Coil shield question

[Chris]

How thick should be the spacing between coil & coil shield? What is the best spacing material?

Chris

[Sid]

Re: Coil shield question

Hi all, shielding is no problem. Make the basket or Lorenz coil, these are minimally influenced by the shield capcitance. Then cover it by a rubber stripe, thick about 2 - 5 mm. Take some cylinder capacitors made of metalised polypropylen, cut ends with contacts and roll off. It is wonderful stuff, thin and firm to be wound round the coil. The delay increase is about two microseconds and the biggest capacitance contribution comes from the coaxial cable. Nice day, Sid.

[Reg]

Re: Coil shield question

Hi Chris,

Hopefully Eric will jump in on this one also. In the mean time I will offer my opinion.

Now, I have not done enough experimenting to say just what is the ideal distance between the coil windings and any shielding used. I just know what works very well for me on the coils I build for my PI's.

A good friend of mine who has built several prototype coils for some of the more sophisticated VLF's has told me that some spacing is also necessary to minimize shielding to coil problems on VLF's also. He mentioned something like 1/8" to maybe up to 1/4" spacing.

On one of the coils for a major VLF, a quick fix to the coil to shield spacing problem was cured by using small pieces of rubber hose normally used for something like car vacuum hose. Small pieces were cut and split and then placed around the windings to provide the proper spacing before the coil was epoxied. On this coil, like most typical VLF coils, the shielding was a carbon based conductive paint.

As for a PI, I have tried different techniques of shielding and thicknesses of spacing but have alwabys gone back to using spirawrap as a spacer and a special copper plated tape for shielding. The spirawrap material I use is about .040 inch thick.

I have mentioned before that I use a special copper plated polyester tape made by 3M for my shielding and it seems to work extremely well. At least it does for me. As Eric mentioned, any copper shielding has to be extremely thin. The tape I use meets that criteria.

Besides the spirawrap, I have also split a plastic hose and used it as a spacer. This had a thickness of maybe .060 inch.

If one were to ask how well my shielding works, I would answer by saying I don't have any capacitance problems and I am able to get my PI's to work with a delay of less than 10 usec with a mono coil. The inductance of my coils is somewhere between 300 and 400 uh, depending upon the coil diameter. With a DD I can adjust the coil to work at even shorter delays.

Reg

[Eric Foster]

Re: Coil shield question

Hi Reg and Chris,

Shielding is very much a cut and try technique at the moment. I'm sure the coil cross section, number of conductors, insulation material, insulation thickness spacing to shield and shielding material conductivity and thickness, could all be calculated, or better still put into a computer program to get an optimum result. However, I find it often quicker to build first with readily available materials, see if it works and worry about the calculations last of all, if ever.

It would be nice, though, if someone developed a coil design program that took all the above factors into account, including optimising the coil diameter for range to different size targets. Bit beyond my capability :-(

Eric

[jackdetect]

Re: Design program

Hi Eric,
Such program can easily write any programmer in simple BASIC, for short time. BUT, you must give to us rules. Maybe Willy Bayout can help also, because he is expert for programming, and also have a lot of knowledge about Pi detectors, I think.
Jackdetect.

[drinnird]

Hi All,

Does there need to be a spacer if im using a spiral coil with insulated speaker wire? Im assuming the insulation thickness should suffice for the spacer?