M9 Versus M8 First Thoughts

[Skull diver]

Although I will not have a chance to test this coil on the seabed until next Monday, an initial test in the air has me very curious...

A few days ago, a user on YouTube complained of considerable noise, very similar to the M11 at the beach with even lower sensitivity, so much so that it could not exceed 20 points.

To my surprise, what I noticed instead is that the footprint of this coil does not seem to respond as strongly as on the 8x5".

Let me explain, apparently, the lateral response is much less pronounced and sensitive, compared to the obvious center part between the two rx and tx parts.

It would appear to be something like a concentric plate, rather than a double-D technically speaking.

With fully open pattern and lower separation speed, the differences are practically zero.

Talking about sensitivity, 22 as last option before falsing yet in the air.

The question certainly arises at this point about effective in depth coverage, rather than mere surface coverage.

It is very likely that the difference in depth is almost zero and that the greater amplitude beyond the elliptical perimeter of the M8 is marginal.

We shall see...

[Chase Goldman]

To me on a recent beach run, it works similar to the 9" on my XP D2.  Did not notice a lack of depth sensitivity, edge sensitivity, or increase in noise susceptibility.  Curious how it performs for you when its submerged.

[Skull diver]

2 hours ago, Chase Goldman said:

To me on a recent beach run, it works similar to the 9" on my XP D2.  Did not notice a lack of depth sensitivity, edge sensitivity, or increase in noise susceptibility.  Curious how it performs for you when its submerged.

In fact, regarding depth in comparison with the M8 it was predictable that there was not much difference.

I found the same sensitivity value with which I use the M8 with no particular interference, but as far as the edge sensitivity is concerned, I see clearly less responsive the M9, probably because of the shape of the coil.

When I was digging with the Deus2 with the 9", I had basically the same depth but the M8 to date is something amazing to dig up really fine and sometimes broken material...

[Redneck]

The 9" coils footprint covers almost double the amount of square inches of soil vs the 5 x 8.  So more ground coverage, also more ground mineral exposure.  Tom Dankowski likes the 5x 8 better that the 9" on the beach. 

[Chase Goldman]

2 hours ago, Redneck said:

The 9" coils footprint covers almost double the amount of square inches of soil vs the 5 x 8.  So more ground coverage, also more ground mineral exposure.  Tom Dankowski likes the 5x 8 better that the 9" on the beach. 

Agree with the above although not sure why ground mineral exposure matters to Dankowski swinging the Manticore on his 0 mineralized sugar sand beaches (salt compensation is fairly easy to manage independent of coil footprint).

Just to clarify the difference between "Ground Coverage" (aka Footprint) and "Swing Coverage" because these terms often get confused as well as how those attributes roughly affect coil performance.

As Redneck said, ground coverage (or footprint) does affect the amount of ground the coil sees and also helps determine relative depth performance.  Bigger footprint generally means greater depth (so theoretically the M9 should be slightly deeper than the M8) but it will also suffer a greater performance decrement in highly mineralized soil and will be slightly less sensitive to micro targets.

The comparative swing coverage of the two coils is 8" for the DD 5.5×8 M8 and 9" for the 9" round DD M9. So the M9 only has an inch of swing coverage advantage vs the M8 (the width of the side-to-side swath of coverage made by the target sensitive center section of the DD coil).

FWIW

[Redneck]

Chase, I can crank up  sensitivity higher on the smaller coil, which makes the detected depth closer to the 9" coil.  

[Chase Goldman]

Good point.  Another general advantage of coils with a smaller footprint but non-issue when submerged in water.

[Skull diver]

On 6/12/2024 at 5:40 PM, Redneck said:

The 9" coils footprint covers almost double the amount of square inches of soil vs the 5 x 8

At first glance, it appears that M9 actually contains two M8s within its perimeter.
I hope I am wrong, but it appears to me that this lateral width looks more like more physical bulk dictated by the shape than actual coverage even deep on the perimeter.
It is a concept I am struggling to explain, but it seems that on the M8 the magnetic field is expanded like a ball around the coil while on the M9 it is much more concentrated in the middle and poorer on the perimeter in terms of sensitivity.

[Dug D]

3 hours ago, Skull diver said:

but it seems that on the M8 the magnetic field is expanded like a ball around the coil

I think I have noticed this while using the M8, detection at the outside edge of the coil, noticed most at the tip and tail.