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Physics/How to Block AM Radio?



Thanks so much in advance! This is a theoretical question. My question is actually pretty tricky. I'm experimenting with a Faraday cage made out of stainless steel mesh with about 100 openings per linear inch. I put an alarm clock/radio inside (battery-powered, with the power cord inside) and sealed the cage. To my surprise, the cage blocks FM, but not AM! All AM radio frequencies, which are supposed to be easier to block, comes through loud and clear, while all FM is totally blocked!

Can you help me understand why this is? I've searched everywhere and all I see is "How to block FM Radio"'s driving me nuts!

Thanks again!


ANSWER: That's appears to be simple, really, wavelength.  Your cage is fine and all, but antennae need to be on the rough order of the wavelength of the wave they interact with.  Radio waves don't interact with your rods and cones in your eyes because the wavelength is too long.  Gamma rays don't interact with things well on the atomic scale because they're too short.  

FM frequencies are in the 100 MHz range, meaning that their wavelengths are on the meter length scale.  A Faraday cage can squelch those frequencies.  AM frequencies are about 100 times lower, meaning that they're about 100x as long and as slow.  Your cage is nowhere near the length scale of AM radio.  Admittedly, it does seem mysterious on first glance how lower frequencies would penetrate something meant to shield against static electricity and low-frequency EM sounds odd, but think about it.  If electrical currents have to run back and forth to transfer charge and create balance of the electrostatic fields, then they have to run currents through the cage in a particular polarization axis.  That, at the right frequency range, could create oscillating magnetic fields inside the cage.  Stainless steel is non-magnetic and not a great conductor, possibly the worst choice of material for your cage.  Mu metal would be better, or some soft steel or other ferromagnetic material.  A bigger Faraday cage made of at least copper or aluminum mesh (for the conductivity) might also be more effective.  Most of the ones I've seen for radio frequencies are the size of a large room, but with additional shielding inside.  We shielded preamplifiers with thick copper shells and even put copper tape at the seams (it made a measurable difference), so I never underestimate surprise sources of noise.

---------- FOLLOW-UP ----------

QUESTION: Thank you so much for your response! You've been EXTREMELY helpful!

I just have a few follow-up questions then.

My goal is to achieve 80 dB's of attenuation or shielding for all electromagnetic frequencies up to 1Ghz, but I can live with 40. Using my stainless steel mesh, I can already achieve E-field reduction of 80 dB's from the FM range to roughly 1 Ghz. So if I understand you correctly, I have to boost the H-field shielding. Could I simply apply a spray-on nickel flake coating to the mesh? And if I do...if the coating is on both sides of the mesh, would it diminish the effectiveness of my stainless steel shielding?

Also, regarding the preamplifiers you mentioned - I'm assuming you used "thick" copper shells for extreme levels of reduction - ie, 125 dB+, right? If I only need 80 dB's, would I need to go to such lengths?

Thank you again!

You don't need to go to such lengths with copper shielding.  Your standard shielding will work just fine, the nickel stuff will help but you should really just focus on using something with a higher conductivity like aluminum (cheap, too).  And nail the corners with copper tape, you'd be surprised how the edges of your box let the field leak in.  It's the shape factor, really, that makes it happen.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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