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

April 19, 2019 10:35AM avatar
George posted this over on the AHRPS Tesoro Electronics Forum in a reply that was started by Joel's post as follows:

Deep coin shooting with a concentric coil???
What would one choose? My tejon was pretty deep with the 9x8 concentric. But still nothing spectacular. Did they ever make a 10" or 11" concentric coil that was good at deep coin shooting. If so what detector coil combo? I would love to have a deep concentric coin shooter that's also a good discriminator.

I tried to split this but all of the replies to George got moved here, but that's alright. It left Kickendirt's initial post on the Tesoro forum and I'd like to keep replies there directed toward search coils and performance as related to Tesoro models. But the subject of search coil size and detection depth and performance associated with smaller-size targets we are mostly after, namely coins, is an important discussion and should be shared by everyone using any make or model detector, so here it is. Let's try to keep the responses on-topic and not drift away.

To Begin: The initial post Joel made was directed toward a Concentric designed search coil as related to Tesoro brand detectors. To be clear, coil size and detection depth also applies to the other often-used coil design, the Double-D. Both the Concentric and Double-D search coils have their strong and weak points, but they do share similar limitations when nit comes to their ability to detect common smaller targets.

Sharing My Replies to George's Post: They are based on over half-a-century of very avid detector use, and working with and discussing search coils with some design engineers. I also had an opportunity of rigging up a search coil testing device to understand any benefits, if any, as well as shortcomings based on search coil sizes. This was especially directed at manufacturer-advertised accessory coils and their supposed increased benefits.

Deeper for what coin?
Kickendirt specifically asked about search coils for Coin Shooting. Since the beginning of the most popular interest in the metal detecting hobby, the bulk of us enjoy it for urban Coin Hunting. There are many of us, and by 'US' I am including myself, who really like to venture out of our urban surroundings to search old ghost towns, homesteads, dance hall sites, old picnic groves, and many other sites that fall into an activity or category commonly called Relic Hunting.

We generally enjoy finding all sorts of interesting smaller-size artifacts which might include decorative buttons, military uniform buttons and insignia, gold and silver jewelry as well as unique and decorative older jewelry, and other unique small artifacts such as sheep ear tags, badges, old bullets and cartridge cases. Many things that can be of various sizes and shapes but generally in the smaller-size category.

But most of the time we set out in the hopes of finding old Coins as well as somewhat similar-shaped Trade Tokens. Those are my primary targets of interest, and coins and tokens are usually round, scalloped or multi-sided, but they are also of a thinner thickness, and they can be made of a wide range of conductivity metals ... yet they share similar effects on a search coil's electromagnetic field.

So while we might be interested in all sorts of interesting found targets, COINS, or similar-sized tokens or medals, tend to be what we are searching for. And most of the time, unless we really get lucky, they are found individually and we should concentrate on a detector and coil's ability to find a singular coin-sized target. Most of them are going to be of smaller size, too.

It was determined long ago that 8" was the ideal size for a dime sized object.
I generally agree, although using factory standard coils, reading up on search coils way back when, and using all sorts of detector makes and models through the years, I feel it was a matter of the design engineers of both detectors and search coils to offer an 'optimal' search coil size that might work best for the 'average' or 'typical' Coin Hunting applications as well as with their detector circuitry design.

For my first three years the kit-built detectors I made had us wind an 8" diameter search coil, but when I started using factory-built detectors in the summer of '68 the coil diameter was 6". Those were BFO detectors, but when I went to TR's in early '71 the 'standard' search coils ranged from 6" to 8" in diameter. By the latter '70s and into the early '80s the most popular 'standard' search coils by major manufacturers included the 7" on Garrett's Ground Hog, Deepseeker and American TR-Disc. models, as well as those by Gold Mountain, a 7¼" on the first offerings from Teknetics on their line of CoinComputer models, both 7" and 8" on different models from Tesoro, and ±8" diameter coils on most models from Compass, D-Tex, Fisher and White's just to name the more popular brands during that era.

Also, at the time, it was most popular to make round-shaped search coils as elliptical-shapes hadn't become a 'fad' like they seem to be today. Nothing wrong with that design, and I even have an elliptical coil mounted on two of my twelve detectors. The rest of them sport a very functional round-shaped search coil. Some are Double-D but most are Concentric designs.

For me and my needs, and to work best in most of the coin producing sites I hunt, my detectors have 5", 6", 6½" and 7" search coils mounted. They all work well for the generally trashier sites I hunt but still do well in 'average' urban Coin Hunting applications. I do have an 8" or two in my Accessory Coil Tote should I feel and urge to use one.

A 12" coil will not detect that dime any deeper, in real ground, BUT, would pick up a larger coin (say a silver dollar) deeper than the 8". This is because of the signal (coin) to noise (ground) ratio. Bigger coil means much more ground in the Tx/Rx field.
Yes, as a general rule this is about correct, but we sure have seen a lot of 'push' within the industry to promote larger-size search coils as 'standard' or over-promoted them as an accessory coil choice.

When I worked at Compass Electronics as their Dealer Coordinator and Marketing Rep I was reviewing their literature and had a discussion with Ron Mack, the company President, about their marketing comments for their two larger-size Accessory Coils. I called his attention to the fact that the majority of metal detecting Hobbyists are Coin Hunters, and that their two larger-size search coils, a 12" and 16" Concentric, had false (let's just say incorrect or misleading) claims in their literature.

