RF Blocking Paint

[DigiTan]

I'm working on a digital control network project that needs to be shielded from noise in the UHF region. I've heard of graphite-laced paints and resins that promise almost a 40dB reduction with a single layer. There's also a few adhesive tapes coated with transition metals that claim comparable results.

This seems like the easiest way to shield an existing project from RF without having to substitute in a metal case. Easy that is until I saw the high retails costs of these paints and adhesives.


Basically, I'm trying to find this stuff for a reasonable price or possibly to make my own as long as it's non-toxic. Does anyone here have any direct experience working with or purchasing these materials?

 

[Nigel Goodwin]

I think, as you've already found, this kind of stuff is expensive!.

Presumably this is a commercial project? (for work etc.), so price shouldn't really be a concern.

 

[RadioRon]

The effectiveness of this sort of thing is entirely dependent on how and where you apply it. It might do absolutely nothing which would be an awful waste of money. How do you plan on using it? I mean, what surfaces will you apply it to, and where are those surfaces relative to the electronics you are trying to protect?

 

[Hero999]

I've got some off cuts of conductive rubber I got from work just for the novelty value. It's made by Chromerics, I don't know if they do conductive paint and resin but I wouldn't be surprised if they do.

If you'ree not that rich then fine wire gauze can be used in some applications as it's cheaper.

 

[DigiTan]

Presumably this is a commercial project? (for work etc.), so price shouldn't really be a concern.

Ah, I wish it were so, but this one's coming out of my own pocket.

I'm planning to apply it to casings for some existing hardware. They're mostly ABS, PVC, and nylon casings. They range from matchbox-sized cases to larger, shoebox-sized ones but they don't seem to have any protection from millimeter waves or anything in that range. As much as possible, I'd like to RF-shield these devices without ruining their aesthetics.


I guess I'll look at Chrometrics too. So far, they list some of the other RF products I've seen so far, plus a few I haven't heard of. If I could find a liter for less than $30 U.S., I might be in business.

 

[RadioRon]

I'm planning to apply it to casings for some existing hardware. They're mostly ABS, PVC, and nylon casings. They range from matchbox-sized cases to larger, shoebox-sized ones but they don't seem to have any protection from millimeter waves or anything in that range. As much as possible, I'd like to RF-shield these devices without ruining their aesthetics.

Do these various pieces of hardware with the ABS, PVC and nylon casings have any cables connecting to them?

 

[DigiTan]

The communication leads are 2-wire shielded, twisted pairs with differential signaling. The power cables I've seen so far are unshielded pairs.

 

[crusty]

duct tape tin foil to your walls.

 

[Nigel Goodwin]

Ah, I wish it were so, but this one's coming out of my own pocket.

I'm planning to apply it to casings for some existing hardware. They're mostly ABS, PVC, and nylon casings. They range from matchbox-sized cases to larger, shoebox-sized ones but they don't seem to have any protection from millimeter waves or anything in that range. As much as possible, I'd like to RF-shield these devices without ruining their aesthetics.

What makes you thing they need shielding?, and how are you going to test them?.

 

[DigiTan]

Right now, my concern is just locating something I can actually use. ...On a cost vs. performance basis. I've seen good reviews in its ability to block wardrivers and EMI. Unfortunately, most of the vendors enforce a large minimum purchase...or at least "large" compared to the half liter I plan to use.

 

[sam2]

For UHF, I would think any metal mesh( window, door screen) would work well, be cheap and easy to use.

Aluminum foil too.

Metalic spray paint, like chrome might work well.

You could make your own paint or epoxt coating by taking aluminum, copper, brass stock( or carbon/ graphite fishing pole- this could be done on a grinder) and sanding or filling it down.You could then mix the powder with what ever coating you want.

All this stuff is work but it is cheap and easy to get.
I wonder if crushed "pencil lead" graphite mixed with paint or epoxy would work well.

 

[quixotron]

I don't know or haven't tried this, but would copper tape be of any use?

If you wrap a good conductor around the exposed surfaces, it should block an rf from entering relatively well, like how a coaxial outer shield works.

Its cheap and can be found anywhere.

You could use rf absorbing foam found in anechoic chamers, but thats really expensive though.

 

[sam2]

Do a search on powderd graphite, its cheap.

 

[quixotron]

I have a question, now that this is brought up.

Would there be a difference in shielding EMI by reflection and absorption?

What I mean is, a sheet of material with conducting properties will usually reflect the em wave/field and attenuate the transmitted wave based on the evanescent wave theory-field decays rapidly throough medium, far less than a wavelength's thickness probaly less than skin depth-optics.

Suppose you have some radiating source and you want to block some device from receiving emi. It just so happens the device is above the source. So if we use some reflecting material, wouldn't it just bounce back some of the radiation towards the source? And if so, would the resulting em field, which is probably shifted out of phase react constructively/destructively with the source fields? Would we need to know, the excitation mode of the source?

