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Dan, you've touched on several different topics. Let's see if I can help: CAUTION .. THIS ONE is LENGTHY..

September 13, 2020 08:04AM avatar
Search coil operation height, ground mineralization, target-masking, target separation, and handling ferrous and non-ferrous targets and their effect on the EMF are each different topics. Unfortunately, several of them can all be related to a single task, and that can complicate the matter of figuring out what to use, how to use it, and what settings are needed. as these are each different topics that come into play.

After reading several comments about how proper coil height is vital to the NBT and weather a detector can do well, does well or not, I wonder how that relates to real world in the field senerio's?
Okay, since you pointed out the Nail Board Test, let's start wit it and go from there.

What is the Nail Board Test and what can we learn from it?

It is a duplicate of an in-the-field encounter where all four Iron Nails and one Indian Head 1¢ were laying on top of the ground just as you see them. All are in the actual physical relationship to each other as I encountered over 26 years ago. Most of the ghost towns I hunt since 1969 have a lot of the area well supplied with Iron Nils from their prior use or from building demolition or tear-down. Most of the time, the Nails are located anywhere from surface to a depth of about 3"-4", although on occasion, if there has ben any activity that disturbed the ground they have been a little deeper. In that same long span of time, most of the Coins I find in those Iron infested sites are also located anywhere from on the surface to a depth of about 3" to 4", and like the Nails and other discards, if there have been any ground disturbance, they might have been repositioned deeper as well. But that is far less frequent than finding them in that upper 4" range.

It is impossible to try and duplicate any of the odd-positions a coin might be in and also duplicate the position of any Iron Nails. Some might be deeper than others in a three-dimensional orientation, or they might be on the same plane as indicated with my Nail Board Test. Coins might be lower-conductive 5¢ pieces to higher-conductive copper and silver coins. They could be larger-size like a silver dollar, or smaller-size like a silver 3¢ piece or nickel 3¢ piece or perhaps a silver Half-Dime.

Nails could all be one size and nice and straight. But more often than not, Nails are found in an assortment of sizes and lengths, and not always straight. You have Nails that are bent in a number of different shapes which have a different effect on an EMF. Therefore, any mix of different Coins along with different Nails of odd shapes, and all of these mixed to layer them from 4" to surface, is going to have a lot of different affects on different detector makes, models and search coils.

Thus, my NBT is used as a basis to see how well a detector and coil combination can do on that specific test, just like it was originally discovered. Could a 4-Nail and 1 Coin 'test' be made more difficult to pass? Of course. But my NBPT is a 'standard' I use because if a detector & coil can't pass this one well, I wouldn't expect it to do well on a tougher test with Nails and Coin at different orientations.

There are a possible 8 hits by sweeping the four marked routes, and for me, it takes 6-out-of-8 to 'Pass' the test, and when I set out to hunt such a challenging site, I will use a detector & coil that gets at least 7-out-of-8 hits on the NBPT.

Why should we be aware of a search coil operating height?

There are several reasons, and we'll touch on these three: Negative effects on the EMF; Falsing; Target detection, separation and isolation.

Our more modern detectors operate with a high level of gain so we can achieve better depth and smaller-target detection, but with that come some trade-offs. One is that today's detectors, compared with those of twenty, thirty, forty or more years ago. are more reactive to interference from near-field influence. It is also more of a concern due to the lower operating frequencies compared to most early-era models, and to the search coil designs and sizes being used. All of these factors come into play, especially with more higher ground mineralization to deal with.

We can sometimes hear the effects of an improper coil search height, and sometimes we can't depending on how the manufacturer designed the circuitry. Detectors can respond in a way to alert us if the coil is being worked too close to challenging ground, or if it is too close to a metal object, and the way this is done can be referred to as 'Overload.' Some makes are designed to produce an 'Overload Audio' to alert us that the search coil is too close to a metal object or that an unseen metal object is rather larger and too close to the search coil. The 'Overload Audio Warning' alerts us to the fact the search coil needs to be lifted a little higher to eliminate the Overload affect on the EMF so we can then have proper function and good-target detection.

Those who search with such a circuitry design, such as found on a Nokta FORS CoRe, Relic, Makro Racer 2, Teknetics T2 and quite a few others, learn that a 'proper' or let's say 'functional' coil height is ±2". Reading the owner manual for many detectors also tell the consumer they should work the coil ±2" off the ground, and using a detector with an audible Overload Alert can be a good thing because it's a constant reminder. But there are other methods that handle the 'Overload' on the EMF, such as sweeping across a target with the search coil too close, and that results in 2 or more multiple-beeps from a single target that is too close to the search coil.

