Coil Design, Separation, Recovery Speed, Sweep Speed, Unmasking- What Are They?

[Critterhunter]

I figured I'd write a quick explanation of separation, recovery speed, sweep speed, and unmasking ability as I see these features get confused more often than not. I'm guilty myself of say mixing up sweep speed and recovery speed, or recovery speed and separation. In some way they can all be related but only for specific reasons.

First off, separation- To understand separation you should first understand coil designs. Separation is based on how big the detection field is in the ground. A DD coil tends to have better separation at any depth based on it's thin detection line from tip to toe of the coil versus the round cone shaped signal that gets thinner as it gets deeper of a concentric. A concentric can separate targets best at it's very tip of the field where it's the deepest in the ground, but maybe still not as good as a DD. A DD's detection field will be just as useful near the coil as it is at it's deepest part, and it's thin line has various other advantages over a concentrics round "ice cream cone" type signal including seeing less ground minerals and offering less potential risk at missing targets.

The other advantage to a DD is that it's equal in depth from tip to toe where as a concentric is only deepest at it's very center in a small spot. For that reason a concentric the same size as a DD can have just a hair more depth, but only in a small spot at it's very center. Concentrics there for need to have the sweep overlapped a lot in order to not miss deeper targets. If there are any kind of ground minerals the DD will quickly get deeper than the same sized concentric. Even with low ground minerals so that the concentric isn't blending or degrading the target signal with all the ground signal it's also taking in, once a concentric gets past maybe 10 or 11" it may still be shallower than a DD due to the DD's ability to maintain sensitivity to small items even at this size of a coil or larger. As a concentric coil gets bigger than say 11 or 12" it starts losing sensitivity to coin/ring sized targets, while DD coils can get as big as 14 or 15" and still retain good small target sensitivity.

With all the above being said about separation it usually means a small coil will get better separation than a larger one as it's detection field is smaller. Some coils can squeeze the DD detection field by being an odd shaped, such as the 15x12 discussed in the other thread or ovals and eclipses. In those cases coils like this can have a thinner field than a smaller coil but go just as deep as a larger coil. Obviously separation is largely based on signal size and not really based on anything else.

Recovery speed is how fast the machine resets once it goes over a target so that it's ready to see the next one. Some say machines with a fast recovery speed separate targets better than ones with a slow recovery speed. While this can be true to some extent, this reason is more based on the sweep speed of the detector than how fast it recovers over a target so that it can see the next one. Slow down your sweep speed and give the machine time to reset and recovery speed shouldn't be a factor in separation. Ask yourself this, would you rather have a fast recovery speed with a big fat detection field or a slow recovery speed with a thin detection field to "see" between two targets? I'd rather have that, and a DD or especially one of these SEF coils will offer you that over a concentric.

Unmasking ability is the ability to see say a coin that is mixed in with a piece of trash or iron. Separation and recovery speed can be a factor in this "unmasking". If you are using a detector with a fast recovery speed it may in fact be fast enough to reset and "unmask" the two targets separately even if you are swinging the coil fast. However, once the two targets "become one" in that they are laying on top of each other or touching no amount of recovery or sweep speed can compensate. Now it's up to two things- the detection field and the detector's ability to analyze the signal and try to separate the coin from the junk. Certain detection fields may hit certain multiple targets better, allowing the detector to pick up the coin and not have it's field deflected by the junk. If the coil's field can only hit the junk and has no eddy currents that are being allowed to also hit the coin then the detector only sees the trash and not the coin. It doesn't even have a chance to try to decipher the two targets from the signal. Some coils seem to hit certain masked targets or coins on edge better than others because of the dynamics of the fields they are transmitting into the ground. These SEF coils are said to have some kind of morphed hybrid detection field that is mostly a DD but also has some concentric traits. For that reason they may unmask coins that these coils like while your "average" concentric or DD coil seems to have a hard time hitting properly.

The other way a masked target like this may be revealed is by how the detector handles a "mixed" signal. That's another place where the Sovereign shines because Iron Mask ON is specially designed to analyze a signal and draw the good target ID out of it while ignoring the bad. It can either do this by totally disregarding the junk signal, or at least not letting it cause the "averaged" two target signal be drawn too far down the conductivity scale to where you wouldn't recognize the value of the good target that is mixed in with it. I've found that I can get either perfect coin IDs or ones only 3 or 4 digits below a 180 signal even when the junk I used to mask the signal was iron or a low conductivity non-ferrous item such as a pulltab. The Sovereign is better at this than any machine I've used, including the Explorer.

