GPX4500....What is the maximum depth

nero_design wrote:(And here we go again) Settle down JP... please! You're quite right on some of the points you raise (including DVD players). But I think you tend to take it personally if anyone happens to disagree with your own way of doing things. Posting negative personal responses will only have the effect of deterring others from posting of their own experiences here.

nero_design wrote:
You guys!???? Why does everyone feel a need to strike up an argument whenever I propose that I've done something different?

With respect Marco, please don't take contrary opinions to yours posted by others as everyone feeling the need to strike up an arguement with you. May I suggest it may be you who "takes it personally if anyone happens to disagree with your own way of doing things"
It is not the world versus Marco, it's a forum where opinions are shared. For the record I was left scratching me head at your post and was wording a reply when I read Narrawas which mirrored my own so I didn't bother.

Now here's where we hold hands and sing Kumbayah

I spent this afternoon verifying that there's absolutely nothing wrong with using the DD coils in Enhance mode. .....Did you catch that, Narrawa? ; )
Yes mate yes i got it, nil intelligence from your above posts! Your detector was looking up at you with squinted eyes mumbling ...ya goose!
Honestly Marco, go back over your posts with pics above & tell me if you can make sense of it? then tell me, were you riding in a truck or taking a shite? Read below.
I was using it in Mono on both days though when recovering targets so there's always a chance it was in Pseudo-Mono at the time.Thers a good chance your control box was on back the front also.
Was you even there that day?

No one said you "cant" use enhance with a DD coil, JP just pointed out why you shouldn't, perhaps you should try cancel with a mono next time, its very good at eliminating ground noise
The point is, alot of people read your posts & take what you say as being gospel.... would you after reading that?

My 4500 is on a plane back to ML ATM, i think iv worn it out, but i could ware out 3 before id have a poofteeth of its many combinations understood, but there are many that i have & yours is not one id be telling others about, unless i took a disliking to them.
Anyway,you have a good heart & thats what counts, so with that its your shout @ the H/E pub whilst i put your control box on the right way round...

PS.Tuna, i will not sing that song whilst holding hands.

Steve,
I was never a girl scout or someone similar, What does Kumbaya mean anyway?
The next line was something like my lord?
Next big meeting of the forum I think it should be sung out loud around the fire. For no other reason than it sounds good.
Brett.

buggered if I know Brett...so I wikkied it....

"Kumbaya" (also spelled Kum Ba Yah) is a spiritual song from the 1930s. It enjoyed newfound popularity during the folk revival of the 1960s and became a standard campfire song in Scouting and other nature-appreciative organizations.

The song was originally associated with human and spiritual unity, closeness and compassion, and it still is, but more recently it is also cited or alluded to in satirical, sarcastic or even cynical ways that suggest blind or false moralizing, hypocrisy, or naively optimistic views of the world and human nature"

bugger now I'm confused, do we sing it or not?
Brett.

Hey Ric, that must have been a disappointing extraction! I bet you had some high hopes there before you unearthed it.

Target yes, depth no. High hopes, no ,gave up on that years ago.
Regards Ric

Thanks very much Jonathan P. for answering my questions clearly, so you NEVER use DD coils since the 4000 came out, very interesting stuff! And all sizes of nuggets down to .1 gram with round coils including the round NF 18" Advantage mono, BUT as you said bigger coils are less sensitive so a round 11" or 12" mono would probably pick up a .1 or .3 gram nugget at deeper depths than a round 18" mono. Also with a smaller round mono it will be more in the center of the coil and less double blip on the edges like a bigger round mono coil. Besides being able to get around trees and scrub better, a smaller round mono easier to pinpoint with and also a smaller hole to dig to get the smaller coil in the hole. But a small round mono has less ground coverage and less depth on about +4 gram or so nuggets.

Along with the round 11" Commander mono, that new round round NF 12" Advantage mono would be good too. CT does not made a GoldStalker round mono in that size. Only in round 6" mono and round 15.5" mono.

Gday

It is commonly heard that mono coils are more sensitive and have better depth than dd coils, of the same size, so with this in mind one would have to wonder at the advantages of using a dd coil on the 4000 and 4500, why not just stick to using a mono coil?

I have touted the use of the Minelab 11" commander mono coil on the 4500 for some time now, based on the fact that I have used it successfully on a few old patches and have found quite a bit of gold with it, with surprising depth on some pieces, no really big pieces to tell about at this stage but a few respectable species, and I dont doubt that I will get some large bigger pieces with it at some stage as well.

