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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.

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

That's what I suspected. I'd appreciate a photo or two, but I think I could work it out without them.

Jim
KC3DPO

(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

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

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

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.

Hi Jim,

Some metal working is required: I've made a slot in the top to access the controls and it works fine. I'll put up some more pictures if you are interested.

Michael (GW7BBY)

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.

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.

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

Mike C.

Here is the one I am using for my H4.



It has some padding, and pockets for cables, etc.

...Bob / AA2FD

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

I like your solution, but how do you access the controls on the Nanovna? It appears they are all inside the case. What is it that I'm missing?

Thanks in advance.

Jim
KC3DPO

Jim, I've used that tin oxide for shielding of a few large products.
Visibly it's transparent which was a requirement. But I've found it's only
marginally effective, but it's better than nothing.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 1/16/22 9:12 AM, W0LEV wrote:

And how are you going to "shield" the touch screen?

in practice, touch screens usually have a transparent conductive layer on the top (indium tin oxide is common).? Or the actual LCD display has an ITO top electrode, and the touch screen is resistive or capacitive over the top of that, and runs at low frequencies which can be filtered (after all, you're probably not touching the screen a million times a second).

And those "thin
traces" to the SMAs? They are likely printed against a ground plane in the
layer beneath the surface layer and likely adjusted in thickness to be
nominally Zo = 50 ohms traces as stripline transmission lines. In the RF
world things are not always what they look like on first inspection.

Yes, but that transmission line still radiates a bit. Not all the field is contained in the dielectric, especially with microstripline. It's going to be a poor radiator/receiver, but that depends on what the need is.

Dave - W?LEV

On Sun, Jan 16, 2022 at 3:06 PM Jeff Green <Jeff.L.Green1970@...>
wrote:

Out and out heresy.
I'm making this a separate post because there have been several threads
covering the emi issues.

I had an interesting conversation with the ee I worked with. She explained
a bit about shielding and filtering for emi/emc.

As a rule of thumb, radiated emissions are a concern at 30MHz and above
and conducted emissions are a concern below 30MHz.

There is some overlap, but, as you go from say 100kHz up to 100MHz, you
will find the emi is caused by conduction at 100kHz and radiation at
100MHz. The transition zone depends on the physical size of the device
under test.

Devices that are physically larger will radiate at lower frequencies.

She used an example. "Say you have an object that is a 1" cube that
produces rf white noise at a constant amplitude. It won't become a problem
until high uhf because the small size simply can't radiate.

As a rule of thumb, significant radiation starts when an object is roughly
1/20 of a wavelength and almost always becomes an issue when an object is
1/10 of a wavelength in size."

The smaller an dut is, the higher the frequency has to be before radiation
becomes a concern.

She suggested I study point sources.

A NanaVNA has a USB cable and two rf cables, this is a near worst case
scenario for radiation because the cables will form the arms of a quasi
dipole.

It is extremely difficult to add enough shielding to control radiation
after a product is designed, and it's "damn near impossible to add enough
filtering adequate to control conducted emissions after a product is
designed."

The best, most effect, way to deal with emi is during the design phase.
Any effort after a product is designed will never be as effective as proper
steps during design and initial testing.

Ferrite inductors can help but, the lower the frequency of the emi you are
trying to attenuate, the more ferrite you will need.

At some point, you can't add enough ferrite to stop the emi.

She added, "Say you make your NanoVNA dead quiet, what about your PC?
Unless you have a laptop or desktop desktop that meets TEMPEST
specifications, your PC will almost certainly produce more emi then your
NanoVNA."

She looked up the NanoVNA and pointed out numerous issues. The main one is
the traces that connect the shells of the SMA connectors. For frequencies
below 30MHz they aren't an issue, as you move up from 30MHz, the narrow
traces will have enough inductance to become an issue for emi.

She is sending me an older edition of
Electromagnetic Compatibility Engineering 1st Edition, by Henry Ott. She
says it's the bible for emi/emc.

She also suggested that we not worry so much about emi because we probably
can't reduce the emi enough to be worth the effort and most likely, well
never be able to see any improvment.

