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RF Charge Build-up on Faraday Cage, and Grounding

Thread starter #1
I am building a Faraday cage from aluminum fly screen to exclude ambient RF. In terms of effect, what are the differences between grounding it and not? If it is not grounded, will a static charge build up that could affect items placed within the cage's internal space?
 

ronsimpson

Well-Known Member
Most Helpful Member
#2
I don't know why your would not ground the cage.
If you don't want to ground it and don't want static charge build up, then ground it with a 1meg resistor. Even a bad ground will blead off the charge.
 

Tony Stewart

Well-Known Member
Most Helpful Member
#3
You will need two layers grounded with x cm separation but no slots on entrance edges. ( extruded Al straps)

Then if you have a Tx inside, there WILL BE standing wave resonances.
 

RadioRon

Well-Known Member
#4
In theory, since the inside of the cage and the outside are completely isolated at RF frequencies, then there should be no effect at all at RF frequencies by grounding it. However, you really should ground the cage for safety reasons. A cage is useful if you can test things inside it and often this means running AC power through an RF filter located at the cage wall. In this case, grounding is absolutely essential to safety. The AC line filter that becomes part of the cage has connections, usually with capacitors, between AC and the cage wall.

The amount of isolation that you get from your cage between the inside and the outside will depend on the frequency band you are building it for, and the quality of your construction. There is a limit to how good it will be with a single wall construction that will depend mostly on how you close the seams in the screen connections. If your frequency range is modest, like say for example 50 to 200 MHz, then you will do pretty well. However, it gets tougher to get good isolation as the frequency goes up. I have built a few at 800 MHz with single wall construction and a door, using copper sheet, and we only got about 40 dB of isolation even after considerable fussing with door seams and such.

Many commercially available screen rooms use double wall construction, as Tony suggests, because they are going for a very broad frequency range and upwards of 80 to 100 dB of isolation. You likely don't need that much, do you?
 

Tony Stewart

Well-Known Member
Most Helpful Member
#5
Before you can design "anything" you must have good specs. In this case for spectral loss s21 and s11 or s22 reflection inside.

I had purchased a 2.5m cube from surplus ATE company in Toronto, dozens of which were used by Stanley Door to test their garage door openers. We used it for microwave Rx tests such as adjacent channel interference CNR levels vs RSSI and BER using a correlation integrate and dump detector. ., but for production, we just removed the antenna and terminated the port to use known leakage level power near threshold..
 
Thread starter #6
Thanks to everyone for their comments. The grounding situation is now clear.

I was not aware of the requirement of two walls for RF. I had assumed that as the frequency increases, or becomes more "beam-like", the fly screen would become more reflective, provided the aperture was about 10x less than the highest frequency to be screened.

What about using solid metal sheeting instead of screen? Would two layers still be needed?
 

RadioRon

Well-Known Member
#7
Two walls is not a requirement. One wall will do if you want 60 dB of isolation. Two walls are essential if you want 90 dB or better.

Isolation becomes more difficult as frequency goes up because imperfect seams, holes and such become larger with respect to the wavelength , and so leakage through them increases.
 

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