The only dd coil use that I have heard about that may be worth a try, is the use of anti interference (salt coils) on the 4500 to help combat the emi issues, apparently some users report that use of these coils, I believe it to be the 14" coiltek, give quieter performance on high emi days, some one else that has tried this might want to add something here about what they found.

cheers

stayyerAU

JP
..twas the GPX factor.... where the conversation was had with Bruce Candy about running in smooth timings with a DD coil....

Ray

stayyerAU wrote:Gday

It is commonly heard that mono coils are more sensitive and have better depth than dd coils, of the same size, so with this in mind one would have to wonder at the advantages of using a dd coil on the 4000 and 4500, why not just stick to using a mono coil?

I have touted the use of the Minelab 11" commander mono coil on the 4500 for some time now, based on the fact that I have used it successfully on a few old patches and have found quite a bit of gold with it, with surprising depth on some pieces, no really big pieces to tell about at this stage but a few respectable species, and I dont doubt that I will get some large bigger pieces with it at some stage as well.

The only dd coil use that I have heard about that may be worth a try, is the use of anti interference (salt coils) on the 4500 to help combat the emi issues, apparently some users report that use of these coils, I believe it to be the 14" coiltek, give quieter performance on high emi days, some one else that has tried this might want to add something here about what they found.

cheers

stayyerAU

I bought an 18" CT round salt (AI) coil specifically to do an area that I was doing in Enhance timings, and the EMI was bad because of the time of year. The salt coil eliminated most of the EMI but the coil signalled on every damn ground noise and hotrock in the vicinity. I thought that I would sacrifice some depth with the salt coil and gain in the EMI stakes and that maybe it would be a better combo than what I was using, but after a few hours I went back to the 20" round NF mono and Enhance timings.

The best coil I have to combat EMI is the 16" NF XP, running in XP mode and mono on the detector. IF I absolutely have the $h!t$ with EMI, that is what I resort to in preference to going home and drinking more rum.

Sorry, yes I know we are a bit off of the original topic Ray.

Thanks stayyerAU, a DD is good to use for newbies on a 4000 or 4500 machine I think. Also less chasing through and after false ground (ghost) noises with a DD, as a DD with less depth, a person can hear-listen for gold through noisy ground better, and sometimes less is more. I heard DD windings "see" approximately 70% less ground effect, but a DD does not have 70% less depth than a Mono, so again sometimes less is more. If a DD sees 70% less ground effect and if only with about 20% less depth, that is a good trade off sometimes, depending on the ground conditions. With the new GPX's now running quieter with Mono's then most people want all the advantage they can get in depth, so a Mono has more absolute depth than a DD.

Then there was that theory that in some grounds a Mono may deflect its signal while a DD's signal would punch down deeper and most of the big nuggets in Victoria were found with DD's, I do not know??

JP said earlier in this thread that a DD is more sensitive(under certain conditions of course), probably means the blade down the middle of a DD is hotter than the outer edges of a Mono, but the DD's hotter middle blade field strength fades out quicker and a Mono maintains the field deeper in the ground. I have heard from Steve Gholson's tests that a DD(no matter which size DD) is more sensitive(hotter) up to about 5"(or so) than a Mono but after about 5" the Mono takes over as hotter, so for shallow exposed bedrock I would use a DD rather than a Mono on a 4000 or 4500. But for any more depth of ground go to a Mono.

You are right Sandy as I have heard the Anti-Interference(Salt) coils do not handle Laterite well and also have about the same ground noise problem as a Mono. Maybe they are worse if the AI coil can not be tuned properly like a Mono coil can. But the AI coils have only 5% less depth than a DD coil of the same size according to CT.

I also have a XP DD in 20"(in fibreglass with the XP switch on the coil itself. I often wondered if the NF fibreglass coils have the same performance as NF plastic coils, or not ??). NF is not making XP's anymore(and probably never will again) as I was told they are either too much effort or too time consuming to make and a second reason is the GPX's have a built in phase switch. Also a DD coil can be used in Cancel Mode for EMI's.

I am trying hard and taking a risk posting this above, and also you should do the same thing in the goldfield and everything you do in life also. "Nothing ventured nothing gained", or "if it going to be, it is up to me". You can make up for not being able to tell the future(like betting on that pony), or not having psychic x-ray eyes for gold by doing this. And yes as Sandy said drink rum for stress relief, or as I say beer, both do help. I take zero-no drugs of any kind, so beer IS my medicine(and weak rum mixes too).

