How can I reduce noise amplification in my Faraday Cage?

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Discussion Overview

The discussion revolves around the unexpected amplification of noise in a homemade Faraday Cage, particularly concerning 50Hz mains pickup. Participants explore the effectiveness of the cage in shielding against this noise, the role of grounding, and the measurement techniques used to assess noise levels.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports that their Faraday Cage seems to amplify noise instead of shielding it, specifically noting an increase in 50Hz mains pickup when the measurement device is placed inside.
  • Another participant questions whether the measurement device itself could be introducing noise, suggesting that its circuitry might be responsible.
  • Some participants argue that Faraday cages do not effectively shield against inductive pickup, implying that the observed noise may be due to ground loops rather than the cage's performance.
  • A participant mentions that connecting the meter directly to the cage and grounding it did not eliminate the noise, and they observe changes in noise levels when opening and closing the cage door.
  • Suggestions are made to use more sensitive instruments, such as an oscilloscope or spectrum analyzer, to better analyze the noise, as a digital voltmeter (DVM) may not provide sufficient detail.
  • One participant notes that the noise levels inside and outside the cage appear similar, and that opening the cage door seems to reduce noise, which raises questions about the cage's shielding effectiveness.
  • Another participant discusses the unpredictability of earth currents around the cage and suggests that a more robust construction, such as a copper box with soldered seams, might be more effective at controlling hum.
  • A later reply indicates that switching the connection of the cage to the guard terminal of the DVM improved the expected behavior, suggesting that grounding configurations may significantly impact noise levels.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the Faraday Cage against 50Hz noise, with some attributing the noise to ground loops and others questioning the measurement methods used. The discussion remains unresolved regarding the best approach to mitigate the noise amplification observed.

Contextual Notes

Participants highlight limitations in measurement techniques and the potential influence of grounding configurations on noise levels. There is also mention of the need for more detailed analysis to understand the noise characteristics better.

jonlg_uk
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Hi everyone, I have recently built a Faraday Cage. However it seems to be working in reverse and amplifying the noise!

My test setup:

I place a 3.3 Mohm Bulk metal precision resistor across the terminals of the meter and measure the pk-to-pk noise using the meter.

I now place the meter inside of the cage (the cage is floating) but now the noise increases!

I now ground the cage and repeat the measurement and it makes no difference.

N.B The noise that I am seeing is 50Hz mains pick up. I have sealed the door of the faraday cage with copper tape and I am quite confident that the sealing is effecting.

Does anyone have any ideas?


I thank you all in advance :cry:
 
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This is not really an answer, but is the meter electronic? Perhaps it's own circuitry is responsible for the noise. A related question is how relatively old (lower frequency) mobile phones operate successfully inside lifts.

P.S. By lifts I mean 'elevators'.
 
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pawprint said:
This is not really an answer, but is the meter electronic? Perhaps it's own circuitry is responsible for the noise. A related question is how relatively old (lower frequency) mobile phones operate successfully inside lifts.

No this is defiantly caused by 50Hz pickup. As I open and close the cage of the door I can see the noise increase on the DVM.

The cage is capable of almost killing mobile 3G mobile phone signals. I suspect the elevators are not sealed sufficiently to shield effectively.
 
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Faraday cages do not work for inductive pickup, meaning they are useless for 50 Hz.
Whenever you see a lot of 50 Hz in a setup it is almost inevitable due to a ground loop. Hence, the cage should not really have any effect on the noise at all, if it does it is probably due to you accidentally creating a loop or perhaps making an existing loop worse (by e.g. separating the cables even further)
 
f95toli said:
Faraday cages do not work for inductive pickup, meaning they are useless for 50 Hz.
Whenever you see a lot of 50 Hz in a setup it is almost inevitable due to a ground loop. Hence, the cage should not really have any effect on the noise at all, if it does it is probably due to you accidentally creating a loop or perhaps making an existing loop worse (by e.g. separating the cables even further)

I have connected the meter directly to the cage and the cage is grounded, so I don't think there is ground loops. I am seeing an increase in 50Hz noise pickup when I place the meter inside the cage, when I open the door to the cage, all the cables remain stationary, but the noise now decreases.
 