Sweep speed is both what the detector needs to remain stable and also what you should sweep it at to allow the detector time to reset between targets. Sweep a machine too fast or too slow and the detector can no longer keep it's self stable in relation to ground minerals or targets. Most say a slow to medium (but still leaning on the slow side) sweep speed is what the Sovereign prefers. However, most also say that once a deep target is found you should wiggle the coil as short and fast as possible directly over the target to pull the best ID out of it. This isn't unique to the Sovereign. On my QXT Pro, which can handle a very slow to very fast sweep speed while hunting, deep targets still require short/fast sweeps (but not wiggles like the Sovereign) to obtain the best target ID.

 

[Critterhunter]

Another thing that comes to mind that I wanted to cover is how recovery speed can determine depth on a machine. This can be largely controlled in some way by the SAT (self adjusting threshold) control on some machines, but can be offered to some extent in a more direct way on some machines that have fast/deep control settings. SAT adjustment is often useful in very bad ground conditions such as beach hunting when the machine is being bumped around by quickly changing ground minerals, hot rocks, or other things. It's not as crucial on BBS and FBS machines because they ignore the ground signal for the most part and so tend to be less affected.

One might think that the optimum ability would be a fast recovery speed speed, or at least a very fast SAT speed, to quickly reset the machine's threshold after passing over a target, being ready to detect the next one. This can be useful for separating multiple targets, but as said in the above message it doesn't really determine if a machine is good or bad at separation, it mainly determines how fast you can sweep before the machine can no longer process targets separately and identify them as such. A slow sweep speed can make a slow recovery speed a non-issue.

Just as a fast recovery speed can offer some advantage (in the ability to sweep fast without blending targets), a slow recovery speed has it's own advantages, and these are not as easily corrected for on a machine with a fast recovery speed by just altering your sweep speed on them. The advantage a slow recovery speed has is in it's ability to "grab on to" and sound off more distinctly to a target. It gives you time to hear the audio of the target and dissect the "meat" of it with your ears. Some machines with slow recovery speeds still don't offer this longer more drawn out audio like the Sovereign does, though. A machine with a fast recovery speed (no hard and fast rule but just in general) will not offer you as much time to hear the target's traits as the audio raises and subsides with the passing of the coil. As the machine fastly resets the audio tends to do so as well, making the space of time you are allowed to hear the target much shorter.

That's one advantage to a slow recovery speed but still not the major one. Where things get really more apparent is on targets at extreme depth. The slow recovery speed allows the machine to have more time to "see" and analyze the target, producing a more noticeable sound off as well. Where as a machine with a real fast recovery speed might "smooth" over the target much like ground tracking can, or at least offer you less chance to notice the signal as you sweep by. It's like trying to read a sign while passing it at 80 miles an hour versus going 30. Slowing down your sweep speed on a fast recovery machine doesn't as easily offer you these same advantages as slowing down a slow sweep machine does to gain the advantages of a fast recovery machine (time to reset and and thus better "separate" or see multiple targets). A machine with a fast recovery speed that is slowed down in sweep speed to try to obtain those benefits might have a new problem in that target quality degrades with a slower sweep speed, or at the very least the machine might begin to lose depth. Even if the machine allows you to slow down it's sweep speed, the rate of it's recovery (or reset) can not be changed on most machines, so it's still trying to "cut the target short" and be ready for it's next signal.

I'd like to hear both supporting and differing views on any of the above topics. There is still great debate even among seasoned detectorists as to exactly what all these traits or functions do and how they effect each other and performance of a machine, so I'm not saying there isn't room for somebody to change my mind on my conclusions with the right information to back it up.

