Electronic – Non-metal cases

I have followed this subject for "many decades".
(I wrote an engineering paper on this just on 40 years ago!)

Working in close proximity to RF generating equipment may have some epidemiological correlation with provable hazards. (eg sucking your microwave oven when it is working is liable to not be wise).

But, due to inverse square law falloff and relatively low transmit power levels, at the typical distances away from cellular towers you are extremely unlikely to encounter hazard levels which are significant compared to any recognised standard.

While you may not trust this assessment it is important to note it because, if you want to be cell-site and other RF source "contamination free" you are going to have to exclude radiation from all similar energy level RF sources.

Screening doors and windows, but not walls, ceiling and floor is not going to produce the sort of reductions that you are liable to want. Normal non-metallic building materials are transparent to RF and if you place mesh across openings only then RF "sees" a see-through house with a few blocks at doors and windows. To do the job properly you need to create a "Faraday Cage". (A FC acts on the electric field but can be effective against RF if proprly built.) A Faraday Cage is achieved with a metal mesh or sheet covering of the whole house - although you can achieve reasonable results with conductors which are separated by distance small compared to the wavelengths involved. If you are dealing with say ~= 900 Mhz cellular, a wavelength of 300/F_MHz metres, and you probably want say 0.1 wavelength separation maximum so a metal grid should have a specing of not more than say 30/F_MHz. For 900 MHz spacing is <= 30/900 = 1/30 metre or about 30 mm. Use of 1 inch mesh or smaller chicken mesh may be reasonably effective. This could be laid in ceiling spaces if accessible. Treating walls may not be so easy. Wrapping your house in chicken mesh may be effective but may attract similar comments to those received when wearing tin-foil hats .

Use of a single antenna indoors is almost certain to NOT be very effective at reducing RF levels. Fastening chicken mesh under the floor is liable to be of more effect. You want to check results with a reputable meter as it would be possible to form a "beam" or waveguide which increased rather than decreased the field inside two planes of mesh - especially so if the transmitter is nearby.

Similar to your indoor antenna but more disributed - If you run multiple long conductors under the floor or in the ceiling you will alter and may well reduce RF fields inside the house. My house has a number of long steel girders running under the floor and it is particularly poor for RF reception. Wireless doorbells have much less range in this house than in others and cellphone operation is also affected. This arrangement "just happens" to work as it does and you'd need to dsign or experimentally produce a similar result.

Multiple low cost dipoles for energy absorption:

As I noted, use of a single antenna indoors is not liable to be highly effective - but also as noted, my underfloor steel beams have a significant effect on RF generally in my house. In your case the high target frequency means it would be easy to construct dipole antennas tuned simply by adjusting their length and with a resistive load - essentially just two wires in series of the correct length with a resistor across the gap between them. A significant number of these scattered about the house could have a significant effect as there is a finite amount of energy available.

You may be able to make these multiband but the simplicity of these means that having ones for each of the bands concerned would probably be easier.

Note that many resistors would be far from pure resistive at these frequencies due to reactive components. Long ago people used carbon composition resistors for RF for this purpose but these are likely to be increasingly hard to obtain and probably are questionable at UHF. There are no doubt materials that will work OK. It may be that a straight Nichrome link of appropriate resistance between two tuned wires would work OK. Dipole feed resistance is traditionally stated as 75 ohms. Will vary somewhat with conductor size and probably with length of interelement central gap.

Taping these on walls or in eg picture frames would probably work, but too close proximity to reinforcing steel in walls will detune them.

It is very easy to make up a few of these, and your field meter will tell you if or how well they are working. A spectrum analyser would probably provide a fae clearer picture of effects at a given frequency.

Biological effects of non ionising radiation:

• The following may be felt to stray too far into the sociological and have little place in engineering design considerations. As long as we only base our engineering in such areas on only what we are sure we know, even when we may be wrong, is to court disaster.

RF Trolls are forever blinded to seeing Unicorns :-)

Whether low level RF or electromagnetic or electric fields constitute or cause or catalyse significant biological hazards has been the subject of much discussion, investigation, research and analysis over many decades.

As Olin notes, some research has indicated that placing a radio transmitter against the side of your head for up to hours a day for many years at a time can be a seriously bad idea. Few would doubt that this might be so and the surprise is more that the results are equivocal and that there is no consistent indication across all studies that low level RF frying of your brain is always demonstrably bad for you. (People who use cellphones for hours a day will often enough exhibit pathological societal behaviour, but the correlation is generally considered to be the inverse of the one of interest here :-) ).

