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Mono Coil construction help
I have been constructing Mono coils as detailed in bbsailor,s excelent article I find the Minelab and after market mono coils read from 280 uH to 330 uH and have a resistance of from .6 to .8 ohms. My coils read from 300 to 328 uH and resistance of 4.4 ohms to 7,3 ohms. the construction details as follows 30 awg stranded silver wire with teflon jacket two layers of PE spiral wrap insulation with Scotch 24 shield tape split in half as shield the coax is 5 feet of 17 pf per foot coax ,the length is for hip mount configuration the sensitivity is hard to test in the urben area where I am confined to for the next month but the coils do not seem to want to ground ballance and scream when brought close to ground where have I gone wrong any help would be appreciated. Regards IBGold.
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Hi,Originally posted by IBGold View Post
if there are false signals when nearing the coil to the ground...you have a bad shield or bad shield connection or both. Scotch 24 shield tape is a good product for shielding. I assume your coax is good too...so check for an interruption in gnd line from control box to the shield over the wounded coil.
Maybe connection was broken or not good.
Other data seems very good...and maybe, after discovering the fault, your coil could break the 10us barrier without any false signal.
Best regards,
Max -
Hi IBGold,
You didn't say which detector you are trying to use your coil on? If you are using it on the Hammerhead, or another built detector, we need to know a few more things such as the delay setting, pulse width, sample width, etc.
If you are trying to use the coil on a ML, I suspect it won't work. ML's are very touchy and require coils that are almost identical to a factory design to work ok.
Since it is about impossible to make a coil work on a ML that is made with 30 awg, I have to assume it is for another detector. So, if that is the case, and your delay is very short, maybe 10 usec to 15 usec, then you will detect the ground. If your coil is right on the edge of not working then it will be hypersensitive and really respond to the ground. To test this, turn the delay back just a little and see if the ground signal doesn't get weaker. It will still be there but will be quite a bit weaker.
Detecting the ground is a common problem when a person builds a coil that allows for a short delay. Shielding doesn't eliminate the ground signal. It does reduce some of the effects, but not all.
Because the ground signal can be very strong, it is common to build and use a DD coil, which does reduce the ground signal a lot, but not completely.
RegComment
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Reg the coils are for Minelab SD/GP detectors what do you suggest to solve the problem and what wire size do you suggest I try. The Minelab ,s resistance of .6 to .8 ohms must be conected but its got me tricked how to get mine that low. Regards IBGoldComment
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Hi IBGold,
I am surprised you were able to get the coil to work at all on a ML. You must be trying it on the GP series and not the SD's. If you were able to get it to work but scream when the coil approached the ground, then the coil is close to working. That surprises me that it might work with that small of wire. My guess is the coil has an overshoot and is hypersensitive because it is on the ragged edge.
Just for fun, you might place a small piece of lead on the coil and try it again. See if the coil seems to get better or worse. Change the size of lead to see if it makes a difference. Sometimes the coil has an overshoot and the lead will reduce that overshoot enough that the coil will work.
Now, I suspect you will have to get closer in a resistance value that is identical to the ML's. This will require using a wire size of about 16 1/2 ga to 17 1/2 ga or so.
You could try both 16 ga and 18 ga and see if either will work. I would try stranded wire to begin with. I would probably try to build a smaller mono, maybe a 11" or even smaller to see if I can get that size to work first.
You can try to build one using basic MTW type insulated wire, but I would prefer the Teflon coated type. This works out better to reduce the capacitance.
I would keep the windings tight, use spiral wrap or ties to snug things up. You could use the same shielding to see if it works on a coil made with larger wire.
The key to making a coil that works is to make the coil you are building match a factory coil as close as possible. One way to do this is if you have made a Hammerhead PI, you could take a ML compatible coil and try it on the HH. Look very carefully at the output of the preamp and then build a coil that matches that same signal as close as possible.
RegComment
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Hi Reg,
I have sent a PM to you,
Please read it
Best regards
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Thank you Reg I will go out and try the lead trick and I will try and get some 16 AWG and 18 AWG silver wire and try again do you recon the coils i have already made would work on a infinium if so I will pass them onto someone who can use them its a pity to waste them. Regards IanComment
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Hi Ian,
I am not sure your coils will work on the Infinium, but one could try them and see. They should work on the Hammerhead and possibly on some of Eric Foster's detectors.
The Hammerhead project is a good one to build. It helps a person to understand more about the PI detector, but would also allow you to compare your home made coils to factory coils. You could see just what differences there are in the response curve of the coil and try to make your coil match that of the factory unit. Once that is done, I suspect the coil will work fine.
RegComment
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Reg wound an 8 inch with 10 strands of 30 AWG works fine I am impressed. Regards Ian IBGoldComment
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Hi IBGold,
It is good to hear you now have a working coil. It is kind of fun making one work, isn't it. I have never built one for a ML since I no longer have a detector available to try one on. However, Eric Foster has built several and he is the one who informed us that they are difficult to get to work. Also, there have been several guys from OZ who have built them and they have indicated the same.
