Commercial Built search coils

[Jim]

Does anyone on the board know where I could get a commercial built search coil for the "Hammer Head" project. I have a home built coil,it is just not good enough for field use. Thanks Jim

[bbsailor]

Re: Commercial Built search coils

Try getting a coil housing at Hays Electronics(http://www.hayselectronics.com/parts.htm). This premade commercial coil housing makes fabricating PI coils very easy.

Just adjust the coil inductance to match you circuit and tweek the damping resistor. Use the same low capacitance lead wire with the coil. Then add a spacer between the shield and the coil using spiral wrap. Connect the coil shield to lead wire ground. Then secure the coil inside the coil housing and fill it with expandable foam or light epoxy filler.

The trick is finding a commercial coil with the same or close characteristics of your homemade coil that you know works. Unless you know the commercial coil parameters including: coil inductance, wire size, number of turns, method of shielding, and the value of any internal damping resistor, you are taking a shot in the dark.

The Garrett Infinium PI does offer a wide range or coils including mono and double D. With the commercial coil, you will need to tweek the damping resistor (on the circuit board) to optimize performance.

Hopefully, the damping resistor is not already mounted inside the commercial coil. Maybe the folks on this forum can add some information about whether there is an internal damping resistor? and the value?

Maybe the folks on this forum who have commercial PI machines could offer input about their coils to help those who might want to use them on the Hammerhead PI?

bbsailor

[Reg]

Re: Commercial Built search coils

Hi Jim,

You should be able to use any ML PI compatible coil on your HH. This would include the ML coils, the Coiltek and Nugget Finder types. Obviously, a mono coil would be required unless you modified your PI to accept a DD type.

The coil current will be quite high and then may require a heavier battery setup and possibly a regulator, but it should work. I have tried one briefly and it did work, but mine is modified a little to use DD coils.

Reg

[jackdetect]

Re: Question

Hi bbsailor,
Thanks for very good link.
But they haven’t coil housing for coplanar coils larger then 7". Also they haven’t DD housing larger then 14"x10".
Is any other link, about similar manufacturer, with some larger coil housing for sale?
Thanks in advance.
jackdetect

[bbsailor]

Re: Question

Sorry, but Hayes Electronics is the only coil housing source that I know of. If you find another one, let us all know.

Go to the following web site: http://www.findmall.com/cgi-bin/forums/pi_classroom/pi_technology.pl

Go to the search page and enter the key search word "coil" and look for posts from Eric Foster and Reg. You will find a wealth of information about optimum PI coils for particular soil types and target types.

bbsailor

[jackdetect]

Re: Question

Thanks anyway. Couple of times, I was try with google, to find coil houses on the net, but without results.
It is more simply, to make Pi coil then Balanced, because of mechanical stability.
For Balanced coil, coil houses must be very stabile in mechanical terms, especially for coils above 11".
If I find something, I shell report it.

Jackdetect.

[Reg]

Re: Hey Charles (NY)

Hi Charles,

Have you considered making coil housings of various sizes with your new vacuum forming machine?

Right now, the sizes are very limited and Bill Hays is about the only one actively making housings, that I know of.

I did have another source but haven't heard from him in quite a while.

It would be nice to have a couple of other larger sizes available, maybe similar to what some of the other manufacturers of PI coils are making, such as Coiltek or Nugget Finder.

Building housings without mounting ears simplifies things and also allows one to build and mount ears of their choice in the location of their choice. I have used such housings where the ears are glued on and made from a U shaped piece of ABS plastic. The U is formed by heating the plastic and pressing it into a form using a male/female combination. This forms the shape and eliminates the shrinkage or thinning that normally happens when vacuum forming.

Anyway, if you are interested, you might post something on this forum? I suspect you will have some people interested.

Reg

[Reg]

Re: Commercial Built search coils

Hi Jim,

You didn't mention just what the problem was with your coil or why it didn't work like you feel it should. Maybe we can help you build one that works a little better.

As BB Sailor mentioned, Bill Hays of Hays Electronics sells different size coil housings that work quite well for PI coils. I know, I have used several of them with great success.

Now, building a coil for the HH project isn't really that hard, It is a little more difficult to assure the coil is properly damped if you do not have a scope. With a scope, things are just a matter of experience or a little experimenting. Personally, I have a damping resistor mounted on the PC board (about 740 ohm or so) which underdampens the coil I then use a small pot to fine tune the damping. I do it this way because I am always changing coils and the fact, I have my delay down at a very short setting. This makes it very difficult to match the coils to all work correctly without some individual adjustments.

