UKD2User

I've noticed with all three (yes, I know, not many !) of the detectors I own, that when I put down my detector (either flat to the ground or perpendicular to the ground) I hear lots more pops and hisses (I like to run my machines pretty hot), but when I start to swing over the ground, that noise (including any more persistent EMI) is suppressed. The way I interpret this is that the digital filtering is able to 'lock on' to the comparatively very large ground response and to average it out, as well as being able to subtract salt(s) conductive ground (a signal which is again comparatively pretty large and fairly constant), and - using clever auto-/correlation type algorithms - EMI can also be removed to a greater extent, than when stationary.

I'm still struggling with this I'm afraid! The copper shielding on the connecting cable is effectively a tubular Faraday cage - it's designed to protect the signal wires inside from extraneous EM interference. To operate effectively, a Faraday cage needs to be made of (ideally, highly) conductive material and each component of it needs a galvanic contact with its neighbour, so that electric current can easily flow between any two points on the surface. The (probably spiral wound, rather than braided like in a coax cable) conductors are effectively acting as one - at the lowish frequencies we're talking about anyway. I tried a little experiment with my trusty digital multimeter: 1) Rather than puncture a real coil cable, I poked a pair of sharp probes, roughly six inches apart, into a scrap piece of shielded audio cable. The resistance was about 30 milliOhms. 2) I dug out all the carbon rods I could find: the lower rod from my Nox 800, a DetectEd lower rod for the D2, a middle shaft for a Nokta Legend/Simplex, a TeleKnox UK aftermarket Equinox shaft, and an XP 'CF@ marked lower rod. I was reluctant to scratch up my precious gear but I made repeated attempts to get a reading with sharp probes about 6 inches apart. The only one to give a reading (the DMM maxed out at 60 MegOhms) was the TeleKnox which gave a reading of about 50 MegOhms. I think that, as a consequence of the way it is made (individual carbon fibres are probably not in such good galvanic contact with their neighbours, and carbon being intrinsically less electrically conductive than copper) a CF rod is a relatively poor electrical conductor and a relatively poor Faraday cage, for that matter. My conclusion from this, admittedly imperfect, bit of experimenting is the the coil cable is about seven or more orders of magnitude more conductive than a CF rod - for low voltage DC at least. This is concordant with my earlier assertion that the copper cable is way more conductive than a CF rod and the effect on the coil ought to be consistent with that. One thing that may be happening with a scoop of 'hot' (and perhaps partially magnetized) dirt is that - because an invariant (or very slowly varying - hand movement may equate to a frequency of about 5 or 10Hz) magnetic field is not suppressed by a Faraday cage (you need mu-metal for that, I think) the very slowly varying magnetic field may be directly influencing the signal wires in the coil cable and causing some of the noise?

Very interesting video. I think the analogy that the rod represents salt water is a good one - it is a fairly 'low conductor'. The thing that still occurs to me watching this is that the coil is static, and therefore the filtering algorithms - which in this 'goldfield' mode are expecting a large response from 'hot' ground, I presume, are not in operation. If you were to attach the rod to the coil with a bit of tape - in the position where it causes this effect - and then were to swing this assembly over the ground, I would expect the algorithms to kick in and the response to quieten down. I may try this experiment myself even if you don't feel it's worth the effort. 👍

Maybe it's my writing style! I understand that you're trying to be helpful to me. Respectfully, you don't know what I know about the subject - in the same way I don't know what you know, although I always consider your posts to be worth reading, as with nearly all the other posters here. My first degree, many years ago, was in electrical engineering and I've written embedded realtime software for a living - including implementing digital signal processing algorithms in software, which are remarkably similar to those that are undoubtedly being used in modern SMF detectors. I don't profess to know any more than other people about any of this - there's always more to learn and I relish learning - particularly about hunting for tiny pieces of gold in very 'hot' ground (although I did find about 15g of mostly 9ct gold on the beach over the weekend 😁). However I do think I have something useful to contribute to the conversation. The thing that's important to me above all is clarity of thought in reaching a better understanding. 👍

I'm not denying that, I'm trying to explain/understand it, Sir 😊

If you look back at my only other post on this thread (not easy I know, for more than one reason!), I made it clear that I've never hunted for small gold, so I know very little about the practicalities of it. I also stated very clearly that I completely respect other people's choices. You stated earlier that you don't like 'black and white thinking' - I'm not really sure what you meant by that. I like thinking, and learning.

Of course there's relative motion between the target (and a very small sample of soil) and the coil - my point is that there is no relative motion between the coil and a (comparatively very large) volume of 'hot' ground and the normal digital processing can't operate properly.

You misunderstand me Steve, I'm trying to understand/explain it, not trying to deny what people are seeing/perceiving.

