Calcoli di attenuazione

CONTINUOUS FIELD

• Condition of un-saturation
   
• To determine the induction inside the material (Bin), we use the following formula: Bin =  k x  Ho x D / t
  (where k is chosen between 1 and 1.5 depending on the other Ho)
       Then we verify that Bin <2/3 Bsat
       
• Calculation of attenuation (Simplified)
  - For an infinite cylinder whose field is perpendicular to the   axis:    At  = μ x t / D
  - For a sphere:
             A = 4/3 x μ x t / D
  - More generally:
             A = k x μ x t / D
             k is a factor depending on the shape of the shield, its
             orientation relative to the field and its degree of opening.

ALTERNATING FIELD

• Alternating fields will create Eddy currents generating a field which opposes the interfering field.
       
• The result will be determined by a parameter called the skin effect: delta = [2,54.105 x ro / (µ x f)]1/2
            (ro = resistivity of the material / f = frequency
      
• This skin effect will then be used in a chart giving coefficients as:  Aac = p x Adc 
  (Aac: alternating field Attenuation)
  (Adc: constant field Attenuation)

Ho: Outfield
At or A: transverse attenuation or attenuation
μ: permeability
t: thickness of the material
D: diameter of the shield

CAUTION

• All formulas are based on:
           - The conservation of flux (field lines in the air are concentrated in the thickness of the shielding material)
           - The principle that the ferromagnetic material deflects the magnetic field lines in a ratio of a given area (in a perpendicular field). However, this ratio depends on the permeability of the material and the size of the screen (a small mumetal cylinder acts differently on its environment than a pure iron screen 2 meters in diameter.
       
• Permeability is not the same:
           - Depending on the field (B-H curve)
           - According to the thickness
           - According to the frequency  
     
• Although this is only one approach, these formulas have solved thousands of designs.

MULTILAYER

• The formula for an infinite cylinder with an axis perpendicular to the field with 2 layers of diameters D1 and D2 (D1         A = 1 + A1 + A2 + A1 x A2 x [1 - (D1 / D2) exp2]
       
• The principle is the same when adding other layers.