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Is it possible to design a device (with transceiver) that detects if it is within a Faraday cage (for its frequency of operation)?

If yes, how it may be created?

tomnexus
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1 Answers1

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Here's one way - if the device can transmit, it can probe its environment. A Faraday cage that's a simple metal box will reflect the signal internally, leading to many closely-spaced modes or standing waves within the box.

For example, here's my NanoVNA measuring the reflection coefficient of a dipole antenna inside a microwave oven (taken through the mesh):
enter image description here

And here it is outside the microwave:
enter image description here
enter image description here

You can see the radical difference in the reflection coefficient - over the 100 MHz that it scans, there are large variations in the reflection. At its peak, the reflection is also a lot larger - the antenna SWR is generally worse inside the oven. Where the antenna position and frequency happens to excite one of the modes of the chamber, the reflection coefficient is smaller as the losses in the walls are increased. Tiny changes in position lead to large changes in the picture, as it excites different modes of the cavity.

Note that a microwave oven only works as a Faraday cage at 2.4 GHz, 7.2 GHz etc, because the door is not a proper seal but a quarter-wave choke. This is a clever way of making an effective RF seal without any metal-to-metal contact. In fact it depends on the insulating layer of powder coating to work. You can test this by comparing the wifi signal strength in side the microwave oven (should be zero) to the cellular phone signal (should be reduced but still there).

This method can only test for a fully reflective chamber. As soon as there's any significant absorption in the chamber - usually anechoic material designed to absorb radio waves, there won't be any standing waves. An anechoic chamber can be and often is fully shielded, so it would still act as a Faraday cage.

Comsol has a nice post about the standing waves in an oven, and some simulation results including this one which illustrates the uneven fields:
enter image description here

tomnexus
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  • I love the (inverse pun intended?) "out of the box" experiment of simply tossing a self-contained VNA+antenna into an [unpowered!] microwave to see what it sees! ~~Surprising to see a NanoVNA that works at 2.4GHz though?~~ n/m apparently that's common with V2 and beyond variants – natevw - AF7TB Mar 17 '22 at 21:25
  • @natevw-AF7TB This one is 4 GHz. I still can't quite believe it. VNAs used to be this heavy box costing much more than a car. Our company could afford just one, second-hand, shared by five engineers. Yes usually the VNA is outside and the probe is inside. I'm sure it disturbed the cavity. – tomnexus Mar 17 '22 at 22:43