Public Shielded Room Work

Karl gmkarl at
Mon Oct 15 07:20:51 PDT 2018

On 10/14/18, Mirimir <mirimir at> wrote:
> I worked on this for a while. I was thinking about compartmentalizing
> services in multiple Raspberry Pi, connected via opto-isolators. Like
> Markus Ottela's Tinfoil Chat,[0] but expanded to work more like Qubes.
> And indeed, the Qubes team have announced work on Qubes Air:[1]
> | This approach even allows us to host each qube (or groups of them)
> | on a physically distinct computer, such as a Raspberry PI or USB
> | Armory. Despite the fact that these are physically separate devices,
> | the Admin API calls, qrexec services, and even GUI virtualization
> | should all work seamlessly across these qubes!
> But, you know, I wondered about EMF cross-talk between qubes. So I
> decided to learn how to measure that :)

How far did you end up getting on this?

>> I don't have any formal training and have developed some cognitive
>> issues, resulting in slow progress, but this is all I am spending my
>> free time on.  I do not work a job, being supported for now by a
>> trust.
> Yeah, me neither. But no trust :(

Maybe we can help each other.  Do you have any experience with funding
platforms like or something?

My trust is small.  I'm happy to share as much as it will allow me to,
but maybe if we could use something general, donations would
eventually come in.

>> I'm currently residing in Green Bank, WV, where emissions are
>> regulated for a radio observatory.  I am trying to develop some
>> relatively simple software using the rtl-sdr to measure the power of a
>> noise source independent of background traffic, so as to quickly and
>> repeatedly measure shielding effectiveness.
> I tried that approach. And it was a nightmare. The setup -- SDR stick,
> upconverter and laptop -- generated far too much EMF noise. I played
> with testing stuff in a Faraday cage. But I didn't manage a signal feed
> to the SDR etc that didn't introduce unacceptable noise.

My current setup is an oscillating noise source powered by a
single-board computer that toggles a relay, turning it on and off at a
consistent rate.

By averaging the noise level when the source is powered, and averaging
the noise level when the source is unpowered, over many thousands of
samples, I believe I can determine the power of the emitter without
regard to background noise by comparing the statistical distributions
of the two sets of samples.

I make the assumption that the foreground signal is the arithmetic sum
of the generated noise and the background noise.

Any thoughts?

> I believe that you'd need professional equipment, which is properly
> shielded, and doesn't bleed noise into the testing environment.

Have you tried or researched any professional equipment to report
back?  I haven't, at this time.

>> I then plan to try to measure a variety of setups ranging from
>> homemade aluminum foil & iron paint to soldered copper and welded or
>> bolted stainless steel, to identify ways for everyday people to
>> cheaply create shielded environments that are actually effective.  I
>> would like to find a way people can use off-the-shelf supplies to make
>> environments that are isolated from DC to light, if desired.
> That is also harder that it might seem. For high frequencies, with very
> small wavelengths, even tiny cracks are enough to leak horribly. You
> need joints with elastic seals, to mitigate against misalignment and
> wear. Such as beryllium copper finger strips, elastic beryllium copper
> tubular braid, etc.

I've looked into that a little.  After skimming through some shielding
books, I've got the following thoughts:

Permanent Seams:
- Alu foil can be stapled or tightly taped (see David Weston's paper
on aluminum foil rooms) to increase its frequency range.  I expect
using a wire brush to remove oxidation, and tightly flattening it,
would help too.  Testing is needed to see if this is worth the effort.
- Metal filings can be mixed with paint.  This allows for tight
sealing, but the conductivity is likely poor.  Advantage is that
sweepings are available for free.  Testing needed.
- Metal can be tightly bolted, as is done for modular rooms.  The
bolts must be frequent and close, to pull microvariations of the metal
into each other.
- Steel can be welded.  This is the gold standard.  Welding is not that hard.
- Copper can be soldered.  This is easier than welding !

Temporary Seams:
- Fingerstock is purchasable and not that expensive, but complicated
and needs cleaning.
- Bolts can be temporary, to bolt a door on as modular walls are
bolted together.  It's laborious, but it's workable and cheap and
doesn't require mail-order.  A robot could tighten and loosen them.
Testing needed.
- A door could perhaps be given pressure that does not penetrate it,
to keep a tight seal, perhaps via an automatic mechanism.  Cleaning
will be needed.  Testing too.
- A copper door could be actually soldered closed, and then desoldered
to open it.  A robotic door could automatically do this.  Very tight.
Research needed.

My understanding is that high frequencies are attenuated mostly by
reflection.  Hence I'd expect these tight seals to be needed mostly
for very thin, highly conductive material, which could keep costs down
if true.


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