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Offline xavierTopic starter
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« on: July 26, 2012, 02:38:28 pm »
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I guys

Well I made a DD coil respected all the instructions got the inductance very close to what it should have been then dosed it with super glue and let it stand for two days as I had other things to do. I then proceeded to shield the two coils but when I measured the inductance it was no where near the required inductance of 6mH and 6.5mH I now have 7.2mH and 7.1mH. What has gone wrong?

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Xavier   

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Offline Homefire
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« Reply #1 on: July 26, 2012, 03:46:02 pm »
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How did you sheild it?

The Henry will change with the over lap for sure.

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Offline GoldDigger1950
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« Reply #2 on: July 26, 2012, 08:09:49 pm »
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Homie is correct. The shielding is the culprit but don't worry. It's self correcting in an AC circuit. The DC measurement is misleading.

Fire away, Captain!

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Offline xavierTopic starter
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« Reply #3 on: July 27, 2012, 11:23:41 am »
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FEUUu thought that I had messed it up real bad LOL. Well all I have to do is wait for the kit to arrive then put it together. Ho one more thing do I have to null the coil with the working frequency or can I use any frequency? I think that I will have to use the working frequency.     

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« Reply #4 on: July 27, 2012, 12:03:46 pm »
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Working it is.

Self Resonance is bester but you can force it to do it's thing.

As long a both transmit and receive shifted together It is self Correcting.

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Offline xavierTopic starter
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« Reply #5 on: July 27, 2012, 12:57:29 pm »
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OK so if I use a frequency generator and I set it to 4MHz put that on the TX and then place the scope on RX then move the RX coil till its at a null will that be OK?  Or is there something else that I will discover? LOL it happens all the time 

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« Reply #6 on: July 27, 2012, 01:35:26 pm »
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4Mhz?Huh?Huh?       Hell it's not that high is it?

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Offline xavierTopic starter
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« Reply #7 on: July 27, 2012, 01:39:01 pm »
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SOoooory I meant 4Khz LOL I do mess up don't I

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Offline GoldDigger1950
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« Reply #8 on: July 27, 2012, 04:40:56 pm »
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Quote:Posted by homefire
Self Resonance is best

No, no, no, no, no!!! Absolutely not.

Resonance yields zero output. What you need is maximum AC resistance which is effectively ANTI-resonance. For an LC circuit with an external antenna, resonance is critical to deliver full power to the antenna - which is NOT in resonance at all. For an LC circuit where the antenna (the coil) IS the antenna, a field must be built up, not made to be zero. Resonance in an LC circuit means all of the energy in the circuit bounces back and forth between the capacitor and the coil making any emissions a null or zero.

This basic fact is an absolute requirement for a field to build up around the coil. This is also why it is so horribly difficult to do calculations in coil design. You have to de-tune the tank circuit (the LC circuit) to get a radiating field from the coil. The estimated de-tuning should be 3db down from optimum to start.

You asked about a Faraday shield and its effect. In a resonance circuit that has a Faraday shield outside the coil, that would have a military grade of shielding applied. In a metal detector, you want the coil's field to penetrate the Faraday shield to the maximum that it can. What the shield does is keep stray energy from being seen as a signal to the receiver. Receiver gain is in the order of 100,000 so a stray signal could blow it out in a heartbeat.

This metal detector design business is very complicated as you may already have guessed by now, Xavier and Homefire.

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It's all about that moment when metal that hasn't seen the light of day for generations frees itself from the soil and presents itself to me.
Let's Talk Treasure!
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« Reply #9 on: July 27, 2012, 05:02:22 pm »
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Information is a good thing. Cool

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