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George, I'll respond to your 5-points.

April 20, 2019 05:42AM avatar
Monte, thanks for the ultra clarifications :-)

In reading that, a couple of important points came to mind.

1) I think "standard" coils started getting larger due to marketing pressure. After all, if 'one company's detector "only" has an 8" coil, if we supply a 9.5" coil, the consumer will think that is better.'
I believe your idea is only partially correct. In my 'Specialty Detector Group' I have four of five models on-hand primarily used for display and demonstration purposes for my seminars. The oldest is a late '60s era Garrett Hunter BFO with a selectable dual coil, the larger, primary-use coil is 8".

I also have a Garrett Master Hunter TR-Disc., from about 1975, with both search coils (which was what the earlier Master Hunter concept included) and the 'standard' use coil is a round-shaped 8" Double-D design. Another Garrett is an American S2 TR-Disc., from ±1980/'81, with a 7" 'standard' coil. I also have a Compass Coin Hustler TR with a 'standard' hard-wired 6" coil, and a White's 6000 Di Series 3 Hip-Mount with the 'standard' 8" coil.

It really was the norm for most typical detectors to come with a 6" to 8" standard coil, and some manufacturers, like the 7 inchish coils on most Garrett's and early Teknetics and some of the Tesoro models. There were some detectors that sported a 6" to 7" standard coil that were more on the low-end side, but most detectors we had came with about an 8" coil by the mid-'80s. Here in the USA we had very few 'foreign-made' detectors except from Minelab, and the general purpose Sovereigns I had even through the '90s and later came with a round 8" coil.

So far as big-name manufacturers going to anything larger as a common size, it was White's move to the 950 Concentric coil in '88, and almost a decade later the FBS models from Minelab had a round 10" or 10½" Double-D coil. While most detector makers still stayed more with an 8" size, we did see some new coil changes, like the 8X9 open-frame Concentric on a few Tesoro models about 2000/'01 while other models kept the popular 8" 'donut' coil.

All the detectors we had worked OK and did, or could do, just fine with an 8" or even a 7", but we saw a real marketing change with the move to larger-size Double-D coils that were open-frame design as well most being more of an elliptical shape. I credit the surge in business and the hyped marketing by Minelab, especially with their FBS models, that got more bigger coils. In 2006 the new Teknetics introduced the T2 with the 11" BiAxial (another term meaning Double-D) coil, which we see comparable coils of about 7X11, 8X11 to even 9X11 of an open-frame design from many detector makers today. Even White's move to the round 10" D2 Double-D was to offer a comparable size and shape in DD form like Minelab was promoting.

2) Another thing to bear in mind, is that the coil housing might be 9.5", but the actual wire coil inside could still be 8"!
Some could, but we usually think of search coil 'size' as a physically measured outside diameter of a coil. We know anything inside will have a smaller winding. One thing that annoyed me, and still does, is how some of the marketing folks had a bright idea to name a coil other than by a physical housing measurement.

The White's Blue Max 600, for example, measured almost exactly 6½" in diameter, but White's still calls it a 6" coil. But then they marketed a name change calling them a 5.3 Black Max, changed that to 5.3 Bullseye (same coil, different label, and the same as the Blue Max 600 except black plastic instead of white) for the 6.6 kHz models, and 5.3 Eclipse for the upper-end models. Bad marketing idea as many people thought, or still think, that it describes a coil that is physically under 5½" in diameter.

Tesoro did the same crazy marketing thing calling their 6" coils a 5.75. What White's and Tesoro were suggesting was that the internal Transmit coil diameter measured 5.3" or 5.75", but it was more of a marketing idea. I was down at the White's factory one day and asked three engineers if they could tell me what the 5.3 term actually measured?

They said the Transmit winding size. I then asked them to be specific and if that meant 5.3" across the inner winding diameter, or if it was 5.3" measured from the outer edge of the Transmit winding, or maybe it was a measurement from the middle of the wire winding to the middle of the winding on the opposite side. Three design engineers and none of them had an answer for me unless they went and measured it.

