4 ms·

One thing about the Faraday cage is that is has a maximum frequency - when the wavelength becomes comparable to the cell size in the wire frame. Any higher freq

One thing about the Faraday cage is that is has a maximum frequency - when the wavelength becomes comparable to the cell size in the wire frame. Any higher frequency will just go through. This is a problem as microwaves are in the centimeter range. The mesh then would have to be dense. That would raise aerodynamic issues. I'm not saying it's not doable. I'm just raising the obvious concerns about basic challenges from the laws of physics.

JoeCoder_ 10y ago
You could put your propellers outside and away from the cage, with their motors inside the cage. This would also greatly reduce the size and weight of the cage.
- Florin_Andrei 10y ago
  That might work. The "cage" would be more like a sleeve in the region of the arm - just creeping out on the arm like a mesh tube (or even continuous foil tube) to protect the wires, electronics, and motors. A small hole would allow the motor hub to stick out of the cage, and the prop would be on top of that, outside the cage. Heck, all the wires, electronics, and motors could be inside the "arm". Then coat that with some EM shield material (like a metal foil). I like it. Of course, from "like it" to "it's actually effective" there's still the nagging little field test to perform. It's the old shell-vs-armor game, since EM radiation has a penetration depth - Faraday cages are not magically 100% effective, make a thin enough one, and a powerful EMP would still do some damage inside. So now you need to carry thicker, heavier EMP armor.
  - JoeAltmaier 10y ago
    I think they are magically 100% effective. Unless the cage is breached, nothing will get through. However, an EMP pulse could melt holes in the foil. However it also means the drone has no GPS; no wireless signaling; no sensors outside the foil at all. It becomes a dumbfire dead-reckoning device.
    - Florin_Andrei 10y ago
      > I think they are magically 100% effective. No, they are not. Even a continuous, zero-hole metallic bubble will leak some radiation inside, depending on frequency, nature of material, wall thickness, etc. Start here: https://en.wikipedia.org/wiki/Penetration_depth https://en.wikipedia.org/wiki/Penetration_depth then continue reading about absorbance, skin effect, etc.
      - JoeAltmaier 10y ago
        I see. Only for zero-resistance metal would skin depth be zero.
        JoeAltmaier 10y ago
        At microwave frequencies, skin depth is essentially zero (microns anyway) for most metals. So until the bag is melted, nothing will get through.
ggoss 10y ago
Wouldn't a large amount of targeted RF incident upon an ungrounded faraday cage induce a large enough charge accumulation to also pose a threat to the drone (and anything around it)?
- Florin_Andrei 10y ago
  It's not ionizing radiation. Even if it did that, with a metallic shield on the outside, any amount of static charge on its surface generates zero field inside - this is a perk of metallic shielding. Two real issues with massive RF energy density: 1. Penetration depth. Even if you have a continuous metallic bubble with no holes, some RF radiation still leaks inside, depending on material nature, thickness, frequency, etc. The amount is small, but it's not zero. 2. If energy density in the RF beam is huge enough, the shield / cage starts heating up. You could conceivably melt it if you take this game far enough, or poke holes in it.
  - ggoss 10y ago
    There is a net zero electric field inside, true, but isn't that because a charge accumulates on the inside surface of the cage that is equal and opposite to that induced on the outside surface? With regard to #1, I agree, but the degree of RF leakage is not universally small - it depends on the frequencies you encounter and the properties of your cage (material, mesh thickness and "pore" size, etc). As frequency increases (and wavelength decreases), your pore size must decrease to effectively block it, eventually becoming a solid shell (through which propellers will, of course, generate no thrust). I agree with #2.
    - Florin_Andrei 10y ago
      > There is a net zero electric field inside, true, but isn't that because a charge accumulates on the inside surface of the cage that is equal and opposite to that induced on the outside surface? No. In a conductor, all free charges migrate to the outside surface.

JoeCoder_ 10y ago

You could put your propellers outside and away from the cage, with their motors inside the cage. This would also greatly reduce the size and weight of the cage.

Florin_Andrei 10y ago
That might work. The "cage" would be more like a sleeve in the region of the arm - just creeping out on the arm like a mesh tube (or even continuous foil tube) to protect the wires, electronics, and motors. A small hole would allow the motor hub to stick out of the cage, and the prop would be on top of that, outside the cage. Heck, all the wires, electronics, and motors could be inside the "arm". Then coat that with some EM shield material (like a metal foil). I like it. Of course, from "like it" to "it's actually effective" there's still the nagging little field test to perform. It's the old shell-vs-armor game, since EM radiation has a penetration depth - Faraday cages are not magically 100% effective, make a thin enough one, and a powerful EMP would still do some damage inside. So now you need to carry thicker, heavier EMP armor.
- JoeAltmaier 10y ago
  I think they are magically 100% effective. Unless the cage is breached, nothing will get through. However, an EMP pulse could melt holes in the foil. However it also means the drone has no GPS; no wireless signaling; no sensors outside the foil at all. It becomes a dumbfire dead-reckoning device.
  - Florin_Andrei 10y ago
    > I think they are magically 100% effective. No, they are not. Even a continuous, zero-hole metallic bubble will leak some radiation inside, depending on frequency, nature of material, wall thickness, etc. Start here: https://en.wikipedia.org/wiki/Penetration_depth https://en.wikipedia.org/wiki/Penetration_depth then continue reading about absorbance, skin effect, etc.
    - JoeAltmaier 10y ago
      I see. Only for zero-resistance metal would skin depth be zero.
      - JoeAltmaier 10y ago
        At microwave frequencies, skin depth is essentially zero (microns anyway) for most metals. So until the bag is melted, nothing will get through.

ggoss 10y ago

Wouldn't a large amount of targeted RF incident upon an ungrounded faraday cage induce a large enough charge accumulation to also pose a threat to the drone (and anything around it)?

Florin_Andrei 10y ago
It's not ionizing radiation. Even if it did that, with a metallic shield on the outside, any amount of static charge on its surface generates zero field inside - this is a perk of metallic shielding. Two real issues with massive RF energy density: 1. Penetration depth. Even if you have a continuous metallic bubble with no holes, some RF radiation still leaks inside, depending on material nature, thickness, frequency, etc. The amount is small, but it's not zero. 2. If energy density in the RF beam is huge enough, the shield / cage starts heating up. You could conceivably melt it if you take this game far enough, or poke holes in it.
- ggoss 10y ago
  There is a net zero electric field inside, true, but isn't that because a charge accumulates on the inside surface of the cage that is equal and opposite to that induced on the outside surface? With regard to #1, I agree, but the degree of RF leakage is not universally small - it depends on the frequencies you encounter and the properties of your cage (material, mesh thickness and "pore" size, etc). As frequency increases (and wavelength decreases), your pore size must decrease to effectively block it, eventually becoming a solid shell (through which propellers will, of course, generate no thrust). I agree with #2.
  - Florin_Andrei 10y ago
    > There is a net zero electric field inside, true, but isn't that because a charge accumulates on the inside surface of the cage that is equal and opposite to that induced on the outside surface? No. In a conductor, all free charges migrate to the outside surface.