Vallon VMH3CS Mine Detector

[Ferric Toes]

By the way , magnetic permeability cannot be "large than needed" in this kind of sensors . Even if we have a ferrite with mu=10000 - it isn't too high ... You see , the overall performance of the MD with ferrite rod coil directly depends on 3 parameters - ferrite rod length , its diameter and magnetic permeability . When I was experimenting with my constant-current PI with ferrite sensor , I had compared 2 coils - one core was made with a bunch of rectangular ferrite rods made for AM radio antennas ( mu=600 ) , and another was glued together 3 pieces of EPCOS ferrite sticks for a SMPS transformers ( mu=2700 ) . And although the first search coil was slightly longer - the second one did have noticeably better sensitivity .... so , as I think , if I find a ferrite with even higher permeability ( 10000 or more ) , I should achieve much more performance . Maybe it's a good idea to try not a ferrite but a kind of amorphous alloy materials , something like this - http://www.nanoamor.com/cat/catalog_amor.pdf - because they can have permeability of 100000 or even more ...

Coincidentally, I only read about amorphous alloys this morning in the following article about a new ferrite. I like the huge pot core - great for searching under wire fences. Only problem would be the weight.

Eric.
http://www.fdk.com/cyber-e/technical/pi_technical06.html

[Chet]

For the first test I wound the 4in x 3/4in rod with as many turns as I could get on and achieved 1.8mH straight off. Resistance was 3.5 ohms so, we are in the right ballpark straight away. I left off the outer shielding at this stage, but had the core grounded with a drain wire running its length. Plugged it into the shaft connector of the Vallon so that it had all the cable capacitance and additional resistance. It ran through all the startup checks and the detector settled down with no problem. The difference I noticed was that it seemed quieter than the other probes I have made. My home workshop is quite noisy electrically, much of it from surrounding shops and light industry. I'll have to check it out away from the premises to be sure, but I definitely gained a couple of inches on a nickel. 10in instead of 8in. Over the weekend I will add the outer shielding and house it in a spare Vallon shaft tube; which happens to be just the right diameter. [ATTACH]36403[/ATTACH]

Eric.

Hi Eric


Thank you for the information in the previous post. Your latest coil (10” on a nickel) wow! That is something to be proud of. What does the field around the side of the coil look like with this kind of probe? Does it still pick up the nickel at several inches?


Have a good day,
Chet

[bbsailor]

[QUOTE=Ferric Toes;218330]For the first test I wound the 4in x 3/4in rod with as many turns as I could get on and achieved 1.8mH straight off. Resistance was 3.5 ohms so, we are in the right ballpark straight away. I left off the outer shielding at this stage, but had the core grounded with a drain wire running its length. Plugged it into the shaft connector of the Vallon so that it had all the cable capacitance and additional resistance. It ran through all the startup checks and the detector settled down with no problem. The difference I noticed was that it seemed quieter than the other probes I have made. My home workshop is quite noisy electrically, much of it from surrounding shops and light industry. I'll have to check it out away from the premises to be sure, but I definitely gained a couple of inches on a nickel. 10in instead of 8in. Over the weekend I will add the outer shielding and house it in a spare Vallon shaft tube; which happens to be just the right diameter.

Eric,

From looking at the photo of your probe and doing some fast back of the envelope calculations I guess that you are using AWG 30 Teflon insulated wire that is .024 inches OD?

thanks

Joseph J. Rogowski

[Ferric Toes]

Eric,

From looking at the photo of your probe and doing some fast back of the envelope calculations I guess that you are using AWG 30 Teflon insulated wire that is .024 inches OD?

thanks

Joseph J. Rogowski

Hi Joe,
The overall diameter of the wire measures 0.022in with the internal tinned copper core being 0.009in (0.25mm). I presume it is Teflon insulation as it is quite difficult to strip. I have a 10,000ft reel of the stuff, bought some years ago on ebay, so it is good to start using it.

Eric.

[Ferric Toes]

Hi Eric
Thank you for the information in the previous post. Your latest coil (10” on a nickel) wow! That is something to be proud of. What does the field around the side of the coil look like with this kind of probe? Does it still pick up the nickel at several inches?
Have a good day,
Chet

The external field is exactly that of a bar magnet. Going out sideways from the middle of the winding, I get 7.5 - 8in on the nickel; the coin being in the same plane as when measured off the end of the rod.