Hello Claimpeg & StayyerAU,
It seems the 4500 depth question has taken a turn to the coils, (mono and DD).
I'd like to go back to the depth and ask a question. Yesterday I buried the 35 gram specie in my back lawn. There was no interference around, and the lawn has no mineralisation. The best depth I could pick up was only 16" with the settings I mentioned in this post. I can not understand why this is. In the bush when I found the specie, it came through as a faint but clear target signal. When I had my 4000 I picked up a 26 gram nugget and the sound was like it covered about a foot of the target area. It was like the ground (LIT UP) all around the nugget. Some one told me this was what they called the HALO effect. Can anyone explain why I could pick up the specie in the bush at 21' deap, but not on my lawn at 21", only 16' deap.
Regards, Grizz

Gday Grizz

You are right about the "Halo" effect, it seems that a long buried target will always give a better response than a newly buried target, whether the surface of the gold very slowly starts to deteriorate and leaches in to the ground surrounding it or whether the nugget actually attracts microscopic gold particles to it I am not sure.

The theory surrounding "Bug Nuggets" seems to suggest that nuggets can actually grow in situ, and attract minute particles of gold from the surrounding ground waters, such as in laterite type ground, I suppose that its also possible that over time a nugget could produce the "halo" effect due to impurities such as copper/silver and other minerals breaking down and moving into the surrounding ground, just like when you have a lump of ferrous material rusting away, when you dig it up you can often see the staining in the surrounding soil as particles of it have deteriorated and washed away from it.

One other thing to consider when you are carrying out depth tests at home and even though you think that your machine is quiet and all, I think that the electronics will still be working harder there to keep it quiet than what it would be out in the bush, due to all the surrounding power lines etc, and that you are surrounded by much more mettalic material as well so this will most likely contribute to de sensitising your coil too, one other thing is that you may not have carried out a ground balance on the machine as you would have in the field, perhaps causing it to be less sensitive.

There are many factors that can effect the overall performance of your detector at home, but if you went out in to the bush to do the same tests away from all this other influence I dont see why you would not get a closer result than 16", its hard to say how great the "halo" effect is?, but I suppose that would change in relation to the target and the condition of the grounds minerals and salts composition etc.


cheers

stayyerAU

Thanks StayyerAU, all very interesting. I like the theory about the small gold particle's. Has any one ever panned the dirt, that they have found a nugget in. Could be quite possible there is a lot of small gold dust around the nugget. Must try it next nugget I dig.
Cheers, Grizz

Hi
A mate in WA got 270 ounces out of a glory hole.Processing the dirt that surrounded it he gained an extra 6 ounces.Nice work if you can find it
Cheers Dig

Hey Beer Beeper.. u wrote..

"Then there was that theory that in some grounds a Mono may deflect its signal while a DD's signal would punch down deeper and most of the big nuggets in Victoria were found with DD's, I do not know??"

Mate where did u get that from as I have suspected this to be the case for some time especially here in Dunolly where I do most of my detecting.... the gold gods are more generous with me when I use a DD coil over a mono.. even thou I have the mono running nice and quite in normal timings (4K)... I thought that it may just b my imagination.

Marco (nero design) wrote a post about it some time ago...


Ray

raymondj wrote:Hey Beer Beeper.. u wrote..

"Then there was that theory that in some grounds a Mono may deflect its signal while a DD's signal would punch down deeper and most of the big nuggets in Victoria were found with DD's, I do not know??"

Mate where did u get that from as I have suspected this to be the case for some time especially here in Dunolly where I do most of my detecting.... the gold gods are more generous with me when I use a DD coil over a mono.. even thou I have the mono running nice and quite in normal timings (4K)... I thought that it may just b my imagination.

Marco (nero design) wrote a post about it some time ago...


Ray

The thing is Ray, a Monoloop and a DD coil all transmit in exactly the same way and sorry to say in a lot of ways receive in the same way too except the DD coils manifest their response where the two coils cross. So how can this bend or warp the signal in the ground? Marco was as far as I know quoting directly from an article written by Jim Foster in the AGG@T magazine a few years back and has then run with the concept.

A DD coil removes a lot of ground noise but still takes advantage of the full spectrum of the Normal timings so on large targets in noisy ground has an advantage over a Mono due to the excess ground noise which will mask or hide the response of a deep large target.

In Victoria back in the SD days, Coiltek discovered an increase in response if they ran a higher voltage (or constant voltage compared to the 6V standard battery of the time) so marketed their 7.4 V system, this had the effect of increasing the ground signal with Monoloop coils ((enhances the ground effect), I tested this myself in Victoria BTW) making them too annoying to persevere with but could be controlled with a DD coil helping to popularise them no end especially the larger ones (24" Coiltek DD springs to mind here).