In the end it really doesn't matter what the actual incremental size of the internal winding is because the user's noggin thinks in terms of how it looks and where it will fit.

3) I did the math on the "volume of a cone", and if you look at the difference between an 8" and a 10" coil, you'll find, the volume of the 10" is 50% more than the 8". That means much more ground seen by the detector with a 10"er. That is OK, as long as the target is larger. I cannot find the reference again, but one of the top engineers mentioned that there is an optimum distance between the ground and the coil, depending upon the amount of mineralization. Reason is, if you "scrub" the ground, you might get 1/2" closer to the target, but the amount of ground the detector sees is considerably increased, actually reducing depth, or overloading the receiver!
I've always enjoyed mathematical problems, to a degree, but in this case, measuring the "volume of a cone" is good if you have a physically designed cone-shaped object, such as to hold ice cream, or make a funnel, or in some cases we might measure an inverted cone in the form of a dunce cap. Those would all be man-made objects designed in such a way as to measure a specific 'volume' of space within their outer designed shape.

However, you can not really measure the "volume of a cone" for an 8" sized search coil because there isn't a cone-shape to measure. Any search coil design, be it round, out-of-round, elliptical, squarish or whatever physical design chosen, will have a measurable volume of space within the confines of the search coil housing, but once you look outside the housing there is not a definitely designed 'cone' to measure.

Some often stated or even printed or sketched designs we read about suggest a Concentric search coil projects a cone-shaped electromagnetic field. It doesn't. Some folks also describe a Double-D coil as having a "knife-blade" or "windshield wiper" shaped EMF that projects straight down from the front and rear of the coil to provide total coverage, front-to-rear, at depth. It doesn't.

The generated electromagnetic field is actually the opposite of what some believe is a projected 'V' shape or 'Cone' shape signal. The magnetic field is working out and away from the Transmit winding, and as the lines of magnetism get farther away from the Tx winding, the weaker signals, there will be more bending of the lines which results in a weaker field or weaker strength, and with a Concentric coil that weakening strength in an uninterrupted field will generally be along the center axis of the search coil. That is why, with a uniform shape and oriented object, we are able to pinpoint a target at the greater distance (aka 'depth') in line with the center-axis of a Concentric coil.

But there is not a measureable projected EMF in the shape of a cone to measure because what we do have to work with is not a projected cone-shape EMF. It is the reactive portion of the generated EMF, and that can easily be distorted. The reactive portion of the EMF will vary based upon:

the design of the search coil and the effects of the size of both the Tx and Rx internal windings

the specific size of the wire windings

the generated transmit power or Sensitivity of operation

the Gain or effect on the receive signal before amplification

the ground mineral make-up to include things like:
a.. being loose or porous to compacted or a solid mass
b.. the amount of iron make-up of the ground, whether it is very dry, damp, wet or water saturated
c.. consideration if the soil or water is of an alkaline or saline content
d.. the orientation or position (closeness) of the search coil's projected EMF in relationship to the ground or other search material/object.

the specific metal detector's circuitry design and how it processes both received signals as well as rejected signals and the settings used

plus we have to remember operator control. Most searches use a motion-based Disc. mode so other influences include:
a.. the search coil maintaining a uniform coil-to-ground relationship
b.. that coil distance remaining consistent or uniform during the entire side-to-side sweep of the search coil presentation
c.. the sweep speed used to achieve the most efficient performance
d.. interfering EMI issues

And naturally the make-up of the encountered target being of a ferrous or non-ferrous metal.

While there is no measurable 'cone' shape, there is a pronounced difference on search coil size and the effects of the mineralized ground as you mentioned. Larger coils will logically 'see' more ground which means they have more ground signal or ground noise to deal with and filter out in order to process the target signal. When used for Coin Hunting, the bigger the coil the more ground noise there will be to deal with, yet the smaller-size coin targets will have a diminished or weaker signal as the coil-to-target depth increases. Stronger ground signals and weaker small-target signals make field performance more challenging.