I will adjust the pots to the point that the damping looks correct, and then I replace the pot with a resistor and mount it in the coil plug. This way, if I change things, I can always get to it.

Is there a particular objective you are trying to obtain? If a general PI is all you are trying to accomplish, then trying to build a coil that will work at the minimum delays is not necessary. In fact, the shorter delays do present a problem since the ground signal becomes much more pronounced at delays less than 40 usec or so. When this happens, then there will be places where the ground signal will be strong enough to present problems. When that happens, the easiest fix is to modify the machine to accept a DD type coil. A DD coil will reduce the ground signals tremendously in most cases.

If you are trying to build a coil that works at very short delays, then extra care has to be taken with all aspects of the coil. Even the shielding becomes a serious factor.

If very short delays are desired and you do not have the time or the patience, then purchasing a coil such as mentioned before might be the easiest answer in the long run. If you are patient, you should be able to find a good used coil for somewhere around $100 to $150 for the standard sizes. A very good size is the 11". This size works very well for most applications. Now, I have tried a couple of different coils built for ML PI's and found they work quite well at the shorter delays. I should mention that the coils I did try were DD types though.

So, if you only need one coil, buying one may be cheaper in the long run. It cost me about $100 to obtain all the items I use to build a coil. Granted, I have enough of some of the items to build more than one coil, but still it isn't cheap.

I hope this helps.

Reg

[Charles (Upstate NY)]

Re: Hey Charles (NY)

Several people have contacted regarding this, at the moment I'm not 100% sure I will be offering coil shells for sale but I'm thinking about it.

It may be cost prohibitive, I cannot make shells that sell for $12 dollars that much I know. My cost in materials is about $7 as I use thicker high quality ABS sheet. I cast the lower rod ears out of urethane verses the molded in ears. Add things like quartz heaters that gobble 36 amps of electricity.

Rough guess, probably about $20 for an average sized shell, $25 for a larger ones, that may be more than people are willing to spend.

On a bright note here's a pic of how I'm going to trim them away from the sheet. Its a hot wire cutting table. I'm told by a coil builder in London these work pretty well.

[jackdetect]

Re: Hey Charles (NY)

Hi Reg and Charles,

Lot of us, are not only a hobby maker.
About 10 years, I was trying to produce nice looking coils, but without good results, and always was sell my detectors with some rude coils.
The price, about 25$, for larger coil houses, is nothing, in comparing with prices of manufacturer coils. Especially because they usually haven’t larger coils for they models.
As you known, coil houses are the most expensive part of any coil.
So, Charles, if you produce some larger coil houses for sale, I shell certainly buy it from you, probably in large number.
Of course, spider houses, for balanced coil, must be very stabile for mechanical vibrations.
I am also, very interested for Mono/DD coil houses about 20” (50cm), if it possible to make.
Regards,
Jackdetect.

[Carl-NC]

Re: Commercial Built search coils

My personal preference is for the Garrett Sea Hunter style, which is a skinny round donut. Also, Eric's coils are made this way. These work MUCH better in the surf than the flat disc style, which flop all over the place.

I have both a Sea Hunter and a Beachscan MKII, and could come up with a nominal value for the damping resistor if you want to go that way.

- Carl

[Reg]

Re: Hey Charles (NY)

Hi Charles,

I can certainly understand the higher price. It would work for me. At times, I would like to have a little heavier coil form.

I know that Bill Hays keeps the price low as his way of sort of repaying the group of people who have supported his business over the years. Bill is a really great guy.

Let me know if you decide to make them and what sizes you will be making.

I kind of like the idea of no ears on the form. This allows me to position them where I want them.

Reg

[imtack]

What dampening resistance would be needed to run a Garrett sea hunter coil on sm pro pi looking at the larger coil but it has two coils 122.8 uh and 126 uh I would connect both in series I think?

[6666]

What dampening resistance would be needed to run a Garrett sea hunter coil on sm pro pi looking at the larger coil but it has two coils 122.8 uh and 126 uh I would connect both in series I think?

those seahunter coils are designed to be operated as separate TX and RX , you need to measure the inductance so the coils add when you connect them in series
t