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I want to design an iPhone case that will include a metal plate on the back of it. Is there any kind of metal that will not interfere with the phone's reception?

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  • \$\begingroup\$ No, Well maybe some very thin layer of a poor conductor. You'd have to be below the skin depth of the metal at the cell phone frequency. \$\endgroup\$ – George Herold Dec 2 '14 at 15:31
  • \$\begingroup\$ Mesh won't interfere - try putting your phone in a microwave & call it. \$\endgroup\$ – John U Dec 2 '14 at 16:17
  • \$\begingroup\$ @GeorgeHerold Will being below the skin depth help? If I put a wire across my car battery's terminals, it's definitely not electrically transparent, despite that its well below the skin depth, which is infinite at car battery frequency. \$\endgroup\$ – Phil Frost Dec 2 '14 at 16:37
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    \$\begingroup\$ @PhilFrost Doing this would not affect the transmission of your car at all, only the electrical system. \$\endgroup\$ – Hörður Mar Tómasson Dec 2 '14 at 16:44
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    \$\begingroup\$ @JohnU: The cellphone signal doesn't pass through the oven's mesh, it leaks around the door seals, which are selective to the oven's operating frequency. See the old discussion here \$\endgroup\$ – Dave Tweed Dec 2 '14 at 19:32
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That depends on what you mean by interfere. If you mean as if there were nothing there but air, then no. But if you mean the cell phone still works, then yes.

By putting metal near the phone, this metal will intercept some of the electrical field emitted by the antenna. Consequently, there will be some RF currents in the metal. These currents will circulate around, and ultimately:

  • be radiated again, or
  • go into losses, making something warm

It's pretty hard to prevent the energy from being re-radiated. As the currents wave through the metal, it's likely they will hit the edge. When they do, the current can't continue flowing, so a voltage develops. You have current, you have voltage -- sounds like an antenna.

In fact if you cut a slot in a waveguide with the right dimensions, it makes a great antenna. It's called, unsurprisingly, a slot antenna.

Depending on the geometry and placement of your metal, it might re-radiate the energy it intercepts quite efficiently. Or, it might not radiate so efficiently, and instead a lot of the current will circulate in the metal and be absorbed by resistive losses. It's going to depend a lot on the specific geometry of the metal.

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In addition to what's been previously stated, you will likely want to reduce the losses from induced currents. Using high-conductivity non-magnetic materials will be beneficial, eg. Gold, Copper, Aluminum. Metals to avoid would be those that interact with magnetic fields in a manner that makes their permeability factor much greater than 1, eg. nickel; such materials negatively impact the skin depth.

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  • \$\begingroup\$ Good points. Also avoid iron, for the same reason as nickel. Besides the decreased skin depth you get significant hysteresis losses -- ferromagnetic metals make really bad RF conductors. \$\endgroup\$ – Phil Frost Dec 3 '14 at 2:08
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No. All kinds of metal have enough conductivity to interfere with the function of a UHF antenna. The placement and dimensions of such a metal plate are much more important than the type of metal.

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