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On 15/1/22 23:47, 2sheds wrote:

I like the Al case, but as somebody mentioned, it likely doesn't justify doubling the price.
When will affordable metal 3D printers be available?
I guess there are work-arounds for EMI shielding that accomplish what we need.

The Sep/Oct 2016 issue of QEX contains an article on metalising the surfaces of 3D-printed plastic microwave horn antennas, the problem being working with curved shapes to improve performance over the more conventional (cheaper...) flat-surface designs that could be made with copper or brass sheet. The authors used MG Chemicals 843-340G Super Shield Silver Coated Copper Conductive Coating <> which can be obtained inexpensively from the usual electronics suppliers. The like-for-like results at 10 GHz (i.e. 3D-printing a test design using flat surfaces) were comparable to the use of foil or solid metal.

If it's good enough to be an antenna conductor, it's almost certainly good enough for EMI shielding. Note that one of the problems that the authors had was gaps in the horn and its coupler enough to see light through, which naturally leaked microwave. A complete seal might be the harder problem.

- Roland 9V1RT

On Sat, Jan 15, 2022 at 11:51 PM, Roland Turner wrote:

If it's good enough to be an antenna conductor, it's almost certainly
good enough for EMI shielding.

I disagree with that blanket statement. The requirements and performance measurements are very different. There are a lot of variable, but for example, a surface that reflects 90% and transmits 10% of incident radiation might work very well as an antenna but very poorly as a shield.

Using your numbers Lou, I would not call a 20dB reduction in E-field "very poorly as a shield". The most difficult part of using any spray is getting it in to tight corners. I''ve done "touch up" on conductive spray painted parts with a small brush and the same paint sprayed on a piece of plastic as a "pallet". You do need to work fast though.

On 1/15/22 10:51 PM, Roland Turner via groups.io wrote:

On 15/1/22 23:47, 2sheds wrote:

I like the Al case, but as somebody mentioned, it likely doesn't justify doubling the price.
When will affordable metal 3D printers be available?
I guess there are work-arounds for EMI shielding that accomplish what we need.

The Sep/Oct 2016 issue of QEX contains an article on metalising the surfaces of 3D-printed plastic microwave horn antennas, the problem being working with curved shapes to improve performance over the more conventional (cheaper...) flat-surface designs that could be made with copper or brass sheet. The authors used MG Chemicals 843-340G Super Shield Silver Coated Copper Conductive Coating <> which can be obtained inexpensively from the usual electronics suppliers. The like-for-like results at 10 GHz (i.e. 3D-printing a test design using flat surfaces) were comparable to the use of foil or solid metal.

If it's good enough to be an antenna conductor, it's almost certainly good enough for EMI shielding. Note that one of the problems that the authors had was gaps in the horn and its coupler enough to see light through, which naturally leaked microwave. A complete seal might be the harder problem.

The other problem is at lower frequencies, where the spray on shield is too thin, relative to skin depth.? This is why nickel is often used - it's magnetic so the increased mu makes the skin depth shallower, so a thinner material can be used.

A lot of EMI treatments are for consumer electronics with high clock frequencies, so they're worried about VHF and UHF - hence the popularity of ferrite materials that absorb well up there. And, for 100 MHz, the skin depth is 1/10th that at 1 MHz, for the same material. So the 2.5 mils for copper becomes 0.25 mils, and a 1 mil thick layer starts to be an effective shield.

I'd call 10dB in power and 20dB in E-field poor shielding and 0.4 dB loss on reflection very good for an antenna. You could take another example, 50%/50%. That's 3 dB each way. We could argue ad infinitum, but In the end, it's whatever works in a given situation. Personally, I would not make the assertion you did, and that was my point. Other's can read this exchange and understand there's a difference of opinion and come to their own conclusion.

Here. I look at it this way. 10% in transmission (power) corresponds to a transmitted E field 32% of the incident E field. A surface that reflects 90% of the incident power still allows 30% of the E field through, neglecting loss in the material itself. Whether that's good or bad depends on the specifics of the situation.

