[hpsdr] [Hpsdr] Call for Comments - ALEX

[Ray Anderson]

>Is there any data on how thick the aluminum shielding would have to be
to
>provide the same magnetic shielding as say 1 mm. thick steel plate?

Here are the skin depths for Copper, Aluminum and Steel (in inches) for
several frequencies.

 Frequency        Skin Depth             Skin Depth            Skin
Depth
                  (Copper)                (Aluminum)            (Steel)
   60Hz            0.335                   0.429                   0.034
  100Hz            0.26                    0.333                   0.026
   1kHz            0.082                   0.105                   0.008
  10kHz            0.026                   0.033                   0.003
 100kHz            0.008                   0.011
0.0008
   1MHz            0.003                   0.003
0.0003
  10MHz            0.0008                  0.001
0.00008


The absorption  attenuation is 8.69 dB per skin depth (so if you want
50dB attenuation you need about 5.75 skin depths of thickness, that
would be 5.75mil of aluminum or 0.46mil of steel).

You can see from the data that you need 12.5 times the thickness of
aluminum as compared to steel for the equivalent magnetic shielding. The
lower the frequency you want to shield the thicker the material needs to
be for the same shielding effectivness.

Also note that the total effective attenuation is greater since there is
also a reflection loss besides the absorption loss. Aluminum has more
reflection loss than steel (at 10MHz steel has about 50 dB loss while
aluminum has around 80dB). Below about 10MHz the reflection loss
predominates, above 10 MHz the absorption loss predominates.

So at 10MHz .46mil of steel has a total of about 100dB loss while
5.75mil of aluminum would have about 130dB. Increase the steel thickness
to the same as we have for aluminum and then the total steel shielding
loss would be a theoretical 675 dB!. At higher frequencies the
absorption loss increases and the reflection loss decreases.

As with all things it is an engineering tradeoff.

-Ray  WB6TPU

 


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