EMC Chamber build log - Page 1
Forum section: electronics / testgear

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:10:48 am
content: Hi all,We recently had to upgrade our in house EMC chamber for pre-compliance testing of our products.The old one was too old (20 years ish) and didn't have the structural support to be upgraded to modern specs.The chamber was teared down and scrapped and our mechanical workshop was cleared to make way for the new (bigger) chamber.I was supposed to do a daily build log of the chamber but other work commitments made it difficult to upload here until today.I was doing work stuff at home to meet a deadline for an exhibition in China. This made sleep my next highest priorityAnyway, I've built up a collection of pictures that will have to be uploaded in separate posts to this thread due to the attachment limits.If anyone would like to know more about whats involved in the process of building an EMC chamber please feel free to ask any questions. I'll answer as many as I can and forward those that I can't to the two EMC companies I hired to provide the chamber and independent testing of the chamber. The EMC consultant has offered to come onto the forums and answer questions directly. You should definitely use the opportunity to pick his brains about EMC.Here are the first few pictures:Mechanical workshop cleared (nearly), external windows & doors removed, external wall (far side) bricked up, near walls (internal partitions from where I took the pictures) are removed to allow room for chamber material.2 months later, EMC chamber material arrives. Area fully cleared and environmental membrane layer laid down.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:14:36 am
content: Chamber floor being bolted together. I've not measured the individual panels but just estimating that the larger ones measure 2mx1m and the smaller ones 1mx1m. Each probably has a 30-40mm folded lip at the boudary. I'm assuming 4-5mm thick which makes each panel somewhere between 75-100Kg (165-220lb). There are two guys doing the build.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:25:12 am
content: Day 3Side walls going up.EMC gasket being persuaded to go in the right position. Panel being bolted through.Next panel being made readyChamber installers on a break.View from the inside looking towards where I normally stand for the pictures. You can see the top left area where the chamber door is going to be.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:38:13 am
content: Inside view of back wall

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:44:27 am
content: Day 4Final layer of side walls fittedInside view looking at back wall (where the antennae are going to be positioned)Inside view looking at front of chamber where the EUT will sit. door opening is on the left of image.Structural steel supports in place ready for ceiling. You can also see the hooks that the knife lock on the door will engage with.As the door handle moves towards is closed position the door is compressed against the chamber wall and EMC seals. More on how this affects EMC shielding performance later.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 02:55:32 am
content: Closer look at the EMC gasket for those who are interested.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 03:07:09 am
content: Day 5,Ceiling panels begin going up.Ventilation panel fitted (extracting air)ahh good ol 2x4 doing its job

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 03:25:48 am
content: Ceiling complete.Door attached.Penetration panel hole cut and fitted. You can also see a black box attached to the side of the chamber. This is a 30A filter to give us a clean supply inside the chamber. The unit takes 230VAC mains and 24VAC & DC low voltage mains. There is a waveguide for optical fibre cable which we use to for isolated comms to the EUT inside the chamber.Phase 1 complete. Ready for the shielding effectiveness test.You can see the transmit horn antenna directing a signal at the penetration panel.The chamber is being powered off an extension cable (real supply not installed yet). This is used to provide a clean supply to the receiver (spectrum analyser) on the inside of the chamber when the door is closed.Once the chamber is complete, we intend to rebuild an internal wall (partition) just where the left side of the ladder is. The wall extends out to where I am standing in the last picture. The area between the partition and the chamber will be where the control equipment and amplifier rack live. Nice and cozy)On the the other side of the wall is the corridor leading to our surface mount and reflow soldering ovens left and the canteen on the right.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2014, 03:46:02 am
content: Cool!Can't imagine that pallet of ferrite tiles (upcoming I hope??) is very cheap!Tim

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 03:55:19 am
content: Quote from: T3sl4co1l on April 11, 2014, 03:46:02 amI think I've got another 2 or 3 days worth of pics to go through yet before I get to the ferrite tiles.Your'e right about the cost of the ferrites. They can be more than 50% of the entire chamber. I managed to get a really good deal but let me make sure with my boss before I start talking about the money side of things.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 06:29:50 pm
content: Shielding effectiveness testing begins.Different antennae are used for certain frequencies in both vertical and horizontal polarisation.The transmit antenna is directed at the receive antenna and both are separated by a set distance.This setup is used take a measurement without shielding.Unfortunately, I cant find the picture I took so just imaging two horn antennas pointed at each other.The receive path is then setup inside the chamber and a measurement is made with the door sealed.Subtract the two measurements and you get shielding effectiveness of the chamber.

