Homemade 12" Whites Coil

I've slowly been gathering information and want to make a 12" coil for the QXT (XLT uses the same 9.5" stock coil). Has anybody winded a 12" concentric coil for them? I've got the inductance and R values for the TX and RX but don't have any idea what number of turns or gauge wire the stock coil uses to act as my guide. I could use any best guess on that as well as any links to concentric coil building. Thanks.


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		Quote:			Posted by critterhunter
	I've slowly been gathering information and want to make a 12" coil for the QXT (XLT uses the same 9.5" stock coil). Has anybody winded a 12" concentric coil for them? I've got the inductance and R values for the TX and RX but don't have any idea what number of turns or gauge wire the stock coil uses to act as my guide. I could use any best guess on that as well as any links to concentric coil building. Thanks.

		Quote:			Posted by GoldDigger1950
	You have come to the right place. Eugene should be along in a few minutes with some resources. I have hankering to help you but it is 3:00am here and I can barely keep my eyes open. Off to bed. If Eugene has not helped you by the time I get up, I shall. Good night!

Hello critterhunter and Everyone . I have enough Whites search-coil information buried somewhere on my computer to get you started . It will take some time [at least one day] to answer you with some pretty accurate information . Concentric coils are a little bit more complex to wind than a simple DD widescan coil . The widescan DD coil will give you more depth but a concentric coil is much more accurate for pin-pointing . Just keep posting on this Forum because you will need 15 posts to download any schematics or technical image files . 15 posts take no-time at all . Post your best metal detecting experiences or any questions and in one day or two you can have 15 posts . I will start looking for the information you need and post at least by tommorrow .
HH and Best Regards............Eugene

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First, I was able to download some attachment files from messages on here already so perhaps the site no longer requires 15 posts?

I do have a background in electronics. I took some classes in high school, built some things on my own over the years as it's a hobby to me, and recently graduated from a heating and air conditioning school. Since 90% of HVAC problems are electrical the school leaned heavily on electrical troubleshooting, sensors, motor windings, etc. I've also been involved in electric RC air planes for about four years and have knowledge on winding brushless (3 wire) motors and so on, so this coil winding business should be well within my skill range.

I've been surfing here and in the Geotech forum and printing out/reading material on winding coils. There is very little Whites specific information out there, as I was only able to find the resistance and inductance values for the Whites 9.5" coil for the XLT, QXT, etc. I was unable to find any wire diameter or number of turns on the stock coil to use as my guide to get started, so both of those numbers are up in the air and I guess would just be found by trial and error? If I'm thinking correctly, the gauge of the wire isn't critical in that the number of turns and length of the wire will somewhat compensate. I would at least think that the length of the wire could compensate since resistance is determined by the gauge of wire and length. Meaning, if my resistance is too low (too large diameter of wire) then I could lengthen it or at least step down in wire diameter to compensate. Any thoughts on that?

I could use a good guess at what wire gauge(s) to use and a starting guess at number of turns to put me in the ball park. By trial and error and taking measurements I can then adjust those things to get the proper readings.

The only information I could find on people winding Whites coils was smaller concentric coils in the 6" or so range but they listed no information for them.

Oscilloscopes...Yes, that is one area where my knowledge if not too strong. I would guess (?) that you don't exactly need one in that you can use a multimeter to read and tune the inductance? And, if I understand what information I've read so far you then use a small tuning (half or one turn) loop between your TX and RX coils (both in series) to null out the final bit? I would assume you'd just do this by firing up the machine with the coil attached and then moving that small tuning loop around until no falsing is heard, then glue in place?

I'll post another message below with the steps, questions, and lack of info I need so far so people can guide me.

One final question: From what I'm reading double D coils do NOT go as deep as concentrics. The only advantage to double D designs is better ground nulling in highly mineralized ground (not a problem in my state so I don't really need one) and a wider sweep area. However, I overlap my sweeps and don't find myself missing deep targets with the concentric on the QXT (other machines do not have the same ability to "hear" and sound off to nearby deep targets for me to further investigate like the QXT does).

Having owned this and numerous other machines over the years, I find the QXT to go deeper than anything I've ever used. The only exception to this rule is the Explorer in highly mineralized ground...but not depth advtange in most ground conditions. And the Etrac a friend just bought. It's got a good 2" depth advantage in non-mineralized ground. That bothered me, as up until then nothing I've used or gone head to head with on undug targets got as deep on targets as the QXT. Now, remember that the ETrac is using an 11.5" coil so it's got a 2" size advantage for deeper targets. That's why I want to build a bigger coil so the QXT Pro I'm using can keep up with this new machine in depth.

