Home » Uncategorized » Last chance for design comments: The Sheet Metal has arrived (18 gauge mild steel, 4ft x 8ft)

Last chance for design comments: The Sheet Metal has arrived (18 gauge mild steel, 4ft x 8ft)

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Here we go: the steel is here. It is 18 gauge (1.27 mm), 4ft by 8ft. I’m still agonizing over the hatch design, and whether I should have an air porthole or not (a person only needs a few minutes at most inside). But we can go ahead on the initial welds.

After three years, the time has come.

Photos to come.



  1. George G. says:


    If the design of a porthole is too difficult, then include a bottle of air in the box (contact a local scuba diving club for advice)

  2. Simon says:

    Hi Glen, just a thought on this issue. I agree entirely that a supply of fresh air is probably essential, but clearly we need to provide it in such a way as to screen any RF signals. I would have thought that some sort of labyrinthine passage made of steel, with a bunch of overlapping baffles and small openings inside it may be a way to do this. Because this may not help with the airflow however, air may need to be sucked out of the box somewhere else (with a small axial flow fan for example) through a similarly baffled route to induce a through flow of air. I assume the box will not be locked if all fails! If there are any doubts about safety I suggest a CO meter in the box to allow the test subject to continuously monitor the air quality.

    • One part of this that intrigues me is how challenging it has been to get decent research or advice on this topic. I’ve dug with my best research tools and found nothing but my own blog posts and very obscure stuff. Surely I’m not the first person to weld up a VLF-proof box. It just can’t be.

  3. Well, you will have a nice barrel to go over Niagara Falls with. I’ve been turning this over in my mind…

    As you know, RF is composed of both electrical fields and magnetic. Propagating over long distances (many wavelengths), it will comprise both electrical and magnetic. Over a short distance (near field region) it can be either depending on what caused it. A magnetic field can be leakage from a large power transformer (which are usually enclosed in a steel can), an induction coil such as a charging coil for an electric car or phone, an induction heater (industrial or stove), or a CRT deflection coil for an old TV or computer monitor. An electrical field can be generated by a discharge from a tesla coil or breakdown in a HV insulator.

    Your box ought to isolate pretty well against EM fields and near field electrical fields if it’s grounded. The magnetic fields ought to be attenuated pretty well, enough to be very noticeable which is all you need to prove.

    Shielding from far field EM waves is usually as easy (?) as making a Faraday cage, because you short out the wave by making holes too small for a wave to pass through. Some leaks through because of evanescent mode effects (don’t ask) so you can get even more attenuation by making another cage inside the first one. I’ve used this in a portable screen room to look at scope signals next to a megawatt-level radar transmitter. At low RF frequencies, the wavelengths are so long that structural steel acts as a screen. This is why AM broadcast band signals disappear under a bridge overpass or inside a commercial building, while FM signals pass through.

    I think that if you step inside your box with an AM radio, the signal will fade without even closing the door. When you close it even lightly (not making multiple ohmic connections as with finger-stock, but just overlapping the steel to make a magnetic “connection”) it will probably be totally gone. At that point, all your VLF signals will be long gone too.

    I presume you have a battery-powered way of measuring VLF signal strength?

  4. George G. says:


    It seems to me that between Simons steel baffles and Jims tech. know how the problem of ventilation is solved.

    GO FOR IT!

    • George G. says:

      Did you take note of Jim’s comment regarding grounding?
      Theory is fine; practical experience is equally important.
      Should you decide later into the experiment that earthing of the box may not be such a bad idea,
      then it won’t be too much of a big deal, just some messed up insulation, paintwork etc.
      Should you want to avoid that, then NOW is the time to provide an earthing terminal, while the box is still in the bare metal stage.

      Please have a good think about it.

      • Yes, it certainly would do no harm. I’ll be making a separate post shortly regarding the hatch to the box, which is the current sticking point the process. Cheers.

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