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 07:01:53 pm
content: Here's Nick, our EMC consultant who's been hired to take independent measurements for us. He can be seen checking the shielding around the door frame.Here's the transmit signal with the two antennas pointed towards each other outside the chamber.You can see the difference in signal strength when the receive antenna is moved inside the chamber and the door closed.We started to experiment with the shielding and Nick showed us what happens when you don't fully lock the chamber door.The knife lock was left about 2-3mm away from its locked position, you couldn't even fit your finger nails in the gap created between the door and the chamber wall. The result was the same as when the receive antenna was pointed directly at the transmit antenna.We then poked a length of 7/0.2 wire through a hole in the ventilation panel. The result was the same as having the door open.Nick was telling me how some people get desperate enough to do this or poke wires through the waveguide tube which is meant for optical fibre cables. This usually happens when the penetration panel doesn't have enough connection points.Moral of the story, if you don't fully lock the chamber door or start putting wires holes not meant to pass wires then you may as well have the door fully open, or no chamber at all. (at least at that test frequency).When installing a chamber, make sure you think carefully about the requirements of the penetration panel. Otherwise you will have to swap panels and this could change the shielding effectiveness.

author: G0HZU
author_rank: Super Contributor
timestamp: April 11, 2014, 07:28:20 pm
content: Where I work we used to have our own EMC Compliance subdivision with various EMC chambers of different size and capability. eg tiled and/or pyramid RAM. I've been in loads of EMC/anechoic chambers over the years so it's interesting to see how a modern one gets built.Are you going to tile the chamber and also fit some pyramid RAM?One comment I have about your new chamber is that the door locking mechanism looks very simple and light. Maybe this is down to modern/better design but the chambers we had at work had huge door latching mechanisms that looked like something from the back of a huge door mechanism from a truck. i.e. you had to use your body weight to move the mechanism and drag the heavy door open.Note: Probably the biggest/most impressive anechoic chamber I visited and worked in was the one at the McAfee Centre/building 600 at Fort Monmouth in New Jersey USA in around 2005. It was huge!

author: G0HZU
author_rank: Super Contributor
timestamp: April 11, 2014, 07:51:40 pm
content: QuoteI wondered if it would be possible to also use ultrasonics to test the new door seal... Many years ago I knew someone who used to sell a simple and low cost seal testing system (for cars or fridges etc)The idea was to put a high power ultrasonic source inside the car or fridge or room and then run a tuned/sensitive detector all along each seam. It could find the slightest leak in the seal within about 2cm as the sensor sniffed along the door or window seals. This is because the ultrasonic energy can leak out of the seal.I know this isn't really testing it at RF but it's a very good method to find 'real' faults in a door seal

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 08:38:17 pm
content: Quote from: G0HZU on April 11, 2014, 07:51:40 pmI've heard of people using ultrasonics to test the integrity of materials (stress, strain, micro-fractures etc) and the integrity of things like welding joints. I'm no expert on it but I guess you could use it to find 'real' faults.The only thing is that it probably only tells you the mechanical nature of the fault and not the impact on the electrical performance or parameters important for an EMC chamber. An example is:it would tell you there is a gap between two panels but it wouldn't show you the effect of the gap on RF radio waves at different frequencies. You would see variations across the spectrum and then huge drops in the shielding effectiveness as the frequency approaches a resonant peak formed by the the chamber & gap dimensions. This probably means you couldn't use ultrasonics to get a full picture of the chamber performance.One of the older EMC experts that I used to work with at my old job told me of how gaps between panels due to EMC gasket failures created electric field strengths inside the chamber that were bigger than the field being produced outside the chamber. This was all down to resonances inside the chamber.BTW - that can catch some people out, if you have a metal enclosure around your product and go to EMC test expecting it to be hit with 10V/m (for example), you could actually expose the electronics to bigger fields inside your enclosure.I used to have paper that described the problem.If anyone's interested I'll dig it out.

author: lewis
author_rank: Frequent Contributor
timestamp: April 11, 2014, 09:02:24 pm
content: Very interesting stuff, I'll be following

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 09:10:57 pm
content: Quote from: G0HZU on April 11, 2014, 07:28:20 pmThe walls and ceiling will have 100% coverage of ferrite tiles. On top of the tiles we will have the traditional pyramidal RAM placed in the center of each wall, ceiling and floor. On top of the RAM we are getting what looks like white polystyrene tiles. I'm not sure if these are RAM also, I'll ask.If you look closely, you can see the door is too large and collides with the low hanging wall. I'll talk more about this later, but this is going to be resolved sometime next week with a new smaller door.The standard door does take some effort to open, but not body weight amounts, (not my body weight anyway).