 

[Nauti]

Brilliant info CH,have you ever thought about writing a book on the subject.....you seem to have a lot of knowledge that would be interesting and beneficial to beginners and experienced detectorists alike.I found your info on recovery speed and how it can degrade a signal quite interesting.A test was carried out over here in England by a well respected detectorist(who owns his own website) on the sovereign gt.He took the gt to a heavily iron infested site which was once the site of Roman occupation and the site was full of iron nails which is typical of these sites.The GT was constantly nulling with virtually no threshold return at all.A site such as this is considered a no go area for machines with a slow recovery speed so the xp goldmax power was also used on the test to compare finds.This machine has the fastest recovery speed of any detector on the market along with the new Deus so in theory it should have performed a lot better than the gt.On this site the gt was used in silent search and in auto sensitivity......basically because in threshold mode the continuous blanking was annoying.After a few hours hunting the finds were compared and it was the gt that just came out on top with a greater number of finds.One of the reasons for this was the fact that the xp user was having some difficulty in identifying all the iron and picking out the good targets suggesting that the ultra fast recovery speed can work against you in some circumstances.
I myself have owned the goldmax and i also own the gt as you know.....i would choose the gt over the goldmax any day as it's a much better machine overall.....ultra fast recovery speed is a bit overated in my experience.
One thing i cannot get my head around is the benefits of one coil configuration over another....dd verses concentric.To be honest if i look back over the finds i have made over the years the concentric coil has been my most useful tool for detecting on inland sites.I know most people suggest the dd has more benefits over a concentric but most of my best finds have been made with a concentric coil on a certain machine.I also have a test bed in my back garden where a hammered coin has been buried at 6 inches in some really bad soil.The soil is contaminated with building sand,cement,bits of brick and small concrete pieces and pieces of iron rubbish.I have tested 4 detectors so far on this target and to date only one can detect the coin.One of the detectors to have failed the test is the gt with both the 10" tornado coil and the joey....the detector that picked up the target,even with the coil some way off the ground,is equiped with a concentric coil.In fact,the only other detector to have even suggested there may be a target in the ground is another with a concentric coil.I am yet to be convinced that searching with a dd is going to get me more finds,what i have experienced so far makes me believe that concentric coils are still a good thing.I think that if all the topics covered here were looked at in a really scientific way,there would be a few surprises coming to the surface and many myths would be busted......up until then i will keep an open mind.Many thanks for the excellent read CH,please keep it up.

 

[Critterhunter]

Will respond later to the above. I need a smoke as I've been on this computer too long. Here's a re-post from the metal detecting forum I posted responding to some mild criticism about coil dynamics...

Of course the article is tailored more to information for Sovereign owners because that's what it was originally intended for. After I wrote it I decided there was enough general information to be of interest to other detector users as well. The fast/slow sweep speed criteria for different machines is also covered in the second message, enough to warn people with fast sweep machines about the problems that may occur if they try to gain the benefits of a slow recovery/sweep machine by slowing theirs down as well. Namely, loss of target quality or depth can result from slowing down a fast sweep and/or recovery machine and even if that's possible the costs may out weigh the benefits. Those advantages are not as easily obtained with a fast machine to mimic a slow one, where as it's very possible to gain the advantages of a fast machine by using a slow sweep/recovery machine properly, mainly by keeping the sweep speed slow enough to allow the unit to reset and "see" between the targets. A lot of people believe recovery speed is what makes separation happen. That's not at all the case. It only allows you to sweep real fast without targets being blended together.

As for the signal dynamics of concentrics and DDs, it is true that many concentrics have more of a half circle or "egg" shaped signal than describing it to be more like an ice cream cone. However, that sort of comparison better illustrates why a concentric can have certain disadvantages to a double D. It still stands that the detection line of a DD coil from tip to toe has more uniform and equal depth across the entire coil length than a concentric. Separation ability of most DDs is also more useful at any depth where as a concentric gets more use of it's field to do this at its deepest part of the detection field. It also is still true that concentrics do not put up as well with ground minerals, costing depth or producing "washed out" or degraded target quality when the minerals present are high enough. A DD's ability to not take in as big of a picture of the ground allows machines to be easier to ground balance as well as decreasing the chance that the more intense ground filtering required to remain stable of a concentric is also costing you target quality or depth. Remember that on most VLF detectors the target's signal must first stand out from the filtered ground signal. Being "drowned" in that matrix more on a concentric means there is more chance for the depth or target signal to degrade.

As I said, there is still great debate among detectorists (including us! ) as to exactly what the benefits of various coils and functions of a machine are, as well as the negatives. I look forward to hearing more differing opinion on these matters to continue the exchange of ideas.

