head or single loop.
The Q is of equal importance when PI metal detectors are concerned.

http://www.nexusdetectors.com/Science1.html

Still would like to know how sweeping the

ground is going to change the Q by a factor of 5.

[JC1]

I worry about Q

On the weekend we tested some coils at the test site and found that a NF 20 inch coil would not work at all on a SD2000 clocked at 1.25 Mhz but sort of worked at 2Mhz but it was on the noisy side. The best coil on the SD2000 was 14 inch experimental NF coil with a Q of 5.2 and inductance of 340uH at 0.35 ohm. This coil was smooth and sensitive on the SD2000 but on a 3000 or 4000 it was so noisy it could not be used. Tested out the 3ft coil and can ping an aluminium test target at 4ft. One thing I noticed is that Litze wire makes a coil noisy in some designs, tinned multi always makes a quiet coil in most designs. To control the Q of the coil try different Faraday shield spacings.

We should not worry about coil Q too much as a 5 in air is most likely a 1 when sweeping the ground.

A PI detector utilising search head consisting of one loop, works (in a few words) in the following way;
The loop in the search head is used to transmit high voltage (few hundred volts) pulses. After each transmitted pulse the
electronic circuit measures the residual signal left in the loop after the pulse. That residual signal will decay in certain manner.
The most important characteristic of the residual signal is the time constant of the decay.
If a metal target is placed in front of the loop the time constant of the decay will change as a result of the same Eddy currents
described in the case with IB detectors. In this case the Eddy currents will have pulse characteristic as the transmitted signals.
However the basic principal stands, meaning that the metal target placed in some proximity to the loop will alter to some degree
the electro-magnetic properties of the loop, which will cause different decay in the residual signal in the loop.
Again, the most important of all characteristics of the mentioned loop is the Q.

The Q factor will define the capability of any loop (coil) to transmit or receive any signals.

The Q of any loop can be maximized in only one way; by using a capacitor with a calculated value, which will form with the loop a
resonance circuit at the desired operating frequency.
A loop tuned in resonance will always have the highest Q and best performance characteristics, compared with any other loop,
which is not tuned.
A loop can also be partially tuned, meaning that the capacitor value in the resonance circuit will be different than the calculated for
total resonance. In such a case the Q will still be better than a loop not tuned at all, but not as good as a loop tuned in total
resonance.

http://www.nexusdetectors.com/Science2.html

[JC1]

The POWER of Nexus

Might as well put up all three pages.

It is not possible for any kind of loop to achieve higher Q than the Q as a result of electro-magnetic resonance.
In comparison to a standard, not tuned in resonance search head system, one tuned in total resonance can exhibit up to 100 times
higher sensitivity to any desired target. That fact alone lead to use of lower electronic amplification, lower electronic instability and
much better resistance to thermal changes for the electronic circuit.
A RX loop tuned in total resonance will act also as 10 th order band pass filter against any external interference, which will
almost eliminate the need of any interference prevention filters.
In comparison to off resonance RX loop, one tuned in total resonance is much more sensitive to phase and amplitude changes,
which fact can guarantee best depth penetration and discrimination accuracy as can be achieved with any metal detector.

http://www.nexusdetectors.com/Science3.html

[IBGold]

Pin Outs

Joecoin the pin outs are for SD and GP detectors dont know about others I have not played with others. Regards Ian.

[Woody.au]

I went a bit overboard when I stated a drop from 5 to 1 , at least it threw up some good debate,

Most ground will not have much of an effect on coil Q but our highly mineralized ground in Australia sure does. I took a 300uh coil .4 ohm Q in air of 4.95 I was using a test frequency of 1 Khz. Q droped 4.8 on normal soil but when I put the coil over ground loaded with Ironstone the Q dropped to 3.72 so on heavy mineralized ground the Q dropped around 25%. There is even worse ground over here that is so loaded with iron that it is impossible to use any type of metal detector. One group did some measurements on some nasty ground and it had the same effect as holding a 10" spanner 4" from the coil.

[Qiaozhi]

I went a bit overboard when I stated a drop from 5 to 1 , at least it threw up some good debate,

Most ground will not have much of an effect on coil Q but our highly mineralized ground in Australia sure does. I took a 300uh coil .4 ohm Q in air of 4.95 I was using a test frequency of 1 Khz. Q droped 4.8 on normal soil but when I put the coil over ground loaded with Ironstone the Q dropped to 3.72 so on heavy mineralized ground the Q dropped around 25%. There is even worse ground over here that is so loaded with iron that it is impossible to use any type of metal detector. One group did some measurements on some nasty ground and it had the same effect as holding a 10" spanner 4" from the coil.

This is exactly what you would expect from theory. The permeability of the mineralized soil would cause a decrease in inductance of the coil. From we can arrange to give: , and becomes 225uH.
Actually the calculates as 4.71, so you may have a small measurement error. If this is reduced by 25% by mineralized soil, then this becomes 3.53.
Make sense?

[JC1]

Thanks

Hi Woody,

Ok, I feel much better now, thought for a moment that

the world had changed without someone sending me

a memo. By the way, another problem I have is that I

have thought all these years that IRON increases

inductance of coils, which in this case would increase the

Q of the detector coil. Or is it backwards for the

southern hemisphere? 10 inch spanner 4 inches away?

That is a hard one to get around !!!