It's been hard to follow this - very interesting - thread over recent days because of the technical problems with this site. I hope those are history. There are a couple of points that I haven't yet seen raised in this thead: I watched the first video linked. The guy is clearly a very experienced detectorist and has lots of knowledge around gold prospecting. He explained that he was new to the Manti and, although he didn't think he was, he was running it quite 'hot' on 'hot' mineralized ground. The detector worked fine, giving clear target signal audio and a useful TID and Target Trace - when swept across the ground + target. The 'problem' came when the machine was laid on the ground - with the coil static - while doing the binary-chop 'scoop and wave' technique (which I see most often on YT used by detectorists using a PI machine, usually while looking for small gold). The Manti, like most of the modern SMF detectors, is not really designed to be used in this static mode. The 'scoop and wave technique' is akin to an 'air test' - and is subject to the same shortcomings - in that it provides a target signal without the normal volume of ground material in each set of digital samples. While being 'swept' the Manti has clever digital filters (using auto-/correlation and other techniques) which are designed to remove ground effects (both magnetic and conductive), EMI and other extraneous signals like 'coil knock' etc. I think that one of the reasons that the machine seems overly sensitive to the carbon lower shaft (and I think there were other influences, as below), in this scenario, when moved by hand in an essentially static situation, is due to this being a 'use case' that the machine hasn't been designed for, and those filters can't operate as designed. Just for reference, my estimates of what's going on in the 8-10 inches around the coil are: 1) Conductivity of human hand ~0.043 S/m and mass, say, 200g (the scoop and wave technique involves waving meat at the coil); 2) Conductivity of carbon fibre ~ 100 S/m and mass, say 30g; 3) Conductivitity of copper-braided connecting cable ~ 60 million S/m and mass, say 100g So - in rough numbers - the conductive effect of the cable is approximately a million times that of a carbon fiber rod, which is about 100,000 times greater than that of a hand. In normal use, the rod is very rigid and is firmly mechanically linked to the coil. The bottom 6-8 inches of the cable can resonate near the coil (I think this was visible on the video, and caused some of the noises). Bottom line - for me at least - a CF lower rod is only any kind of issue when you're using a motion detector in a non-motion situation, and the coil connecting cable is by by the greatest 'influence'! Being unfamiliar with gold hunting, I'm unsure why prospectors don't seem to use a hand-held (and most likely PI, non-motion, based) pinpointer, with many preferring to scoop and wave.

They look like fishing pliers - for removing fish hooks etc. 👍

Carbon conducts electricity. Some electronic components are made from carbon e.g. 'resistors' (ie items with a carefully tuned conductivity). Here in southern England: 1) There is no naturally occurring gold, although some rivers/beaches have some gold dust deposits resulting from erosion of jewellery, if you can find them. There are often tiny flakes of other metals like alumin(i)um which can be a nuisance, especially on the beach. So I have no experience of hunting tiny nuggets - of gold anyway. 2) On land, we suffer in places from 'coke' - or waste from partially burnt wood/coal. For example, where steam engines were used to work farmland. Coke is basically carbon. Such coke has a relatively low conductivity, giving a TID in the low 20's on the Deus 2 which can easily be notched out and/or ignored. 3) A carbon lower shaft, or a copper cable, or any other conductive component near the coil (I think we can ignore anything more than about a foot from the coil) will, as NASA Tom has indicated, have two effects: a) A static effect, which alters the way electromagnetic fields behave around the coil. b) A dynamic effect, which only occurs where there is some relative movement between the coil and the conductive component. This effect will show that the component has a specific electromagnetic signature - a specific TID, in other words, just like a piece of coke, or a rusty nail. Both these static and dynamic effects can easily be filtered, and are being filtered in different ways, by modern signal processing algorithms. Detectors already have to cope with ground effects, mineralisation etc. I am happy to use CF shafts for their lightness and rigidity. I completely respect other people's right to choose differently. I would be interested to some methodical 'scientific' evidence that there is any 'problem' using CF in real life, but I'm not holding my breath! PS I suppose it could be argued that there might be an additional benefit to the Deus 1 and 2 'cable free' design! PPS I had always thought that 'coil knock' came from working at high sensitivity where resonances within the coil itself cause the effect - NASA Tom has commented on this in the past on his forum.

I appreciate the problem you illustrate, but the batteries are commodity items and changing them is a relatively straightforward task for someone with even fairly basic technical knowledge - considerably easier than changing a cellphone battery I would say.

This is the only thing I disagree with/understand differently. Adjacent iron does make a non-ferrous target look/sound worse (more ferrous) than it otherwise would. 'Masking' is the term I (and I believe the majority of other people who think about such things) use to describe this effect. Whatever techniques you use, and whatever they're called, some will be better, and some will be worse at winkling out 'masked' non-ferrous targets in the ground. Do we agree on the above?

If you were doing air tests then it's difficult to make any meaningful inferences. I find that - in the ground - on good targets in benign conditions at least - the D2 TIDs are every bit as stable as the Nox800 ones - even taking into account that the D2 has two or three times more numbers to choose from! ? PS Park mode for the D2 is not super-sensitive to small low-conductors, by default.

I don't think that it's a problem in this case. A magnet moving relative to the ferrite rod antenna would definitely be a question mark though. ??