6mm metal mesh failed to attenuate a 2.4ghz RF signal

[nova2]

I purchased some galvanized "hardware cloth" 24'' x 5'
23 gauge with 1/4'' (~6mm) gaps

however it appears to have no major effect at all on the signal.
I covered up the 2.4ghz transmitter alright, except near the concrete ground there was very very little open space.

any ideas why this has failed to work?

edit: i'll probably later be testing a mesh with much smaller gaps

 

[Modelworks]

Did you ground the mesh ?

 

[bobdole369]

^^^^ errrr

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forum will probably screw it up.

 

[bobdole369]

Also note that your not going to see it go away, But your field strength meter will see its effect.

 

[nova2]

it could be possible the ground connection is at fault in some way, i'll be sure to check it out later

 

[Born2bwire]

Originally posted by: Modelworks
Did you ground the mesh ?

You really shouldn't have to ground. The main thing you need to do is have a closed surface around the source. Going to larger mesh gap can only increase the transmitted field.

I think the first thing you should check is to make sure that the mesh has a high enough conductivity (which I highly doubt this is the problem) and/or can truly block the signal. Just wrap some leftover mesh all around your transmitter and see if that does anything. Easiest way of doing it is make a cylinder and cut two sheets for the end caps. Then seal it together using metal (copper preferable) tape and run your wires up from holes in the mesh underneath the transmitter (and seal up around the holes with metal tape again).

EDIT: Taking a look at what you are using, I don't know how much of an improvement you will get with that galvanized mesh. Doing a quick back of the envelope calculation, you are only going to reduce the signal strength by about 45%, or around 3 dB with that large of a gap size. Most RFI screens are made of copper and have a very fine meshing.

A quick look at one site: http://www.twpinc.com/twpinc/c...~category_id=TWPCAT_11
shows that the largest gap would result in a reduction of around 67%.

 

[nova2]

born said "Doing a quick back of the envelope calculation, you are only going to reduce the signal strength by about 45%, or around 3 dB with that large of a gap size"

are you certain that is correct? (only 3 dB)

making note of this
http://www.safelivingtechnolog...rf_fiberglass_mesh.htm
its gaps are 5.5mm, but yet, the claimed attenuation is 23 - 13 dB

in other (good) news, i've found this website (via google query: Mesh RF Shielding)
lessemf.com/fabric.html
one of their cheaper products claims 20-35 dB attenuation (nice!). Obviously the gap/aperture size of their items are much much smaller.
I may end up going with them, unless I find better prices else where.

 

[Born2bwire]

Originally posted by: nova2
born said "Doing a quick back of the envelope calculation, you are only going to reduce the signal strength by about 45%, or around 3 dB with that large of a gap size"

are you certain that is correct? (only 3 dB)

making note of this
http://www.safelivingtechnolog...rf_fiberglass_mesh.htm
its gaps are 5.5mm, but yet, the claimed attenuation is 23 - 13 dB

in other (good) news, i've found this website (via google query: Mesh RF Shielding)
lessemf.com/fabric.html
one of their cheaper products claims 20-35 dB attenuation (nice!). Obviously the gap/aperture size of their items are much much smaller.
I may end up going with them, unless I find better prices else where.

It also depends on the thickness of the wire and that isn't given on their site. If the mesh was 3.5 mm thick, then that would be about 17.5 dB down. And like I said, it was a simple back of the envelope calculation. A real calculation would be to do a periodic modal analysis. I've done this for wire fences but I used the software to calculate the attenuation. Here I'm just looking at the attenuation of the fundamental mode for a rectangular wave guide. That estimate neglects the physical area that the mesh occupies. So for example, if I take that into account for your original mesh... ummm.... that would give an additional reduction of 1 or 2 dB. For the prior estimate of a thickness of 3.5mm, it would reduce it down to maybe 2 mm or 12 AWG. The estimate for the linked mesh would be around 8 dB down. So it's a pretty rough estimate.

Personally I would just make sure that if you make a crude Faraday Cage around your transmitter with the material that it does attenuate the signal and then try to debug your original setup.

