Author Topic: DIY RF absorber for the interior of a fully shielded lab  (Read 1482 times)

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Offline rhb

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Re: DIY RF absorber for the interior of a fully shielded lab
« Reply #25 on: May 20, 2020, 03:34:05 pm »
Well, at least this is better than bags of charcoal and other nonsense.

The pyramidal absorbers are using the shape to effect a graded transition in impedance.  The earliest ones were horsehair coated with a conductive mixture. Varying the composition of a uniform layer will accomplish the same thing. And be much less fragile.  But will still need a 1/4" wallboard covering for durability.

Commercial TEMPEST grade chambers use plywood panels with galvanized steel sheet on both sides clamped into channels with bolts at very close spacing.  I'm using HVAC sheet because it is easy to get and cheap.  Tinned steel would almost certainly require buying a coil which I could not handle and would be far more than I needed.  I'd love to use copper sheet, but that would raise the price tag by $4-5K to about double the current budget.  The HVAC is about 1/3 the cost, the ESD flooring about 1/3 and the metal sheet and studs about 1/3.  The effectiveness of metal mesh drops off rapidly with frequency,so an actual screen room is not desirable.

With the seams fluxed with acid flux, a thin flat strip of solder placed between the sheets and some strong neodymium magnets to draw the sheets together I don't think I'll have any problems with a 150 watt iron.  I've got drawplates and a rolling machine, so bespoke solder is not a problem.

I'm *not* considering RF paint.  It's ridiculously expensive and wouldn't work well anyway.  *If* I do this I'll be spraying mixtures of sheetrock mud, powdered iron and charcoal with latex paint added to the mud as a binder to make the mixture less brittle.  Each layer will be 1 mm or so starting with the maximum amount of iron and charcoal and grading to a minimum.  It won't be very effective at low frequencies because it will only be 2-3 cm thick.  I was making fireworks at age 12.  I learned to recognize and handle explosive mixtures long ago.

Before I go to all  effort to coat the walls and ceiling I'll be testing the reflection transfer function with an 8510C.

Non-linear junction detectors will find anything except the great seal bug which was purely passive.  You can see Gene Hackman's character use one at the end of "The Conversation".  I think Snowden deserves to live in Russia for the rest of his life for revealing the USB cable bugs.  An FET smaller than a grain of silt and dipole of #40 AWG magnet wire.  Executed well you could never find them by physical inspection and so cheap to make NSA sold them by the bag to CIA.  A non-linear junction detector would find them, so the cat is out of the bag :-(

The LED lamps in my current space have a large Corcom EMI filter to suppress conducted EMI and 1/8" hardware cloth covering it.  It's effective enough it is now below the level of other EMI.  I stopped hunting and suppressing other sources when I bought more test gear and ran out of space, requiring a new lab area.

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