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JSPI: using transmitter as discriminator

Hi Robert,
First of all nice pictures and great effort. I guess the part I'm not understanding is the large reactive ferrous signal, when the coil current has barely got moving. Is this really true?
JC
 
Damn it, I wish I were clever enough to make plots like that. The fact is that I never was much of a math type. The waveforms agree very well with what I saw and what Eric just posted, Dave. * * *
 
Forgot to finish the deal for those of you may not have caught on. In the smaller diameter wire, the eddy currents will die faster and you won't see them so much. Neat huh? Now nobody can use Litz wire in their detector unless they can think of another reason.
JC
 
Yeah, I know that's what he claims. But there a couple flaws in the ointment.
1. Whenever I've used Litz, it was for the reason the manufacturers of Litz say to use it-- to reduce AC losses. If, in the process, the other benefit Candy claims also accrues, that's just a freebie. Candy made a discovery, not an invention.
2. I have never seen any evidence actually testing coils in black sand, that Litz offers any improvement in clean ground balance, over plain magnet wire. (Yeah, it surprised me too.)
I know two additional reasons to use Litz. They're discoveries, not inventions.
--Dave J.
 
In general, receiver resistance has no effect on sensitivity. However, if you get more than a couple hundred ohms, it can impair the preamp noise figure due to thermal resistance noise, which, I'm proud to say, is also called
"JOHNSON NOISE"!!
--Dave J.
 
Hi Dave,
Hey I'm not defending nothing, just trying to shed some light (and lighter) for those who were sounding like they had no idea what the deal was, and that Candy was patenting Litz. I think some German named Litz did that.
So what are the other reasons for using this fine, but pricey, heavy, harder to terminate wire?
JC
 
I've had the best luck with DD coils of the simple coils. Don't know why the metal detector community took so long to catch on to this.
I wouldn't want any of my friends to see me nowadays with some kinda stack coil thing.
Actually better is some form of two recieve coils wound or hooked up common mode to cancel far field effects, but these are also ackward.
JC
 
JC
Yes, the reactive signal is proportional to the rate of change of the coil current. The fact that the coil current is still near zero does not mater. It is the slope of the current that maters.
Robert
 
I see the transmitter spikes, but do you have any idea what the current during the on period looks like. Did it rise quickly, during the time of the spikes, or the normal slow L/R profile over the 350 us.?
JC
 
Hi JC
Tinned hook up seems to just as well as Litz wire
Regards Frank Wallis
 
Hi JC
Could explain your thoughts a bit more on two receiver coils
Regards Frank Wallis
 
I would have to agree Frank, - and god knows Ive made a bunch of prototypes. In fact they do seem more stable and sharper on response, and the reason for that I dont understand fully.
Keep tinkering. john
 
Like I've mentioned before I have one coil which is 52 inch diameter coil which has about 300 feet of wire in it. I see a difference. For small coils where there isn't much wire, can't see how it would so much. But then whats going to change, in other words what would look for? Is the fidelity of the decay correct? Gee this would get involved.
Dave Johnson mentioned that is has been used in md world in BC (before Candy), why?
But then the audio guys get excited with things like monster wire and seem to think the increase in surface area helps the tunes at audio freq. Oxygen free copper is another one. Just tell em, if you want the music to sound better, drink another beer.:)
But the Candy thing makes some possible sense. And I have heard on the forums that they actually use some litz wire(part of what you pay for), but don't know if it helps or not. Since they don't talk about using it in adds, seems kinda strange they would use something that costs more that doesn't do anything.
JC
 
Hi John,
Do you know of anything which can be done which will improve the performance over the commerical coils that come with the machines.
JC
 
Hi Frank,
Eric has talked about this before, and has a post here recently mentioning a three coil setup.
In roughly one plane, you take two recieve coils and set them side by side. Then wire them up so they cancel each other. Then you can put the transmit coil (third coil) so it goes all the way around both them, or just half way across both, or whatever, just so long as it covers both the same.
Now when the coil is energized the field induced in both cancels (null) and the coils cancel any external fields (external noise) that are far away.
So how do you ever see anything? Well it a target is placed under one receive coil it will see it and the other one won't. So you get a signal. the problem is the other coil will see it some and you will lose some sensitivity over not doing this.
The good news is in noisy environments this may be you only choice, or if you are testing on a bench, but now you got three coils and the whole thing is getting big, and heavy, etc.
Hope you could follow this, there are a bunch of ways to do this, the above is just one.
JC
 
You can do noise cancelling with two coils, or even just one. I used a two coil setup on an agricultural detector that was mounted on the front of a forage harvester. The bottom coil was 5ft x18in and was TX/RX1, which did the detecting. Rigidly mounted 18in above was an identical coil that was RX2. The two signals were subtracted at the front end amplifier. Power line noise, low frequency rf and movement in the earth's field were effectively cancelled. You get a bit of cancellation of the signal from large deep objects, but not much, and because of the low noise you can wind up the gain.
The other single coil way is to use a rectangular figure of eight coil. the field pattern is such that it doesn't go quite so deep for a given size coil but it is less cumbersome.
Eric.
 
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