author: Tabs
author_rank: Regular Contributor
timestamp: April 11, 2014, 10:03:39 pm
content: Day 6 (Weekend - Saturday 5th April)Most high end chambers achieve shielding effectiveness around 120dB or more.Placing too high a spec on this requirement can drive up the price of the chamber. This is especially true if you want to maintain that level of shielding up 18Ghz or so; which is what you need if you do military EMC testing to MIL-STD-461, DEFSTAN 59-411 or STANAG.Most commercial products don't need that level of performance from an EMC chamber. In our case we test to EN61326 for EU and CISPR 11 for FCC when we sell into the American market. I went with an 80dB shielding spec to 3Ghz after consulting with Nick, the chamber providers and some commercial test houses were we know helpful people. 3Ghz is the max we need to test at the moment.The chamber is capable of going up to 18Ghz.This was one of the ways we were able to keep the cost of the chamber down. Shielding effectiveness at this stage is really down to how well you tighten the bolts and the EMC gasket performance. The expectation was that it would be very easy to achieve better performance than the requirement I set.In a few years, we may be required to test to 6Ghz due to newer mobile phone and wifi standards. But these requirements will take a long time to supersede the current standards and it was cheaper to purchase equipment up to 3Ghz and wait to see what happens.The shielding tests were completed and only two problems were located:1) missing EMC gasket between two panels2) a single nut & bolt not tightened to the right settings to give the required sealing.Nick only had access to two faces of the chamber. The 1st was direct line of sight access to the control area where the penetration panel is. The second was through a breeze block wall in the room next to the chamber.The rf signal passes through the wall and hits the chamber side wall. The signal travels around the chamber since its acting like a Faraday cage. If there are any gaps in the shielding the rf leaks into the chamber where Nick was able to detect it.Both problems were fixed easily.The final measurement for shielding effectiveness came to 110-115dB. Nick said if he brought his higher end pre-amp we could probably squeeze a few more dBs out of the chamber. At this point we've pretty much got a pre-compliant chamber with shielding effectiveness on par with full blown test houses.

author: Tabs
author_rank: Regular Contributor
timestamp: April 12, 2014, 12:13:11 pm
content: Some of you may have noticed from the later pictures that the door is too tall for the opening in which it was meant to swing.The original intention was for the door to swing under the low hanging wall. I measured a good 30mm clearance between what I thought was the top of the door and the bottom of the low hanging wall. I hadn't realised the door was raised by 100mm because it wasn't indicated on the plans.A perfect combination of assumptions and misinterpretation. Shit happens.The door was only able to open through an arc of 400mm before it collided with the wall and this meant it would be difficult to transport the material for the next stage of the build into the chamber.It also had the potential to delay the schedule by 3 weeks because the shielding test couldn't be completed without the door.Its always better to get the shielding test done before the next stage starts. Can you imagine fitting all the ferrite tiles, absorbers, wiring etc and then do the test only to find out that 1 bolt wasn't tight enough and you missed an EMC gasketWe decided to complete the chamber with the larger door so that Nick could come in test the chamber. The idea was that he could find problems in other areas that we could fix then and there. After the testing, the large door was going to come off and the next stage would start. All the material could now be moved in. The area around the door frame would be left until the last possible minute. This allowed the 2nd stage to stay on track whilst a shorter door and blanking panel were made up and delivered.The solution was proposed by the chamber installers who are contracted by the company who supply the chamber. They are on track to deliver the new door mid next week and at no charge to my company. All very professionally handled.If you need a chamber check outhttp://www.globalemc.co.uk/highly recommended.Anyway, I arrived back to work the following Monday to this:2x4 batons were laid along the floor, ceiling and side walls. 10mm plywood was screwed into the batons on the floor. The chamber smelt of fresh sawdust.

author: Tabs
author_rank: Regular Contributor
timestamp: April 12, 2014, 12:43:59 pm
content: More plywood going on.The wood is hydroscopic so its water content changes throughout the day with ambient environmental conditions like humidity and temperature. Not the best thing for creating uniform field strengths inside the chamber.Hence the aluminium foil lining glued on top of the plywood.

