Using Ferrite To Increase Frequency Of A Coil/Detector?

[Ham]

Has anybody experimented with ferrite material on their coils? I'm aware that placing ferrite material near or in the middle of a coil will raise the inductance. If i were to place a piece of ferrite material on or above a metal detector coil could i raise the frequency of the coil/detector? I'm also aware that brass material placed on or near a coil will lower the inductance.

[Skippy]

Short answer: No, avoid sticking lumps of metal anywhere near a coil, except for a few specific reasons.
It might help if you told us more info. VLF machine, I presume? Any specific model? Concentric coil? DD coil? BFO maybe?

[Ham]

Using a tesoro cebola with a concentric coil. For experimental purposes i want to raise the frequency about 2 khz.

[Skippy]

Definitely NO then. I thought you might be wanting less than 1% change ( eg. to avoid a frequency clash with another identical machine). Changing the operating freq of a machine is a major project.

[Silver Dollar]

Ferrite is a bad idea as it increases your null residual signal.

You could use a smaller cap for C20 in this schematic then switch
back to the same value for stock frequency.



The original has a freq tweaker switch which could be reconfigured to
change the frequency further away.

The RX is series resonant so I don't think you need to change anything there...

[Ham]

I don't want to make component changes. I'm going to leave well enough alone. Thank you.

[golfnut]

Its not too practical, but u could swap the coil over to change freq

[Ham]

What i really wanted to do is convert my tesoro compadre or cibola to a frequency of or near 16.5 khz. This is the frequency that "George Payne" claims that nickels & gold jewelry correspond to best when using a metal detector. Just trying to fine tune my detectors if that's possible.

[Skippy]

Fine-tuning is not necessary, your machine will detect US 5 cent "nickel" coins without any problem, just as it detects US quarter dollar coins easily, even though they are "best-detected" at 2KHz-ish frequencies, a significant way from what your detector operates at.

[Carl-NC]

Ham, you won't see a bit of difference with the Compadre at 16.5k instead of 12k, and even less difference with the Cibola at 16.5k instead of 14.5k. If you really want to prove it to yourself, I think the transmitter on the Compadre is free-running (I don't recall if it is on the Cibola) so you can easily increase the frequency by reducing the tank cap. Don't ask me which cap or by how much, you'll have to trace out the TX circuit and figure it out cause I don't know.

Edit: I see Silver Dollar provided you with the right info above.

[Ham]

I don't agree or disagree with what you gentlemen say about frequency. Here is more information pertaining to the subject... http://compass-metal-detector-forum....td7591388.html

[Ham]

I am also aware that the majority of metal detector enthusiasts have found gold jewelry with detectors whose frequency are far from 16.5 khz! I just wonder "if" having a detector with a frequency of or near 16.5 khz is an advantage specifically for gold jewelry?

[Skippy]

Where to start...firstly, the 16.5KHz figure is for a US 5c "nickel" coin, NOT for "gold jewellery", the "best detected" frequency for gold finger rings will cover a range of 30KHz (small ladies) to 5KHz (large mans). I presume you were not looking for other gold jewellery, like chains, ear-rings? If you were, you might want to look at buying a higher operating freq machine, like a Fisher Goldbug2 (72KHz).
Secondly, depending on how the specific machine in question achieves discrimination has an effect on your choice of machine frequency. It is often said that the machine needs to operate at a noticeably higher frequency tha the target, eg. a 20KHz machine would find 16KHz targets easiest. I suspect that this doesn't apply to modern DSP-based machines( like the Teknetics T2).
Then there's the "16.5" figure. You are making assumptions of precision here, that are unfounded. It's not some finely-tuned characteristic, it's about as flat and untuned as there is. Just try measuring the corner-frequency of a "nickel" - it's easy to come up with a figure in the 16/17 area, but as for any more accuracy, it's tough. [Not to mention that buried cupro-nickel coins corrode and the freq changes, usually higher]
It's generally reported that womens finger rings are more common losses than mens, so having a machine that operates in the 12K - 25K would be a better choice than, say a 4.8KHz Fisher 1266 etc, but any more than that is not going to help.
This finger ring chart (thanks to IvanLL) might help enlighten you:
http://nqminersden.com/wp-content/up...uctivities.pdf

[Ham]

Skippy......My "assumptions" were based on what George Payne wrote concerning 16.5 khz= 5c/gold jewelery. I will admit that his findings are approximate & not an exact figure. As you stated there are other factors involved. Thank you & the others for your input.

[Carl-NC]

Skippy's right... 50kHz for small thin chains, 5kHz for large fat rings, and everything in between. And no one has mentioned the effects of ground mineralization vs frequency. Again, if you want to find out for yourself, play with C20.