EMI question

Hi All,,
EMI gets a mention every-so-often on the forum and I ask: how does one recognize it,, what effects does it have on the audio or what effects does it have on detectors in general,, is there any preventive of it???? Are certain coils prone to getting it and which are they??? I'll be out there shortly with an older machine and I'd like to cover all bases... Please help.. noyungan..

A good start...........................https://en.wikipedia.org/wiki/Electromagnetic_interference  

........LINK........

Hi Noyungan,

Emi, well you will notice it. It is that constant chatter of the threshold. Some days the threshold tone is noce and smooth, others a bit rough and occasionally just about completely unusable. I have had many occasion when I think that there is something wrong with the detector, but then it gets to about 3pm and the emi just goes away like it was never there in the first place.

About all you can do is use autotune or manual tune if your detector has this feature and try to get it reasonably stable, but when the emi is bad, you might end up doing this every couple of minutes which ends up very frustrating.

On days when it is bad I will end up just putting the detector away for a while and wander around and check out nearby areas for all the signs we like to see.

Cheers Mick

G'day noyungan

The Link geof_junk gave you sums it all up, so does Mick, but I'll try to give you an explanation in layman's terms.
The presence of EMI, simply makes detectors noisy, drowning out the possibility of hearing positive metallic ground signals.
After switching on your detector & ground balancing the coil, lift your coil up  from the ground to waist height, (face of coil at 90 degrees to the air, not horizontally level with, or facing the ground) & listen for any unwanted EMI noise coming through your speaker (a constant humming/warbling noise)  That excessive unwanted noise coming from/through the air, is external EMI which is picked up by your coil. The strongest point or direction of EMI is noticed whilst rotating your coil in a 360 degree fashion. You will notice  EMI getting stronger or weaker. (stronger when facing where the EMI is coming from, weaker when moving your coil at an angle away from where EMI is being emitted  from.  
If after ground balancing your coil to a quite and stable operating procedure & you find it running quiet/smooth after repeating the above "presence of EMI procedure" there is little or no EMI present or nearby to affect your detector.
All Minelab PI's before the release of the well insulated for EMI GPZ 7000, were prone to picking up EMI, anywhere EMI was found to be present.
EMI If present, is most notable during detecting, when lifting your coil up side ways from the ground (for instance when scanning a gully side wall, or when scanning over ground, whilst walking on an uphill incline.
Coils act like a radio or TV antenna. They transmit signals as well as receive them.  Mono coils are more prone to EMI because of their more sensitive winding's & greater transmit/receive capabilities. DD coils to a much lesser extent.  The detector is also prone to picking up EMI, when working close to or nearby overhead power lines, but you'll find a nulling effect, (the minimization of EMI) when working in line & or right bellow them. Working close to or over underground power cables/telephone lines also produce EMI, disrupting the working operation of the detector.

Cheers Kon.

A few things to quieten your machine, just in case it isn't EMI.

1) Ground balance the beejeezes out of your machine, I mean constantly.
2) Put your iPhone in airplane mode if possible.
3) Empty any ferrite dust that might be trapped in your coil skid plate.

Cheers, Harry

Many thanks to Geof,Kon, Mick and Harry, for their helpful info which gives me a better understanding of what can be a b----- nuisance to detecting where it is imperative to have clear threshold hum.. One never thinks that those invisible radio waves can get into the tector but as you said the machine can behave like an antennae and unless one knows a bit about EMI one could get pretty frustrated... Many thanks fellers.... noyungan

noyungan wrote:Hi All,,
        EMI gets a mention every-so-often on the forum and I ask: how does one recognize it,, what effects does it have on the audio or what effects does it have on detectors in general,, is there any preventive of it???? Are certain coils prone to getting it and which are they??? I'll be out there shortly with an older machine and I'd like to cover all bases... Please help.. noyungan..
       

This may be of some help.
https://www.tekneticsdirect.com/the-tek-files/electrical-interference

One item of these so called modern times that messes up all of my tectas when bench testing in the house is the long life fluro lights that everybody uses these days.
These things cause a lot of chatter and warbling noises from the detectors. Also some of the lithium battery chargers are a source of strong EMI. My Enecharger NC1500U has to be switched off at the wall socket if my detectors are within 5 mtrs of it while bench testing.

Hello Adrian,
I have just read your post and yes I read the link as well.. I must say that your link was easy to read, less uni language!!! All the posts on this topic are making me more aware of detecting problems, and it sort of amazes me that electronic technicians have not developed some means of nulling it.. Perhaps there is some means available and I have not done enough research to hit upon it... It'd be a fair cow to have to forgo a day's detecting because of EMI after travelling many miles to wave the wand... One can't find the treasure sitting at home grisling!!!.... noyungan

Cutting the Uni crap from explanations is easy for me because I don't know much Uni crap.   .The down side of that is that I often end up writing crap.    