author: Tabs
author_rank: Regular Contributor
timestamp: April 12, 2014, 09:19:36 pm
content: Tuesday 8th April,Aluminium lining has been glued to the inside surfaces and ferrite tiles are going upWhen its all complete, 1.5 tonnes of ferrite tiles will cover every inch of the inside.As you can see from the last picture, there are two types of tiles going on.I'm told the dark gray tiles are Samwha tiles. These are the high quality tiles. After a quick google search I think I have these ones:SFA600 or SFA600Ahttp://www.samwha.co.kr/SW_catalogue/catImage/37/Ferrite_Absorber.pdfEach individual tile is 100mm sqaure. They are mounted on a board in a 6x6 grid.I'm told each tile costs $5 US direct from the manufacturer, don't know how much we paid for each.I worked out (roughly) that we would need a little over 10,000 tiles to fill the inside of the chamber so $50k just on tiles would not leave me with anything for the equipment.Global EMC offered some 2nd hand tiles (which are hard to come by; its not like people scrap a ferrite lined chamber on a daily basis). These are the light gray tiles in the photo above.I'm told they are some sort of Phillips tiles but not sure which specific type.The good tiles are going to be placed strategically on the important walls and then less strategically on the side walls. The Phillips tiles will then cover the remaining area on each surface. Using the 2nd hand tiles let us keep the cost down.When Nick comes in again to do the anechoic tests, we will see if this payed off.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 05:29:09 pm
content: Thursday 10thInstallation of ferrite tiles complete (except the area around the door frame)You can see the ferrites are practically edge to edge.The chamber dimensions are designed around the dimensions of the ferrite tiles. This means the chamber size can only change in increments of 100mm. This guarantees edge to edge coverage of ferrite tiles without having to cut them. These things are very brittle so cutting them would be next to impossible and probably result in a lot of wastage.This is another way to keep the costs down.In the final photo, you can see the area where the penetration panel and consumer unit sit. There's a temporary socket placed inside which was used by Nick during the shielding tests.

author: Vgkid
author_rank: Super Contributor
timestamp: April 13, 2014, 05:42:04 pm
content: This is a really interesting read, keep us updated.How are you doing the ventillation?

author: BravoV
author_rank: Super Contributor
timestamp: April 13, 2014, 05:57:28 pm
content: Why there are ferrite tiles with holes in the middle, while others don't have it ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2014, 06:21:59 pm
content: Ferrite is actually very easy to cut on a carbide abrasive saw, like used for bathroom tiles (which are a stronger ceramic). At a PPoE, we once needed shielding and only had bricks, not tiles or plates; had a shop guy get rather dirty (the black cooling water splashes everywhere..) making slabs. They were all kinds of rough and uneven, but that didn't matter. A lapidary saw would be much nicer, if probably slower.Could probably also cut them like glass or tile, using a carbide scribe to draw straight lines to (hopefully) fracture on.Tim

author: SeanB
author_rank: Super Contributor
timestamp: April 13, 2014, 06:33:22 pm
content: Granite guys here cut the blocks with a small angle grinder with a diamond blade in it. lots of water and grey mess, but it cuts well enough. They use a larger grinder and large slow moving wire saws to cut the blocks into slabs, and a final operation is polishing to a fine gloss finish.I drilled granite with a masonry drill cooled with water, and it is doable if rather slow. I was not going to polish to a fine finish so cheated with clear matt varnish instead, a lot less work ( about 10 hours of polishing) involved, and a good enough finish over the dull grey it was.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 06:42:15 pm
content: Quote from: Vgkid on April 13, 2014, 05:42:04 pmThanks Vgkid,If you look back at day 4, the last two pictures show the roll of EMC gasket.The wider field of view shot shows the intake grill, for the ventilation, leaning against the chamber at the bottom near the door frame.The grill is positioned there. You can see it in its installed position in one of the photos showing Nick's transmit antenna pointing at the door.This isn't the standard position for the intake grill and I can't remember the reason we moved it to its current location.If I remember correctly, would normally go higher up on that end piece, I think above the height of the door.The exhaust grill for ventilation is contained in the ceiling on the diagonally opposite corner. You can see it in the shots of the ceiling going up.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 06:53:02 pm
content: Quote from: BravoV on April 13, 2014, 05:57:28 pmWell spotted!. I missed this subtlety.The center hole is used to screw the board, on which the ferrites are mounted, into the underlying plywood.I'm not 100% sure on this, but I'm assuming its because the ferrite tile panels (6x6 grid) that carry load (the absorbers) need more screws to take the weight. In our case, we will be using absorbers at the center of each face (including the bottom face). We will use small absorbers so I doubt we need a screw in every tile center. I guess the tile manufacturer can't know before hand what load will be attached to the tiles so gives you a dedicated hole per tile.It looks like the tile panels that don't have a hole in every tile are only fitted at the edges of each face. I know the absorbers won't cover these areas so only need screwing down in the corners.I'll ask on Monday when I get in.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 07:01:43 pm
content: Quote from: T3sl4co1l on April 13, 2014, 06:21:59 pmand SeanB,Yes, agree. Not sure why I thought it would be difficult. Using proper tile cutting equipment didn't cross my mindThe water cooling would also help control the dust and smoke which may set off the alarms otherwise.The installers did have to cut the metal holes for vents and penetration panels outside.As long as the installers were careful the water wouldn't be too much of a problem.I think it still makes financial sense not to have to cut the tiles (thus creating a bespoke design). Its just easier on time, transport costs and produces less mess and waste.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 07:22:10 pm
content: Friday 11thFoam absorbers starting to go in.The ones shown in the pictures are the small one. Judging by the size on the pictures these look like 100mm square x 200mm deep.They are glued to the ferrite tiles.I believe we have two more sizes of absorbers 400mm and 500mm deep.I'll confirm once the installers open the boxes.

