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Offline Super Pierrot
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« Reply #820 on: May 13, 2018, 02:56:59 pm »
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I will try graphite shield on a 5 feet frame. It's interesting to test.. Wink

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Offline Super Pierrot
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« Reply #821 on: May 16, 2018, 01:43:34 pm »
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Hello everybody.
Since I work on the EMI/RFI problem, I start to discover som realities.
Experts from this forum can correct my summary to avoid broadcasting
wrong informations. Idiot

It is impossible to shield a coil against low frequencies magnetic fields
like them from power lines (50/60Hz)
I didn't try the graphite spray shielding for reducing ground effect.
It should be useless for detecting big iron objects with a frame coil.
I should can operate with a very long delay before sampling so the
ground signal would decay much faster than a big iron object.
I think about lowering the coil resistor value from actual 6 ohms
to 3ohms (I need to adjust coil turns and wire section to observe a
good tuning).
The decay time to 0v should still be faster than a long sampling delay.
I could tune the damping resistor value to adjust.
The idea is improving strength of magnetic field emitted by the coil
to receive ŕ stronger field from the object. So I could reduce sensitivity
behind the chattering level from power lines and keep a good detecting
ability for big iron objects.
What about the theory? Thank you. Wink

Ps: still hard to undertand the tuning from each potentiometer on the delta pulse.
can somebody confirm or join a link?
Noise= pulse width?
Signal= delay?
Sens?
Gain?
 Pray







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Offline Emanuel Pires
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« Reply #822 on: May 16, 2018, 09:38:12 pm »
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Hi, what version of delta pulse do you have? You can send some photo

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« Reply #823 on: May 16, 2018, 11:12:36 pm »
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Hi. it's the Delta Pulse 2 from the download section

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« Reply #824 on: May 17, 2018, 07:20:35 am »
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It is impossible to shield a coil against low frequencies magnetic fields like them from power lines (50/60Hz)
--

seems you like ask same one quetion much times.
so my answer again - NOT. you can not shield a coil from EM field be cause the Nature of EMF of your coil and EMF from power lines IS SAME.
your coil is THE receiver antenna for EMF from power lines.

what you can do... 1. decrease a size of coil, the power of TX, the sensitivity. 2. tune frequency on sure 100 Hz (or 120Hz) to decrease the interference to minimum.
but you CAN NOT to delete the interference to zero and CAN NOT to delete EMF itself. 3. blackout the power line in your seeking places.  

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! No longer available

 Shocked

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« Last Edit: May 17, 2018, 07:23:30 am by NE555 »
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Offline Super Pierrot
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« Reply #825 on: May 17, 2018, 02:28:18 pm »
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I dont like asking the same question.
Please accept it's not my first language. I was misunderstanding EMFI and ESFI acronyms.
Technical langage is far way more difficult to understand than: "Where is Brian? Brian is in the kitchen"
Thank you for understanding. Merci pour votre compréhension.

Do you know why Eric Fosters says:

"The important thing was to avoid the same frequency as power lines, 50Hz in the UK, 60Hz in the US. Multiples of the power frequencies also had to be avoided e.g. 100 or 120. Severe beat note interference would result if the detector and power frequencies coincided; that is if you were anywhere near power lines as is often the case in the UK."





Posted on: May 17, 2018, 12:44:11 pm
"blackout the power line in your seeking places"
LOL. I thought about it but the line is coming from a nuclear power plant Grin Grin


Posted on: May 17, 2018, 02:01:25 pm
As you say, tuning frequency on 100Hz (or even 50Hz I think) should be efficient. Lets imagine a power line emitting 50Hz magnetic field and delta pulse tuned to 50 Hz frequency.
If the delta pulse is sampling the signal while the power line magnetic field is at the bottom of the wave, it should't be any interference.

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« Reply #826 on: May 17, 2018, 06:01:00 pm »
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To receive the signal at the bottom of the wave you would have to synchronize the detector signal to the power line signal. The detector frequency may drift with temp, etc. This would require the detector to have a high stability oscillator and phase lock loop frequency control. If not synced then you would hear the difference between the 2 which would be off frequency to the detector receiver causing degraded sensitivity. Easiest thing is to just stay away from the power grid frequencies and harmonics.

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« Reply #827 on: May 17, 2018, 08:25:22 pm »
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Mudflap is right, as is Eric Foster. Something is being lost in translation to what NE555 is saying. Stay the hell away from the resonant frequencies and you will be fine. The main priority is to do your testing and setup in an interference free environment, so as not to be distracted or confused by the resulting performance changes.
When you go out in the field where there is interference, you will then be very aware something is wrong and may need to be remedied.

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« Reply #828 on: May 17, 2018, 11:30:53 pm »
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 AKa Harmonics ! Cheesy

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« Reply #829 on: May 17, 2018, 11:35:45 pm »
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Thank you guys, it's nice from you sharring your knowledge Wink Wink
I think everybody is right and meaby there are two ways to work against EMFI.
As you say Mudflap, for tuning on 100Hz without drifting it requires a very hight quality and stability circuit.
The most important to progress is understanding. Smiley

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