However, the concerns and examination are far more widespread than just the above. As Olin says, the power densities are far (far) lower from a cell site than from a cranialy attached phone. Of greater concern would be higher power transmitters. However, in this area, too, there have been conflicting study results with no absolute certainty that there are not effects in some cases. I'd personally comment that in most cases what effects there may be are so much "in the noise" that prudent avoidance (as opposed to active mitigation) and being careful with your health generally is liable to produce much greater whole of life gains than concentrating on this one aspect.

In some cases there may be genetic predispositions that increase effects but there is a very very grave risk of falling prey to confirmation bias and quackery from people who claim to be experts but who bias their 'results' either unintentionally or with good intentions or to make a profit or just maliciously. Of these "confirmation bias" is the most insidious to deal with as you are trying to objectively examine your own thinking from within your own system. I understand Olin's perspectives of Unicorns and Elvis's - but he is as blind to the possible realities as those at the other extreme who imagine effects that are not there. Engineering and scientific rigour is essential - but this includes not dismissing, as of right, claims which appear to have no reasonable basis in reality. Essentially all new Scientific discoveries come from areas with "no reasonable basis in reality" - if it were not so we would be using them already.

In most cases where there is no good reason, based on scientific and engineering knowledge, that some effect should exist, then it probably doesn't. There will always be some who think such things exist regardless of proof or reasonableness, and there are large 'industries' built up around encouraging such false beliefs for whatever reason. Those involved in such promotions may be well intended or charlatans or conmen. BUT in some cases, regardless of all that we know and understand and rgardless of reasonableness and confirmation bias and conmen and the rest, there will be genuine effects which we have missed or are yet to discover. Amongst the over-unity magnetic motors, antigravity machines, magnetic bracelets, crystals, mainifestly swamped by noise biological effects, Unicorns and Elvises we may find with surprise effects which are unknown and real but usually swamped by noise or very well know effects that arrive by a different path.

An example of the later, which may or may not be significant but which helps illustrate how Unicorns may suddenly appear is "magnetic lensing" of incoming extra-terrestial ionising radiation by local em fields from power lines. It is widely suggested that the magnetic fields from HV transmission lines are not large enough to cause claimed increases in birth defects and cancer in adjoining residences. One plausible mechanism, no doubt by now both well proven by many scientiifc studies and completely discredited by an equal number of other scientific studies, is that the fields serve as a lens to deflect charged incoming ions and charged particles such that those that would have fallen within the area near the lines instead are deflected to fall in narrower areas outside the high intensity field area - ie just outside the line of the power lines. Whether in this case this does prove to be a significant factor in birth defects and cancer is not the point. What matters is that a mechanism does exist that may allow power lines to cause birth defects and cancer due to their 50 or 60 Hz magnetic fields, based entirely on a plausible main-stream engineering basis. Whethr it does then becomes a matter for investigation. Until such potentially feasible mechanisms are proposed it's all Uncorns and Elvis's.

I personally think that there is a high prospect that Icarus's concerns are not well founded and that, even given a high degree of apparent correlation with family health and genetics with electrosmog, that the correlation is illusory and more likely to be caused by some other effect. Example only! - eg the family may prove to all have low level gluten intolerance or some other such effect with accompanying health effects. BUT as long as it may be that low level RF fields are the cause of family health issues, rejecting them along with the Unicorns is doing not only Icarus and his family but society as a whole a grave disservice. Engineering prudence is allowable, as is even strong scepticism, but outright rejection, ridicule and rudeness goes beyond the pale.

Dipole antenna length calculator:

A dipole can be as simple as a length of wire cut in the middle.
The middle may be com=nnected to a feed line to a receiver or transmitter, or have a resistor connected between the two halves to received dissipate energy, or have a back to bak diode pair connected across the gap to, um, er ... .

To make an antenna resonant at a frequency of interest use two lengths of wire, each wire cut to about 120,000/F_Mhz millimetreslong. eg at 900 Mhz each wire = 120,000/900 = 133 mm long. (Or take a length of wire twice as long and cut it in the middle - instant dipole. These may be about 5% too long and wire diameter also affects length very slightly. Play ...

To receive and dissipate energy connect a resistor - ideally 75 ohms at the frequency of interest between the two halves. Nichrome wire is probably a good choice for the resistor as you can get a defined resistance with some ease. he wire can be twisted with the dipole ends but the actual resistive part needs to be straight.