author: lewis
author_rank: Frequent Contributor
timestamp: April 13, 2014, 07:32:21 pm
content: Looks very cool. Do the tile gaps between the walls and ceiling need to be filled?

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 08:05:32 pm
content: Friday's are half days here,I took these pictures just as I was leaving.The ground plane was installed and the bottom face was ready to have ferrites and absorbers placed on them.This will probably happen over the weekend.In this picture you can see the 2nd penetration panel that I wanted to get installed. I'm standing were the consumer unit and temporary AC supply is. The far wall is where the antenna is going to be placed.The primary plan is to have a cable running along the left wall down the full length of the chamber and then across to the antenna.The EUT will be placed on platform where I am standing.The 2nd penetration panel is for when I get desperate. I need to squeeze every last Watt out of the 1Ghz amp to generate a 10V/m field and cable losses are playing a big role in this. Above 1Ghz, i need to create 3V/m and this is done by a 2nd lower wattage amplifier. This was done to keep costs down. An amp that can produce 10V/m + 80% Modulation (18V/m) to 1GHz at a 3m separation between the antenna and EUT can easily reach upwards of 30-40k (£). The plan was to re-use our existing AR amplifier and buy a new amp for the 1-3Ghz range.At the moment I haven't ordered the cables yet, I've been advised by Nick to look at the ultra low loss cables from Rosenburger/Micro-Coax 's UTiFLEX range. Specifically the UFB311Ahttps://assy.micro-coax.com/MCDlgCableDetails.aspx?Cable=UFB311AI got a quote from Micro-Coax's online store and it costs a little under £1000 ($1400 ish). That'll be the most expensive cable I've ever orderedThe second picture is taken from where the antenna will be placed. You can see the incomplete section left around the door frame.When the new door arrives, this area will catch up to the rest of the build.

author: Tabs
author_rank: Regular Contributor
timestamp: April 13, 2014, 08:14:09 pm
content: Quote from: lewis on April 13, 2014, 07:32:21 pmAgain, not 100% sure but I'm guessing no. The tiles are required for the low frequency rf range of the test spectrum. The absorbers take over at around 800Mhz to 1Ghz. I don't think the low frequencies will see the gap so won't get a chance to go through it and reflect off metal sheet behind the tiles. Since the absorbers are only being placed in the center of each face, I'm guessing this is where the important area is to absorb the high frequencies and the edges may be less important.The anechoic tests that Nick will do at the end will tell us if this is a problem. Nick did suggest we could re-use the rectangular absorbers from the older chamber in these areas if we needed to. Again lets wait and see.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 14, 2014, 09:30:26 am
content: Strange. All the anechoic chambers I've been in have had the absorbers from floor to ceiling and on the door also. As for the roof I can't actually remember.I remember seeing a G6 armoured vehicle inside a huge anechoic chamber years ago. It was quite a sight.

author: 1design
author_rank: Regular Contributor
timestamp: April 14, 2014, 03:49:29 pm
content: For the cable you might also want to take a look at the Huber+Suhner Sucoflex 406.BR,Miha

author: Tabs
author_rank: Regular Contributor
timestamp: April 14, 2014, 06:43:01 pm
content: Quote from: 1design on April 14, 2014, 03:49:29 pmThe full list of cables recommended by Nick were:Rosenberger Microcoax UFB311A, Huber & Suhner Sucoflex cable, Gore cables or ‘True Blue’ cablesThanks for the recommendation,Have you used the SucoFlex?