There is now a very viable function in the GPX series which should be providing far better depth with DD coils if savvy operators where switched on, or if the original concept had merit. This is the new Sharp timings which provide a 20% increase in power from the coil which is far more than was ever provided by the higher voltage adds ons of the SD days, but after experiencing the joys of Smooth and now Enhance I seriously doubt it, which strongly suggests there was always much more performance with Monoloop coils just that people did not like the noise.

JP

Jonathan Porter wrote:Marco Nero was quoting directly from an article written by Jim Foster in the AGG@T magazine a few years back and then ran with the concept.

Not quite. What happened was that I observed a common link between Coil-Type Vs Targets-Recorded after taking on a bet with a fellow employee... and then dug up the article by Jim Forster and found that it supported my theory for modern detectors, even though it was written quite some years ago when the GP 3000 was king. Jim was kind enough to drop by and comment on the subject when it was in swing earlier this year. As for DD's Vs Monos, the situation was more pronounced with a couple of different DD's from Coiltek which generated a slightly denser field compared to Monos (at the expense of 20%+ of the battery life). I've since confirmed this with Coiltek earlier in the year.

Ray: Jim also wrote that Minelab had sent a rep out to talk to detectorists to try and enlighten them on the subject of Monoloop coils... claiming that Monoloop coils would be deeper-seeking than the (then) popular DD coils....but the prospectors kind of laughed and went back to their Double-Ds because the Monos were pretty noisy on the GPs. It's not until now, with the GPX detectors on the market, that people are really embracing their Monoloop coils.

On the Halo Effect: I'm sure it can probably occur when residual alluvial gold fills the soil surrounding a nugget... but the term was originally used to describe the effect that occurred when a metal coin began to corrode and its metals leached into the surrounding soil. This led to the target appearing larger to the detector than it really was ...until it was dug up.

The point is Marco , both coils are essentially the same from a transmit point of view and also in the receive, so how the heck can the DD coil avoid or facilitate the bending of the signal? The truth of the matter is a similar sized DD coil is essentially half the transmit and half the receive of a similar sized Monoloop with the added constraint of having the two windings near each other which requires them to Null each other out before anything can be passed onto the operator. Due to the smaller transmit and receive windings however a similar sized DD coil will actually have more sensitivity which overcomes some of the Null.

Due to the Null a DD coil will essentially ignore the majority of surface feedback (ground effect if you like) from the ground, hence their popularity (not much different to what the new Smooth and Enhance timings are doing in the GPX range with Mono coils). The Coiltek constant voltage systems took advantage of the "direct to coil coupling" of the SD range which had the effect of increasing the magnetic field out of the coil, this did not mean however the operator was gaining a similar increase in depth, in some cases they actually got less depth depending on the ground conditions. This higher voltage technique was/is not as useful in the GP range of detectors due to the Dual Voltage system, as in the coil produces the exact same sized field as before just that the audio sounds a little brighter.

The GP range of detectors have far more sensitivity than the SD range thanks to the Dual Voltage system which also helped popularise the use of DD coils in noisy ground types such as the Golden Triangle, however more than 95% of coils sales these days are Monoloop coils due to the new timings of the GPX range working so well in ground that has been frequented regularly since the GP extreme's release in late 2000, the exact same technology that is present today in the GPX range (namely DVT). Obviously the new timings are working much better than the old constant voltage/DD combo options of yesteryear. I should also point out Marco this is in the same ground you theorise the DD coils are punching into deeper due to the warping of the magnetic field etc, yet the operators are using Monoloop coils which kind of negates the argument don't you think?

JP

Hi Nero , JP
I don't agree it's as simple as JP says, by saying a mono with the largest transmit and receive area as measured against a DD with the two smaller windings has a sensitivity /detection depth directly proportional to these coil winding diameters.
Something else is going on here and a good example is put on a Coiltek 250mm Elliptical DD and see how deep it goes,if this detection depth was directly proportional then a 24" round mono would detect at an easy 2X the depth it actually does .
I only purchased this small coil to pick up the small surface bits and was shocked how deep it went for such a small coil and being a DD at that.
Mark

JP ,I seem to remember Jim saying that the more complex winding of the DD coil meant that it produced a denser field (due in part to the related energy drain). There's definitely something else going on here. Why else are operators claiming that they find more large gold with their older SD/GP detectors (using DD coils). The field emissions may be textbook behavior, but the observations by others indicates otherwise. So regardless of how many people say "this can't be true", there's an awful lot who come forward and state that they own both the old SD/GPs and the new GPX and they still get more of the big stuff with an older detector. The ONLY obvious difference in all cases is the coil used in relation to larger, deeper targets. We know that the characteristics of Monoloops and DD coil fields are different to begin with.

