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Author Topic: Suggestions on homebrew coil shielding?  (Read 483 times)
KC9KEP
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« on: October 31, 2009, 05:32:42 AM »

Hello all,

I am planning on building an ARRL 1965 side-band transmitter :-)

I see that they’ve used old Calumet baking powder cans to house the plug-in coils.  

(If I recall, Calumet came in a red can with an Indian Chief image.)

My question is: is it an acceptable practice to use steel cans for coil shielding?  I thought that shielding should be made of aluminum?

I would think that steel cans would change the resonance and Q of the coils.

For that matter, I’d think that any kind of metal could change the coil’s characteristics if in close proximity?

Can anyone offer any suggestions on preferred shielding cans?

Thanks in advance for the great replies that I know will ensue :-)

--Tom Nickel

KC9KEP
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W8JI
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« Reply #1 on: October 31, 2009, 06:25:53 AM »

Tom,

There is a common myth that the shield material will affect inductance in different was, but in practice that is not true.

Any reasonably conductive material, steel, aluminum, copper, brass, zinc, or silver, will cause coil Q and inductance to decrease. The exception to this is if the inductor is in a "transmission line" mode where it acts more like a small antenna. in cases like that (helical resonators) a reasonably conductive shield actually improves Q.

Some materials will de-Q the inductor a little more than others, but they all have basically the same effect. For optimum Q we would have to not have the covering act like a shorted turn and not add shunt C throughout and across the winding.

Try this experiment. Measure reactance of a solenoid inductor at radio frequencies and then slide a steel nail into the axis. The inductance will DECREASE. It will decrease with iron, steel, copper, tin, or any other metal including mercury.

The reason it all behaves this way are eddy currents in the material's surface. So long as there is surface room for significant eddy currents to flow, the counter magnetic forces provide a shorted turn effect. This is frequency sensitive. If we broke the core into smaller and small cross section insulated pieces, we would reduce eddy currents at higher and higher frequencies. Eventually if we used a very fine powder, any iron present would start increasing inductance.

Other than this the only difference steel and iron would make is an increase in the effective RF resistance, but with a large surface area shield that is small.

For example the tank inductor Q of one of the coils in an AL1200 is about 350 in open air. When installed in a steel cabinet and chassis it is about 325. When the cabinet and chassis is changed to aluminum, it is still around 325. You will see the largest material type effect if it is a core of small area.

I had to measure all this when deciding if a change from steel to aluminum or stainless steel would have an effect.

Tom
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W8JI
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« Reply #2 on: October 31, 2009, 06:26:43 AM »

that should be affect inductance in different ways, not was.
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W5FYI
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« Reply #3 on: October 31, 2009, 07:42:59 AM »

One of the benefits of using a steel shield is that it would tend to confine magnetic flux to (and within) the shield. Mu metal is an alloy often used for such confinement, however it is usually used at much lower frequencies, like A.C. and audio.

If I recall correctly, weren't the old Calumet cans foil-coated cardboard or aluminum? An email to Calument, or a visit to an antique junque store should provide answers. I do remember that baking powder was a mixture of baking soda, salt and cream of tarter, and the idea of a foil-coated container was to keep humidity from ruining its "rising" properties.
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WX7G
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« Reply #4 on: October 31, 2009, 07:58:42 AM »

W5FTI: Aluminim will confine the magnetic fields just fine if it is several skin depths thick. At 3.5 MHz one skin depth in aluminum is about 0.002".
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W8JI
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« Reply #5 on: October 31, 2009, 08:01:48 PM »

WX7G said it all.

Once the shield wall is several skin depths thick nothing passes through.

Doesn't matter if the wall is aluminum, steel, copper or silver.

While field impedance can be changed in the very near field with different voltage and current distribution changes, it is physically impossible to take a time-varying electric field to zero without also taking the time-varying magnetic field to zero. If you shield one, you shield both.

MU metal is used for 60 Hz frequencies where skin depth is pretty deep, not RF.

Tom
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