3 Coils On Legend / Depth Comparison

[Digalicious]

Compared using SMF in low mineralized ground, high EMI, max sensitivity, a 6" round DD coil, a 9.5x6 DD coil, and an 11" round DD coil.

I started with the 6" coil and buried the coin to a depth in which the 6" just barely gave me a repeatable "dig me" tone, then tried the 9.5x6 and the 11" coil on the coin. The 9.5x6 went about 1" deeper than the 6" round, and the 11" round went about 1.5" deeper than the 9.5x6.

However:

In the EMI with max sensitivity, the 6" was quiet, the 9.5x6 fairly quiet, but the 11" was ear piercing noisy. When I reduced the sensitivity on the 11" so that it was fairly quiet, I could not detect the coin at all. 

What this means, is that with having to significantly reduce the sensitivity when using the 11" coil, both the smaller coils will at least match the depth of the 11", and often exceed the depth of the 11" coil. In addition, with the smaller coils, I get far superior unmasking and separation performance, and I don't have to worry about my arm falling off

EDIT: By saying "going deeper", I meant I could raise the coil by that amount off the ground and still get a good repeatable tone.

[Digalicious]

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.

[Ridge Runner]

What detector are you using? If you said I can’t find it.

 Chuck 

[Digalicious]

10 minutes ago, Ridge Runner said:

What detector are you using? If you said I can’t find it.

 Chuck 

Hi Chuck.

I'm using a Legend. I didn't post what detector I'm currently using, because I think the results would be similar with any detector and its various coils.

[Ridge Runner]

I thank you for letting me know. I have one standing in the corner of the room.

it would be interesting to see if other detectors in the same test would come to the same conclusion as you did with the Legend.

 Chuck 

[PimentoUK]

Quote:"I didn't post what detector I'm using, ... I think the results would be similar with any detector and its various coils"

The machine used IS important. Many modern machines, such as the Equinox / Manticore / Deus / Deus2 ,and more, have electronics inside the coil. The Equinox/Manti have a pre-amplifier, plus a 'security chip' that ( as well as security) identifies itself as a "15cm circular DD" etc. The Deus1 & 2 have half the detector in the coil. As a result, you should not expect them to behave the same as a simpler detector with 'plain' exchangeable coils.
For example:
The Equinox small coil may have a higher gain pre-amplifier. The control box may read the coil ID data, and implement a higher sensitivity or gain , to give an accurate "depth gauge" readout, or to compensate for the inherent lower signal levels produced by a smaller coil.
The Deus doesn't even have to use consistent coil winding inductance/resistance values across the range of sizes, as both the transmit and receive electronics are integrated.

As this thread is about the Legend, I can't say what N-M have done with their coils. There may be X-rays out there ( I'll check "Strick's" thread later ) that give some clues?

[Digalicious]

Pimento,

I could be wrong, but I don't see how a preamp in the coil, or a security chip in the coil, would affect the coil's susceptibility to EMI and ground impedance. More specifically, despite a preamp or security chip, does the fact not remain, that using a larger coil in EMI, means reducing the sensitivity much more so than when using a smaller coil?

[PimentoUK]

How coils pick up a target signal, how they pick up the ground signal, and how they pick up EMI, is a complex issue. And it varies from one machine/manufacturer to another. So it's important to state that this is a test of the Legend. Plus, it's probably wise to state what revision of firmware it's running, as performance could well vary as a result of this ( in the past and for future revisions )

Regarding:"I don't see how a preamp in the coil, or a 'security chip' in the coil, would affect the coil's susceptibility to EMI and ground impedance"

The pre-amp DIRECTLY affects pickup of targets, the ground, and EMI.
So for example if a 5 inch coil has a pre-amp gain of 20 , and they make a 10 inch coil with a gain 14 ( ie. 0.7 x the small coil gain ), then:
The larger coil will likely pick up about 2.5 times the EMI, multiplied by 0.7 = 1.75 times total. So it still picks up more interference, but not as bad as would be expected ( based on typical 'simple' detector behaviour ).
Likewise, the larger coil picks up a stronger ground signal. This is part of the reason the manufacturer may choose to make the pre-amp gain lower.

And on the subject of ground signal pick-up:
Quote:"By saying "going deeper", I meant I could raise the coil by that amount off the ground and still get a good repeatable tone."
This shows up a significant flaw in your tests. If you raise your coil, not only do you reduce the target signal, you also reduce the ground signal. Considering a primary function of the detectors circuitry is to pick out the  ((target)) signal from the ((target plus ground)) signal, changing both target and ground signal is not equivalent to changing just one alone ( by burying the target deeper ).

[Digalicious]

Thanks for that explanation. However, despite a preamp or security chip, does the fact not remain, that using a larger coil in EMI, means reducing the sensitivity much more so than when using a smaller coil?

Also, the raising of the coil was a very slight (slightly lazy lol) flaw. It wasn't a "significant" flaw, as the vast majority of the field below the coil was still in the ground for each coil.

[midalake]

10 hours ago, Digalicious said:

I didn't post what detector I'm currently using, because I think the results would be similar with any detector and its various coils.

44 minutes ago, Digalicious said:

could be wrong, but I don't see how a preamp in the coil, or a security chip in the coil, would affect the coil's susceptibility to EMI and ground impedance.

I think you are wrong, and it is a very narrow focused, test you did. 

In four YEARS of running the Equinox on a daily basis the 11" and 15" were able to run 1-point of discrimination difference depending on the conditions that meant 21 on the 15" and 22 on the 11". also, if conditions were better, I could run 22 and 23 sensitivity.  I am not missing ANYTHING at these levels! 

Your theory would be completely incorrect with a Tarsacci, as the 12" coil could run at the same sensitivity and that coil could clearly detect deeper than the 11X9. 

After this next season of beach hunting, I will be able to confirm the differences of the Deus2 with 9" and 13x11" Coils. I am betting my sensitivity levels will be close! 

I will say as a general rule a larger coil will not be able to run at the same sensitivity as a smaller coil. But small and large coils are also used for different reasons!