Use an average size coin, like a US 5¢ piece, and sweep back and forth,over it, making complete sweeps across the coin and not a 'wiggle', with the coil maybe ½" away and listen to the audio response. Most likely a double or multiple-beep response. Slowly raise the coil while sweeping back-and-forth across the coin until you get to the coil height that only produces a single-beep on each sweep. At that point you have found the 'functional' coil sweep height for best performance and good-target detection while eliminating the Overload issue.

Be aware, however, that some makes and models were designed such that a near-proximity target, or really bad ground, might not produce an audible Overload Alert or a multiple-beep response, but simply give NO response. Since so many detectors made today operate in a silent-search motion Discriminate mode, we don't hear a nulling of a Threshold, and they just cut-off any target response if the object is too large or too close to the search coil in the intense portion of the EMF .... so you hear nothing. Personally, I like to have some forms of audible Overload Alert, be it a deigned-in ringing audio or just hearing the double or multiple beeps if the coil is worked too closely.

Falsing caused by search coil height? Yes, it can be an issue. One audible alert is the 'Overload' related issue of multiple-beeps if the target is too close to the coil. Another, and related to the audio, is the falsing we see in the visual Target ID response. If the coil is not at a proper, functional search height, the visual ID can be more jumpy and erratic. Again, when we hear or see such a potential problem, we simply need to raise the search coil a little to restore a proper coil position and the audio and visual responses will improve and be more clear or accurate.

Search coil position is also important for Target Detection, Separation and Isolation. Again, two or more of these three things are related, yet a little different. If we have a trashier place to hunt, be it ferrous junk like Nails or modern trash such as Pull-Tabs or Bottle Caps or Foil, we want to be able to detect the desired targets, be it coins or trade tokens or good jewelry, in amongst the trash. If we have multiple targets, desired or junk, that are too close to each other, it can create several problems for us to address.

If targets are too close, the search coil size and type and physical design are important. Usually, smaller-size search coils can work better in a trashier environment because they have a smaller-size EMF, and it is also a more intense EMF or 'tighter' EMF. This can let us 'cherry-pick' a location and 'separate' good targets from nearby bad targets ... especially if there is reasonable spacing between the two located targets. A mid-size or larger-size search coil has difficulty separating two closely-spaced targets, often only producing a blended audio response or blended audio Tone ID from the two different metal objects. Also, even if just one individual target, a bigger size coil can not 'isolate' or 'pinpoint' as well as a smaller-size search coil.

Additionally, a larger-size coil has a larger-size EMF and, in some conditions, it can be more affected by a near-proximity target and require a slightly increased coil-to-target (or coil-to-ground) position to eliminate any Overload behavior.

Back to 'separation' for a moment. Search coil shape and type also come into consideration, and we'll use the NBPT as an example this time. There is only 1 Coin in the centered #1 position, and the 4 Iron Nails, of different sizes and positions around the coin, create several challenges:

• We have to handle the ferrous-base junk, and there are four piece and they are at different positions from the coin on the different sweep directions.

• We are not encountering this like we would a single coin and a single Iron Nail that are close to each other. In those conditions, we might sweep toward the Non-Ferrous Coin and then cross it to encounter the Ferrous Nail, or we might be sweeping from the opposite direction and encounter the Nail first and then the Coin. Just those two sweep directions can leave us with a different audio response and visual Target ID response to try and interpret. In the NBT it is an even greater challenge because no matter from which direction you sweep over the test piece, you will be encountering an Iron Nail first, then the Coin, then dealing with another Iron Nail after passing over the coin.

A third issue is the orientations where a sweep route doesn't only present a Nail before and after th Coin, but the size and shape and internal design or type of coil can also be influenced by the nails to the front and to the rear of the Coin that are still influencing the EMF during the detection of the Coin. Thus problems not just from the left and right sides of the coil's EMF, but how the Nails influence the detector process that are towards to 'toe' and 'heel' of the search coil.

So we can now see that search coil size is an important consideration, and smaller coils generally excel in this case. But search coil height is also important to note and we should evaluate the performance of any detector we use with all the coils available for it. Keep in mind, also, the type of coil design, being either a Concentric coil or a Double-D coil. I use both types of coils based on what the detectors were designed to work better with and what worked best for me. If I had a choice, we often we don't based on what the manufacturer decides to make, is for a smaller-size Concentric coil. Why? Because ewe can usually get better Discrimination and thus a more accurate Target ID and visual Target ID with a Concentric coil. It's due to the consistent or uniform EMF providing similar performance when the coil is swept from either direction compared with a DD coil design.

What can we learn about a particular metal detector and / or search coil?