 

[Critterhunter]

Thanks. I wrote a book, well mostly done, for the QXT Pro but never finished it. I'll probably one day finish it and throw it up on the web for free as there is very little info for the QXT out there. It's a very overlooked sleeper in the Whites line up. Up until the Sovereign (& head to head with an Etrac in the field) I hadn't found any machine that went as deep as it on silver/copper coins.

Hmmm...That's an interesting test. Why isn't the Sovereign hitting that coin? There could be several reasons that come to mind. The first and most obvious would be that whatever machine could see the coin might be set at the perfect ground balance to neutralize the soil. That still wouldn't explain why the BBS technology of the Sovereign isn't able to also see through it. Are you turning the Sovereign on with it's coil laying on that ground and then sweeping around for a few minutes so that it has time to adjust it's self to the ground conditions? Then the next most obvious question would be what size is the concentric coil that did well and are you using a similar size DD? Another possibility is that with all that construction material you listed being present in the soil there might be empty pockets of air mixed in with it. If that's the case then that would explain a lot, since the Minelabs do not like disturbed ground or seeing air in the detection field. I know that guys who hunt recently plowed fields prefer other machines because of this.

 

[Nauti]

I think you may have answered the question....it could certainly be the freshly disturbed/air pocket scenario.I've certainly hit coins deeper than that in the field.The concentric was an 8" and the machine balanced very slightly positive.The coils tested on the sov were the 10" tornado and the joey coil.It's a shame though that the gt does not like freshly disturbed land....this is the type of land that yields most of the historic finds over here.

 

[Critterhunter]

You could always run it in All Metal Fixed (after ground balance has been set by tracking or the pump method). I would figure that All Metal won't have any issues with air pockets in disturbed ground. Then just switch to discriminate to check targets. Remember that a Minelab's harder time with air/freshly moved ground is compared to it's optimum performance in normal soil. I bet even with it's less performance in freshly moved ground or soil with air pockets it's still getting deeper than most machines.

Try sweeping the GT around for a bit in discriminate so it can ease into the ground conditions and then check that coin. Give it a minute or two before trying. Turn it on with the coil on the ground. Also try All Metal Fixed and see what that does and PP as well. It'd be interesting to see the results!

Also, a more direct comparison would be to strap an 8" Coinsearch or the new Tornado 8" (or even an S-8, stay away from the old BBS 8") onto the GT and see what that does. It might then hit the coin due to less masking or at the very least taking in less ground matrix signal (and trapped air) with the coin. In highly mineralized or bad ground such as that the smaller the coil the better it will perform, even offering more depth than your typical larger coil. However, I sure would like to see what the 12x10 or 15x12 SEF can do with that coin you got there.

 

[Nauti]

I've got to say i'm tempted by the 12x10......sounds like a good option.

 

[Critterhunter]

If you've already got a 10" then go for the 15x12. You might be let down by the size of the 12x10 compared to the 10". Just too similar in size. Why "duplicate" a coil if you already have one near that size, though I do hear the 12x10 even is deeper than the 10" and the 11" Pro Coil. These SEFs are fantastic coils. I'd really love to hear how one of them do with that coin of yours!

 

[Nauti]

Watch this space

 

[Critterhunter]

Cool!

 

[Critterhunter]

Just bumping this thread to the top due to recent discussions on other threads...

 

[Critterhunter]

Somebody was asking me about a few of these questions on a GT and I figured I'd just bump up this old article I did for them to read.

 

[TURNMASTER]

Interesting read. I prefer the DD coils hear in the Great Northwest. Mostly hunt with the Xterra 705 in Ground Tracking. I really like the 6" DD coil for old home sites. The Concentric that came with the machine did not stay on long, I swapped to the 5x10 DD that I bought with it, better separation better ground handling. I found a bunch more coins in ground that I had already covered. The issue was mostly Iron trash of many types.

So Nauti Nell,
You make an interesting test that should not be overlooked. I think the reason the 8" concentric can see the coin may in fact be due to the diminishing size of the field. Based on an ice cream cone shaped detection field the effective diameter at 6" depth would be in the 3' to 4' range. Depending on the pattern of the random scattering of debris the field size at depth may allow that coil to be interfered with less than a 8 or 10 inch DD coil .

I have thought about spending more time with the concentrics to see if I can coax more coins out them, but every time I put them on I revert back to the DD coils. So little separation, so many nails.

The technical side of this hobby fascinates me. If I could do this for a (good) living I would

HH
Jeff