author: Tabs
author_rank: Regular Contributor
timestamp: April 14, 2014, 06:55:40 pm
content: Quote from: Richard Head on April 14, 2014, 09:30:26 amThe chamber we are installing will be fully anechoic once complete. This means it will have absorbers on the top and bottom and 4 side walls.I'm guessing the absorbers in the chambers you've been in were deep which is why you saw them from floor to ceiling, it may not have had tiles behind them. Dave did a EMC chamber facility tourhttp://www.eevblog.com/2011/09/22/eevblog-202-emc-rf-anechoic-test-facility-tour/in which no tiles were used in one the larger chambers.Our chamber is referred to as a compact 3m chamber (Most EMC chamber manufacturers refer to the chamber size by the separation distance between the EUT and antenna). I think the standard calls for 1m separation between the EUT and nearest wall and 1m between the antenna and its nearest wall. Taking these distances into account and including antenna size resulted in a requirement for 7m total length which is just about the biggest size we could fit in the area we selected. This means we have to use shallow pyramidal absorbers backed by ferrite tiles which go from floor to ceiling. Getting bigger pyramidal absorbers would have meant compromises on separation distance.

author: Nick Smith
author_rank: Newbie
timestamp: April 15, 2014, 04:50:39 pm
content: Hi, I'm Nick - as mentioned by Tabs in some of his posts. I'm new around here and I've been waiting for Tabs to send me a link to this thread, but I gave up waiting and sought it out for myselfAnyway, I'm happy to answer any questions on anechoic chambers - I've spent most of the past 20 years or so hidden away in one chamber or another and have validated chambers for most of the major chamber suppliers over the years.I've read the whole thread and would like to pick up on a few of the comments...Somebody commented about the coverage of the pyramidal absorber not being 100% and that this had differed to other chambers they had seen. The absorber coverage depends on the intended use of the chamber and a chamber for measuring antenna patterns or performing Radar Cross Section measurements would general have full coverage of pyramidal absorber but chambers for EMC measurements are not usually fully covered with pyramidal absorber. Commercial EMC chambers require ferrite tiles in order to get the performance they require down to 30 MHz for commercial testing - unfortunately, putting anything in front of the ferrite tiles will degrade the performance of the tiles and this includes pyramidal absorber.The pyramidal absorber used in EMC chambers is a Hybrid absorber that is matched to work on top of ferrite tiles - this is done by carefully controlling the carbon doping, however the whole hybrid absorber solution is a bit of a compromise in order to get wider frequency range - the higher frequency performance is not as good as a 'fully loaded' pyramidal absorber that would be used in an antenna chamber and the lower frequency performance is not quite as good as a plane ferrite tile. If you use conventional (non-hybrid) pyramidal absorbers, then the ferrite performance is badly affected.So the whole absorber layout is a bit of a balancing act and it is generally better to only apply pyramidal absorber where it is most needed. In an EMC chamber at higher frequencies, we generally use log-periodic or horn antenna, which are quite directional - this means that some areas of the room do not require pyramidal absorber, whereas if you want to measure antenna patterns with a 360 degree azimuth rotation, you will need absorber coverage everywhere because at some time during an antenna measurement your antenna will be aimed at the walls.One final point to note, even for EMC testing not all standards require testing down to 30 MHz and for some (non-commercial) testing, using conventional pyramidal absorber without ferrite is a good solution - the walls in these are then general completely covered in absorber.Next - holes in the ferrite tiles. As Tabs commented, the tiles have holes so that they can be screwed to the walls. Some manufacturers screw every tile and some pre-glue the tiles to backing boards and then screw the boards (using screws through the tiles at the corners of the boards).Years ago, it was commonplace to use solid tiles (with no holes) except where holes were required, however after measuring many chambers, it appears that the holes make little or no difference to the final measured result (or not enough difference to make them fail their validation tests anyway). So nowadays, some chamber manufacturers just use tiles with holes. This means they only need to stock one type of tile and they probably get better volume pricing. It also give more flexibility if additional screws are required other than the corner screws. I think they generally put a few more screws on the ceiling panels because it will spoil your day if a 600mm square panel of ferrite tiles falls on you.Next - The gaps in the tiles between the walls and between the walls and ceiling. These make little difference to the performance (again, not enough to cause a failure on the validation tests). Once upon a time, chamber manufacturers ensured that there were no gaps and that the tiles were basically touching. This requires lots of tile cutting, which is time consuming and messy (it needs a wet diamond wheel tile cutter). The tiles are 100mm square and some bigger chambers may be 18 meters long, 12 meters wide and 6 meters high (or even larger) - that's a lot of tile cutting if you have to cut into every corner!There is a limit to the size of gap that you can get away with, but 20 or 30mm generally makes no significant difference so nowadays they seem to try and minimise the amount of tile-cutting required.The foil backing behind the tiles is not related to the wood being hygroscopic as Tabs suggested. The (RF) wave is attenuated as it passes through the tile, it then reflects and is attenuated as it returns through the tile. The spacing between the tile and the reflector affects the phase relationship between the direct and reflected waves and this consequently changes the performance (due to constructive or destructive interference of the two waves). Obviously this is frequency dependant, but If the spacing between the tile and the reflector is too large, then you end up with a chamber that doesn't perform well at some frequencies. Without the foil, the reflector would have been the shield itself and with this type of shield construction, the shield is spaced 60 or 70 mm away from the tile, so the foil is used so that the dielectric spacing between the tile and the reflector can be controlled. This spacing is actually quite critical and can be used to 'tune' the broadband performance of the tile. Traditionally, this was used to improve the higher frequency performance of the ferrites so that these rooms could be used to 1 GHz. Nowadays, the hybrid absorber takes over from the ferrites around 700 or 800 MHz and the fine tuning of the tiles is less important, however if the dielectric spacing is too large then you will not get the required performance from the tile.I think that's about it from me for now - I've written a bit more than I intended too...I'm happy to answer any other questions and if anybody has any shielding effectiveness measurement requirements or needs an anechoic chamber measuring, then please get in touch!(I'm not sure whether it's against forum rules here to post a link to my company's website so I won't. Tabs might post a link though...)