Self-adhesive copper foil may be alternative that offers better shielding. I have used it on many plastic enclosures. I can solder bridge between sheets to cover areas wider than 4 inches.

--
N0YWB

How about aluminum 'duct' tape for shielding? What are the differences compared to copper tape?

Mike C.

On 1/16/22 11:22 AM, N0YWB wrote:

Self-adhesive copper foil may be alternative that offers better shielding. I have used it on many plastic enclosures. I can solder bridge between sheets to cover areas wider than 4 inches.

That's about 0.7 mil thick.? You might be able to find it with conductive adhesive - in which case you don't need to solder.? The 3M stuff is, this doesn't say one way or another, so I'd assume not.

On 1/16/22 12:19 PM, Mike C. wrote:

How about aluminum 'duct' tape for shielding? What are the differences compared to copper tape?

It works ok (skin depth isn't much different), but the real issue is that usually, the adhesive is not conductive.? Folks use this to make toroids for Tesla Coils, but in that application, the HV punches right through the adhesive. And, the fact that aluminum has an insulating surface film makes it tough to make good seams. However, if you do a rolled & crimped seam (hard to draw with text), it can work ok (because that makes the "slit" very long).

I've not had good luck at UHF and up, and haven't tried at lower frequencies.

The real problem is the aluminum oxide film, though.

Mike C.

On 1/16/2022 2:22 PM, N0YWB wrote:

Self-adhesive copper foil may be alternative that offers better shielding. I have used it on many plastic enclosures. I can solder bridge between sheets to cover areas wider than 4 inches.

I'm not sure I would call $45 for a 12 oz can inexpensive unless all alternatives cost more. The article refers to professional applications which can easily absorb such costs.
McDermid Chemical which was famous for chemical polishing of metals and innovating a practical method of plating plastic surfaces used a buildup of multiple layers to finally achieve a conductive surface. I tried that with painting a surface first with plastic-compatible paints like Krylon Fusion to create a surface that other coatings could adhere to and then adding increasingly conductive layers. I had variable success. I'm still working on it.
Even sticking copper or other conductive screens might have more success. The conductive surfaces need to be inter-connected and grounded to prevent them from becoming responsive resonant surfaces which defeats the purpose.
And, of course, shielding the rest of the box isn't worth it if RF leaks out or comes in the screen.
I'm an organic chemist with rudimentary skills in the ham world (electronics) but sometimes the chemistry is helpful on the macro scale.

I endorse N0YWB's recommendation of using self-adhesive copper sheet. On the usual Chinese sales websites one can easily find it, in rolls of various widths, with conductive glue. This copper tape is very good! The conductivity of the glue is surprisingly good. I cover larger areas by overlapping the tape just a bit, and it works pretty much like a continuous surface. The copper film is thin enough to easily adapt it to complex shapes, but of course it will crinkle. It will in fact crinkle as soon as you remove the backing, in any case, so don't expect a perfectly smooth, mirror-like surface, but it does work really well as shielding material.

I cover the plastic cases, and stick the tape to the connector bodies too, to make proper cable entries into the boxes.

I have also made rather complex antennas, like Yagis and phased arrays, for UHF and microwaves, by sticking narrow strips of this tape to a suitable sheet of dielectric base material. It's the quick and dirty alternative to PCB antennas. One can even carefully solder wires to the installed tape, without the glue coming off!

Instead the aluminium sheet I got from China all comes with non-conductive glue, and since it also can't be soldered, it's a poor choice. Use copper, and make sure you get the version with conductive glue.

Yes, it's thin. So don't expect it to shield against low-frequency magnetic fields.

XQ6FOD

On 16/1/22 22:11, Lou W7HV via groups.io wrote:

On Sat, Jan 15, 2022 at 11:51 PM, Roland Turner wrote:

If it's good enough to be an antenna conductor, it's almost certainly good enough for EMI shielding.

I disagree with that blanket statement. The requirements and performance measurements are very different. There are a lot of variable, but for example, a surface that reflects 90% and transmits 10% of incident radiation might work very well as an antenna but very poorly as a shield.

