Need advice for building a Faraday cage. - Page 1
Forum section: electronics / rf-microwave

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: May 17, 2025, 10:31:29 pm
content: Hello,I'm trying to shield a small PCB from about 60hz-4Khz RF interference in water.I've tried reading up on how to make a proper Faraday cage, but the info I've found online is incomplete.1: How large should the hole be? Too small is okay, right?2: How thick should the copper wire be?3: What's the difference between a cage of soldered together wire and an aluminum or steel sheet?Thanks!EDIT: To be clear, I'm asking about protecting against both E and H fields as I'll be within 1 meter of the source of the EMI.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 17, 2025, 10:50:06 pm
content: You state that you have RF interference, but mention 60 Hz to 4 kHz, which are typically thought to be well below RF frequencies. At your upper mentioned frequency the wavelength is about 750 km. The gradient in the electric field across any reasonable size box is going to be very small. I suspect that your problem lies in the wiring/grounding not induced voltages in your circuit card.But to your specific question, given the wavelengths involved hole size won't matter. Skin depth will only a tiny fraction of a wavelength, but that will be so large that there is no possibility of making the enclosure thick enough to matter.You can think of the problem statically. In other words how to avoid electrical potentials developing, either through induced charge (basically your circuit, shield and surroundings acting as a capacitor, and possibly magnetically induced in any "loops" of wire. The latter is the type of thing that twisted pair wiring helps with. These thoughts will inform your questions about soldered copper vs aluminum or sheet steel. As long as the pieces are electrically tied it shouldn't matter (again at these frequencies).If your RF really is at much higher frequencies, say a MHz or above then other responses would be appropriate.

author: radiolistener
author_rank: Super Contributor
timestamp: May 18, 2025, 12:11:59 am
content: Quote from: ballsystemlord on May 17, 2025, 10:31:29 pmThe size of the openings in a Faraday cage should be significantly smaller than the wavelength of the highest frequency you want to block. However, even more important is the distance between the Faraday cage, the shielded circuit, and the interference source.For reference, in air:- The wavelength at 4 kHz is approximately 75'000 meters.- At 60 Hz, it's around 4'996'540 meters.In water, electromagnetic wave propagation is much slower, resulting in shorter wavelengths.At distances shorter than the wavelength, the dominant coupling mechanism is not radiation but capacitive and inductive coupling - similar to how energy transfers in a transformer or capacitor.At less than wavelength distance the shielding effect is less about blocking radiated waves and more about minimizing capacitive and inductive coupling. So even with no direct radiation, interference can still couple through shield.In short:Openings in the shielding surface should be as small as possible, especially when dealing with high-frequency interference. Ideally you should avoid it. However, the relative positioning of the EMI source, the shielded device, and the shield itself is even more critical - particularly at low frequencies or in the near-field region.Quote from: ballsystemlord on May 17, 2025, 10:31:29 pmit depends on how much field attenuation you need and what kind of field you need to attenuate.Quote from: ballsystemlord on May 17, 2025, 10:31:29 pmThe main difference between a cage made of soldered wire (a mesh) and a solid aluminum or steel sheet lies in the effectiveness of shielding - especially across different frequencies and field types (electric vs magnetic).At low frequencies, the material matters more:- Steel (with higher magnetic permeability) is generally more effective at attenuating low-frequency magnetic fields.- Copper or aluminum (with higher conductivity) are better at attenuating electric fields.As for construction:- A solid sheet provides better overall shielding, especially at higher frequencies, where electromagnetic waves can pass through holes or gaps more easily.- A wire mesh or perforated surface allows more high-frequency leakage due to the openings, which reduce the effective shielding performance. But they are more lightweight.However, the specific effectiveness always depends on the exact geometry, frequency, and coupling mechanism - so there's no one-size-fits-all answer. In general, for best shielding across a wide frequency range, solid conductive enclosures are preferred.Quote from: ballsystemlord on May 17, 2025, 10:31:29 pmAt distances within approximately 1/6 of the wavelength (or more precisely, within the reactive near-field region defined by approximation R < 0.62*sqrt(D^3 / wavelength)​, where D is the size of the source radiator), the fields behave as reactive near fields, meaning electric and magnetic components are strongly coupled.In this zone, conventional shielding becomes almost useless. Instead, techniques such as tight grounding, differential signaling, and careful PCB layout play a more critical role than passive shielding alone.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: May 18, 2025, 03:56:04 am
content: Great answers, not what I was expecting, but still great.I guess I should have explained the whole setup so you'd see where I'm coming from. It's a bit long though... I'll try and shorten it as much as possible...There is a 3-phase submersible pump motor my father is in charge of maintaining. For whatever reason, it doesn't pump very well. It's brand new (because the old wet-end was failing and so the decision was made to buy both pump and wet-end new), the wet-end is brand new, the pump drive is brand new. It doesn't pump well.The manufacturer, Fuji Electric, has been contacted and the whole programming portion of the drive has been gone through without finding anything.My father even turned the pump in for warranty but nothing was found. The only differences in testing are, they have native 3-phase incoming, whereas the pump drive is doing a conversion, and they only tested it for 5 minutes which is insufficient to show symptoms.No one touches the drive, pump, or anything associated without Dad's knowledge.The symptoms are, you install it, it works okay-ish to perfectly. 2 days later, it's performing horribly. This is a 100% repeatable thing. The pump is rated at 51A but only draws about 30A-35A. The resistance of the pump plus the wiring is 0.6ohms according to an uncalibrated DMM and 0.5ohms according to the pump drive.I volunteered to help him.I looked at the incoming and outgoing waveforms with my oscilloscope using a single differential probe and a single current clamp. Nothing stands out. It's perfect 240v in to the pump drive and very clean PWM 240v out under load.I tested the pump under no load with a laser tachometer and it reaches full speed under no load.I need to test the pumps speed underwater as that would narrow down the problem to either the pump and drive or the wet-end. A non-contact tachometer would not only take many days to arrive but would also cost an arm and a leg and not be water proof.So, I decided to make my own magnetic tachometer. I tested it with a 12v motor and it works.My concern is that it might pickup on the large pumps EM fields. The pump rotates at 3,600 RPM. Rounding up that would be 60hz-4Khz that I want to shield against.Now I'd go into the field and test this, come back, and do revisions as necessary, but as should be obvious from what I wrote above this entire process is taking way too long, quite a lot of extra money has been spent on a new pump drive, and no progress has been made at all except to say that the problem is "weird".The circuit I'm employing is just a simple hall effect sensor with an LM358 (normally sold for use with an arduino). My DMM can do the frequency measurement and I can run cables up to my waist for a power pack. I intend to seal the sensor and PCB in silastic tomorrow.A megger is in the works to test the pump as well.Thanks!

