The Legend And Emi

[CTidwell]

I was under the impression that EMI was a relatively constant thing.  As in, if it's there, it's constant (more or less).

Reason I bring this up is I was at a site that had Hella EMI with a large telecommunications tower and power station about 100 yards away.  The Legend actually performed well even at full power (sens 30), but what I found odd is that while swinging the coil, most of the EMI was not detected.  Just the occasional pop or beep.  As soon as I stopped swinging, however (to dig a target or something), the EMI would show up in all it's glory!  Happened on Multi Freq as well as all single freqs.  Even after noise cancelling.  Is that normal to be virtually gone while swinging the coil then slam the detector as soon as it's stationary?  Or is this something the Legend is able to do in order to combat EMI?
 

[Digalicious]

In my EMI tests, I've also found that EMI noise was reduced significantly once the coil is in motion on the ground. The amount of reduction was based on the strength of the EMI. What I also found out, is that the EMI noise had little or nothing to do with the orientation of the coil. For example, with the coil off the ground, the EMI noise remained constant with the coil stationed vertically or horizontally.

I first thought that the ground noise was masking the EMI noise. On the other hand, I've got extremely mild ground, so I don't really have ground noise to begin with. As such, my next best guess was that the EMI noise was significantly reduced because with the coil on the ground, EMI can only affect the half of the electromagnetic field that is above the coil, but the EMI can't penetrate the ground to affect the other half of the field that is under the coil. If that is true, I need an "Auto Mute" feature when my detector is horizontal 😁
 

[GB_Amateur]

5 hours ago, CTidwell said:

I was under the impression that EMI was a relatively constant thing.  As in, if it's there, it's constant (more or less).

Not in general.  (There are many forms of EMI.)

5 hours ago, CTidwell said:

The Legend actually performed well even at full power (sens 30), but what I found odd is that while swinging the coil, most of the EMI was not detected.  Just the occasional pop or beep.  As soon as I stopped swinging, however (to dig a target or something), the EMI would show up in all it's glory!  Happened on Multi Freq as well as all single freqs.  Even after noise cancelling.  Is that normal to be virtually gone while swinging the coil then slam the detector as soon as it's stationary?  Or is this something the Legend is able to do in order to combat EMI?

That's typical of every IB/VLF detector I've owned, including the ones without any kind of EMI mitigation other than the user manually turning down the sensitvity.  I thinks it's a signal-to-noise issue.  If EMI is bad enough even swinging the coil doesn't help.  But for less intense EMI, when swinging along the ground's surface, the natural mineralization 'signal' can overpower the EMI.  And a strong target overpowers both the EMI and the ground noise.

If you remember tuning a radio, a weak radio station will be deciperable but at the same time you'll hear scratchy noise.  And away from any detectable channel you just hear the noise.  OTOH a strong station will come in loud and clear with no noticeable background noise.

I suspect a detector engineer can give info on the electronics properties of the various internal components and how they are configured to take the max advantage of the strongest signal. Maybe one of them will comment.

[Jeff McClendon]

I am definitely not a detector specialist or engineer.

This is my take on the OP's post. If I am mostly wrong just let me know. I like to learn too.

The Legend is a very high gain detector. Add to that, turning up the sensitivity to at or near maximum. When the coil is horizontal on or very near the ground the Legend and any other VLF, will detect any changes in the electromagnetic field of metallic targets of many sizes and depths that it stimulates in the ground. Having the coil well off the ground or perpendicular to it......a VLF is still trying to detect any changes in the electromagnetic fields that it stimulates. Correct me if I am wrong, but I'm guessing that the transmit part of the detecting process doesn't stimulate EMI to any great degree. The receive part however will definitely pick up any EMI interference that is strong enough to be received.

At least from my experience, most airborne EMI registers as a very deep target on the VLF detectors I own that have a depth gauge. In other words, detectors that can detect EMI through the receive part of the process see it as a very small target. The hotter they are at detecting very small targets, the more EMI can potentially be an issue. Reducing sensitivity can help.

Yes, VLF detectors can pickup EMI through the top of the coil too.

EMI coming from the ground from buried power lines and from buried wireless systems for sprinklers, water lines, sewer lines, etc. are the worst sources as far as being the hardest to deal with from my experience.

I also find that being close enough to large power lines to pick up EMI is more of a problem at an angle to the power lines than it is directly under them at least on single frequency detectors. I need to test that more with SMF VLFs.

[Digalicious]

1 hour ago, Jeff McClendon said:

At least from my experience, most airborne EMI registers as a very deep target on the VLF detectors I own that have a depth gauge. 

I also find that being close enough to large power lines to pick up EMI is more of a problem at an angle to the power lines than it is directly under them at least on single frequency detectors. I need to test that more with SMF VLFs.

Regarding your first paragraph:

That's strange, because my experience has been the opposite. On two SMF detectors and one SF detector, EMI doesn't even register on the depth meters. Granted, I'll occasionally see EMI bury the depth meter, but for the most part, it doesn't budge it.

Regarding your second paragraph:

When I first started investigating EMI, I did come across a couple of posts that people did experience less EMI from power lines when at a 45 degree away from them. In my tests, I did experience some noise reduction at 45 degrees, but it wasn't enough of a reduction to confidently conclude that it was a result of the 45, or simply a matter of moving further away from the EMI source.

When you mentioned single frequencies, I realized that when I did the coil orientation test and found no noise difference, I didn't try the same test in SF. Anyway, I just got back in from my backyard from such a test. I was right under 3 power lines testing the SF modes. The coil remained at about 3ft above the ground and was stationed vertically. I then pivoted the detector up, so that the coil was horizontal. Low and behold, there was a significant reduction in EMI that did not occur in the same test with SMF.

What I get from all of this, is that the relationship between metal detectors and EMI is very complex due to all the variables. Variables such as the size and type of coil, detector frequency, type of EMI, intensity of EMI, and possibly even whether or not the power lines are stacked vertically or spread out horizontally. Add to all that those pesky harmonics, and it's not hard to see how experiences can differ greatly based on those variables.



 

[CTidwell]

A lot of great information shared here.  Very interesting.  Thanks to everyone for their input.  Seems that there really is no simple answer. hahaha

[JCR]

Jeff’s post is spot on from my experience.

Increasing the Audio Gain will also cause any EMI to be more prominent. Faster Recovery settings also will.

I prefer changing the frequency to dropping Sensitivity below 22 or so, but it is a good thing to know that even 16 is surprisingly deep.

[CTidwell]

Have a look at this.
 

 

[JCR]

That's a good one.  I like his Channel & watch it on a regular basis.

[Digalicious]

In the video, Liam is using the LG30 (12x9) coil, which has similar depth to the stock 11" round coil.

The small, paper thin, hammered coin at 8" is producing a solid hit with a good solid TID. How is that even possible with such a low (12) sensitivity setting?

In my air tests, going from the maximum sensitivity of 30, down to 16, results in a depth drop of 4". Yet, he's going down to 12 on the sensitivity and still solidly hits that tiny coin at 8"? What the heck am I missing here?