She suggested we all study the manual at:


to see the steps required to really deal with emi. She also thought we'd
probably never see the difference in our test results if we were able to
make a case with perfect shielding because of the small size of the NanoVNA
and the conducted emi from the usb and rf cables.

"Far better to concentrate on a case that is robust enough to protect the
NanoVNA."

She is a smart gal, PHDs in EE and physics. Extra class ham, really smart.

"For the price, the NanoVNA is an amazing value, just don't expect the
same results you'd get with a Agilent 8563EC Spectrum Analyzer, used ones
start at 10K."

And how are you going to "shield" the touch screen? And those "thin
traces" to the SMAs? They are likely printed against a ground plane in the
layer beneath the surface layer and likely adjusted in thickness to be
nominally Zo = 50 ohms traces as stripline transmission lines. In the RF
world things are not always what they look like on first inspection.

Dave - W?LEV

On Sun, Jan 16, 2022 at 3:06 PM Jeff Green <Jeff.L.Green1970@...>
wrote:

Out and out heresy.
I'm making this a separate post because there have been several threads
covering the emi issues.

I had an interesting conversation with the ee I worked with. She explained
a bit about shielding and filtering for emi/emc.

As a rule of thumb, radiated emissions are a concern at 30MHz and above
and conducted emissions are a concern below 30MHz.

There is some overlap, but, as you go from say 100kHz up to 100MHz, you
will find the emi is caused by conduction at 100kHz and radiation at
100MHz. The transition zone depends on the physical size of the device
under test.

Devices that are physically larger will radiate at lower frequencies.

She used an example. "Say you have an object that is a 1" cube that
produces rf white noise at a constant amplitude. It won't become a problem
until high uhf because the small size simply can't radiate.

As a rule of thumb, significant radiation starts when an object is roughly
1/20 of a wavelength and almost always becomes an issue when an object is
1/10 of a wavelength in size."

The smaller an dut is, the higher the frequency has to be before radiation
becomes a concern.

She suggested I study point sources.

A NanaVNA has a USB cable and two rf cables, this is a near worst case
scenario for radiation because the cables will form the arms of a quasi
dipole.

It is extremely difficult to add enough shielding to control radiation
after a product is designed, and it's "damn near impossible to add enough
filtering adequate to control conducted emissions after a product is
designed."

The best, most effect, way to deal with emi is during the design phase.
Any effort after a product is designed will never be as effective as proper
steps during design and initial testing.

Ferrite inductors can help but, the lower the frequency of the emi you are
trying to attenuate, the more ferrite you will need.

At some point, you can't add enough ferrite to stop the emi.

She added, "Say you make your NanoVNA dead quiet, what about your PC?
Unless you have a laptop or desktop desktop that meets TEMPEST
specifications, your PC will almost certainly produce more emi then your
NanoVNA."

She looked up the NanoVNA and pointed out numerous issues. The main one is
the traces that connect the shells of the SMA connectors. For frequencies
below 30MHz they aren't an issue, as you move up from 30MHz, the narrow
traces will have enough inductance to become an issue for emi.

She is sending me an older edition of
Electromagnetic Compatibility Engineering 1st Edition, by Henry Ott. She
says it's the bible for emi/emc.

She also suggested that we not worry so much about emi because we probably
can't reduce the emi enough to be worth the effort and most likely, well
never be able to see any improvment.

She suggested we all study the manual at:


to see the steps required to really deal with emi. She also thought we'd
probably never see the difference in our test results if we were able to
make a case with perfect shielding because of the small size of the NanoVNA
and the conducted emi from the usb and rf cables.

"Far better to concentrate on a case that is robust enough to protect the
NanoVNA."

She is a smart gal, PHDs in EE and physics. Extra class ham, really smart.

"For the price, the NanoVNA is an amazing value, just don't expect the
same results you'd get with a Agilent 8563EC Spectrum Analyzer, used ones
start at 10K."

--
*Dave - W?LEV*
*Just Let Darwin Work*

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.

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.

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.