Now, I did build a coil for a guy who has a GS 5 which uses the same connector. His friend tried that coil on a GP series unit. It worked but was extremely quiet indicating it was not working correctly. That particular coil was a rectangular designed DD coil and had a transmit winding of 300 uh made with 26 awg wire. So, it might have worked if the wire transmit wire size was a little larger. At least, that is what your tests indicate.
I am curious, which ML PI are you using to do your tests?
Now, that you have a working coil, you can start thinking of how to package it to make it as light as possible. You might look at the coil forms made by Bill Hays. He has a wide selection of housings to choose from.
A while back, BJBJ posted some info about using two part foam as a means of keeping coil windings from moving. On a mono coil, the movement isn't that much of a problem, but keeping the coils light is something desirable. Now, I have been using the 2 part foam for about three years now and it does stiffen the coils and makes them very light.
As BJBJ indicated, working with the two part foam is tricky and somewhat difficult. What I have done in the past is usually not drill the holes, but will do more of it in the future. I have drilled holes on a couple of very light weight housings, but on my Hays, I have relied on having a good clamping system. Unfortunately, I have more problems with that aspect and will drill holes in the future. The reason is the foam is extremely strong and when it expands, it will do its best to push the two halves of the housing apart. In the past, I used some rather crude wooden clamps to try to keep the housings from expanding too much. Unfortunately, one of them broke in the middle of making a coil and the results were I had a functional but ugly coil. So, I tried aluminum flat to build my next clamps and found it flexes too much. So, I need stiffer flat or angle braces also, or both.
Anyway, getting back to your coil building, keep us posted. It sounds like you are having success and most likely having fun.
RegComment
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Reg I am using GP3500 for testing but will try my modded SD2000 I build my own housings my first was a 28 inch x 18 inch fibreglass spider but after that I built a Vacuum former and somr formers mostly spider elipticals but I will try a rectangular when I get some more wire.I have been a radio ham for 37+ years and built my first VLF/IB Detector in the seventies and now revisiting the hobby as I have Retired early, The biggest problem in Aust is getting parts. Allan from West Aust emailed me and was very helpful on the phone re coils for the Minelab thanks again for your advise. Regards ianComment
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Info for Minelab Mono coils
Construction of Monos for the Minelab detectors can be difficult to make work if you don't have the correct coil parameters. If you construct a coil for an SD2000 for example it may not work well with an SD3500. These are the parameters you should be aware off for a 12 inch coil for example.
SD2000
Inductance 300 uH, Q 4.2 Shield to coil capacitance approx 250 pF.
SD2200 and above
Inductance 300uH Q 3.8 Shield to coil capacitance approx 250 pF.
If you use a SD2000 mono on a SD3500 machine, the operation of the coil will be very noisy as the Q or gain of the coil is comparatively higher than designed for. As the shield to coil capacitance increases then this must be conpensated for by an increase in Q. ie the DC resistance will be lower.
The original 18 inch mono for the SD2000 had a Q of 4.5 and I have measured a shield to coil capacitance in excess of 450 pF.
I normally construct my coils using self made Litz wire and optimise the number of strands to give me the required Q. I recently constructed a 21 inch coil for the SD2000 which had the following parameters:
Inductance 300uH, Q 4.1 Shield to coil capacitance 403 pF.
With a coil of this size minimizing the shield to coil capacitance is difficult.
One further point , if you mininize the shield to coil capacitance without making an appropriate reduction in the Q then the coil will be noisy. ie to much coil gain.
Regards,
StefanComment
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Thanks for the tip Stephan another thing to check coil Q, I check the coils self resonance / inductance / coil to shield capacitance ect but my capacitance is lower as I use Teflon coated stranded silver wire to wind my coils. Regards IBGold.Comment
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Info for Minelab coils.
Hi IBGold,
I check for self resonance etc as BBSailor recommends and make the coils with a low interwinding capacitance by paying special attention to winding of the coil. I make a coil winding former and do not bulk wind. I place 4 turns on the first layer then place a spacer and then wind another 4 turns etc. This way you get a rectangular coil. The reason for the spacer is to minimise the interwinding capacitance between the layers. To further minimise the interwinding capacitance between adjacent turns I wind a cotton/polyester tape on the litz wire to provide additional spacing. This coil will self resonate well over 1 Mhz. After applying the shield, the self resonance will drop down to approx 800 khz.
Before applying the shield I place a spiral wrap over the total coil winding and then wind the shield over this.
For an 18 inch coil there would be 4 layers + 1 layer with 1 turn giving a total of 17 turns.
I have considered using teflon insulated wire but I get very good results using the above method and can ensure that I can get exactly the same Q for all size coils. The other point I was trying to show is that by reducing the shield capacitance you get a more sensitive coil however the Minelab detector expects a certain signal range. If you give the coil more gain the detector will drive you crazy on highly mineralized ground. I had to de-sensitize my 12 inch coil for this reason. ie gain to high.
I have passed this info on to show that there are various methods one can employ to get good results.
Regards,
StefanComment
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SD coils
Hi Stephan
Thank for your help on coil winding
Just a short question .. What material do you use for the shield and technique on its winding.
Am currently using Minlab coils on my HH but would like to better my coil making technique. I find that a good quality audio video cable has lowest capacitance if any one interested.
cheers
gefComment
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