Emi Revisit

[kac]

EMI effects the coil more than the control box and cable unless those are unshielded.  The coil cable usually has an emi shielding much like a cable tv coax cable has. The coil is the most vulnerable because it's signal is sent not only towards the ground but on top of the coil. This why larger coils are more prone to interference.

The other contributing factor is the frequency of the machine and the frequency of the power grid, wifi, cell towers etc. nearby. It is all about harmonics.

I'm not completely sure but think pi machines are more sensitive to emi than a vlf (ib).

[CPT_GhostLight]

4 hours ago, midalake said:

 

 

I guess the reason is:  Has anyone tried an EMI Bag over their detector or one of the multiple EMI reducing products on the coil?

I'm guessing that you are detecting an area with unusually high EMI. The best weapons against EMI are reducing your sensitivity and noise cancelling. Also a smaller coil is less susceptible to EMI than a larger coil. Covering the Control box and cable with a Faraday cage type bag may offer some reduction of EMI, but covering the coil with it would reduce or disable your coil's ability to function.  

[nordic]

Perhaps a silly question, do MDs have to comply with any EMI restrictions? Any reason they emit similar amount of power? Looking at the EQX, it seems like it is working at about 70% of what power components could possibly handle.

[midalake]

5 hours ago, CPT_GhostLight said:

but covering the coil with it would reduce or disable your coil's ability to function.  

What about a layer of EMI coating on the coil except for the part facing down? 

How would this effect coil performance?

[GB_Amateur]

5 hours ago, phrunt said:

Coil designs usually include some shielded paper (like Minelab Commander coils) or a shielding paint on the coils housing and how many layers of it they've done....

Great, detailed writeup, Simon.  I think you pretty much 'shielded' (err, covered) the entire subject.

I think the primpary purpose of the conductive coating on the inside of the coil housing is to minimize/eliminate electrostatic buildup affecting the performance, e.g. from the coil's housing rubbing vegetation, etc.  When I modified the 150 (mm) coil for my White's TDI/SPP which included replacing the bottom half of the enclosure I had to paint it with conductive (graphite loaded) paint.  It's been a while but I think I added coats of the paint until it reached a certain resistance/resistivity.  I assume that is what they do during coil manufacture except they shortcut my process by knowing how many coats (how much thickness) are required to attain that R/rho.

 

[mn90403]

12 minutes ago, phrunt said:

toil foil

?? 😊

[GB_Amateur]

16 minutes ago, phrunt said:

A little experiment, tape tinfoil over the top of your coil....

Do you really mean tin or do you Kiwis (and/or Aussies) consider that a synonym for aluminum?  😉  If the layer is thin it could flex (e.g. in the wind) which is going to mess with the performance.  You can buy copper sheet here in the USA from companies like McMaster Carr (heck, maybe even Amazon) in thicknesses that will be remain more stationary than store-bought aluminum foil.

You make it sound like you've tried this.  In another recent thread (also see this one) it was touched upon whether altering the field above the coil would help getting under a metal fence.  (In fact is was your thread!)  If your proposed experiment really works is it a solution for the under-the-fence problem?

[kac]

6 hours ago, midalake said:

What about a layer of EMI coating on the coil except for the part facing down? 

How would this effect coil performance?

I experimented with creating an EMI barrier on my Multi Kruzer stock coil. I had some hot stamp foil that I applied a spray adhesive to the back then transfered the micro thin aluminum based metal on the perimeter of the coil.

It did reduce the emi effects and did increase id accurace as the machine was able to stabilize and not have a flutter as it tried to lock in on a number.

The performance results were interesting. Using gen mode 99 gain with a silver dime I tested the distance (air test) to where the numbers would fall off.

Without shielding at 5khz, 14khz, 19khz

TID's 24-68, 39-70, 64-80

Depth 6.75, 6.5, 6

With shielding

TID's 58-70, 60-81, 73-86

Depth 7.5, 6.25, 5.5

Proportionally the lower frequency performs how it should and is deeper than the higher frequencies. You may ask why the numbers jump so much and it is because I was at the fall off on the machine in all metal mode in a high emi environment. In the field the machine did perform much better.

So my stupid theory is the top side of the coil is more of an issue in emi than coil cable, connector or control box housing. Bigger coils act like bigger antenaes and are more prone to emi. EMI in general especially on machines with high gain and have less filtering and/or poor shielding in general will be unstable. Add in the factor of harmonics and trying to find that frequency that just fits in between can be impossible in some cases and to me is just a band aid to the problem and doesn't solve it.

Recently I purchased a Superfly coil which is 12" wide vs the stock 7" wide and it actually has less EMI interferance but belive that it may have better shielding or more shielding than a stock coil. This isn't necessarily a flaw with the stock coil but rather it doesn't take in account the extreme conditions that are in some areas.

If you guys want to fiddle and experiment try maybe just a single copper wire and simply tape it on top so it does not wiggle or move. Any movement will cause the machine to go out of balance and sound off.

Curious what others come up with.

[midalake]

2 minutes ago, kac said:

If you guys want to fiddle and experiment try maybe just a single copper wire and simply tape it on top so it does not wiggle or move. Any movement will cause the machine to go out of balance and sound off.

Curious what others come up with.

What are your thoughts on EMI liquid products that are non-metallic?? 

Graphite is one? Others? 

[kac]

I'm sure they work and thought of getting some to paint the inside of my Tejon control box but the emi on that machine isn't too bad compared to the high gain Kruzer. They make a silver paint too but it is really pricey, bottle might cost as much as an average coil.

I think this helps disrupt the emi but feel if it could be grounded it would work better. Maybe some experts can chime in.