Note that what was discussed in the article was a driven element of an antenna (a microwave horn), not a parasitic (like a parabolic reflector for example). The numbers that you suggest would make for a very poor driven element and — if the authors' results are to be believed — are not what they saw.

I do take your point though, I said "conductor" rather than "driven element". I intended the latter.


- Roland 9V1RT

(apologies for the previous post; there was some fat-fingering on my part :-) )

On 17/1/22 00:21, Jim Lux wrote:

The other problem is at lower frequencies, where the spray on shield is
too thin, relative to skin depth.? This is why nickel is often used -
it's magnetic so the increased mu makes the skin depth shallower, so a
thinner material can be used.

I didn't know this, thanks.


- Roland 9V1RT

If someone has the skills to create a 3D model for the case, printing with both RF-absorbing carbon-fiber-fill FFF filament along with using cavity-style infill seems it might be a nice upgrade on the noise floor by both shielding against outside interference and suppressing noise generated from components within the device itself. We could also take the opportunity to add additional features like a compartment to store the calibration pieces.

Does anyone know of a 3d model available anywhere ?

-=dave - AI4ME

On 1/16/22 4:21 PM, Roland Turner via groups.io wrote:

(apologies for the previous post; there was some fat-fingering on my part :-) )

On 17/1/22 00:21, Jim Lux wrote:

The other problem is at lower frequencies, where the spray on shield is
too thin, relative to skin depth.? This is why nickel is often used -
it's magnetic so the increased mu makes the skin depth shallower, so a
thinner material can be used.

I didn't know this, thanks.

That bites microwave designers a lot - you have a gold plating over nickel, which "sticks" a lot better to many things, and doesn't have a diffusion problem like gold or silver directly on aluminum or copper. If the skin depth is greater than the thickness of the gold, then current flows in the nickel, which is lossy and the skin depth is shallow, so the current is entirely contained within the nickel.

(you'll see ENIG on PCBs - Electroless Nickel Immersion Gold - 2.5- 5 micron nickel with 0.5 to 0.23 micron gold - it's RoHS compatible, easy to solder to, etc.?? At 10 GHz, skin depth in gold is about 0.8 microns, in Nickel, about 0.1 micron, so pretty much none of the RF actually flows in the copper trace underneath)

Keeping this discussion more VNA like - you could probably measure this by looking at loss vs frequency. Skin depth goes as the square root, so you can look for S21 or S11 that is 5 dB/decade.

On 1/16/22 4:59 PM, Dave Johnson wrote:

If someone has the skills to create a 3D model for the case, printing with both RF-absorbing carbon-fiber-fill FFF filament along with using cavity-style infill seems it might be a nice upgrade on the noise floor by both shielding against outside interference and suppressing noise generated from components within the device itself. We could also take the opportunity to add additional features like a compartment to store the calibration pieces.

I'm not sure the noise floor is greatly affected by shielding; it would be dominated by the noise of the input amplifier (if any - the original NanoVNA just feeds the mixer through a pad, with no gain ahead of it), the phase noise of the synthesizer, and the ADC noise (including any contribution from the sampling clock).? The latter is about 85 dB SNR (datasheet number) but that's best case. The measurements sum 48 samples (depending on firmware), so that picks up another 7-8 dB


Shielding might help interference, maybe, but it *is* a narrow band receiver that steps across the band. Unless you're in a really noisy environment, you'd probably only see a single data point affected. You'd also have potential harmonics to worry about.

Silver is the best conductor of all. I used to specify "microwave silver" plating which was supposed to be 99.999% silver, but that comes out looking frosty and not shiny like platers are used to so they tend to put brighteners in the mix (molasses, for one!) which degrades the conductivity. Things got better after we educated our plater about changing the recipe.

There are other barrier metals such as titanium, tungsten, palladium or platinum, or proprietary combinations of them; of course they are orders of magnitude more costly than nickel. But we used them in the semiconductor industry because it was required for reliability.
73, Don Brant N2VGU

Actually diamond is the best conductor by a factor of about 5. Yeah, not easy to crush and turn into a sprayable paste. We'll keep trying though.

Mike C.