JP, I agree that your explanation appears logical. But when coil makers and detector manufacturers have noticed the same thing, it's probably time to consider the possibility that there may be exceptions to the applied rules. It seems others in this thread have observed this same phenomena first hand. And whilst you say it can't he happening, it seems that it is. Until I can find another explanation for the high number of large, deep targets being found with DD coils versus the Monoloops, it may be worth keeping an open mind. I work with high intensity electromagnetic fields at home in experiments using high tension field generators and I'm occasionally surprised to discover unusual and unexpected behaviors when fields of different frequencies overlap. Fields are easily deflected or coupled. The odds are real good that I'll never be having kids.

Mark: the smaller coils tend to have a strong and dense field emission - especially the smaller DD coils. It makes them a great coil for super-small targets although sometimes they punch sharp and deep. I found the elliptical 250mm Coiltek DD to be a super sensitive coil although I liked their 6" Goldstalker (round/mono) just a tiny bit more for my own uses. The Americans sure love using these as Sniper Coils.

JP, Marco...

I see the logic in what both of u are saying... JP what u r saying makes sense, how could the signal b deflected??? however I understand what Marco's has written also. Here in the triangle and I am only referring to this part of the world there seems to be more deeper targets located with DD coils then Mono's. I am referring to large deeper nuggets and strangely mainly with 2200's then later machines. That is big nuggets at extreme depth. Again I am only referring to Victoria and in particular around Dunolly. I am not aware of any BIG finds with the 4500 at depth here with a mono coil. (There are areas hiding such large nuggets still at depth)

My own experience is that larger nuggets at depth have been located with a DD coil on a GP3ooo and GPX4000 (45 grams).. smaller nuggets up to say 5 grams with a 16inch N/F Mono. I thought it was my imagination/coincidence till I read a thread by Beer Beeper and others and it started me thinking. I am just asking is there something else we need to take into the equation or is it just coincidential... I dont know.

Thanks

Ray

Something has been missed & that is most of the older detectors don't have the s/s mode or enhance, this is why bigger gold at depth is being found with older machines.
Most operators of the 4000/4500 are spoilt into using the quieter timing, & as you know you will lose depth on s/s & enhance.
On the 4000/4500 there are the forgotten timings of Normal & s/extra, for the operators to go back to using them is a hard thing to do after being spoilt.
I try hard to fight this erg but over most of the ground i detect the enhance timings are a winner because of the noise factor when using the other timings.

Narrawa... no I dont use smooth timings here for the reasons u set out... the trade off is to great in loss of depth so thats not it.

Ray

MS wrote:Hi Nero , JP
I don't agree it's as simple as JP says, by saying a mono with the largest transmit and receive area as measured against a DD with the two smaller windings has a sensitivity /detection depth directly proportional to these coil winding diameters.
Something else is going on here and a good example is put on a Coiltek 250mm Elliptical DD and see how deep it goes,if this detection depth was directly proportional then a 24" round mono would detect at an easy 2X the depth it actually does .
I only purchased this small coil to pick up the small surface bits and was shocked how deep it went for such a small coil and being a DD at that.
Mark

Mark there is an inverse square law relating to the depth attainable with all detectors, as such an 18" monoloop will not necessarily attain twice the depth of a 9" Monoloop etc. My younger brother once found a 33 ounce nugget at close to 3 feet with an 8" Monoloop, my deepest digs were on nuggets ranging from 70 grams through to 83 ounces all around the 3 foot mark with coils ranging in size from 18" (all Monoloops) through to 25". Can you see the correlation here?

The point is depth is relative and varies widely depending on mainly the level of mineralisation followed closely by the style of coil, depth of target, orientation of target and also Halo effect (which is I feel a combination of weathered oxidizable metals such as copper and silver leaching into the surrounding soils and perhaps even gold particles attracted to the nugget itself (the nugget acting as a nuclei) that have then not been disturbed in millennia (in other words the ground is electrically or magnetically virgin)).

Marco DD coils are an old and proven design , the original prototype SD detectors were designed around them, it wasn't until later during the testing phases (maybe a year or two later) that Bruce designed the concept of a Monoloop coil to provide even more depth. I would say the ability to transmit and receive off a single winding through switching is far more complex than the original DD design wouldn't you?