I'm mainly a coin hunter and I've always heard that 12" was the max coil size to still keep sensitivity to coin sized targets. However, I see there is some debate these days and some are saying 14" coils are the max size for coins now. Any thoughts on this?

I will wind a double D if it's that much easier and perhaps something in a 14" or so would compensate for any extra depth a 12" concentric will have? Still, I'd prefer a concentric in either a 12 or perhaps 14" if you think that would still be sensitive to coin size targets.

I'm getting kicked off the computer in a minute so if I don't get back on today I'll post more info later about materials, steps, etc that I have so you can fill in any blanks.

By the way, I can send the coil to a friend who has an oscilloscope if that's a must but do I have to send him the machine too or can he tune it without the unit?


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Yes, I've already printed out and read that article (I was able to download it just now to check). So I guess that's another name for a concentric coil, eh? The next message will be in more detail with steps with what I understand about the process so far in the hopes that others can fill in the blanks. But first, off topic with the next subject for a minute...

The QXT (Quantum XT) line of detectors were designed with the help of some guys who left Fisher and joined Whites in the 90s. At the time the machines that set the standards in depth were the CZ line by Fisher. They got probably deeper than all the other machines on the market in most people's opinions but the problem was they had very basic discrimination and harmonic ability. The QXT was kind'a a morphing of the best of both worlds...the excellent depth of the Fishers combined with the excellent discrimination and audio harmonics of the Whites.

Set up properly this machine was the deepest machine I ever laid my hands on. Mainly, I hunt in discrimination mode (discrimination mode goes as deep as all metal on this machine) but don't discriminate out any of the discrimination (notch) zones, of which there are eight. By including all zones including iron and ground you maxamize depth and minimize target masking. I would then assign high tones to the zones I was interested in and then listen for those in the midst of unwanted (low) tones. That, combined with the very fast response between targets also made this machine deadly on coins mixed in with trash. I could pull silver (high tones) next to trash targets easily. In fact, I tried the 5.3" (6?) coil and found it had no better target seperation on this machine because the QXT was already so fast at recovery response. However, I would like to try winding say a 3" coil to try to really laser some guys out of very heavy trash at certain spots this is unhuntable with any machine I've used.

With the above settings I also would turn OFF noise reduction (this is sort'a like a hot rock reject function and will quiet the machine down in bad soil, but I leave it turned OFF as it provides slightly more depth). I also can crank the sensitivity up to 18 on a QXT Pro (about 16 on a regular QXT). Ground tracking would be turned off (it's not as good on the QXT as it is on the 6000 di pro and can drift) and make sure to ground balance the machine properly in a clean spot, rechecking every so often). All other functions would be set to default by first loading the COIN program and then making all the above adjustments.

As I said, the only machine I've owned or gone head to head with on undug targets that would get better depth was the Explorer and stock 10.5" coil in highly mineralized ground. In normal ground I saw no depth advantage over the QXT. However, the new Etrac with 11.5" coil has a good 2" or more better depth on targets. Two different targets, silver dimes at 9.5", rang and ID'd loud and clear on his machine. I was able to get slight audio and occasional coin ID on the QXT over those targets but to be honest they were so fringe that they were even more "fringe" then the "phantom" targets I would normaly dig in the hopes for deep silver that is just outside of my machines normal abilities. I would say that I have a max depth on silver dimes of about 7.5 to 8.5" on a QXT in most ground conditions.

I've owned a few Explorers over the years and they do have some advantages...mainly ability in highly mineralized ground and very percise "splitting hairs" to, for instance, try to edit out pull tabs while accepting most gold rings. However, they are picky about their sweep speed (the QXT has a very forgiving range of sweep speeds but needs a fast tight sweep over a real deep target to try to pull a coin ID out of it). The Explorer visual ID also floats around too much, often causing me to suspect a bad target where as the QXT is more rock solid with it's indications. Beyond that, the Explorer is of course heavy and will wear me out both physically and mentaly (you have to stay on top of it all the time to work it properly), where as the QXT doesn't fight me with what I'm trying to do. Much more "easy going" when hunting with it.

On another topic, the only major complaint I have with the QXT is it only has eight zones of discrimination. I can use the audio and visual response to get a pretty good idea of what the target is...Pennies bounce between COIN and PENNY zone. Dimes stay more in the COIN zone but will hit the penny zone here and there. Quarters sound more big, flat, and fuzzy. Silver dimes and quarters have a slightly higher pitch to them, etc. Audio harmonics of trash targets such as bottle caps and iron are easy to hear and avoid when wanted. Even gold rings in the foil/nickle range have a nice soft and round sound to them while pull tables will sound a bit harsh and sharp.