You also pointed out the negative effects or 'scrubbing' a search coil or working to too close to the mineralized ground. That's why most of the savvy detector manufacturers point out in their User Manual that a search coil should be worked at a distance of ~1" to ~2", and that can be critical based on how a particular model handles 'overload.' We can get an 'overload' effect if a metal object is too large and too close to the coil such that it warps the field and saturates it with an intense signal. We can also have an 'overload' effect if the search coil is too close to the ground mineral such that is saturates or warps the close-proximity of the strongest EMF very close to the search coil.

Some detector models will alert the operator with an audible sound, such as a ringing or static noise type sound to let us know we have an overload effect and need to raise the search coil away from the ground a sufficient distance to restore proper operation. Unfortunately, many detector models to not have an audible alert of this condition and they simply shut-down and remain silent and will not/can not produce an audible response on a nearby target due to the overload saturation.

4) This from Dave Johnson, another top engineer in metal detection engineering: "Searchcoil size Most standard searchcoils are approximately 8 inches (20 cm) in diameter if round, or approximately 10 inches (25 cm) in length if elliptical. Larger searchcoils allow covering more area with each sweep, and offer a slight increase in depth on medium and large size targets. Unfortunately they are heavier, more difficult to pinpoint with, tend to lose small targets, and provide poor target separation. Small searchcoils provide superior target separation (important in trashy areas) and ability to detect smaller targets (important in gold prospecting). Of course they don’t cover as much ground as a standard size coil. However (and this may surprise you) small searchcoils usually have nearly as much depth capability as standard size searchcoils."
A logical and correct write-up by Dave Johnson, and I've read or heard similar comments from other search coil design engineers. A larger size coil might provide a little better coverage of a site from a side-to-side sweep, but for best efficiency ample overlapping is still very important. And there is usually going to be more improved coverage for shallower targets, in the surface to about 3" - 4" depth range, but not a remarkable improvement in small-target depth.

I do have a few standard and one larger-than-standard size coil in my Accessory Tote, but they are reserved for specific applications, especially for use when searching for medium to larger-size targets. I do NOT keep all the standard size coils, or even buy a new detector with a 'standard' coil, because I very seldom use them. Most places I hunt have a lot of trash, building rubble or dense brush and smaller to mid-sized search coils have been serving my needs quite well for over five decades. I use them almost exclusively.

And yes, the 6" and 7" coils tend to perform very close to what I'd achieve from an 8" coil, but they fit places better, weight less and are much better balanced and comfortable to use. Also, the 5" and 6" coils can give me better depth in a modestly littered site and unmask targets in iron debris much better than an 8" coil can.

5) Again, the coil size vs. target size is heavily influenced by the amount of mineralization in the area you are hunting.
There was a 3-year period when I had the opportunity to travel to, and hunt in, quite a few US states from coast-to-coast, and I really envy those who get to work a detector in the almost neutral ground in Florida to the very low to moderate mineralized soils in Texas, Oklahoma, Kansas, and many other mid-west to eastern states. Especially when I compare them to many of the very mineralized ground environments I search in Utah, Idaho, Nevada, Oregon and some parts of other western US states. I've hunted with friends who have visited me out here from several of those other states and were shocked at the challenges we deal with, the ground noise they encounter, and the reduced depth of detection they experienced compared to most places they call home.

All this rambling from a guy who swears by his 5" to 7" search coils and works them an inch to two inches off the ground, and has for many decades with great success.thumbs up


"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
Fisher: F-44, Nokta-Makro: FORS CoRe and FORS Relic, Teknetics: Omega 8000,
Tesoro: Bandido II µMAX and Silver Sabre µMAX, White's: XLT, XP: ORX
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Note: Detectors are listed alphabetically by Brand. Models are chosen based on search site conditions.
*** All working well today to make memories for tomorrow. ***
Subject Author Views Posted

Important Topic by George: Deep coin shooting with a concentric coil???

glabelle 198 April 16, 2019 06:50AM

Related Reference if you're interested -

UtahRich 111 April 22, 2019 11:47PM

Related Reference of interest? Well, I'm not, and I don't mind commenting on it, either.