The fourth physical factor is the type of core material used with the coil. Figure 2-8 shows two coils: Coil (A) with an air core, and coil (B) with a soft-iron core. The magnetic core of coil (B) is a better path for magnetic lines of force than is the nonmagnetic core of coil (A). The soft-iron magnetic core's high permeability has less reluctance to the magnetic flux, resulting in more magnetic lines of force. This increase in the magnetic lines of force increases the number of lines of force cutting each loop of the coil, thus increasing the inductance of the coil. It should now be apparent that the inductance of a coil increases directly as the permeability of the core material increases.

http://www.tpub.com/neets/book2/2b.htm

[Qiaozhi]

This is exactly what you would expect from theory. The permeability of the mineralized soil would cause a decrease in inductance of the coil. From we can arrange to give: , and becomes 225uH.
Actually the calculates as 4.71, so you may have a small measurement error. If this is reduced by 25% by mineralized soil, then this becomes 3.53.
Make sense?

No - doesn't make sense. Dammit - I wrote decrease when I meant to write increase. Don't you just hate it when that happens?

In order to explain this apparent anomaly, we probably need to take self-resonance effects into account.

[JC1]

Post It Up

Ok,

I'll post this three places, everybody read it that wants

to know stuff about coils and Q. Probably on the forum

somewhere/sometime before but here it is again.

George Payne on fixed (preset) vs adjustable GB
and coil design

http://jb-ms.com/Baron/gb.htm

[Woody.au]

We all know that poking some iron or ferrite type material into or next to a coil will increase the inductance. Our "lossy" mineralization can actually drop the Q of a coil.

We all read text books that tell us what to expect but sometimes we are not dealing with text book examples. I have seen the effect of Q reduction righ in front of me while carrying out tests, so can't argue with "seeing is believing".

Maybe Q meters in the Southern Hemispere read backwards

[JC1]

Could be

We all know that poking some iron or ferrite type material into or next to a coil will increase the inductance. Our "lossy" mineralization can actually drop the Q of a coil.

We all read text books that tell us what to expect but sometimes we are not dealing with text book examples. I have seen the effect of Q reduction righ in front of me while carrying out tests, so can't argue with "seeing is believing".

Maybe Q meters in the Southern Hemispere read backwards

Well now I read the books years ago, but I have quite a

collection of test objects that I have used over the years

and iron does seem to make the inductance increase.

So that much of the book learning seemed to pan out.

So did a whole lot of the rest of it.

Now your Q meter may be southern hemisphere sensitive.

Or your ground may really be something very special.

Anyway, keep up the good research.

GDay.

[Qiaozhi]

We all know that poking some iron or ferrite type material into or next to a coil will increase the inductance. Our "lossy" mineralization can actually drop the Q of a coil.

We all read text books that tell us what to expect but sometimes we are not dealing with text book examples. I have seen the effect of Q reduction righ in front of me while carrying out tests, so can't argue with "seeing is believing".

Maybe Q meters in the Southern Hemispere read backwards

Although the Q of a coil is a measure of its quality, a tuned circuit also has a Q factor. I believe you said you were using a fixed test frequency of 1kHz in your measurement.

If you are already driving the coil at a frequency that is higher than its resonant frequency, then placing an iron target next to the coil with increase its inductance, and consequently lower the resonant frequency. This has the effect of moving the resonant point even further away and therefore reducing the measurement of Q. These meters usually have a means of tuning the coil to its resonant point, rather than using a fixed frequency.

What is the resonant frequency of your search coil?

[garyq]

Gary get a multi-meter **** Smith Q-1559 about $90. will do your resistance and inductance measurements get a coil socket and bridge pins 1 and 5 together and solder a wire to 5 this becomes the common wire solder wires to pins 4 and 2 pin4 is the TX and pin 2 the RX now strip about 10 mm on wire ends solder seperatly now you have a test rig push the soldered ends into the inductance socket as required to test both mono and DD coils of coarse momo only use 4 and5/1 DD use TX 4 and 5/1 and RX 2and 5/1 hope that all makes sense. REgards Ian.

Thanks again Ian for your assistance. I was not sure if the Q-1559 meter was capable of giving a enough accurate readings due to being a 3.5 digit type instead of a 4 or 4.5 digit as “carl” made mention off.

Well not only is Q an important letter in my surname since it starts with a Q it seems it has a more important part to play in a coils performance, as I try to get my head around the various replies here.

Regards,
GaryQ.

[JC1]

Small meters

Hi Qiaozhi,

Alot of the handheld Inductance meters only have

one frequency and that is usually 1 kHz. Actually

some of the "pro" {expensive} meters or bridges

can do various frequencies.

And there are dedicated instruments just for Q.

But some of the handheld

RLC meters actually measure inductance OK but usually

mess up Q if they have that option.

So measure inductance and know the freq of the meter

and calculate Q, then you can test the Q of the meter and

see if it is right, is what I recommend.

[Qiaozhi]

Hi Qiaozhi,

Alot of the handheld Inductance meters only have

one frequency and that is usually 1 kHz. Actually

some of the "pro" {expensive} meters or bridges

can do various frequencies.

And there are dedicated instruments just for Q.

But some of the handheld

RLC meters actually measure inductance OK but usually

mess up Q if they have that option.

So measure inductance and know the freq of the meter

and calculate Q, then you can test the Q of the meter and

see if it is right, is what I recommend.

I don't own a Q meter, but your explanation above is as I suspected.
From what I've read, with the more flexible units, you tune the coil to resonance to get the correct reading. If you were then to place some iron near the coil, you would need to retune, otherwise the reading is incorrect. In fact, by not retuning, the coil will move out of resonance and the Q reading will be reduced. Sound familiar?