We can learn a lot about how the engineers designed the detector and their performance at handling Iron trash as well as something about how quick they can respond-and-recover after encountering a non-ferrous target, a ferrous target, and two or more target close together. All detectors are definitely not made the same. Some of the things we learn regarding search coils is understanding a functional search coil height, and also how they behave or are influenced by different types of metal. Remember that Iron / Ferrous targets have a different effect on the EMF than do same-size Non-Ferrous targets. Because Iron has a greater effect, it can make it more difficult for the detector circuitry to process a Discriminated or Ferrous response and still quickly-recover to be able to give a positive respond to a nearby Non-Ferrous target.

Those who own and use only one detector and only one or two search coils can spend their time learning it well to know the strengths and weaknesses it might have. Those of us who own and use two or three or more makes and models have the work cut out for us because we need to learn each of them, well, and then learn their strengths and weaknesses between each other so that we might select the best coil for all-purpose performance for each model for certain applications. That will then let us pick-and-choose which detector and coil might serve our needs the best at a given site.

Since we never know how deep the next target will be, will this affect the detector's ability to pull out that good target next to iron or is it just the coil height to the ground that makes it better or worse on the NBT-therefore if the NBT was burried 3" under the ground but one maintained proper coil to ground height would the results be the same, meaning favorable?
We could put a coin on the NB at a 3" depth in the ground and test a detector and coil on it, and the results are going to be similar to what we would get in any natural encounter afield. That is, we would still be dealing with the near-position Iron Nails to the Copper 1¢ Coin ... and adding the ground mineral to deal with. Also, if buried and out-of-sight we would also learn the importance of using a slow and methodical sweep speed and efficient over-lapping. A lazy sweep that isn't overlapping sufficiently might only encounter the coin at the fringe of a sweep and not have the search coil's center-axis swept directly over the coin.

Good over-lapping of 50% or more of the coil would enhance our search performance and results, but using a sloppy site coverage sweep won't help. Also, many detectors will not work well with a fast sweep speed in a more mineralized environment as they can't process the ground signal and pass along a target response. They require a slower sweep mode for best efficiency. Again, that is something we need to learn about any make or model detector we use. What are the sweep speed requirements to work, and the limitation where they start to not work so well.

Then remember, if the search coil is too far from a mixed array of targets like that, there will be a decrease in overall performance because the EMF will be larger and yet weaker at that greater distance and without a tight EMF, we won't have the 'separation' we need, and might not be able to isolate a non-ferrous target in a mix of multiple ferrous targets. That why, when you get right down to it. anything we learn and do is going to sometimes be helpful if we want to isolate some potential 'keepers' and just 'cherry-pick' a site, but if we really want to have the best results to find all potentially good targets, we really need to clean a site out. Remove the trash in order to achieve performance on smaller-size or deeper-positioned desirables..

I suspect there is something I'm not quite getting here.?
I think you 'got it' but just couldn't put your finger on the reasons. Using my 'Separation Stick' points out what it takes to achieve close-target 'separation', and also how search coil height and coil size can loose performance as the coil-to-target distance increases. Of course learning the differences can be a little tricky, too. Let me explain ... or at least try to.

My 'Separation Stick' is comprised of 7 Trade Tokens that are about the size of a US 5¢ Coin, most are higher conductive brass type and one is a lower-conductive token that is close to th conductivity of a 5¢. I used two wooden paint sticks that are 12" in length. One trade Token is positioned on either end of the 12' stick so that the Token's center is on the '0' line and 12" line, and the other 5 Tokens are centered at the 2", 4", 6", 8" and 10" marks. That means the 7 Trade Tokens are positioned just 2" from center-to-center, resulting in a tough challenge for any detector to handle with a quick-response and fast recovery to produce 7 individual target hits on a complete side-to-side sweep.

Not all of my detectors, and there are 10 units in my primary-use 'team' (today at least) will give me 7 individual hits when sweeping across that stick. It's a tough 'test' that I use to give me some clues as to how well a detector might work in a very trashy site. I do not have a Teknetics T2+ in my group of detectors now, but I did this spring and with the 5" DD coil the T2+, all of the T2's as a matter-of-fact, can produce a blistering fast response on all 7 tokens. I used to use a T2 then T2 'Classic' w/5" DD coil as my Relic Hunting dense-trash combination through the end of 2014. Back then, not too many competitive detectors had a Disc. mode that could match the T2's quickness on that type of test, other than my Tesoro's with their 6" Concentric coil..

In my current Detector Team I can get 7 hits using my CoRe w/'OOR' DD, Relic w/5" DD, Simplex + w/5X9½ DD, Bandido II µMAX and Silver Sabre µMAX w/6" Concentric, and yet I can't with the 6½" Concentric on my MX-7. Another detector that is very surprising on the 'Separation Stick' is the Vista 'X' w/5¾" DD coil. It is also extremely quick to respond, recover, and respond to the next target. And even more of a surprise to me is the new Garrett Apex w/6X11 DD 'Viper' coil because it, also, gives me 7 very clean, separate hits w/o any Overload issues. I can't wait to see how well a smaller-size coil will perform.

