DD or OO Coil Overlap Calculation

[der_fisherman]

I asked who uses High Frequencies, 50KHZ is low man, low.........like VERY low......

[Skippy]

!

[Carl-NC]

I asked who uses High Frequencies, 50KHZ is low man, low.........like VERY low......

Induction metal detectors have historically peaked out at about 100kHz (older BFOs and TRs). Current Fisher GoldBug 2 runs at 70kHz. Higher frequencies (500MHz and into the GHz range) are used by GPR.

[simonbaker]

At high frequencies capacitive coupling will move the null point around. A clever designer can compensate for this. I've designed a coil which holds a pretty deep null from 1kHz to 50kHz, without compensation.

I wonder if capacitive coupling should always be ignored, and we always want the "magnetic" null, regardless of other coupling.

This would make an interesting procedure for nulling coils -- always do it at a very low frequency.

This relates to my speculation that the residual null voltage has magnetic and non-magnetic components (coupling), and we only care about the magnetic component when ground-balancing -- which we accomplish without realizing it by using ferrite or pumping the head over the ground and adjusting the GB sync pulse until the dynamic effect disappears. I conjecture that the ground or ferrite doesn't affect capacitively coupled components of the null signal much -- but need practical tests to see if this has any basis at all.

-SB

[simonbaker]

I think simple linear scaling up/down of a given searchcoil dimensions is all you need to do. Changes in operating frequency, number of turns, wire gauge etc should matter very little.

Yes -- 32 /410 = Your Overlap / Your OD.

Your Overlap = Your OD * 32 / 410

Your Overlap = .08 * Your OD

[der_fisherman]

Induction metal detectors have historically peaked out at about 100kHz (older BFOs and TRs). Current Fisher GoldBug 2 runs at 70kHz. Higher frequencies (500MHz and into the GHz range) are used by GPR.

I am sorry to pick on you or disagree in any way with you, but you are mistaken, none of the frequencies you quote are "High". Special metal detectors may use "high-er" than your average MD frequency, as maybe older ones, and types that rarely produce good results (BFOs for example), but none of this is high frequency......some may "creep" into the medium wave radio frequencies, just! That is still not "high", its medium at best.

If you look here, you will find a great discussion of the long wave band in which area some detectors work:-

http://en.wikipedia.org/wiki/Longwave

In Europe, the long wave is considered to be between 30–300 kHz.

PIs, for example, are very variable in frequency, but are often so low, they are in the Audio range, look here for the Audio range:-

http://en.wikipedia.org/wiki/Audio_frequency

Simply because battery life is improved when using a low(er) frequency, plus you get a larger gap between cycles, allowing more time to analyze the captured signal and even allowing some PIs to discriminate (I am told!) quite reasonably. Usually with some sort of µprocessor.

Audio is up to about 16KHz or so.....which ties in well with many MDs range of frequencies.

The µprocessor used may have a processor clock in the MHz range for this purpose.....but still not for the search coil, only for data processing...

For radio work high frequency would be say still be considered to be between 1 and 50 MHz and are still called "high" and have been for many years....though technology today has progressed even further in radio communications....

There is a good Wiki about this here:-

http://en.wikipedia.org/wiki/High_frequency

But today, a high frequency could be considered to be in the Gigaherz range technically speaking.

I personally have never heard of a metal detector using such frequencies in even the MHz range, let alone in the Giga- range, but that does not mean they are never used, maybe for some specialist usage with maybe looking for tiny metallic particles (guessing only there!) in food, but they don't fit in with our hobby of metal detecting.

We are a "low, low frequency" hobby branch.......even at "our" highest point !!!

I hope that you understand better the relationship between various frequencies today and what is low and what is high.....we must not lose sight of the "big picture" when just looking at our hobby specs....

I wish you a great day/weekend and hope that we can conclude this discussion here and now. It was very interesting while it lasted.

Regards

Andy

[Qiaozhi]

I am sorry to pick on you or disagree in any way with you, but you are mistaken, none of the frequencies you quote are "High".

Andy - I think you're being somewhat pedantic with your reply.

When Carl mentioned "high frequencies" in his original response, ...

At high frequencies capacitive coupling will move the null point around. A clever designer can compensate for this. I've designed a coil which holds a pretty deep null from 1kHz to 50kHz, without compensation.

... he was speaking in relative terms. He meant that 50kHz is a high frequency for a metal detector, not that 50kHz is a high frequency per se.

[der_fisherman]

Andy - I think you're being somewhat pedantic with your reply.

When Carl mentioned "high frequencies" in his original response, ...

... he was speaking in relative terms. He meant that 50kHz is a high frequency for a metal detector, not that 50kHz is a high frequency per se.

AH!

Thanks for clearing that up, I misunderstood the direction he was trying to take, My bad.

He could have easily written it exactly as you did, which precludes any misunderstanding very well.....
regards
Andy

[Carl-NC]

Yup, frequencies are relative. "High frequency" means entirely different things to submarine communications, induction metal detectors, CB radios, cell phones, terahertz imagers, and fiber optics. We're talking metal detectors here.

[der_fisherman]

Yup, frequencies are relative. "High frequency" means entirely different things to submarine communications, induction metal detectors, CB radios, cell phones, terahertz imagers, and fiber optics. We're talking metal detectors here.

Correct, but if you had posted in an exact manner in the first place, we would have understood that immediately.....

Accuracy cannot be calculated in $$$$$$$$'s

regards

Andy