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Hi ,Carl, your idea of building a BFO with two coils to compensate ground mineralization reasembles me a DD coils used in TR detectors. What are you going to achieve? Two generators in direct proximity will surely influence each other (causing even lock ups).I think BFO methods have no future. The inductance changes are considerably smaller that resistance changes in the search coil. The drift is unavoidable also. You won't turn back the clock.As far as the one turn coil is concerned, I'd like to point out that the TM808 and similar two-box MDs have such coils, but there are ferrite transformers on it. By the way, my most intersting finds I made with a self-made BFO. But it was in baygone days.For now,Piotr
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Re: two-coil BFOs
Piotr, The main reason I built the BFO was for the learning experience, and at the time I did not own one. I built the detector for a fraction of the cost of a commercial unit, and it works as well. I agree, they probably are outdated, but it is still enjoyable experimenting with the concept.
The plans for the two box detector, a transmit/receive citcuit, calls for multiple turn coils, I may try single turn coils like I used for the BFO. I also plan to make it capable of operating at various frequencies. Part of the reason for building it myself is for the enjoyment and the other part is the cost savings. Tom G -
Re: two-coil BFOs
>Hi ,Carl, your idea of building a BFO with two coils to compensate ground mineralization reasembles me a DD coils used in TR detectors. What are you going to achieve? Two generators in direct proximity will surely influence each other (causing even lock ups).I think BFO methods have no future. The inductance changes are considerably smaller that resistance changes in the search coil. The drift is unavoidable also. You won't turn back the clock.As far as the one turn coil is concerned, I'd like to point out that the TM808 and similar two-box MDs have such coils, but there are ferrite transformers on it. By the way, my most intersting finds I made with a self-made BFO. But it was in baygone days.For now,Piotr
I absolutely agree that there is no real future for BFOs, but there are still some interesting things you can do with them (design-wise), and like Tom says below, it's a great learning experience. Plus, I want to have one or two good BFO designs for the web page so others can start with something really simple.
You are also right that a two-coil BFO will suffer from frequency lockup unless you run one oscillator at a higher (harmonic) frequency. I think it is also possible to use this setup to compensate for drift, not sure.
As far as single turn coils go, my thought is that the negatives outweigh the positives. Yes, it does give better ground capacitance rejection but a good Faraday shield will do this, too. Plus, a single turn coil creates a much weaker magnetic field than a multi-turn coil, thus you have to drive more current through it to compensate which wastes power. Not so sure you even need to worry about ground capacitance with a TR-type detector, frequency drift is not an issue with these is it?
- CarlComment
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Re: two-coil BFOs
Hi Tom and Carl,The two box I mentioned has multiturn coils in fact but is it is wound on a small ferrite core coupled with a metal frame (which is secondary winding of the transformer) so the magnetic filed is the same as on a big multiturn coil wound directly on a frame.This is quite good idea (I was not aware of it until recently). PiotrComment
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Re: Two box
>Hi Tom and Carl,The two box I mentioned has multiturn coils in fact but is it is wound on a small ferrite core coupled with a metal frame (which is secondary winding of the transformer) so the magnetic filed is the same as on a big multiturn coil wound directly on a frame.This is quite good idea (I was not aware of it until recently). Piotr
I think I need a sketch of this to clarify. Would seem like the metal frame would generate eddy currents and destroy the field.
- CarlComment
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Re: Two box
Carl,
Sorry, I cannot provide you with additional info at this moment.
PiotrComment
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Carl, BFO question
Carl,
Mineralized soil was the downfall of my BFO back in the 70's. I found I could compensate a little by increasing the 'motorboat' RPM's. Why did this improve usability? Was it just that sensitivity was decreased or was something else going on? JJComment
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Re: Carl, BFO question
>Carl,
>Mineralized soil was the downfall of my BFO back in the 70's. I found I could compensate a little by increasing the 'motorboat' RPM's. Why did this improve usability? Was it just that sensitivity was decreased or was something else going on? JJ
Decreased sensitivity, yes. When you are motorboating down at a 10s of Hertz then a few Hz change from mineralization is very noticeable. Increase the motorboating and that same few Hz change becomes less noticeable. So are faint targets.
- CarlComment
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Re: Slow motorboating
>>Carl,
>>Mineralized soil was the downfall of my BFO back in the 70's. I found I could compensate a little by increasing the 'motorboat' RPM's. Why did this improve usability? Was it just that sensitivity was decreased or was something else going on? JJ
>Decreased sensitivity, yes. When you are motorboating down at a 10s of Hertz then a few Hz change from mineralization is very noticeable. Increase the motorboating and that same few Hz change becomes less noticeable. So are faint targets.
>- Carl
My best find ever with a D-Tex BFO unit, was when I had it adjusted down to really slow motorboating. I could just about count the "ticks", it was so slow. The ground wasn't too mineralized, and I was hunting real slow, in a very old city square park. I got what I thought was a slight increase in motorboating, and began to investigate. Turned out to be a nickel 3-cent piece, about 4 inches down. Nice find, but of course being the nickel variety, rather than the silver type, it was brown and of not much real collector value.
Most areas, of course, were too mineralized to operate at that slow a motorboating speed.Comment
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Re: Slow motorboating
Sam,
Do you still have your old D-tex? What model and year did you aquire it? I just aquired in a trade one of the old Fisher T-10's. I think it's from 1961...the one with the wooden coil. Old, but that baby is in perfect unused condition, not a scratch anywhere even on the bottom of the coil. Now if I can only get a 51 volt battery to power it up. Jungle Jim.Comment
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Re: Slow motorboating
It shouldn't be much problem to build a 12V to 51 Volt converter/power supply. Do you know how much current it draws? How much space is there for the battery?
>Now if I can only get a 51 volt battery to power it up. Jungle Jim.Comment
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