RF Shielding Effectiveness - Page 1
Forum section: electronics / rf-microwave

author: imzack
author_rank: Newbie
timestamp: October 01, 2020, 09:04:20 pm
content: I have some RF Components, and am trying to figure out how much effective shielding we are getting with process variations taken into account.Questions:Is there a dB attenuation value that corresponds to thickness of aluminum?(ie, a foil thin aluminum provides XX dB of attenuation, while a 5inch wall provides xxdBs)Also using EMI gasketing; Its extruded, and can have gaps;I have been seeing only rules of thumb, saying gaps in gasketing only matter if gaps are >1/10th of a wavelength.Is there also a way to calculate how attenuation of your emi gasket would change, as your gap size increases?

author: bob91343
author_rank: Super Contributor
timestamp: October 01, 2020, 11:14:17 pm
content: Honestly, I feel this topic is too complex for the forum. There are volumes available on the subject, and much calculation and experimentation has been done and documented.Just one guy's opinion.

author: 0culus
author_rank: Super Contributor
timestamp: October 01, 2020, 11:23:46 pm
content: Honestly I kind of agree with Bob...while there are "ground rules", this area of RF is particularly fraught with the problem of being more of an art than a science...every application is a special snowflake and will require it's own solution pretty much. Experience is key.

author: coppercone2
author_rank: Super Contributor
timestamp: October 02, 2020, 02:04:21 am
content: doubt it will cook book well in regards to a cost standpoint (it won't get you what you think you are buying)

author: ejeffrey
author_rank: Super Contributor
timestamp: October 02, 2020, 02:56:48 am
content: There are rules of thumb for these questions but you need to have a solid understanding of the principles before you can use them effectively and know when they are relevant. There are some standard books on EMI, try Henry Ott's book.Quote from: imzack on October 01, 2020, 09:04:20 pmSkin depth in aluminum @ 100 MHz is about 8 micron. From there you can calculate the theoretical shielding of a chunk of aluminum. Other than a thin foil, the answer is going to be essentially infinite. When talking about good metals, the limiting factor is almost always holes and seams, not the bulk shielding.QuoteYou can, you basically treat the "gap" as a waveguide. If it is small, it will be below cutoff. Thus when you solve the wave equation, the modes do not oscillate, but fall off exponentially. You can turn this into an attenuation spec. Note that to be below cutoff, the gap must be much less than the wavelength in the X and Y directions. A long narrow seam is often a big problem.

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2020, 10:53:52 am
content: Quote from: imzack on October 01, 2020, 09:04:20 pmyes, such equations that allows to estimate shield attenuation in dB are exist.But it depends on which kind of field you're want to attenuate and which frequency.- plane wave EM field (far field region from the source at about lambda/2 distance and above)- E field (near field region from the source)- H field (near field region from the source)

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2020, 11:07:47 am
content: In short, shield attenuation depends on three components:Shield Efficiency in dB = A(dB) + R(dB) + B(dB)1) Absorption loss:A(dB) = -20*log10( exp( t / δ ) )wheret - shield thickness (in meters)δ - skin depth (in meters)δ = 1 / sqrt( pi * F * μ * σ )whereF - frequency (in Hz)μ - shield permeabilityσ - shield conductivityFor most non-ferromagnetic substances (such as wood, plastic, glass, bone, copper aluminium, air and water) have a permeability almost equal to μ0.2) Reflection loss:R(dB) = -20 * log10( (1+k)^2 / (4 * k) )wherek - impedance ratiok = Zshield / ZwaveZshield = sqrt( 2 * pi * F * μ / σ )Zwave depends on kind of field from which you're want to shield:- plane wave (far field region): Zwave = Z0 = sqrt( μ0 / ε0 ) = 376.73 - H wave at near field can be approximated as: Zwave ~ 2 * pi * F * μ0 * r- E wave at near field can be approximated as: Zwave ~ 1 / sqrt( 2 * pi * F * ε0 * r )whereμ0 - environment permeability for vacuum μ0 = 4 * pi * 1e-7 = 1.25664e-06 [H/m]ε0 - environment permittivity for vacuum ε0 = 1 / (μ0 * c^2) = 8.85419e-12 [F/m]r - distance between shield and source (in meters)c - speed of light constant, c = 299792458 [m/s]3) Multiple reflection correction term:B(dB) = 20 * log10( 1 - exp( -2 * t / δ ) * (k-1)^2 / (k+1)^2wheret - shield thickness (in meters)k - impedance ratioδ - skin depth (in meters)

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2020, 11:31:54 am
content: for aluminum at 20 °C: σ = 3.77e+7 [S/m]for silver at 20 °C: σ = 6.30e+7 [S/m]for copper at 20 °C: σ = 5.96e+07 [S/m]other metals you can found here:https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

author: radiolistener
author_rank: Super Contributor
timestamp: October 02, 2020, 12:06:34 pm
content: The most complicated thing is to shield H wave at near field region of the source.

author: cdev
author_rank: Super Contributor
timestamp: October 05, 2020, 02:09:42 pm
content: Related queation.. PC cases and RF shielding.Are there any good cheap, simple, full tower ATX or mini-ATX PC cases with good RF shielding that also have decent airflow and an effective air intake filter thats washable?