 

[wwswimming]

on one test i had a chance to witness in the EMI lab at TRW Space Park,
we obtained 143 dB of isolation up to 2.5 GHz. (signal came crawling
out of the noise floor at 2.5 GHz). i designed the device being tested,
& watched a co-worker do the test.

it was a blast !

it would be very helpful to see a picture of your test set-up. if there
are IP issues involved, a diagram, something.

can you obtain some perforated aluminum sheet with similar size
holes & bend it into a box & weld the edges to provide a baseline ?

where the wires of the cloth come together to make a 1/4" aperture,
there should be a genuine contact, as if they were soldered.

1/4" apertures should be quite enough, if they're making good contact.

 

[ScottMac]

If there is a ~5" (roughly the wavelength at 2.4GHz 4.92") gap in any orientation (like the circumference at the base that "almost reaches the concrete", then you will pass the signal through.

Concrete is usually a pretty good bounce for 2.4 (unless it's wet).

A Faraday cage doesn't need to be grounded, it is a path of least resistance; like the metal shell of resistance a car provides to lightning (while being suspended from ground via "rubber" tires).

 

[Modelworks]

The reason I said ground it is because if it has gaps you could be making a reflector instead of a cage.

 

[Born2bwire]

Originally posted by: Modelworks
The reason I said ground it is because if it has gaps you could be making a reflector instead of a cage.

You do not need to ground it to make it a reflector. Any conductive surface will reflect regardless of its grounding. Off hand, I would say that the grounding will affect static charge build-up and might also be a liability since a ground wire might act as a radiator if it is not properly shielded outside of the cage.

 

[Modelworks]

Originally posted by: Born2bwire

Originally posted by: Modelworks
The reason I said ground it is because if it has gaps you could be making a reflector instead of a cage.

You do not need to ground it to make it a reflector. Any conductive surface will reflect regardless of its grounding. Off hand, I would say that the grounding will affect static charge build-up and might also be a liability since a ground wire might act as a radiator if it is not properly shielded outside of the cage.

In a perfectly designed cage it wouldn't be needed, but if you have gaps, I would highly recommend it.

Everyone I know that uses faraday cages for everything from tesla coils to microwave transmitters specs that the cage be grounded.

The reason is that a charge can build up on the outside of the cage that begins to emit the same type of waves you are trying to block, defeating the purpose of the cage. It only becomes a radiator when it is not grounded.

 

[Born2bwire]

Originally posted by: Modelworks

Originally posted by: Born2bwire

Originally posted by: Modelworks
The reason I said ground it is because if it has gaps you could be making a reflector instead of a cage.

You do not need to ground it to make it a reflector. Any conductive surface will reflect regardless of its grounding. Off hand, I would say that the grounding will affect static charge build-up and might also be a liability since a ground wire might act as a radiator if it is not properly shielded outside of the cage.

In a perfectly designed cage it wouldn't be needed, but if you have gaps, I would highly recommend it.

Everyone I know that uses faraday cages for everything from tesla coils to microwave transmitters specs that the cage be grounded.

The reason is that a charge can build up on the outside of the cage that begins to emit the same type of waves you are trying to block, defeating the purpose of the cage. It only becomes a radiator when it is not grounded.

Yes, like I said, grounding would remove static charge buildup, but the grounding lines can radiate since they will be carrying currents that are oscillating at the incident frequency. You always have to be careful about shielding anything that connects to and from the cage. The main problem with gaps is that you can allow surface wave modes to propagate out onto the outer conductor which are not cancelling out the inner conductors waves. Not to mention the fact that when the currents hit the discontinuity of the gap they will radiate off energy akin to what we see in microstrip antennas. There are probably other "practice as opposed to theory" reasons for grounding though. The main thing is that he needs to seal up all the seams on both sides with conductor tape or solder and he should also probably place a sheet of conductor on the bottom of the cage. It's probably easier for him to make a very careful and ideal cage and then work backwards from there to see how his current setup is lacking.

 

[s7]

Found your query while searching for 'Faraday Cages'. Did you solve the problem? If not I may have a possible solution.