You need to look at the 'noise' in more detail. Have you tried a receiver in there, or an oscilloscope on max gain? A DVM is far too crude an instrument. A spectrum analyser may be over the top but could you not borrow a small hand-held scanner? Without more information you could be chasing your tail.
What did you build your Faraday Cage for?
 
sophiecentaur said:
You need to look at the 'noise' in more detail. Have you tried a receiver in there, or an oscilloscope on max gain? A DVM is far too crude an instrument. A spectrum analyser may be over the top but could you not borrow a small hand-held scanner? Without more information you could be chasing your tail.
What did you build your Faraday Cage for?

This is a 8 1/2 Digit Meter, the best on the market, it is no ordinary DVM. It is connected to a PC via GPIB and we are able to monitor the signal waveform on the PC. The cage is built out of 6mm chicken wire.
 
And does the meter also measure frequency / spectrum? It may be 'extraordinary' as a DVM but if you need another instrument, it won't do the job. How is your "noise" defined? What bandwidth etc.?
 
*******************
UPDATE
*******************

I would like to add a update. I have conducted some experiments. I noticed the following, atm I have a resistor connected to the HI and LO terminal of the DVM, I am using this as a fixed noise source. When I connect a short from the LO terminal to Ground the 50 Hz noise decreases by half.

I have also noticed that the 50 hz pickup is the same outside the cage as it is inside the cage. However whenever I open the (trap) door of the cage the 50Hz pickup decreases dramatically. Its like the door is providing extra shielding when it is open?!?1? I am very confused.
 
  • #10
IS this Noise, Hum or what? How does your system distinguish?
 
  • #11
sophiecentaur said:
And does the meter also measure frequency / spectrum? It may be 'extraordinary' as a DVM but if you need another instrument, it won't do the job. How is your "noise" defined? What bandwidth etc.?

The PC that the meter is connected to has a function that will allow me to export the data to excel. I can then perform fast Fourier transform on it to obtain the frequency spectrum. I have tried placing a spectrum analyzer inside the cage and I could not detect any noticeable changes in 50Hz pickup. In fact despite my spectrum analyzer having a 30Hz -3 GHz BW, I could not even pickup 50Hz when it was outside the cage.

My spectrum analyzer has no antenna so that is probably the reason why it isn't picking up anything.
 
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  • #12
sophiecentaur said:
IS this Noise, Hum or what? How does your system distinguish?

Its defiantly 50Hz pickup, the DVM samples many times per second and the oscilloscope software is showing a 50Hz wave form.
 
  • #13
So we are looking at Hum and not noise. As I suspected,, your equipment just can't show noise (at least not above a few Hz bandwidth), but this may not be relevant.

Earth currents can flow all around your cage in a totally unpredictable way so there is just no way of knowing what levels of hum you could be picking up. A faraday cage - certainly not a homemade one of chicken wire is really not the best way to control Hum. A copper box with soldered seams might work quite well but the door would need a very good contact all way round, with spring fingers etc.. Then you would need to be getting signals in and out - which is another path for noise and Hum.
As usual, I would suggest going back to your original requirement and address that first. Then decide whether you need this cage and what would be the required spec. This puts me in mind of enthusiastic guys who think they are acoustically treating a room by lining it with egg boxes.

Re your comment about the spectrum analyser. Why would you have an antenna on it if you were interested in the signals from your DVM? Better to put a balanced antenna inside your cage and connect that directly to the spectrum analyser.
 
  • #14
Thanks for your help. I think it was some type of current loop. On my DVM I have a HI and LO terminal and a terminal. There is also a Earth terminal. I had the cage connected to the Earth terminal and not the guard terminal. I don't know why but when I switched it to the guard terminal, the cage started behaving as you would expect. Now when I open the door the noise goes up and when I close it the nose goes down.
 

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