Chance PI coil...

[Goaty]

That extra length of shield wire run to the control box will add more capacitance than if it is simply connected to the ground side of the coil at the coil.

Yes, you're are probably quite right! I guess I can always cut it off flush with the coil housing and seal it over with glass if it not effective. It would be interesting to see if the un-connected shield still has any effect.

Also, I would be interested to know if you measure the resonant frequency of your coils and, if so, how you do it. I found an earlier post in this section describing an oscillator to test coils and may have a go at building it when time permits.

[baum7154]

My plan is to try a mix of black acrylic artists paint with varying proportions of graphite powder as the shield coating. The artists acrylic is water based and probably contains carbon black as the pigment and this should contribute to increasing it's conductivity.

I was also planning to bring the wire from the shield up to the detector control box so I compare the difference in coil characteristics with the shield connected and dis-connected to the board ground. I'm hoping this tell if the shield is having any effect on the the 3DSS coil.

BTW, I forgot to mention that I sprayed a light coating of spray adhesive over the windings before wrapping them with the PE foam. It made the coil more rigid but did not noticeably change its inductance. I guess I should have also checked to see how much it changed it's resonant frequency as well.

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Goaty,

That extra length of shield wire run to the control box will add more capacitance than if it is simply connected to the ground side of the coil at the coil.

I don't think the spray adhesive will be a problem since it doesn't flood the coil.
Dan

[Goaty]

My plan is to try a mix of black acrylic artists paint with varying proportions of graphite powder as the shield coating. The artists acrylic is water based and probably contains carbon black as the pigment and this should contribute to increasing it's conductivity.

I was also planning to bring the wire from the shield up to the detector control box so I compare the difference in coil characteristics with the shield connected and dis-connected to the board ground. I'm hoping this tell if the shield is having any effect on the the 3DSS coil.

BTW, I forgot to mention that I sprayed a light coating of spray adhesive over the windings before wrapping them with the PE foam. It made the coil more rigid but did not noticeably change its inductance. I guess I should have also checked to see how much it changed it's resonant frequency as well.

[6666]

Hi 6666,

Yes, that was my impression from that one coil. Since that time I have built several 3DSS coils and never added any additional shielding and they have worked fine here.

Dan

Interesting, one of my many projects is to mix up 4 batches of graphite and wood varnish, 1:1, 1:2,1:3,1:4 and see what effect each batch has on resistance and target signal strength, got to talk to Green about the last part.

[baum7154]

Hi 6666,

Yes, that was my impression from that one coil. Since that time I have built several 3DSS coils and never added any additional shielding and they have worked fine here.

Dan

[Goaty]

Hi Dan

I'm curious why you are adding more shielding to the 3DSS Self Shielding coil?

I plan to use this second coil with the MPP I am currently building and want to experiment with the sample delays just to see what it will do.

Is it giving you lots of false tones at the beach?

The first coil is now on my Surf PI 1.2 as I was not overly happy with the Bara performance and want to tweak it a bit. It does chirp and warble a bit on the SP as I prefer to keep the audio threshold adjusted so I always hear some tones, but is generally fairly stable. It works quite well in wet or dry sand and submerged in shallow water.

I'm also curious what sample delay you are running?

I'm afraid I have not measured the delay on the SP but it's probably quite long. It was a pre-made board which I bought on-line as I was too lazy/impatient/cheapskate to buy a kit and build it up. Now I have it working and am having so much fun using it I am reluctant to pull it out of its housing to make any measurements - I much prefer it to the X-terra as I can just wander down to the local beach in the evening, turn it on and go hunting and I don't mind digging everything (even lots of ringpulls and bottle tops!). I guess it's just as much the pleasure of being on the beach.

Are you working a salt water beach?

Salt water (in Oz)

Also, thanks for the info on your shielding resistance as it will give me something to work towards when I start experimenting. At the moment I have so many jobs to tackle I seem to be jumping from one thing to another. This retirement lifestyle is hard work - I've been at it for several years now and seem to be getting busier every year. May have to get a job to have a rest .

Anyway, I will try to post more info and photos as I progress.

[6666]

Hi Goaty,

Looks good and should present a smooth finish when fiberglassed.

I'm curious why you are adding more shielding to the 3DSS Self Shielding coil? Is it giving you lots of false tones at the beach? I'm also curious what sample delay you are running? Are you working a salt water beach?

Please be aware that adding a shield will add capacitance and slow down the coil but that is ok if you are running sample delays of about 15 to 18us. Graphite is the best shield material in my opinion. I have a 3DSS coil that is shielded with graphite measuring about 1200 ohms @ 1" gap. if I had it to do over I'd go for 2000 ohms at 1" instead. It appears that the coil lost some sensitivity to small fast targets with the shield and will not operate down at 10us.

Dan

Hi Dan are you saying that 1200 Ohms per inch is too low ?

[baum7154]

Hi Goaty,

Looks good and should present a smooth finish when fiberglassed.