They stated the 12" coil gave a 20% increase in depth and the big 16" diameter search coils had a 40% increase in depth. I let him know that while most coins are typically located in the surface to 4" range, a 'deeper' coin is realistically going to be in the 5" to 8" depth range. And while we do occasionally hear of surprising claims of a 14" US Dime being found, I know that to be very unrealistic. It's either a coin that fell into a deeper hole, a poor judge or bad guess of distance, or a far-fetched fib. Usually the latter.

But I have made coin finds, suing 6" to 8" coils, to an honest 8" to 11" depth with those deeper coins being larger-sizes to include a 2¢ piece, a 50¢ piece and a big silver Morgan $1 coin. But in all the time I have been avidly hunting, there have been no more than 10-12 coins I've found at honest depths to 10" or more.

So using a normally deep 8" coin and a very deep 10" coin for comparisons, I pointed out to Ron that their 20% increase with a 12" coil would be an extra 1.6" to 2" more than their 8" coil, and 3.2" to 4" more depth with their 16" coil. Those were apparently unrealistic depth enhancements for day-to-day Coin Hunting.

I set out to disprove those claims as I had the opportunity and time to design a rig that would hold a search coil and move in side-to-side with a 'normal' sweep speed for different detector models, then compare the performance of a standard 8" coil against the results from the 12" and 16" search coils. When I finished my testing I gave the report to Ron and also demonstrated the results.

Keeping in mind that the majority of the coins we are going to recover are of the smaller category, such as 1¢ and 10¢ denominations, so I used both a Penny and Dime to compare results on the most common coins. I also used some smaller-size US coils we might find while Relic Hunting such as a small nickel 3¢ and much smaller silver 3¢ piece as well as a silver Half-Dime. I couldn't ignore the other larger size coins, to include a 5¢, 25¢, 2¢, 50¢ and $1 coin in those progressive sizes.

While the 12" coil showed a slight depth increase over the 8" coil on a Dime and Penny, it was only up-to a 10% increase at best. The larger-size coins, which are seldom encountered came closer to the 20% claim thy made, but only closer. The 16" search coil, at best, had an increase on the largest-sized coins just to about a 20% or so over the 8" coil. The 1¢ copper or zinc pennies and a 10¢ clad and silver Dime's game a comparable audible response almost matching the 8" search coils performance. By 'almost' I mean that on some tests the 8" coil gave a slightly better response.

Tests were done using the coins positioned directly below the center-axis of the search coils, and done as both an 'air test' as well as by placing a box of clean, metal-free dirt between the coil and sample coin target. The better performance was with the 'air test' as the dirt used was reasonably mineralized making more of a challenge.

And Yes, the detectors had the Ground Balance adjusted for the mineralized dirt just in case you're wondering.

Here's another very thoughtful comment made by Alan Holcombe, White's marketing guy and former CEO at a Texas Council of Treasure Clubs presentation back in '88 I believe. White's Electronics had been selling their detectors with a standard 8" coil for quite a while and offered a 10" Accessory Coil. But when they brought out their Eagle II and 6000 Di Pro SL with the new coil change, they featured the then new '950' Concentric coil as 'standard' and discontinued the 8" coil as a standard offering.

That new 950 was almost the size of their accessory 10" coil, and when manufacturer's rep had their opportunity to get in from of the audience and announce their new products, Alan started out, first thing, by describing their newest models came with a new 'standard' 950 coil instead of the former standard 8" coil. He said one reason was because so many people in the hobby had been asking about getting more depth and wanting a bigger-size search coil to get better depth.

Alan continued by saying this: "Personally, I think they made a mistake." And he continued by saying the 8" coil was really about the best coil and worked well for most needs (and remember, most hunting is Coin Hunting), and if someone wanted a little better depth they could just buy the optional search coil.

Bigger-sized coils are better used for bigger-sized targets, not smaller-sizes. Things like a jar of coins, a hidden metal box or other larger-size metal object, but not for typically smaller-size common targets.

That is why to find a buried treasure chest, you would use a 2-box detector, which could detect the box feet deep. But it could not detect a dime laying on the surface!
Correct. The 2-Box type detectors are for larger targets that are often buried deeper. They are useless for smaller-size / coin-sized targets.

Small coins - 8". Large coins - larger, to a point. I really don't know what the optimum size coil would be for a silver dollar coin. Maybe Monte knows.
Yes, the targets as to be reasonably larger for a bigger size search coils to really show any appreciable increase in detection depth, all the while having to also deal with more ground signal. Quite a challenge and trade off to deal with.

So, there's what Monte knows, at leas t this Monte, and it's all been experienced-based. There are some other considerations that come into play when selecting a search coil that can serve you anyone best on a day-to-day basis, but there is so much marketing hype out there that it is difficult to get a lot of folks to appreciate a smaller-to-normal size search coils for most hunting, and I am referring to coils in the 5' to 8" range.

I am a slow typist. haven't had time to proof read this lengthy reply and I'm tardy getting to an appointment and then out detecting so I'll close it here. More responses to this very interesting topic later today. It's nice outside cool smiley so Rikki & I are headed out for a while ... with a Relic w/5", F44 w/5", Silver Sabre µMAX w/6" and an Omega 8000 and F44 w/7". No need for an over-size or exaggerated-size coil for my Coin Hunting needs.


"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
Killer B's 'Hornet' and White's 'Pro Star'
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 74 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 73 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 77 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 100 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 103 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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