Smooth regular low frequency (20hz to around 50khz emi signals can be almost eliminated. But it is the sporadic wide band sudden bursts that come from lightning, car ignition sytems, military low frequency transmissions, Deep geological survey equipment, telephone and mbl phone tower transmissions and bazillions of other bits of electronic equipment that are difficult to eliminate.
A metal detector is simply a vlf transmitter and receiver and the coil is the antenna. Any changing electromagnetic signal will generate voltage and current in the coil which results in a signal being detected by the receiver and converted to an audio response in the head phones.
This is also what a detected metal target does/causes.

Designing an emi cancelling cct that can differentiate between a signal generated in the coil by  a metal target and a signal generated by any of the above mentioned causes of emi is a tad tricky to do without eliminating the target signal as well. The more sensitive the detector the more responsive to EMI it will be.
The larger the coil the more sensitive to EMI. ....The higher the frequency that the detector operates at the less it is effected by EMI.

So at the moment with the current range of publicly available metal detectors to ordinary prospectors all we can do to reduce EMI is to A. use a smaller coil, B. Change from mono to a DD coil.  C. Run the detector at a low sensitivity setting or go to another area and try again. D. Equipping yourself with a high end metal detector that has some EMI cancel capability.

I have only ever had to give up on a high EMI area on two occasions in 58 years of detecting.. Once you have learnt the sounds of your tecta you will be able to pick out the target signals. ....If the EMI doesn't drive you nuts first.

Adrian
I am very new to detecting and have found your explanation of EMI to be very informative and helpful to me.

There are other sources of interference that your detector can experience as well, some may or may not fall into the category of emi but are probably just described as that due to lack of realising what it is and where it is coming from.

One source of interference can possibly come from yourself and the positioning of electrical devices such as phones, uhf radio, gps etc that are on your person, as apart from the coil receiving sporadic signals an unshielded control box or cracked coil lead may pick up signals from some devices as well, and some interference also can be experienced when some types of pin pointers are used.

Other sources can be metallic objects that you are carrying, while not in the category of emi they can interfere all the same, metallic object that are carried close to or on the front of your body can be seen by the coil, more particularly when using big coils, maybe not enough to generate a signal but as you walk about the movement of these objects can generate a low level response, and as the coil is seeing these object it is believed that it can have the effect of de sensitizing the coil as well, this is the reasoning behind the use of fibreglass otto type top shafts on pi detectors.

Some other sources that will give you emi issues are long lines of wire fencing, these can carry interference from many klms away, I cant recall what it is called but it is similar in effect to what you experience when you get close to an electric fence, sort of a pulsing and sporadic spiking noises, obviously if it gets worse the closer to the fence you get you either have to move away or run your detector with a anti interference (Salt) coil coil or on cancel mode.

Another is experienced on a hot dry windy day, and more so when you are detecting in long grassy areas, and I think that it is from static build-up in the coil,detector or body, and its the discharge of the static that causes sporadic spiking sounds much like when you get lightning in the distance, maybe it can be exaggerated by wearing certain clothing types that also will add to the static build-up.

In some places we have noticed that we get random spikes and warbling sounds that we have associated with being under a commercial airline flight path, and think it has something to do with the planes radar system sometimes you hear it but cant see the plane and can only hear a very low sound overhead so that why we are thinking the planes are doing it.

cheers

au-fever

I have sometimes wondered if a metal detector could respond to the electromagnetic energy released from rocks just prior to and during earthquakes??
As rocks become stressed due to changing extreme pressure the atomic bonds between atoms is stretched and eventually breaks. Electromagnetic energy is released during this process and should be able to be detected prior to an earthquake if the detection equip is sensitive enough.
The ground around Merredin WA is always moving. I wonder what the EMI is like in that area??

I suppose that we are already detecting the released electromagnetic energy anyway as the earths tectonic plates are always shifting somewhere so the energy release would be pretty constant, that's the M in emi (Magnetic interference) that's coming from the ground, but it seems its less detectable on a cold day or during the night, but as the ground warms up it becomes a lot more active and seems to allow the electromagnetic energy to flow better and that's likely why on hotter days we have more issues getting our detectors to ground balance and to keep a steady threshold.

