coil shielding

[woodbob123]

My latest method for shielding let me know what you think.

First wrap coil with Mylar with metalic side out you can pull it tight
Second using thin parcel tape cover sticky side with a layer of graphite powder rub it in
Third wrap tightly over mylar with graphite on the inside.
connection can be made by striping a short length of wire spread out strands and laying it between mylar and graphite.

[golfnut]

My latest method for shielding let me know what you think.

First wrap coil with Mylar with metalic side out you can pull it tight
Second using thin parcel tape cover sticky side with a layer of graphite powder rub it in
Third wrap tightly over mylar with graphite on the inside.
connection can be made by striping a short length of wire spread out strands and laying it between mylar and graphite.

Hi Bob, Sounds pretty good to me.

My last coil had graphite in the shell and was ok.

The bulk of commercial coils use graphite/carbon in the shell. Im not sure that those businesses would survive if they employed teams of people winding foil or space blanket strips all day!

Steve

[woodbob123]

Always found making the connection to mylar or graphite a problem this way solves that. commercial coils are made to a price I would go bust on my first one, experimented a few months ago and as the shielding got thinner (started with kitchen foil) depth seemed to improve that was on a Surf coil 10 inch, have to improve where I can lol

[seadoggy]

how to shield coil whit kitchen foil??any one know?

[Davor]

You most probably are making a DD coil, so in fact you have two coils. Each coil must be wrapped separately, and they must be insulated at points where one coil overlaps the other. You must also leave an unwrapped ring on each coil so that the foil does not form a shorted loop. It is of less importance where you leave your coil unwrapped, but it is usually on the far side from a point coil wires go out. It is the best if wires for coil connections are not originating from the coils' overlap, as the overlap is very sensitive to vibrating metal parts.


Hr/BiH: Najvjerojatnije radiš DD sondu pa zapravo imaš dvije zavojnice. Svaku trebaš omotati zasebno, a treba ih izolirati na mjestima gdje se jedna zavojnica preklapa s drugom. Moraš ostaviti i neomotani prsten na svakoj zavojnici kako folija ne bi tvorila kratko spojenu petlju. Nije posebno važno gdje se ovaj neomotani prsten nalazi, a obično ga ostave na suprotnom kraju od onoga gdje žice od zavojnice idu van. Najbolje je da te žice ne izlaze na preklopu zavojnica, jer je preklop vrlo osjetljiv na titrajuće metalne dijelove.

[WM6]

You must also leave an unwrapped ring on each coil so that the foil does not form a shorted loop.

Moraš ostaviti i neomotani prsten na svakoj zavojnici kako folija ne bi tvorila kratko spojenu petlju.

.

Davor, probably you mean "unwrapped part of ring" - or a gap (neomotani dio prstena - odnosno procep)?

Or you mean to left one turn of wire (ring) in air for later fine tuning?

[Davor]

I meant unwrapped part, a gap. Just leave a small part of a coil unwrapped. Shielding is better with unwrapped portion being as small as possible, hence I said "a ring". A part I perhaps missed is that both coils' shields are supposed to be attached together in a single point, but in practical sense just connect together the wires that are attached to each shield. Just wrap a piece of non insulated wire, preferably tinned, while you are wrapping a foil. Otherwise keep shields mutually insulated. Poor contacts tend to become annoyingly noisy. Aluminium to copper contact deteriorates over time, hence the tinned wire preference.

Hr/Bih Mislio sam na neomotani dio, usjek. Samo ostavi jedan mali dio zavojnice neomotan. Oklapanje je bolje kad je neomotani dio što manji, zato kažem "prsten". Dio koji sam vjerojatno zaboravio je da se oklopi zavojnica trebaju spojiti u jednoj točci, ali u praksi to znači međusobno spojiti žice koje su spojene svaka na svoj oklop. Samo zamotaj komadić neizolirane žice, po mogućnosti pokositrene (kalajsane), dok omataš zavojnicu folijom. Drugdje bi oklopi trebali biti međusobno izolirani. Loši kontakti obično posanu neugodan izvor šuma. Kontakt aluminija i bakra se vremenom pogoršava, zato preporuka za pokositrenu žicu.