Just to clarify what a coil actually does, the detector generates a magnetic field (in the case of the DVT machines the fields are created at two or more voltages) this magnetic field is transferred to the ground via the coil, the ground and also any metal targets in the ground are excited by this field, through timings the bulk of the excitation of the ground (especially the "close to coil" surface minerals) are allowed to dissipate away (the ground creates or forms eddy currents which decay away very quickly). If a target is present the magnetic field transmitted from the coil excites the target which then creates eddy currents of its own which start flowing through it, these in turn create or are seen by the coils own field as a very faint disturbance which is then picked up by the detectors extremely sensitive receiver either through clever switching in a Monoloop coils case or on a dedicated receive winding in the case of a DD.

If as is suggested the field is warped or bent somehow either during the transmit stage or the receive then it would happen with both coil styles not just with the so called Complex DD coil, in other words Marco it would be across the board. Logic would tell you the DD transmit suffers somewhat from the proximity of the second winding as well as the receive, however thanks to this relationship the DD coil is able to ignore a lot of surface saturation of the transmit field thanks to the nulling effect of the second winding. Just because a Coiltek DD Pro coil uses or has more current draw does not mean it produces a denser magnetic field, regardless of what Jim Foster would have us believe, remember the GP and GPX range feature DVT which regulates what power is supplied to the coil, these are very precise measurements which if tampered with can throw out the whole detector (something I think modders have been doing with the faster clock speeds).

Regards

JP

A mono coil transmits and receives from one winding. A DD coil just simply transmits from one winding and receives from the other. In other words, a mono and a DD operate the same except the receive coil in the DD is parked off to one side of the transmit coil's ground signal. The two DD windings are also smaller than a mono of the same shell diameter so in some cases the smaller equivalent diameter will be what is required for a particular nugget at depth and sometimes it won't.

Coil makers usually state the diameter of the coil housing and not the diameter or dimensions of the internal windings and the "D" shape can vary somewhat so it gets a bit hard to do accurate comparisons, but the TX and RX coils in a 17.75" DD (actual winding size) are each around 17.75"x10.75" and a bit closer to an elliptical shape than a D. In this example, the circumference of each winding is around 46" which is equivalent to a round mono with a true winding diameter of approx 14.6".

Because it is parked off to one side, the receive coil in a DD only sees a fraction of the ground signal seen by the transmit coil (or a mono) and is therefore quieter but there's nothing in the theory to suggest a DD's transmit field penetrates a deep mineralised layer better than a mono of the same size (and shape) as the DD transmit winding. The D shape distorts the field somewhat but so does an elliptical shape and ground minerals.

It is said that a DD has a blade like search pattern but this is only true for shallow small targets, ie, small nuggets that would normally only be detected at the edge of a mono coil. It doesn't actually transmit a blade like field. The transmit field is effectively the same as a mono's or perhaps closer to that of an elliptical coil. The blade doesn't actually exist or make any contribution whatsoever.

A large mono will give two signals when passed over a very small nugget, one from the left edge and another from the right edge but a DD will only give one signal at the centre.

It's easier to see what occurs if we draw an edge on view of a DD looking from the rear. Two overlapping lines with the transmit coil on the left and receive coil on the right.

If we transmit and receive from the LH winding (a mono) we will obtain a signal from a very small nugget at the left edge and another from the right edge of this winding (or anywhere on the circumference), but if we transmit from the LH winding and receive from the RH winding then the signal at the LH edge of the transmit coil disappears because the small nugget is then too far from the edge of the receive coil winding. We do though obtain a signal when the same nugget is within the so-called blade region because it is then very close to the LH edge of the receive coil and maximum signal occurs when the nugget is in the centre of the overlap region or where the winding cross at the toe and heal.

A very small shallow nugget will only give a signal at the edge of a large mono but a slightly larger one will give a signal across the whole face and this can result in a broad ground type signal which may be easily ignored when patch hunting. The signal for the same nugget is always very distinct and sharp though when using a DD and the reasons are as outlined above. This sometimes explains why some rather obvious shallow ~1 grammers are ignored by operators using large monos but are easily detected with DDs.

If the DD coil is too large or the nugget is too small then the small nugget may not give a signal at the centre of the coil but may towards the toe or heal and this is because the path to and from the nugget is shorter where the winding cross and the field is a little more concentrated by the pinched D sections. You could argue you are effectively swinging three coils when swinging a large DD or elliptical coil, one large and a smaller one at the toe and the heal.

The reason why a large symmetrical nugget (at depth) gives the best signal when under the centre of a DD becomes obvious if you look at the edge-on diagram and draw lines from the centre of each coil to a large symmetrical target at full depth. A target directly under the centre of the so-called blade gives the shortest path from the TX (transmit) coil to the target and from the target back to the RX (receive) coil.