The hotter the day the more erratic the detector becomes and that's why I always think that its a waste of time detecting when its like that as it probably has the effect of making the detector less sensitive, as the electronics are trying to deal with all the activity in the ground, and in the air, so feint signals are drowned out or simply not recognised as targets by the detector anyway, that's why we find that very early mornings and detecting in the night are much more productive in the warmer months and we do better in the winter months as we have more detectable day light hours available.

cheers

au-fever

Some interesting info re electricity flowing through the earth. (Telluric currents)

Telluric currents are phenomena observed in the Earth's crust and mantle. In September 1862, an experiment to specifically address Earth currents was carried out in the Munich Alps (Lamont, 1862).Including minor processes, there are at least 32 different mechanisms which cause telluric currents. The strongest are primarily geomagnetically induced currents, which are induced by changes in the outer part of the Earth's magnetic field, which are usually caused by interactions between the solar wind and the magnetosphere or solar radiation effects on the ionosphere. Telluric currents flow in the surface layers of the earth. The electric potential on the Earth's surface can be measured at different points, enabling the calculation of the magnitudes and directions of the telluric currents and hence the Earth's conductance. These currents are known to have diurnal characteristics wherein the general direction of flow is towards the sun. Telluric currents continuously move between the sunlit and shadowed sides of the earth, toward the equator on the side of the earth facing the sun (that is, during the day), and toward the poles on the night side of the planet.

Both telluric and magnetotelluric methods are used for exploring the structure beneath the Earth's surface (such as in industrial prospecting). For mineral exploration the targets are any subsurface structure with a distinguishable resistance in comparison to its surroundings. Uses include geothermal exploration, mining exploration, petroleum exploration, mapping of fault zones, ground water exploration and monitoring, investigation of magma chambers, and investigation of boundaries of tectonic plates. Earth batteries tap a useful low voltage current from telluric currents, and were used for telegraph systems as far back as the 1840s.

In industrial prospecting activity that uses the telluric current method, electrodes are properly located on the ground to sense the voltage difference between locations caused by the oscillatory telluric currents. It is recognized that a low frequency window (LFW) exists when telluric currents pass through the earth's substrata. In the frequencies of the LFW, the earth acts as a conductor.

Well, well, well,, I am simply gob-smacked by the informative, fascinating explanations put forward by Adrian and Au-fever.. while not overlooking all the other posters !!! The world is truly wonderous how it has,,I think inherent,, electrical/magnetic currents that laymen like me are completely unaware of until one goes metal detecting and I suppose mankind will never produce a detector capable of ignoring the lot...We'll just have to live with it and do our best.... To get explanations simply put as you have gives understanding and that's a bonus.. I feel a whole lot better now.. Thank you..noyungan

The electric fence one had me going years ago when I first encountered it. SWER (Single Wire earth return ) power line have mainly gone these days but were a problem in remote area. Down wind of High Voltage Transmission Line (220,500 KV) due to ozone.

Along with others' comments,, I enjoyed reading your's Adrian on the EMI topic,, but I would ask regarding you saying EMI in the 50hz/50khz can be somewhat eliminated.... Would that be so if one is using a "filter" such as I have come across only yesterday and advertised by a well known moderator... Do such "filters" really clean up the airwaves???? Wondering,,,,noyungan

If you are referring to ferrite cores that clamp around the coil cable. then it is a bit iffy as to whether they achieve anything at all.
Having said that, I have these gadgets fitted to my tectas and haven't bothered to remove them. They don't do any harm, but if they do anything in relation to blocking LF  and mains interference and lightning I have not noticed it.

I guess it is possible that a capacitive filter at the HP connection to the control box could do something to limit low frequency EMI from getting through to the electronics.  Remember the cross over networks in early day quadraphonic HI FI audio systems. These capacitive and inductive filters were used to direct/block particular audio frequencies to the various speakers. Woofer, Mid range and tweeter.

Most of the ML upper end detectors have shielded control boxes to prevent EMI from effecting the electronics directly. It is however near on impossible to eliminate all EMI.

With regard to the ferrite magnets clamped around coil cables, it is my belief that they may be counterproductive.
PI detectors essentially transmit a square wave made up of the fundamental frequency plus an infinite number of odd harmonics.
Placing a ferrite ring around the coil lead will attenuate both the transmit and the receive signal at the higher frequencies resulting in less power applied to the coil or an attenuated receive signal.
Happy to be shown to be wrong.

Terrt

Hi, again,,
Would placing ferrite magnets around the coil cable be an attraction to the search coil,, especially a larger coil???

I think you will find that they are not magnetised at all and when placed at the control box coil connection they will not affect the coil. However The coil can detect them from a couple of feet away ( vlf Induction balance type) so do not clamp them around the coil cable at the coil end. From my experience with these gadgets it seems that they have zero effect on EMI in metal detectors. These things are fitted to computer cables and other electronic equipment including some of my watch timing instruments so I guess somebody thinks they are beneficial in some way. They are cheap enough so give one a try out and make up your own mind about them.

Also you may find that PI metal detectors do not detect these ferrite filters, they are mineral iron, although I cannot speak for the ML GPX and the Z type, but many vlf metal detectors will likely give a  response from a couple of feet away from the coil.