[Henrikras]

Dear all
If you do the shielding by tape, you must make a gap. But when you using spray in the Shell, do you allso need to make some sort of gap...! If yes, how can this Be done?
I've just sprayed a 1/2 bottle graphite 33 in my 13" Shell from Georgi, and the resistor still up about 3-5 Kilo ohm.(app.500ohm 1 cm)
Do I need more spray, or could this Be enough ? ( I think you Davor said something like "one can of spray, to one shield)
Ps. My hand didn't make any signal trying the without any shielding.
Thanks.
Henrik.

[dfbowers]

If you are using graphite a break may not be required as the resistance is fairly high. I make a gap anyway by putting a small strip of making tape down before applying graphite and then peel it off later.

Don

[Henrikras]

Thank you Don.
Its my first coil. So fare I found the accurate mH and the resonans was Bulls eye, and I could allso find the nul when it was nicely in the Shell. (There is some hope that it might work ).
The last part closing with the epoxy and still keep the 100% "nul", properly is very difficult.
Henrik.

[dfbowers]

Thank you Don.
Its my first coil. So fare I found the accurate mH and the resonans was Bulls eye, and I could allso find the nul when it was nicely in the Shell. (There is some hope that it might work ).
The last part closing with the epoxy and still keep the 100% "nul", properly is very difficult.
Henrik.

A little trick.. Cut a 1" x 1" square aluminum foil sheet and move it around the surface of the coil. This can also help fine tune your null . : )

[Henrikras]

Thanks Don.
I surpose that I must use a plastic or Wood long thing to hold this 1"x1" alu foil, and move it around the coil.
But can I measure the nul at the plug ? You know the coil is so sensitive in vibration and touch, measuring at the wire in the Shell.
Henrik

[Ferric Toes]

A little trick.. Cut a 1" x 1" square aluminum foil sheet and move it around the surface of the coil. This can also help fine tune your null . : )

I cemented a 30mm x 10mm piece of pvc pipe (A) just over the crossover of a DD coil. This was on the top surface of the top coil shell. I them machined a piece of pvc solid rod to just fit into the ring, also 10mm high (B). A horizontal hole was drilled through B and a small piece of ferrite rod inserted and so that it was off centre. Rotating B would then give an accurate null. Try it all first without glueing to find the best position for the ring. Next glue the piece of ferrite when a suitable size has been found. When the null is found the inner can be glued with something that doesn't set too quickly, so that final adjustment can be made. I made one version as an adjustable knob, so that it could be tuned on mineralised soil for experiments.

Eric.

[Davor]

But can I measure the nul at the plug ? You know the coil is so sensitive in vibration and touch, measuring at the wire in the Shell.

Your best choice is a coarse null at the plug and then "in vivo" by measuring for a null at the preamp output. It is usually a 1k resistor there, and such measurements are not too prone to odd couplings etc.
Both Ferric Toes and dfbowers give you approaches to easy coil nulling. Your best choice for a start is using brute force and convince a null out of your coil just by squeezing and stretching here and there, and work your way from outside to the overlap. put a drop of crazy glue as you advance. This way you'll be able to achieve the best null a coil geometry can offer. This exact result is achievable with a small piece of ferrite applied somewhere near the overlap. A ferrite reactance is always in phase with Tx, so there you have it. There will be a small unbalanced minimum signal that you'll not be able to make any smaller with geometry or ferrite. Both geometry and a ferrite reach only up to the exact same minimum.

Now comes the small piece of foil trick. At thin foils eddy currents rotate the response to Tx some ~90° away and dfbowers' trick becomes handy. With a small piece of foil you'll be able to achieve a deeper null than with geometry or ferrite alone. My choice of foil is a small piece of milk carton. It contains aluminium foil at 1/3 thickness of a household foil, so it is closer to the 90° optimum.

Whatever you see around as "this commercial coil gives a signal at (some degree) to the Tx" and the others do quite opposite, truth is that the best null is a bare minimum that you can squeeze from the coil. It just can't be done by geometry alone. A piece of foil helps.

[Henrikras]

Thank you so much...
I will try.
Henrik