But to accomplish getting 7 clean hits on those 7 targets that are so close together, it requires the search coil to be worked very close to the 12" string of Tokens. Depending on the detector, the coil, and the settings used, especially the Sensitivity level, this can be a questionable or tricky coil height. For example, my Nokta CoRe w/'OOR' DD coil easily produces 7 clan hits IF the coil is close, but raised just enough to overcome the Overload audio, and the same for the Relic w/5" DD or the Teknetics T2 series w/5" DD. Too close and you get an Overload signal and need to raise the search coil just a little to overcome that.

I have tried some detectors that sounded-like they were giving good multiple hits on each token, however the extra 'beeps' were due to their Overload issue of having the search coil too close to the target. For that type model you need to sweep over an individual Token or Coin and determine how much coil height is required to defeat the multiple 'beeping' from a too-close Overload. The Vista X w/5¾" DD and Bandido II µMAX w/6" Concentric can be worked very close to the string on Tokens w/o the Overload issue.

In the end, with any make or model detector and any coil that gets mounted .... what they can do is going to be iffy once a mixture of targets are present and close-by. By the orientation and size and shape of the different targets present. The depth of the targets and the ground mineral make-up for any given site. And then we can't forget th operator's role in using the best adjustment settings possible along with a proper and efficient sweep speed. Because ANY test that ANYONE conducts is simply that .... a 'Test.'

My NB is a 'Test' to give me some information on how well a detector & coil can handle a situation with multiple Iron Nail in a close relationship to a single US Copper or Zinc 1¢. My 'Separation Sticks' help me to learn what a detector might be able to do in an ideal multi-target encounter in the way of Response-and-Recovery, but that's it. I only get a rough idea of what it might do, but it only provides that 'separation' performance with a smaller-size coil and when the coil-to-target distance is rather close. It lets me know that in dense trash a quick-working detector and smaller-size coil will have an 'edge' over a slower-responding detector and with a larger-size coil.

Then, as with any other 'test', we can check to see where there is a change in performance. In this case, if the search coil distance from the 'Separation Sticks' is increased, the strip of Tokens is now farther from the tight, intense portion of the EMF and that means the bigger-size portion of the EMF is seeing multiple targets at the same time. The result? It won't be possible to 'isolate' a single Token, and that means there won't be any 'separation' of nearby targets when the coil-to-target distance is beyond its efficient range to do so.

I know this is long as it took me and my two typing fingers a long time to do it. And it might leave some a bit confused as it is much easier to talk-to and then demonstrate to someone one-on-one, but if you read it through, and then work with each of your detectors to get a handle on what they can and can't do, and what they can do well, it will make more sense.


"Your EYES ... the only 100% accurate form of Discrimination!"

Stinkwater Wells Trading Post
Metal Detector Evaluations and Product Reviews
monte@ahrps.org ... or ... monte@stinkwaterwells.com
Detectors: Fisher F5; Garrett Apex; Minelab Vanquish 540; Nokta / Makro FORS CoRe, FORS Relic & Simplex +;
Teknetics T2+; Tesoro Bandido II µMAX & Silver Sabre µMAX; White's XLT
Pinpointers: Pulse-Dive -- ProPointer AT .. Headphones: 'Hornet' & 'Wasp' -- MS-3 Z-Lynk -- ML-80 -- N/M Green Edition
Note: Detectors are listed alphabetically by Brand. Models are chosen based on search site conditions.
*** All working well today to make memories for tomorrow. ***

Edited 1 time(s). Last edit at 09/13/2020 07:24PM by Monte.
Subject Author Views Posted

Coil height

Dan'o 78 September 12, 2020 10:33AM

Dan, you've touched on several different topics. Let's see if I can help: CAUTION .. THIS ONE is LENGTHY..

Monte 68 September 13, 2020 08:04AM

I just made a couple of corrections / additions to the above post.

Monte 38 September 13, 2020 07:28PM

Re: Dan, you've touched on several different topics. Let's see if I can help: CAUTION .. THIS ONE is LENGTHY..

SvenS 48 September 13, 2020 10:56AM

Re: Dan, you've touched on several different topics. Let's see if I can help: CAUTION .. THIS ONE is LENGTHY..

Dan'o 44 September 13, 2020 09:15AM

Re: Coil height, my coil height varies according to conditions.....

Hombre 66 September 12, 2020 11:35AM

Re: Coil height

Digstrashtomuch 51 September 12, 2020 11:02AM

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