A large nugget's signal at maximum detectable depth will normally be at the centre of the coil but this is influenced by the shape of the nugget for both a DD and a mono. A large flat nugget inclined at say 40 degrees will give a signal displaced in the direction the upper surface is facing (perpendicular to the face) and an inclined steel spike gives a signal displaced in the direction the upper end is pointing. This results in an error when pinpointing if the target is deep and we then have to "move" the hole as it deepens. The error becomes smaller though when we enlarge the hole and bring the coil closer to the nugget. Note that the path to and from an inclined flat nugget is greater than if it is flat-on at the same depth in the same ground. This results in what some call a suck and blow signal in one direction and a single response if we check it at 180 degrees.
So for large nuggets at depth, a DD and mono are effectively the same except the DD transmit and receive coils are smaller for the same shell diameter and the DD suffers less from ground noise because the receive coil is positioned off to one side of the much larger ground signal seen by the transmit coil. If an 18" DD is found to be optimal for a given nugget at depth then you may find that a 14-16" mono is also, but the DD will always suffer less target-masking ground noise than the mono in squeaky ground. If you have drawn lines from the transmit coil to the nugget and from the target back to the receive coil then I think you must conclude that there is a slight to medium loss for all but the tiddlers compared to when using a mono with the same dimensions as the individual coils in a DD, but again, the mono will be noisier.

A particular coil might draw more current at the same applied voltage but this doesn't necessarily mean it has a "more intense field". We can make a coil draw more current simply by dropping the inductance but we are then only swapping coil turns for current. One particular third party mono has 1.5 times the resistance of a ML mono and yet it was, and still is, one of the most popular coils today so I wouldn't get too carried away with claims regarding a "more intense field".

Rob.

When NASA prepared to send its first man into space, a *whopping 98% of doctors said that it could not be done.
They said that the human body would cease to operate, the lungs would cease to breath, the intestines and organs would shut down, the eyes would change shape and distort the vision and the heart could not pump blood without the effect of gravity. These included some of the world's experts in the field of medicine. And lets not forget that the Van Allen Radiation Belts around the Earth should have killed any astronauts passing through them on the way to the Moon.

That said, this thread is about people's experiences with detecting deeper targets and it is certainly worth noting that others have noticed a difference between the coil types and their success rate. When I speak to other full-time operators, manufacturers and numerous dealers, they all seem to say the same thing: that they have a higher success rate on larger and deeper targets with DD coils. Jim's article appeared to legitimize this observation with an explanation. One which also supported recent overall retrieval statistics. To say that Monoloops and DD coils are the same is a bit of an understatement. Monoloops can't be used to discriminate targets nearly as effectively (if at all) as a DD coil. They're also unable to handle highly mineralized soils as well (or as quietly) as a DD coil either - which is why Coiltek are about to release a new batch of DD Goldstalker coils shortly for GPX users. Rob has just explained that the DD sees less ground than a Monoloop and therefore less mineralization. Otherwise we could throw away all of the DD coils altogether and run with the slightly lighter and faster-swinging Monoloops. But Rob: How do you account for Jim's claim that no large nuggets were found in the entirety of Australia by a Monoloop in the year given? And why do GPX users produce less of the larger,deeper targets today when compared to those using older detectors who are forced to use a DD coil. I've seen some big successes with monos on GPXs so it's obvious they can do the job. But the balance seems to favor Jims observations.

Remember that Jim was talking specifically about Gold trapped within or below a Second Layer of gravel wash... behind or beneath a second layer of mineralization. You're saying that there's no reason for mineralized soils in dual layers to deflect signal from a Monoloop coil. Jim suggested that the Monoloop emission field would pass through one layer but not so much through a second layer whilst sending the signal back through the first layer and back into the coil. There's a difference in field shape and density between Mono & DD. There's also a difference between the transmit and receive function of each type of coil as well. So I would expect to see a difference in results, not the same degree of signal return.

I accept that what you say is the accepted view. All I can go by is other people's observations and experiences and look to see if they support current theories... or not. In the end, it may come down to using what seems to work for you and your location. This isn't a big deal mind you. And it's not a personal attack on anyone's coil of preference (which seems to be a round Mono coil in most instances). But at the moment, 'it is what it is', as they say.

* Souce: 'When We Left The Earth' - Discovery Channel's NASA FILES - Episode 1.

