DIY RF absorber for the interior of a fully shielded lab

[rhb]

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.

Have Fun!
Reg

[abinashsubash]

so did you make RF pyramidal absorber?
if yes can I know how
I have an exact requirement for converting normal RF shield room to anechoic chamber

[rhb]

I have not made any pyramidal absorbers and don't have any immediate plans to do so.

The purpose of the pyramidal form is to provide a gradient without having to make graded material.   There are lots of other ways to make an absorbing  panel.

If I were going to build an anechoic chamber, I'd use a mixture of drywall mud, latex paint, iron powder and charcoal with lightweight filler and spray it on the surfaces in graded layers with thin sheets of gypsum board to reinforce it every few inches of depth.  It's cheap, fireproof and durable.  More work though as you have to research the mixture to get the correct properties and verify it was fireproof.   But also vastly more durable a surface. 

Another option would be to blow compressed air into the mixture instead of the beads.

Some of the early commercial stuff was animal hair dipped in a mixture of  carbon and iron powder and then placed in pyramidal forms to harden.

Have Fun!
Reg

[rfguy2020]

I have also considering to make own pyramid absorber. I would like to have compact chamber for antenna measurements at around 900MHz. I think I would need about 300mm thick pyramid absorber. Making own absorber probably does not make much sense, but it is interesting topic. High quality absorbers from well known manufacturer are very expensive. Cheap absorbers from unknown manufacturer can be good, but there is risk that you don't get what you expect.

There is rather detailed document about manufacturing process (page 7) from ETS Lindgren:
http://www.ets-lindgren.com/sites/etsauthor/General_Brochures/Top%2010%20Anechoic%20Absorber%20Considerations.pdf
 
It looks like fairly simple. I did quick test from old mattress. I was using electric paint from Bare Conductive. I mixed some water to the paint. Then I soak the piece of mattress to this solution. To speed up the process I was making curing at elevated temperature. Outcome looked pretty good and feels and looks like commercial absorber. I didn't perform any RF testing, but the foam had similar DC resistance than commercial absorber measured with multimeter.

Cutting pyramids is a bit challenging with DIY tools. My absorber foam was shrinking a bit after curing, so it need to be cut from all sides. I think some kind of special scroll saw would be needed to cut shapes from the foam block.

Another challenge is to find thick mattress foam. I think 150mm foam is easy to find. I'm not sure, what would happen if layers of foam is glued together. I guess some solvent based glue would work in soaking process. Glue layer might have some impact to RF properties.

Final issue would be the characterization of own absorber. Probably the easiest way is to use the NRL arch method, which is basically bistatic measurement. The reflection of absorber compared with the perfect electric conductor. It would take big effort to make prototype sample for the measurement.

It would help if there would be proven formula of the soaking solution and curing process. I'm not sure if some iron oxide or graphite or some other material should be added to formula?   

[coppercone2]

may I suggest some short term investments in a financial market to get enough capital to do this with proper materials, these ideas are janky junk

something is not right when you are using mattresses as a building material as an improvement to loading a basement with bags of charcoal 

being in these rooms is NOT cool enough to warrant building a mattress crack fort, I would expect to find a faraday cage built of wire with little pieces of someones wall paper and alabaster on it in such a structure for some reason

[rfguy2020]

may I suggest some short term investments in a financial market to get enough capital to do this with proper materials, these ideas are janky junk

something is not right when you are using mattresses as a building material as an improvement to loading a basement with bags of charcoal 

being in these rooms is NOT cool enough to warrant building a mattress crack fort

You are right. It does not make sense to make own absorber. Same situation is with many DIY projects. It is better to use commercially available products. Sometimes it is just good to understand how things are done and learn from the process.

[coppercone2]

they are made in a factory with carefully tuned expensive spray guns and skilled operators making stuff when trial and error figures out the enormous amount of bullshit problems involved (a good spray gun costs 5k+ for painting a car even).

[rfguy2020]

they are made in a factory with carefully tuned expensive spray guns and skilled operators making stuff when trial and error figures out the enormous amount of bullshit problems involved (a good spray gun costs 5k+ for painting a car even).

Good point. You can do some rust repair for your car with cheap spray can, if you don't have much requirements for the quality.

[coppercone2]

even if you know what you are doing it ends up looking wrong, some car restorers and collectors are able to spot this kinda stuff from miles away, even if you used a cheap paint gun.. some how they see through the paint correction also

to cut foam anyway you use a hot wire cutter, https://www.amazon.com/Proxxon-37080-Wire-Cutter-THERMOCUT/dp/B0017NS8H6

The factory might have special giant specialty blades though

[rfguy2020]

even if you know what you are doing it ends up looking wrong, some car restorers and collectors are able to spot this kinda stuff from miles away, even if you used a cheap paint gun.. some how they see through the paint correction also

to cut foam anyway you use a hot wire cutter, https://www.amazon.com/Proxxon-37080-Wire-Cutter-THERMOCUT/dp/B0017NS8H6

The factory might have special giant specialty blades though

I don't think you can use hot wire cutter for polyurethane foam. Hot wire works for polystyrene foam. Polyurethane would make nasty fumes and cutting result is not good based on my knowledge. Also surface of the absorber will be more dense due to melted plastics. Mattress foam (polyurethane) should be cut mechanically as far as I know.

[coppercone2]

forgive me, I was focused on imagining someones family finding a satanic worship pit lined with several tons of charcoal and mattresses in the basement

[coppercone2]

looks like Matt Groening came up with this image, prophetic as usual. missed the charcoal though