Skin effect will probably play havoc with resistor calculations - discuss.
When RF flows in a conductor it tends to flow only in the outer "skin" in a thin layer. It will mean you probably need a shorter Nichrome link than expected. As the Nichrome wire is thin compared with most antenna elements the effect may not be as pronounced as in many cases.

For dipol length either use my formula above or one of these online calculators. Some pages also give extra information on dipole antennas.

Some extra comment
Some good extra comment
Basic but useful
Shows effect of diameter or wire - limited choice
Similar
Similar

Details:

Does the wire's gauge-size, material or insulated/naked matter ?
Any specific value of resistor to use ?
I believe nothing to be grounded ?

Some of those questions are covered in the text above or by reference material.

Wire does not need to be insulated if free space mounted. If mounted on a wall or similar then insulation may have some advantages if the wall was somewhat conductive.

As noted, the feed point is notionally 75 ohms.
75 ohm resistance at frequency concerned gives maximu power ransfer.
Note that many resistors will be far from a pure resistance at the frequencies concerned and so may be unsuitable.
As noted, use of nichrome wire seems a good idea.
Due to skin effect it will need to be less than 75 ohms.
Try varying lesser resistances to see effect.
Electric radiator element wire may be a source of Nichrome if otherwise unavailable.

What country are you in?

Well, the go-to book is Henry Ott's Electromagnetic Compatibility Engineering, but that's probably overkill for you. You're pretty much a hobbyist that doesn't need FCC certifications.

So is it ok if I provide the case as it is?

Yep.

The aluminium is punched with some air vents on the rear (4mmx12mm holes), some on the base (same hole dimensions) where the motherboard would sit and some where two 2.5" hard drives would sit (again same hole dimensions).

The emissions from the box are related to the largest holes or arrays of holes. Which in your case are the entirely open sides.

I've heard of nickel based paints that can be sprayed to the plastic parts (on the inside) but is this safe?

Yes, they're as safe as any other paints (i.e. clean up your messes and don't eat them). They've been in use for decades. There are also copper- and silver-based paints. The latter paints are more conductive than nickel; sometimes, you want lower conductivity so the shield acts as a damper.

Does the paint need to be conductive?

Yes. Not conductive is not there. Also, there needs to be a good connection to the other parts of the shield. A wire isn't that great; you really want a metal spring-finger EMI gasket or something similar. The joints are holes in the shield if they're not connected. Again, all the details are in Ott's book.

Is there a risk of short circuiting the components inside, fire or other?

Yes there is, if you have components resting against the case. This is a bad idea in any case design.

If sprayed, would the plastic parts need to be grounded and to what?

Basically, a complete shield works off of the principles of Gauss' Law: A closed conductive surface with a single point charge can represent the same surface with numerous point charges; you just add them up. Put that together with Kirchoff's Voltage Law and connect the conductive surface to your ground, and any charge movement in the surface doesn't matter to the outside world.

Of course, this perfect world falls apart when you put a hole in the closed conductive surface, but then it turns out that for long wavelengths (i.e. low frequencies), if the holes are small enough and spaced out right, it's the same as a closed surface.

Thus you want your shield to minimize the size and number of openings in this conductive shell, and you want to prefer more small holes to fewer bigger holes.

What about shielding tape, is that any good?

It's fine for plugging gaps, but be careful: some tapes have conductive adhesive and some don't. I prefer to avoid tape if possible because you have to put new tape down every time you open that joint. Spring fingers don't have that problem.

Speaking of tape, though: There are insulating tapes (I prefer the Kapton stuff) that can come in handy for insulating spray-on shield rom nearby components. So long as the components aren't so hot as to melt the tape, you're good.

Would I or the user need to cover the plastic parts entirely?

As much as possible. Anything uncovered is a hole in the shield.

What about the bits where aluminium and plastic join?

Spring fingers or clips or something.

And the vents in the plastic parts?

Nothing you can do there that is practical.

And finally how do I calculate the exact area required for the air vents so that it doesn't affect RFI/EMI too much?

With finite element analysis software. To me, that would be a lot more effort than you need.

Overall, I think you can just go build your case as-is and not worry. I've seen unshielded cases made entirely of acrylic (clear plastic), which has no shielding properties whatsoever, for sale in local stores. Also, FCC and other regulatory testing only applies to completely assembled devices, not components that go in the box.