author: Tabs
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:44:00 pm
content: Quote from: Nick Smith on April 15, 2014, 04:50:39 pmHi Nick,Welcome to the EEVBlog forums.Apologies for not sending out the link earlier.Thanks for the input & corrections.Here's a link to Nicks company website:http://www.wave-scientific.com/I think its ok for the link to go in the thread.If Dave or an admin takes issue with it please let me know and i'll modify the post.

author: Tabs
author_rank: Regular Contributor
timestamp: April 19, 2014, 12:34:47 pm
content: Wednesday 16th,New shorter door arrives and is installed. The ground plane near the door is ripped up to remove the old door frame. You can see a small 100mm blanking panel at the top.

author: Tabs
author_rank: Regular Contributor
timestamp: April 19, 2014, 12:41:28 pm
content: Plywood fitted around door area and foil lining glued into place.500mm absorber being fitted to main wallmain wall complete and ferrite tiles added around door frame.

author: SeanB
author_rank: Super Contributor
timestamp: April 19, 2014, 01:44:23 pm
content: At least you did not do as I saw in one building and cut the beam away. That was done so the forklift could pass under it, and they were stacking on the floor above as well with crates 2m high. I was always wondering when I was there if the floor would decide that that was the day it was tired.

author: nctnico
author_rank: Super Contributor
timestamp: April 19, 2014, 03:52:52 pm
content: Quote from: Tabs on April 11, 2014, 07:01:53 pmPutting ferrites onto a cable going into the chamber works very well to keep unwanted signals out. A couple of months ago I attended some EMC tests. We got a spike at 90MHz I couldn't explain. It turned out the tech forgot to put the filter on a cable going into the chamber.

author: lpc32
author_rank: Frequent Contributor
timestamp: April 20, 2014, 12:10:32 am
content: Interesting. What's the temperature in there?BTW, very artistic/mysterious those glare/blur photos.

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 12:35:00 am
content: Quote from: nctnico on April 19, 2014, 03:52:52 pmI'm not sure what your'e describing nctnico. I think your describing a penetration panel designed to accept your products cables and which allows you to monitor/control the product/EUT. These signals are usually filtered and the chamber boundary and any screens also attach here. If you tech forgot to add the filters in this setup then I think you resolved it correctly.What I was describing was when people make use of vent holes or the waveguide to pass cables through the chamber boundary when the penetration panel doesn't have the interface they need.Although you are right (in principle) that you can use a ferrite clamp on a cable to resolve problems like what you describe in the setup above, I wouldn't recommend it for general use when you don't have a compatible penetration panel. For me, the difficulty would be showing the 90Mhz spike was from the EUT (using an accepted EMC standard) and not and external signal being coupled into the chamber along the wire. The antenna wouldn't be able to differentiate between external source or EUT. At worst you may have added a ferrite clamp to your BOM for the product cable when in fact it wasn't needed. If the 90Mhz was from the EUT it's better to find the source (using something to act as a field probe) and fix it (time/money dependent).Its just something that I would say causes more headaches for keeping traceable records and proving your EUT complies with EMC regs. I know in europe, if you self declare with the technical file approach, you need to keep the file on hand for 10 years from the point the product was last placed on the market.