I'm curious why you are adding more shielding to the 3DSS Self Shielding coil? Is it giving you lots of false tones at the beach? I'm also curious what sample delay you are running? Are you working a salt water beach?

Please be aware that adding a shield will add capacitance and slow down the coil but that is ok if you are running sample delays of about 15 to 18us. Graphite is the best shield material in my opinion. I have a 3DSS coil that is shielded with graphite measuring about 1200 ohms @ 1" gap. if I had it to do over I'd go for 2000 ohms at 1" instead. It appears that the coil lost some sensitivity to small fast targets with the shield and will not operate down at 10us.

Dan

[Goaty]

I am part way through making a 3DSS coil for my MPP. This will be the second coil of this type that I have made. The first was of similar dimensions but wound with 0.5mm enamelled Cu wire and wrapped with 2 layers of 3mm PE packaging foam then covered with epoxy FG. In spite of having no shielding it works quite well with the bara on the beach.
With the second coil I plan to try some shielding with graphite paint but still have not quite worked out the details of how I will do it. I also used PVC insulated stranded copper wire to wind it. This wire is quite thin and was stripped from an old 7.5m long telephone extension cord, hence the different colour wire in the photos as I had to join some on to get the required 30turns with an inductance of around 320uH (at the moment!). At least with the last turn or two of a different colour I will know which wire wire is the shield, unlike the first coil where the label came off during glassing - I'm still not sure if I have it connected with the right polarity!
I have included a couple of photos of the construction so far and the different method I used this time to apply the PE foam. Instead of wrapping it, which made the finished coil look very uneven, this time I pulled lengths of foam strip reasonably tight around the circumference and held the inside edges together with sticky tape. As can be seen in the photos the resulting covering was much smoother and should give a better finish to the fg. For the shield I am hoping to use graphite paint applied to a wrap of paper masking tape with a thin cu drain wire embedded in it.
At the moment I am waiting for a bag of graphite powder to arrive from China and then I will experiment with various paint mixtures so it could be a while before I get to finish it

[baum7154]

I would still use the 3mm foam as it will help give the round shape you seek while providing good dielectric properties for lower capacitance. As far as neutral bouyancy goes any non metallic ballast would be required. Silica Sand is a possibility. However I have not built any purely underwater search coils. One note; don't be tempted to flood the 3DSS coil air space with any wax, epoxy, glue, or anything else. Filling the airspace raises capacitance and ruins the coil speed.


Dan

[ionut_mtb]

Dan what i have to use as spacer instead of foam to give round shape of the coil, for neutral boiancy?

[baum7154]

Well Rinkerdink as it turns out your choice of the MiniPulse is the best ANALOG detector kit I know of because of its 2 stage preamp. If you ever decide to go after small/ fast targets like 5 grain gold nuggets this is the analog detector I would choose and build a fast 3DSS coil for it.

Good luck,

Dan

[Rinkerdink]

If you will not be detecting very small gold at 6 us to 10us sample delays you will be fine using solid enameled wire on the narrower plantilla form. I doubt that the solid wire with enamel insulation will operate at sample delays below 12us depending on wire size and coil inductance. However for best performance on all targets,the use of thin stranded 600 volt 24awg or even Litz wire on the 1" wide wire form mentioned on every other page of this thread is recommended for fastest coil performance. Coil capacitance is the enemy of coil speed and to keep it minimized, inter-wire spacing using thick 600 volt or greater insulation and reasonably thin wire conductors is a must. The 0.5mm wire you mention is about 24awg. Eddy currents in solid wire will be greater than in stranded wire of the same gauge. Eddy currents are also the enemy of very short sample delays.

By the way the 'Plantilla' coil is a 3DSS coil and should exhibit self shielding properties. The solid wire of the Plantilla coil will not lend itself to use of it as the feed line and other likely types of feed line are big contributors of capacitance to the coil/feed system. You may minimize capacitance of your Plantilla coil with the layering of some thin closed cell polyethylene foam between the winding layers to give more spacing between the crossing wires.

Best of luck,

Dan

Thanks Dan,
this is a fun project and i have pretty low expectations, if I can pick up anything with the detector I'll be thrilled! I bought the MPP kit from Silverdog after having a metal detecting interest piqued but with no money to buy a decent detector. Little did I know the coil is the hardest part 😄. I'll have a go with the plantilla coil with the enamelled copper. I was hoping to twist and heat shrink the cable ends with a view to connecting directly to the detector with no join but I'm guessing the capacitance will be pretty high if I do this? Hoping to find a few coins and general trash is all 🙂

[6666]

Hi 6666,

Yes, it was just regular strength alum foil that is .001" thick. FYI the heavy duty stuff is .002" thick and consequently is much easier to detect.

Regards,

Dan

Thanks I've been doing some tests with 1/2x1/2" .001, and just noticed your results, cheers.

[baum7154]

Hi 6666,

Yes, it was just regular strength alum foil that is .001" thick. FYI the heavy duty stuff is .002" thick and consequently is much easier to detect.

Regards,

Dan