This is what Black plastic enclosed ferrite cores.are supposed to do.

Help reduce EMI on AC power lines.
Help reduce noise on USB, Firewire, Phone, Power Cord, Coaxial, Audio, Video etc cables.
Just clip it on and you will get clearer signal and faster. data transfer ....(If yer lucky)

Thanks Adrian I may give it a try when I get going.... noyungan

bicter wrote:With regard to the ferrite magnets clamped around coil cables, it is my belief that they may be counterproductive.
PI detectors essentially transmit a square wave made up of the fundamental frequency plus an infinite number of odd harmonics.
Placing a ferrite ring around the coil lead will attenuate both the transmit and the receive signal at the higher frequencies resulting in less power applied to the coil or an attenuated receive signal.
Happy to be shown to be wrong.

Terrt

I have seen the effects of adding extra ferrite to a coil lead. I was sent an "emi reducing device" for a look see, which contained some ferrite rings.
This device extended the coil settling time, causing the first sample to be taken while the receiver was still overloaded, thus making it very difficult/impossible to ground balance in hotter ground. The effect was quite impressive when we compared with and without!

I wouldn't add any ferrite to your coil cable.

The only reason a ferrite core is installed inside your detector(yes there is a ferrite core already that the coil cables run though) is to keep radiated emissions below the limit for what ever standard it needs to pass during EMC testing.

EMI sucks!

Cheers Mick

Hi Mick.
Last night I ran a few more checks with and without the ferrites on the coil cable of my Infinium, Sand Shark and The Vallon VMH3CS.
This was done inside my house where EMI is bloody awful and with and without the ferrite fitted to the coil cable at the control box end of the cable. I still did not notice any difference in the performance of the detectors or any reduction in the local EMI effect. Even with the coils orientated in the direction of least EMI,
I must admit that I have never checked the GB ability of the Infinium when fitted with the ferrite and is Probably because I have not noticed any change.

Mechanic wrote:
I have seen the effects of adding extra ferrite to a coil lead. I was sent an "emi reducing device" for a look see, which contained some ferrite rings.
This device extended the coil settling time, causing the first sample to be taken while the receiver was still overloaded, thus making it very difficult/impossible to ground balance in hotter ground. The effect was quite impressive when we compared with and without!

I wouldn't add any ferrite to your coil cable.

The only reason a ferrite core is installed inside your detector(yes there is a ferrite core already that the coil cables run though) is to keep radiated emissions below the limit for what ever standard it needs to pass during EMC testing.

EMI sucks!

Cheers Mick

Mick,
Agreed
Good to see your experience gels with the theory.
Ferrites are a relatively broad band attenuator and consequently must affect both the TX & RX signals in addition to attenuating EMI. This must result in a poorer performing detector. The reduction in performance will probably only be noticeable on very small/deep targets
When you tested the "EMI reducing filter" did you observe any reduction in target response?

Terry

Interesting posts fellers, don't think I'll bother with ferrite but try to better my ability with the wand.... noyungan

Hi Terry,

I don't think I specifically did target depth tests with it. After I saw what happened to the settling time of the coil/receiver, interfering/overloading the first sample, I didn't see much point to depth tests. Though, this would have made fine gold of the 5k less sensitive for sure, as it relies on the very early sample to ping those tiny bits.
For a detector to ground balance properly, all of the receive samples need to be taken at a time when the receiver is not overloaded, otherwise the ground balance calcualtion will not be correct and a false target(ground noise) will appear where there normally would not be one.
This is easily seen when you find a hot patch of ground that the detector sounds off like crazy on when the coil is skidding along the ground, yet, you lift the coil up an inch or 3 and you get nothing.
Another time when this is obvious is when you can't get the detector to gb all the way down to the ground. It migh balance to within several inches of the ground, but no matter how hard you try when you go lower the detector wails. This is normally due to the receiver overloading while the samples(particularly the early samples) are being taken.

Sorry for the techo overload for those whos heads are now spinning, feeling as though you have just been violated!

These detectors are really fussy things and it is very hard to find things that actually improve them.

Cheers Mick

I definitely agree with your last comment Mick.

I reckon that the ML GPX and the Z are a very different type of PI?....Are they really PI??
When you can use a piece of ferrite to ground balance them, it is certain that the detectors are not the usual run of the mill PI.
My Infinium, Sand Shark and the vallon and Sea hunter  do not respond to ferrite rings or the EMI reducing cores whereas all of my VLFs scream at them and can ground balance over them.

Thanks Mechanic for the info and I think I've got it right to thinking that not only does one when GBing need to be sure one is not over a target but also one needs to avoid GBing over a hot bit of ironstone..... Did I get that right ??? noyungan

Spot on dude.