Monte 98 April 23, 2019 07:55PM

Coils and Coins -

UtahRich 73 April 23, 2019 08:46PM

Re: Related Reference if you're interested -

glabelle 85 April 23, 2019 07:11AM

Coil Sizes and Depth on Coins -

UtahRich 72 April 23, 2019 08:40PM

Re: Coil Sizes and Depth on Coins -

glabelle 56 April 23, 2019 09:37PM

Coil Sizes and Depth on Coins -

UtahRich 55 April 23, 2019 09:44PM

Re: Related Reference if you're interested -

OregonGregg 76 April 23, 2019 05:45AM

Re: Related Reference if you're interested -

UtahRich 60 April 23, 2019 08:54PM

Pardon me George and others, but this is a great topic.

Monte 128 April 19, 2019 10:35AM

George, this is a great topic. Question for Harold,ILL

UtahRich 99 April 19, 2019 05:45PM

Comments for Rich (Utah) from Monte.

Monte 99 April 20, 2019 08:40AM

What is DEEP ? - Some Detecting Options

UtahRich 104 April 20, 2019 07:43PM

Re: What is DEEP ? - Some Detecting Options reply Rich Attachments

Kickindirt 87 April 20, 2019 09:12PM

Re: What is DEEP ? - Some Detecting Options

OregonGregg 84 April 20, 2019 08:06PM

Re: What is DEEP ? - Some Detecting Options

UtahRich 67 April 20, 2019 09:22PM

Re: What is DEEP ? - Some Detecting Options Attachments

OregonGregg 76 April 20, 2019 09:35PM

Re: Pardon me George and others, but this is a great topic.

glabelle 85 April 19, 2019 04:43PM

George, I'll respond to your 5-points.

Monte 99 April 20, 2019 05:42AM

In a nutshell . . . . .

UtahRich 65 April 19, 2019 07:48PM

Re: In a nutshell . . . . .

glabelle 83 April 19, 2019 08:42PM

Preferences -

UtahRich 63 April 19, 2019 11:48PM

A few last comments on coils and absolute depth Attachments

glabelle 84 April 20, 2019 09:24AM

George, a simple reason I described the 'cone.'

Monte 83 April 20, 2019 03:59PM

Adjacency - And why is that still there thinking

UtahRich 72 April 20, 2019 06:13PM

Re: Adjacency - And why is that still there thinking Great Post Richthumbs up

Druid 64 April 21, 2019 09:49AM

Re: A few last comments on coils and absolute depth

jmaryt 55 April 20, 2019 10:28AM

Re: Deep coin shooting with a concentric coil???

Kickindirt 102 April 16, 2019 06:46PM

Re: Deep coin shooting with a concentric coil???

glabelle 94 April 16, 2019 09:57PM

Balancing to ignore the ground?

Monte 86 April 20, 2019 09:22AM

Re: Balancing to ignore the ground?

glabelle 84 April 20, 2019 09:34AM

Re: Deep coin shooting with a concentric coil???

Kickindirt 84 April 17, 2019 11:05AM

Just a thought or two regarding coil size selection.

Monte 76 April 20, 2019 09:59AM

Re: Deep coin shooting with a concentric coil???

glabelle 92 April 17, 2019 04:13PM

What should I use to look for deep silver dimes?

UtahRich 116 April 16, 2019 10:20PM

Re: What should I use to look for deep silver dimes?

glabelle 89 April 17, 2019 09:21AM

Re: What should I use to look for deep silver dimes?

Harold,ILL. 103 April 17, 2019 08:06AM

Re: What should I use to look for deep silver dimes?

jmaryt 85 April 17, 2019 10:44AM

Re: What should I use to look for deep silver dimes?

UtahRich 100 April 17, 2019 04:12PM

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