3 Coils On Legend / Depth Comparison

[Jeff McClendon]

12 hours ago, Digalicious said:

I just wanted to clarify that the 11" coil will detect a coin sized object approximately 3-4" further than the 6" coil, and approximately 2" further than the 9.5x6" coil. BUT, that is in the air (no ground mineralization) and without reducing the sensitivity due to EMI.

To get a quiet and stable detector in EMI, the sensitivity has to be reduced a heck of a lot more with the 11" coil, compared to the smaller coils. When that sensitivity / depth loss due to EMI is taken into account, the 11" loses its depth advantage. On top of that, the depth loss due to ground mineralization should be higher with the larger coil.

I have no problem with your testing, results and conclusions as far as EMI and the Legend is concerned. Reducing coil size is another way to deal with interference from EMI that really works.

As far as ground mineralization........No, I do not agree that depth loss due to ground mineralization should be higher with the larger coil in general. That totally depends on the individual coil, the detector being used, the settings being used and especially the size of the target. That is an entirely different issue.

EMI is a return signal issue. 

Ground mineralization is a transmit and return signal issue.

 

[Digalicious]

Midalake,

Isn't the Tarsacci a SF detector? If so, your point isn't relevant to my test, because SF's are far less susceptible to EMI than SMF.

Regarding the Nox:

Try out the 11" coil and the 6" coil in high EMI. The 11" will be far more noisy and unstable than the 6", resulting in having to reduce the sensitivity significantly on the 11", compared to having to reduce the sensitivity on the 6" either very little, or possibly not at all. Now in the air, and with no sensitivity adjustment for EMI, that 11" coil will detect a coin about 4" deeper than the 6" coil. In the ground though, with the major sensitivity drop on the 11", that 4" depth advantage is often completely negated.



 

[Digalicious]

3 minutes ago, Jeff McClendon said:

I have no problem with your testing, results and conclusions as far as EMI and the Legend is concerned. Reducing coil size is another way to deal with interference from EMI that really works.

As far as ground mineralization........No, I do not agree that depth loss due to ground mineralization should be higher with the larger coil in general. That totally depends on the individual coil, the detector being used, the settings being used and especially the size of the target. That is an entirely different issue.

 

 

 

 

Yes Jeff.

I wasn't so sure about the mineralization aspect. Reason being, the actual percentage loss of depth due to mineralization, should be equal across all coil sizes.

[midalake]

10 minutes ago, Digalicious said:

Try out the 11" coil and the 6" coil in high EMI.

Well my reference to conditions is about EMI. If I can run the Equinox at 21-22 with the 11 and 15" coils, WHY would I ever want to try a 6" coil on a salt beach? 

What if I told you for the block and a half that I have EMI issues on, the Deus2 is unaffected. {Which it is}  What detector would you pick and how would this affect your test? 

This is why it was important to say what detector you were using. It also only confirms what you experience in one location. ONLY!   Not general rules on how all SMF will work. 

As for the Tarsacci, it sounds like it might be the detector of choice for your location? 

[Digalicious]

20 minutes ago, midalake said:

 

What if I told you for the block and a half that I have EMI issues on, the Deus2 is unaffected. {Which it is}   

 

I can't speak to your specific EMI issue, but in the countless videos I've seen, the D2 is just as noisy in high EMI as any other SMF detector. A good example of this, is all of Calabash's videos with the D2. It's so noisy in his EMI, that the commenters often point out how noisy it is (again, just like the other SMF detectors he tests). I don't even know why he keeps doing the noise cancel when he's testing the SMF detectors, because every time he does it, it does nothing. 

 

20 minutes ago, midalake said:

 

As for the Tarsacci, it sounds like it might be the detector of choice for your location? 

I use SMF almost exclusively, because I find that it has more accurate ID's than SF. I imagine that has something to do with SMF being able to obtain more varied information from the target. As such, I would rather use SMF in high EMI with a smaller coil, than use a larger coil with a SF.