Eric.

[Ferric Toes]

4in x 3/4in probe core test. 2

I have now applied the helical outer shield of lead tape. This is 0.1mm thick and adhesive backed, so it is easy to apply. I ceased using this some years ago but still have some left, so I might as well use it up in these experiments. The first picture shows the set-up for measuring the range to a nickel off the side of the coil and the orientation of the coin. There are three lobes of detection for a coin in this orientation; one each, top and bottom and the other all around the side, with a sharp minimum as you move from one lobe to the other. With a sphere you get a fairly uniform sensitivity all around the core, with no dips.
Second picture shows connection from lead tape to core ground wire.

[Brian Deese]

Hello Eric its good to see you play, I have been away from Geotech for a while also.I just signed on and found your post. I have a Vallon VMC1 that I have been very impressed with,I havent used it in the field yet though. If there is anything I can do for you regarding that model I am at your disposal.I made a video of air testing a us nickel this morning. I'm in the city,and I just set it on the parking lot,so its not the greatest test im sure. Youtube killed the video you could read the numbers on the ruler before upload. But the results were 10" very strong signal,11" good repeatable digable signal,just loses it at 12" the sensitivity set just to the first notch of 10 any more and it falsed from interference. https://youtu.be/BJb7XAZut7Q

[Ferric Toes]

Hello Eric its good to see you play, I have been away from Geotech for a while also.I just signed on and found your post. I have a Vallon VMC1 that I have been very impressed with,I havent used it in the field yet though. If there is anything I can do for you regarding that model I am at your disposal.I made a video of air testing a us nickel this morning. I'm in the city,and I just set it on the parking lot,so its not the greatest test im sure. Youtube killed the video you could read the numbers on the ruler before upload. But the results were 10" very strong signal,11" good repeatable digable signal,just loses it at 12" the sensitivity set just to the first notch of 10 any more and it falsed from interference. https://youtu.be/BJb7XAZut7Q

Hi Brian,
I am not sure if the VMC1 is electronically the same as the VMH3CS. Maybe it is and it is programmed differently, or in fact there is no difference. I have been informed that the VMH3CS is programmed differently for different customers and markets, so there are a few unknowns.
Electrical and electronic noise can make a difference to the achievable ranges. My workshop is noisy and I have have a communications receiver that reaches as low as 75kHz. I cannot use it at all on a wire antenna up to about 2MHZ for all the noise, so I use a directional frame antenna that I can rotate and find a null for noise or a max for wanted signal. The frame antenna is actually a shielded PI metal detector coil! In spite of the shield I can still get good signals from short wave stations up to 10mHZ, then it falls off rapidly. Cell phone transmissions you would not think to cause interference as they can be 100 times higher in frequency, but the high field strength from the repeaters and the non-linearity of the detector RX results in demodulation to interference frequencies. The best situation is to get away from power lines, cell phone antennas, computers, fluorescent lights, etc, etc. One useful hint is that the vertical orientation of the search coil is the worst for picking up interference; always have it horizontal.

Eric.

[Brian Deese]

Hi Brian,
I am not sure if the VMC1 is electronically the same as the VMH3CS. Maybe it is and it is programmed differently, or in fact there is no difference. I have been informed that the VMH3CS is programmed differently for different customers and markets, so there are a few unknowns.
Electrical and electronic noise can make a difference to the achievable ranges. My workshop is noisy and I have have a communications receiver that reaches as low as 75kHz. I cannot use it at all on a wire antenna up to about 2MHZ for all the noise, so I use a directional frame antenna that I can rotate and find a null for noise or a max for wanted signal. The frame antenna is actually a shielded PI metal detector coil! In spite of the shield I can still get good signals from short wave stations up to 10mHZ, then it falls off rapidly. Cell phone transmissions you would not think to cause interference as they can be 100 times higher in frequency, but the high field strength from the repeaters and the non-linearity of the detector RX results in demodulation to interference frequencies. The best situation is to get away from power lines, cell phone antennas, computers, fluorescent lights, etc, etc. One useful hint is that the vertical orientation of the search coil is the worst for picking up interference; always have it horizontal.

