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Offline lucky_devilTopic starter
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« on: August 27, 2014, 04:25:50 pm »
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  I just built a SM Pro Pi kit I bought off ebay. It has great sensitivity but prone to mineralisation, as with most pi's. I modded it to use DD coil. Couple questions. Is it necessary to shield the TX coil since it only transmits?  Also, how to determine damping resistor value. According to the mod a 1k 1/4 watt for the RX coil is ok. The TX needs to be at least 2 watts and vary's in value. I have a pc scope but not familiar with how to use it or were to probe the board. Any how to's around to do this?
   Thanks

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« Reply #1 on: August 27, 2014, 06:05:16 pm »
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   I would shield both.   Damp Both.   On geotech there is a procedure to damp DD coils.  They do one side with the other side loaded with a known resistor then do the other side.

Use the signal from the Amplifier out put to ground.   Be sure to limit signal  or toast your mic input. 



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« Reply #2 on: August 29, 2014, 10:04:53 am »
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Just use a 15 Meg resistor on your probe just to be sure. As for shielding the TX coil there is only one reason why I could think that it's better to shield it and that would be to maintain the same inductance change in both RX and TX coils (shielding will cause a slight change in inductance) other than that a waist of time in my opinion.

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« Reply #3 on: August 29, 2014, 10:13:56 am »
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I wouldn't say it is a complete waste of time.  Depending on how sound you want your system to be.  Shielding prevents a host of things, crosstalk, interferences, bleedovers, etc. etc.  But I come from a pretty strong RF background and you can't ever have enough shielding and grounding.  I would do it just to make sure.  Can't ever go wrong with it.  Even if it does seem a bit un-necessary.  IMHO

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« Reply #4 on: August 31, 2014, 06:12:44 pm »
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Shielding electronics in a EMI proof box is one thing, but when it comes to VLF or PI coils the correct techniques have to be used,
so you dont excessively  attenuate the tiny target signals we are dealing with.

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« Reply #5 on: September 02, 2014, 05:00:21 am »
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Yes but would you say that one needs to shield the TX coil? I for one can think of one good reason only, to keep the inductance change uniform (in a VLF) but completely useless in a PI if it has a TX and RX coil.

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« Reply #6 on: September 02, 2014, 08:41:43 am »
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hello you lucky devil

did you think you can achieve an advantage to use a tx/rx coil for the simple surf pi?

what kind of advantage is it?

wouldnt it be 10 times easier do make a fast and shielded monoloop coil for the surf?

but back to your question i would shield tx and rx side but dont forget to make spacers between the coil core and the shield

i have done some new videos last weekend, it is not surf pi but look, an i can also make a spiral coil that works a lot better for small pieces of gold

which you much luck and i hope you keep us up to date and stay here at side of us

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« Reply #7 on: September 02, 2014, 10:45:56 am »
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   Air Test means nothing with a target in the ground.    Minerals do funny things.  When Wet they can help you. When Dry they hide things.

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Offline lucky_devilTopic starter
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« Reply #8 on: September 08, 2014, 10:21:37 pm »
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Quote from: xavier link=topic=57644.msg281029#msg281029

date=1409655621
Yes but would you say that one needs to shield the TX

coil? I for one can think of one good reason only, to keep the

inductance change uniform (in a VLF) but completely useless in a PI if

it has a TX and RX coil.



  That's exactly what I was thinking. And the reason I asked.  I've tried and tested a few variations of DoubleD configurations and the best I can tell the RX & TX inductance  doesn't appear to be critical if they're uniform or not.  Based on the coil specs I seen somewhere the Surf Pro came with a 230uh coil.     I also read that  the less windings produced higher (current ? may not be the correct term ).   And as  far as RX, the more windings seem to produce better results.  So my thinking is to use this formula , shield the RX only.  And, the fact that when you overlap two coils with all the spacers and shielding it produces something that is a bit large for a standard housing.






Quote:Posted by bernte_one
hello you lucky devil

did you think you can achieve an advantage to use a tx/rx coil for the simple surf pi?

what kind of advantage is it?

wouldnt it be 10 times easier do make a fast and shielded monoloop coil for the surf?



bernte,
    The reason for the DoubleD is because mineralization here is a big problem inland where I mainly search. The SM Pro works great with mono at swimming areas.
  I have a fist size "hot" rock that I use to test with. By changing to the DD its like adding Ground balance.  
      I have a fast , shielded mono coil allready. But the DD isn't hard to make either. The balancing isn't critical like on a VLF or IB.  And by separating the RX and TX I gain full control over the pulse delay without the need to make a Fast coil.
 


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« Last Edit: September 08, 2014, 10:30:37 pm by lucky_devil »
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Offline Ridge Runner
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« Reply #9 on: September 08, 2014, 10:36:06 pm »
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Quote:Posted by lucky_devil
 I just built a SM Pro Pi kit I bought off ebay. It has great sensitivity but prone to mineralisation, as with most pi's. I modded it to use DD coil. Couple questions. Is it necessary to shield the TX coil since it only transmits?  Also, how to determine damping resistor value. According to the mod a 1k 1/4 watt for the RX coil is ok. The TX needs to be at least 2 watts and vary's in value. I have a pc scope but not familiar with how to use it or were to probe the board. Any how to's around to do this?
   Thanks


Mate I think you have that back to front, Mineralisation does not worry PI Machines that's the whole Idea of them, In fact a PI machine can usually reach down more than twice their AIR TEST distance set up right, Or have I misread your Post,

AU

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« Last Edit: September 08, 2014, 10:38:05 pm by Ridge Runner »
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