Nero,
The roles are reversed at pulse switch off and the nugget becomes the transmitter and any nearby coil becomes the receiver. The latter might be the transmit coil or any coil parked off to one side. It is very difficult to see how a more intense field can be involved here so this just leaves a small benefit from using a DD’s short time constant receive coil versus using the mono's long time constant transmit coil as the receiver.

The fact is though, there is no blade like field and no “more intense field” involved here other than a small coil has more intense local field than a large coil. The tx field obviously passes thru all layers and the nugget’s field obviously passes back thru all layers to the observing coil whether it is a DD’s receive coil or a mono. This surely must be obvious?

These debates didn't occur back in the vlf days when most of us just swung the stock coil. Things only became complicated with the huge array of coils on offer for ML PI detectors.

Nuggets aren't coins or spheres or man made blocks of metal of a known shape and size, they are nature's non-ferrous junk and come in a variety of twisted shapes and sizes. Some are relatively solid lumps, some are more like a bunch of smaller nuggets loosely joined together and others are thin flakes or slabs. This means we would need to wind a coil to obtain maximum depth on each and every nugget buried at that angle in that ground at that depth and any other coil may miss it by a fraction of a millimetre to many centimetres.

I think the DD's success can be partially explained by history and chance. The 2000 was released with an 11"DD and an 18" mono. The 11"DD was good on very noisy ground but the 18" mono covered more ground and was better on a reasonable percentage of medium to larger nuggets. The 11" mono then appeared and it easily beat the 18" mono on the medium size bits, which meant the 18" was a bit too large for these and it seemed rather obvious that more coils were needed to fill in the gaps. (This hit home when I received a good signal from a 1/2 ounce nugget at depth with the original ML 11" mono but none when I checked it with the old ML 18" mono. You could argue here that if an 18" mono and an 18"DD performed equally well on a particular largish nugget at depth then the optimal mono coil would be one somewhere between the dimensions of the 18" DD's transmit coil and the 18" Mono).

Coiltek then released their earlier coils and the 14" DD became a favourite for general use and the 18" and 24" DDs became the flavour for larger nuggets. Note though that all of these DD coils are actually equivalent to smaller monos but don't suffer the same level of ground noise and hot rock problems. These large DDs became a fad in one area in Victoria and some locals thought you were a bit strange if you were swinging anything else. A flaw in this thinking was revealed though when blissfully ignorant tourists began digging large nuggets from "flogged" ground when using medium to not-so-large monos.
One local caught on quickly and began using large custom mono coils (up to 30”) to good effect on some flogged ground.

Big DD coils are just smaller monos and often a smaller mono coil will do better on a medium size nugget than a large coil. Small coils leave deep large nuggets in the ground and large coils leave deep small to medium nuggets in the ground and sometimes we get more accurate information from tourists than the locals.
BTW, I have looked at the Smooth Mode/Enhance patent and it would seem to be a good idea to stick with monos when using these modes. I might be able to clarify this a bit better in the future but it appears so from the text.
I suppose we will know more when the new DD coils hit the market.

You said, “Monoloops can't be used to discriminate targets nearly as effectively (if at all) as a DD coil”. This is true because a balanced coil is needed to obtain the information to discriminate.

Rob.

Hi Rob, liked your explanations and the overview of trends in the coils favoured over the years.
At the moment many people are finding the small 8 inch monos can uncover literally hundreds of nuggets less then 0.1 that abound on the fringes of many quartz reefs and shallow bedrock areas and others are discovering the new 12 inch round mono is good at getting 0.5 gram (or therabouts) pieces at good depth off old patches.

I find the debate interesting but plan to just keep learning to master the 4500 settings to get the best settings and mono coil size i need to increase depth or match the conditions for every spot i detect. I have a have a couple DD coils under the bed...they are for sale.

I think mono's give you the best chance of finding deep gold in all but a few exception conditions. The next step for getting deep biggish gold won't be coil selection as much as Minelab developing new circuity/software with a frequency and settings specially designed for goin deep where a monkey can just flick the switch or dial the programmed setting/frequency.

Tributer

I wouldn’t mind betting we see a drift back to DD coil’s in some cases, considering the number of people using SS and Enhance exclusively. It will only take one operator to find one large nugget with a new DD on worked ground in normal mode to convince some to drop their monos, stop using the new timings and buy a new DD. We are really a rather fickle bunch when it comes to looking for an advantage and we are all open to fads and the placebo effect, which doesn’t always help.
Tourists can often do well by finding out if there is a local fad regarding using a specific coil in an area and use anything but. This doesn’t always work but it has paid off for some. We will always leave gold behind and the only one who doesn’t is the guy who gets the very last bit.

Rob.