[PimentoUK]

Quote:"The raising of the coil ... wasn't a "significant" flaw, as the vast majority of the field below the coil was still in the ground for each coil."

That sounds like a very hand-waving guesstimated assumption to me. Not backed up by any technical measurements taken on a real detector, or any ( difficult ) mathematical modelling of search-coil behaviour.

Like anything technical related to detector operation, the truth is never straightforwards. Ground signal does drop off as the coil is raised .... but not anywhere as severe as the target signal drops off. And when a coil is very close to the ground, the ground signal behaves rather unexpectedly, and can actually 'plateau' or fall in level. But ... the phase angle of the ground signal also varies too: raise the coil 20 inches, you'll get a huge phase shift from 'ferrite-like' to 'salt-like'. It's really the "wobble" in ground signal that causes trouble to a detector. Some perfect uniform flat ground, and a perfectly swept coil, would give nearer air-depth results.

If you're comparing different size coils, are you comparing them at the same height? Would it be more correct to compare a 10 inch coil at 1 inch height, to a 5 inch coil at 0.5 inch height, for example? Or do you keep a fixed 1 inch, as that's how you would use a coil in real world hunting?

It's also worth noting that EMI pickup is coil height dependant. It seems that the ground provides some degree of shielding, perhaps it depends on whether the EMI is vertically or horizontally  polarized. 

[Digalicious]

7 minutes ago, PimentoUK said:

 

It's also worth noting that EMI pickup is coil height dependant. It seems that the ground provides some degree of shielding, perhaps it depends on whether the EMI is vertically or horizontally  polarized. 

Agreed that the ground itself does provide a good amount of shielding.

7 minutes ago, PimentoUK said:

Quote:"The raising of the coil ... wasn't a "significant" flaw, as the vast majority of the field below the coil was still in the ground for each coil."

That sounds like a very hand-waving guesstimated assumption to me.

It is, but only to a very small degree, because the ground was encompassing most of the field with each coil tested. As such, that's not nearly enough of a difference to negate my results, or disprove my main point.

[mn90403]

A coil scraper type of detectorist may get different results because there would be less air between the coil and ground.  Maybe the target needs to be reburied to get true depth difference in your test?  If you are waving above a target, then it would show how 'forgiving' a coil would be if your swing was 'off' and still find the target.

This in part is the old statistics argument.  If you give me a set of results then I can prove multiple conclusions.

Where do you hold your coils when you noise cancel? 

[Digalicious]

Mn,

When I do such testing, I always move the coil in a way to give it the best chance at hitting the target. More specifically, I move the coil both vertically and horizontally to try and hit the target.

Also, my conclusion is based in mathematics and empirical evidence. That is, the more the sensitivity has to be reduced, the more the depth loss...and when sensitivity has to be reduced significantly with a large coil due to EMI, it's usually better to use a smaller coil at or near maximum sensitivity. 

[PimentoUK]

Quote:"It is [an assumption], but only to a very small degree, because the ground was encompassing most of the field with each coil tested."

I've no idea what "encompassing" is defined as, I would assume even with the coil pressed on the dirt, it would only encompass half of the field, the half below the coil.

But anyhow, I've located some ground pickup measurements I did on my Fisher F75 11" x 7" DD coil. This behaves much like a 7" coil ( long, narrow coils tend to have their characteristics dominated by the 'narrow' dimension ).

Assuming your 'standard' coil height is 1 inch, and you state the 11 inch coil was raised 2.5 inches above this, to 3.5 inches:
I'll convert that to 25mm and 87mm metric.
My measurements show a ground signal level of:
25mm ... 7.6 mV
87mm ... 2.8mV
that is a drop to 37% of the signal at 25mm height.
I would expect an 11 inch coil to be less affected proportionally by such height changes than a 7 inch one, so perhaps a 45% to 50% drop would be an estimate? This then permits a target signal 45/50% weaker to be picked out. This roughly equates to 15% more target depth ( ie coil to target depth ).