author: Andy Chee
author_rank: Super Contributor
timestamp: May 18, 2025, 08:13:15 pm
content: If the pump is performing horribly, I would suspect mechanical issues rather than electrical.

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2025, 08:37:14 pm
content: the number one thing that makes pumps do weird shit is air bubbles and voids. Are you mounting it level? They kind of suckWhen its misbehaving have you tried turning it off and on again with a short delay? Sometimes you need to turn em on slow or run it on slow to get rid of bubblesI think trying to do magnetic shielding for some probe is going to be a waste of your time lolAir makes weird noises, nonlinear rotations, jerking and stuff like thatIf the things are mechanically correct the speed should not matter very much and just vary flow a little

author: coppercone2
author_rank: Super Contributor
timestamp: May 18, 2025, 08:47:55 pm
content: To test a pump you get a timer and buckets. You pump water into bucket for x amount of time then weigh the bucket and get your flow rate. or you use a a calibrated flow meter but a scale is much easier. If its huge you can try to use volume measurement (some kind of metered buckets like a graduated cylinder), but weight is damn preciseYou do this on the bench and in the system. More then likely I suspect there is a restriction or some thing going on with the impeller that makes it act up, or the pump can't handle the system that its being used for.It's like measuring coulombs.This is like the first test you do with a pump.Keep in mind will pump lessup. They have spec for flow @ depth @ tubing thickness.Pumps can have very impressive performance on a bench and then totally disappoint you when you realize how much restriction and gravity you need to fight (water is heavy). If your system is level then you are more or less fighting the tubing restriction. If its pumping up you have gravity and restriction.Different pump is optimized for flowing to low restriction or high restriction (i.e. gear pump for getting water far and other pump for getting water alot and cheap). Extremely easy to find a pump that impresses you and realize that its completely not going to work for the application you have in mind even if its a fire hydrant.Then you can theoretically measure resistance of the pipe work via differential pressure measurement (pressure drop @ flow) but it gets really involved to try to measure these parameters because its just not done.The number one problem is that people find a cheap pump that is like a sump pump that drains a little flood basin quickly by lifting water a few feet, and then think it will work 5 meters under water the same way, and then they get disappointed it just trickles.... because it just not built to push that water like that.Have you compared the curves for the new and old pump? They have flow rate at elevation for some tubing thickness.In this case the pump in the diagram pumps725gpmat 50 feet of head and basicallynothingat 90 feet of head. Pump replacement is easy to screw up if you eyeball it.. its not linear and it could be much more so depending on the system its in i believe