I think thats why in anechoic chambers, the walls are lined with rf absorbing material. The foam absorbs the field pattern in a controlled manner, interfering with the source as due to reflection as minimum as possible.

The foam would have some type of loading effects though, right?

This is an interesting subject matter.

 

[justDIY]

I'm planning to apply it to casings for some existing hardware. They're mostly ABS, PVC, and nylon casings. They range from matchbox-sized cases to larger, shoebox-sized ones but they don't seem to have any protection from millimeter waves or anything in that range.

wait, millimeter waves? that's well beyond the traditional definition of UHF. Aren't mm waves like 100ghz+?

 

[quixotron]

wait, millimeter waves? that's well beyond the traditional definition of UHF. Aren't mm waves like 100ghz+?

Upper TV UHF, defined as channels 14-83 extends from 470-890 Mhz. US and EU cell phone bands are slightly higher.

lambda(890Mhz)= .34 meters=340 mm

f(10mm wave)= 30 Ghz

In free space.

So yeah u-waves roughly begin at 3Ghz band up to 300Ghz, bordering near infra-red spectrum. We can't see infra-red but there are some cool gadgets that simulate it. Like the predator...

 

[justDIY]

Upper TV UHF, defined as channels 14-83 extends from 470-890 Mhz. US and EU cell phone bands are slightly higher.

lambda(890Mhz)= .34 meters=340 mm

f(10mm wave)= 30 Ghz

In free space.

So yeah u-waves roughly begin at 3Ghz band up to 300Ghz, bordering near infra-red spectrum. We can't see infra-red but there are some cool gadgets that simulate it. Like the predator...

cool - thanks for the nice explanation

 

[jpanhalt]

So yeah u-waves roughly begin at 3Ghz band up to 300Ghz, bordering near infra-red spectrum.

I think you mean the far infrared. The nomenclature for IR and UV ligt is referenced to visible light. Thus, near infrared is short wavelegnth (i.e., beginning just below the visible) and far infrared is long wavelength and borders on the EM spectrum (e.g., mm waves). For ultraviolet, near UV is long wavelength (around 350 to 400 nm)and far UV is short wavelength (say, less than 200 nm). Wikipedia gives more specific breakpoints for each. John

 

[quixotron]

I think you mean the far infrared. The nomenclature for IR and UV ligt is referenced to visible light. Thus, near infrared is short wavelegnth (i.e., beginning just below the visible) and far infrared is long wavelength and borders on the EM spectrum (e.g., mm waves). For ultraviolet, near UV is long wavelength (around 350 to 400 nm)and far UV is short wavelength (say, less than 200 nm). Wikipedia gives more specific breakpoints for each. John

yes. if you want to take me to court, thats true. what i meant that the EHF range is near the infrared range, which it is. where on the inrared range, on the far/lower end. happy?

 

[RadioRon]

I think you may be disappointed with results if you approach your shielding work with the idea that RF behaves like optical energy, that is, light rays. For example, a sheet of metal, say 1ft x 1 ft, would stop a light beam from a laser pointer pretty effectively, but it will do almost nothing to stop an RF signal at 800 MHz. The reason is that the piece of metal is fairly small, and RF energy will sweep around it more like waves of water than beams of light. The RF will be "stopped" in the center of the metal by inducing a current in the metal, but then this current will conduct out to the edges, flow around to the other side, and in the process reradiate. So it will appear to pass right through the metal. Some RF will also refract off the sharp edges of the metal and bend inwards, also filling in behind the metal. You have to imagine RF energy as more like waves of water rather than beams of light, and remember the old physics lab experiments where you observe waves in a shallow wave tank and how they bend around things.

When visualizing that old wave tank, imagine that the distance between the wave peaks is, say, 1 inch, and your shielded box is also about 1 inch across (analagous to a 12 inch box reacting to 800 MHz waves). The box in this case will be almost transparent to the waves as they sweep around it and fill in behind it.

So, the effectiveness of a metal shield made from a good conductor is dependent on the size of the shield compared to a wavelength. If your box is relatively small in wavelength terms, then the RF will sneak around it looking for any opening, then go through that opening. This is especially true if there is a wire passing through a hole. That wire makes a much lower impedance path for RF to sneak in than a simple hole presents. This means that not only do you have to worry about holes and slots in your box, but you have to be 100 times more worried about wires that go in and out of that box.

So, if all you do is paint a box, but not seal up the seams and holes with metallic conductor gaskets or such things, then the RF will sneak in/out anyway and you will be dissappointed. If you don't use filtering for each conductor that passes in or out of the box, you will also be dissapointed with your paint job.

Doing a proper job of protecting a circuit inside a box from bombardement by UHF rf energy is an advanced art. There are many details I have not mentioned.