Still, it would be nice to be able to get a numerical number for the various targets much as the MXT does. I was wondering if anybody has tried to reprogram the software on the QXT to also display the numerical number of the target. It would seem rather easy to do as I would guess that the computer already obtains a number for the target and then decides which zone that number falls into, but I could be wrong about this. I'm not a fan of the MXT line (14khz doesn't hit as hard on silver/copper than the lower frequency of the QXT/XLT), but if I was primarily into ring hunting I might buy one. I'm happy that the QXT/XLT line use a lower frequency in that they are a little hotter on silver/copper as that's mainly what I'm after.

I'll use a double D design if you think it will go as deep as a concentric in non-mineralized soil but from all the reading I've done they don't. I also don't like the harder pin pointing they have. Still, if say a 14" double D would probably match a 12" concentric in depth on coins then I'll wind one of those. Would like opinions on that.


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After skimming the article I thought it was in a concentric configuration. My mistake.

One other last point about why I like the abilities of the QXT. These machines feature what is called a flowing sizing display. Coin sized targets show up as two bars at normal sweep speed where as something like the top of a can will have 3 or 4, bigger targets more, etc. It makes it easy to quickly determine the size of the target without having to pinpoint and try to outline it to see just how big it is. I prefer this flowing sizing display over the Garrets forum of sizing which forces targets into predetermined sizing blocks to try to indicate the size of the target.

Anyway, I just thought you guys would like a little history for reference so you can see where I'm coming from. I'm no newbie to metal detectors or electronics, but there are several blank spots in my knowledge of winding these coils.

I'd still like to hear opinions on say a double D 14" versus a 12" concentric. Which will get better depth. I'm not interested in the ground nulling abilities of the double D as we have very little problem with that in my state. The trouble with pinpointing with them is also something I don't care for. However, I may take your suggestion and try winding a double D first before doing a concentric. Will a 14" double D still have good sensitivity to coins?

Here goes...Corrections or advice welcome...

Supplies:

Coil: From Hayes...a 12" Concentric and/or 14" Double D housing

RF Shielding: Best I hear is that Scotch (3M?) "24" (or whatever it's called) tape. Anybody have a source to buy a single role?

Cable: I hear S-Video cables work good...2 coax shielding type? Other sources?

Plug: I do have a web source for this plug but I'd prefer getting it at a local store. Isn't this a microphone or CB plug? Is it four pin or five (not sure if the ground is the fifth pin or if it uses the shielding in the cable for it's ground)?

Wire: Depending on gauge I can probably get it at Radio Shack? I still need a best guess as to wire gauge to use.

Capacitors: I'm not sure what raiting and type they are so I could use input on this.

Number Of Turns: Again, anybody have a good starting guess for TX, RX, a bucking coil?

I'm going to assume the double D design would still need the capacitors but (haven't read up on their design yet) that don't don't use a bucking coil?

Process:

Wrap your TX/RX/and then bucking coil onto a piece of plywood using wood dowl rods as the guide. Since no metal is present and the coil is well off the ground there should be no problem with interference.

While winding the TX check it for resistance and LCR readings and get to specs. Too much resistance- use larger wire. Too little- use smaller wire. LCR Readings too high or too low- wind with more or less turns.

Do the same process with RX.

With the bucking coil I'm a bit confused as to what to do. How do you get in the general ball park with the gauge and number of turns in order to tune out the TX? I'm guesssing there is a formula for this based on the wire gauge/number of turns for the TX, since R and inductance values are only listed for the RX/TX?

To tune the coil you use either a half or full small loop in between the TX/RX, which are both winded in series? Move it around until it nulls out. If you don't have an oscilloscope can you do this with a meter? Perhaps by getting the RX to read as near to zero voltage as possible? How close to zero is good?

Once all is well with all three coils you remove them from the plywood and then epoxy in place in the coil. I would assume you then re-adjust the tuning loop and then epoxy it in place? Any better process here?

Like I said, I can use any all all advice and procedures for doing all of the above. If somebody could walk me through what they do and how they do it for a Concentric and a Double D coil that'd be great. Thanks again for all the help.

One final question: A friend has an oscilloscope but lives in another state. If I have to use one to get the proper tuning done on the coil does he need the machine as well, or can the coil be powered up and tuned without it?




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