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: May 19, 2025, 04:38:04 am
content: As mentioned, it's a 3-phase pump. So, think a little bit bigger than buckets. Think about 100psi at about 165GPM.As for being level, the pump is at about an 5 degree angle and is rated for between 0-90 degrees.The wet-end has a check valve, so unless it was physically pushed open and then screwed onto it's 3" stainless nipple underwater (who even does that?), the wet-end will have air in it to start out, but not much as the intake will let most of the air out.The pump itself doesn't make much noise.Here are the links in case you're curious.https://product-selection.grundfos.com/us/products/sp-sp-g/sp/160s150-7-93143957?pumpsystemid=2673530305&tab=variant-curveshttps://www.rcworst.com/franklin-electric-2366038120-sand-fighter-motor-w/o-subtrol-6-15-hp-230-volts-60-hz-three-phase-3-wire.html(A distributor site that has the manual).https://www.franklinwater.com/products/submersible-motors-and-control-boxes/6-inch-submersible-motors/6-inch-induction-motor/6-inch-three-phase-motors-sku-collection/2366038120/

author: coppercone2
author_rank: Super Contributor
timestamp: May 19, 2025, 05:06:08 am
content: in that case you need a flow meter or barrels / totes and some kinda industrial scaleI don't know what tricks there are to bigger pumps, but for smaller ones you treat it like a circuit component basically, but with fluidic principles instead of electrical onesAt high pressure and flow it seems difficult to get it into a container without it like effecting the pump via the usual tricks. I found flow meters to be crappy. actually I totally hate them.Haha I wonder what kind of shit goes on at NASA or hydroelectric facilities when they have to deal with their insane pumps and flow rates. (saturn turbo pump was 42000gpm)

author: coppercone2
author_rank: Super Contributor
timestamp: May 19, 2025, 08:20:56 am
content: the first link has the curvecan you compare it to the curve of the old pump (overlay?)I don't know why it has 3 curves though or what they mean.Also what is it submerged in. Are you sure its not like sucking up silt or something and clogging after two days? Maybe the reservoir is malfunctioningtwo days is a really long time for electronics to malfunction. The only thing that usually takes a while is heat. I don't know if it can take that long but what I was thinking that if the pump is like over stressed, and barely putting out enough water to begin with (i.e. it "looks" good because no one measured it, even when its basically tapped out), then you get some little build up of heat that causes it to lose performance to a noticeable level because it was barely passing to begin with. But I never even heard of a similar problem to that. A DC motor loses power when its hot. I think maybe some is the magnets some is the winding. I don't know much about AC motors, but they do have windings.

author: ballsystemlord
author_rank: Frequent Contributor
timestamp: May 20, 2025, 02:02:54 am
content: Quote from: coppercone2 on May 19, 2025, 08:20:56 amYes, the old pump put out less pressure at equal water flow (Because the wet-end was for a 7.5HP motor but had a 15HP motor attached. E.G. it was grossly undersized and the performance of the system was barely passable as a result.)Quote from: coppercone2 on May 19, 2025, 08:20:56 amWell, the curve I'm interested in shows the flow rate vs. the pressure. I'm attaching an image translated to PSI and GPM.Quote from: coppercone2 on May 19, 2025, 08:20:56 amThe pump's water flow rate is roughly measurable. I'm getting about 90GPM at 35PSI. That data point is not on the graph because the performance is supposed to be a lot better than that.Heat shouldn't be a concern because the pump is in a lake. It has a 4" intake connected to an 8"-4' filter with 1/8" holes. The filter is not clogged.We tested it with a megger today and got about 30Mohms across each leg to ground through the submersible wiring. The pump is 230v, so anything above 2Mohms is good.But I think I'm closer to finding the problem. The motor's phases are out of balance. Also, the phase imbalance rotates between the phases. For example, phase-1 will draw ~32A while the other two draw 27A. Then phase-3 will draw 32A and phase 2 will draw 28A and phase 1 will draw 27A. Then phase-2 will draw 32A etc...It's the weirdest thing I've ever seen. No one around here has a clue.

author: coppercone2
author_rank: Super Contributor
timestamp: May 20, 2025, 07:36:00 am
content: why is there three curvesI only saw those graphs with 1anyway I don't know its too big and also AC. everything i know about this one is unrelated and impractical in this scaleBut I do want to say I think I recall a situation where there was a pump (not submerged) that ran a little system and the flow would get messed up when it hot (low flow), it got blamed on the magnet getting hot. but this is a bench top setup. Those pumps have a fair amount of isolation between the motor and the pump so they don't get much cooling through the isolated shaft. In this case it was a simple current draw too high situationwhat is alot easier to do then try to measure RPM is to stick a themrocouple to the side and check. But its odd that it would get so hot if its not running hard.I believe these motors get hotter if the voltage is higher ?But I don't know why there would be a phase imbalancedid you check if the line voltage is what it says? Maybe there is other equipment effecting a phase. This sounds like a problem with the motor or the electrical since it only turns one thingpump too big is a new one. its always too small (under specified) because of cost lol. If its too big you can have problems too but I don't know what they are. Typically you would add a restriction on ithttps://www.grundfos.com/us/support/faq/what-happens-if-the-pump-is-operating-too-far-to-the-left-on-the-curvehttps://www.pumpsandsystems.com/why-left-can-be-dangerous