Eric.

Horizontal,and on a board to elevate it right off the ground. I think I read somewhere that was your recommended test procedure. I'll take it out of the city and have at it again. Pretty fancy for a mine detector though.I have tried a couple other mine detectors and they are awful.I like the Vallon I would like one like yours,the collapsible vmc1 is heavy on your arm.I think you did right putting it on a belt.

[ivconic]

Some problems I have come across with certain ferrite rods that cannot be determined by reading specifications are -
1. Some rods ring mechanically and cannot be damped out.
2. Some rods are microphonic such that if you hit a rock they give a false signal.
3. Some rods respond to the earth's field even when the detector has earth's field cancellation. This is not just swinging the rod but the baseline zero changes if the rod is positioned NS and then changed to EW; acting a bit like a fluxgate sensor.
4. Rods from the same batch respond slightly different.

Right now I have two seemingly identical rods bought at the same time, wound and shielded the same and tested on the Vallon. One is noisier than the other.

Eric.

Yes i know exactly what do you mean.
In the past i tried to examine closer some core materials.
Right now i am thinking on the same method, but considering the size and volume of the rods; a bit larger coil diameter will be needed to perform this simple experiment:

Point is to establish some sort of "reference", a referent feature for classifying rods.

[Chet]

I have now applied the helical outer shield of lead tape. This is 0.1mm thick and adhesive backed, so it is easy to apply. I ceased using this some years ago but still have some left, so I might as well use it up in these experiments. The first picture shows the set-up for measuring the range to a nickel off the side of the coil and the orientation of the coin. There are three lobes of detection for a coin in this orientation; one each, top and bottom and the other all around the side, with a sharp minimum as you move from one lobe to the other. With a sphere you get a fairly uniform sensitivity all around the core, with no dips.
Second picture shows connection from lead tape to core ground wire.
[ATTACH]36409[/ATTACH]
[ATTACH]36408[/ATTACH]

Hi Eric


Thank you for the nickel test, lobe information and photos. I really like your probe and its great performance.


Have a good day,
Chet

[Ferric Toes]

Hi Eric

Thank you for the nickel test, lobe information and photos. I really like your probe and its great performance.

Have a good day,
Chet

I realised yesterday evening that I have been disregarding my own advice. That is, to have the detector coil horizontal to minimise noise pickup (coil axis vertical).
I have been testing the probes laying horizontal, when they should be vertical. For broadcast radio use, a ferrite antenna is horizontally mounted to get maximum signal from the magnetic component of the carrier wave. Also radio direction finding frame antennas are vertically mounted on a swivel base. That is not what you want for a ground searching metal detector as minimum r.f pickup is the desired situation both for open coil and ferrite probe.
Sometimes probes are used in a horizontal situation so I am going to put together a differential arrangement which is noise cancelling in all directions. Will that work on the Vallon I wonder? I will consult my favourite reference work 'The Electronics Empiricist'.

The picture shows a detector coil in use for radio reception.

Eric.

[ivconic]

...The picture shows a detector coil in use for radio reception. [ATTACH]36415[/ATTACH]
Eric.

Collins or what?
...
btw http://www.radista.info/en_hf_radio.html

[STAVROSNU]

Dear Eric,

I have just sent you a private message with the requested informations.

[Ferric Toes]

[QUOTE=ivconic;218363]
Collins or what?
/QUOTE]
No, they belong to me.
The l.f. receiver to which the search coil is connected is out of the picture and under the nearest receiver and power supply. To take the picture from the other side and it would have been dominated by the face of a Morbiere grandfather clock close to the coil. The picture was to illustrate that the vertical plane was good for the reception of radio signals and not good for a metal detector. Also an alternative use for a Vallon shaft which has been doing this job long before I managed to get a couple of control units to play with. BTW. some search coils by other manufacturers make better radio antennas than my coil, due to the use of graphite paint shielding which has less attenuation unless you spray on several coats.
A free ferrite rod if you can identify the receivers and the manufacturer. Clue - they are not American.

Eric.