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 12:42:21 am
content: Quote from: lpc32 on April 20, 2014, 12:10:32 amThe chamber is located down a corridor from the main product area where we have a lot of wave soldering machines. I'm told the temperatures used to get very warm in the summer but I haven't experienced the problem because i've not been there long. Since my arrival, we have had new air-conditioning installed. Its a heat exchange system which takes heat from our reflow ovens in SM and distributes it through the building in winter or takes heat out of the building in the summer.The chamber should be at what ever the ambient temperature of the environment is. There are no heat sources radiating onto the chamber and most of our products are less than 15-20W so there shouldn't be any heat building up inside.

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 12:54:46 am
content: Extra artistic/mysterious glare/blur photo for Ipc32.Thursday 17th2nd pic - electrical conduit and sockets being routed and made ready for electrician to run his cables throughThere will be sockets at the front (for EUT) and at the back where the antenna is (for camera), light switch near door and an emergency light above the door.3rd pic - white polystyrene tiles are glued into shape and fitted on top of blue pyramidal absorbers.

author: lpc32
author_rank: Frequent Contributor
timestamp: April 25, 2014, 02:01:24 pm
content: Quote from: Tabs on April 25, 2014, 12:54:46 amOh my god, it's full of glares!BTW, not bad for a cellphone camera.

author: LapTop006
author_rank: Unknown
timestamp: April 25, 2014, 02:33:21 pm
content: Quote from: Tabs on April 25, 2014, 12:54:46 amAre those just impact protection, or do they actually serve an RF purpose?

author: PedroDaGr8
author_rank: Super Contributor
timestamp: April 25, 2014, 02:35:57 pm
content: This thread is badass. I never realized how much work went into making a chamber like this. Very impressive and thanks for posting it.

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 03:22:22 pm
content: Quote from: LapTop006 on April 25, 2014, 02:33:21 pmThe white tiles are made from polystyrene which Nick tells me is completely transparent to RF so they offer no technical/performance benefit to the chamber characteristics. They are added purely for protection and decorative purposes. They make the chamber brighter to work in and protect the tips of the foam absorbers.Currently, all the pictures I've taken inside the chamber are lit with the actual lights that were intended to be installed in the chamber (I thought they were temporary work lights). They were plugged into a temporary extension chord because the chamber hasn't been wired by the electrician yet.The lights were trac pro 70w SON lights which are sodium based and give a very yellow/orange light similar to street lights here in the UK. I asked Global to replace them with something whiter because it was too gloomy inside. Global don't normally use SON lights and were happy to replace them.The final lights (metal halide) are still wired up using a temporary connection but give a much better light.See the pictures below:Tuesday 22nd April (After bank holiday)Lights changed to metal halide, ferite tiles added to floor and pyramidal absorbers placed on top of those. Nick arrives to begin next stage of testing (All the pictures below are from Nick and show the test setup he is using. The equipment is his as well)White tiles fitted on top of all side walls, with tiles missing in some areas (Global sent a full complement, but murphys law got us). Replacement tiles were sent down free of charge. Global had to deliver some of Nicks test gear so they sent the replacement tiles in the same shipment.The picture is taken at the corner where the door is and is looking towards the back of the chamber where my antenna will be placed.Here's another picture showing the same view. This time you can see a walk way on the right that leads up to the antenna area.The ferrite tiles and pyramid absorbers on the floor are not fixed in place. This allows us to move the floor around.Nicks tests will assume the floor is going to stay in this position. Global EMC call this an offset chamber which means the antenna, floor absorbers and EUT are not on the center line of the chamber. This allows us to have the walk way on the right. Nick will check to make sure we can establish a uniform field around the EUT.Another view of the antenna area, taken from the EUT position.Final picture showing one of Nicks setups. This time taken from the antenna area, looking towards the EUT area.

author: lewis
author_rank: Frequent Contributor
timestamp: April 25, 2014, 03:36:25 pm
content: That looks awesome. What measurement setup are you using?Are you likely to get interference from the ballasts in the halide luminaries?

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 03:59:14 pm
content: Quote from: lewis on April 25, 2014, 03:36:25 pmI'll leave Nick to talk about his setup since I don't know it in enough detail.In normal operation, the lights are switched off during emissions and immunity testing and no one will be inside the chamber. I've been inside chambers whilst doing mil-std-461 tests but this is when we start approaching the higher Ghz. By the time we hit 18Ghz the chamber doors were wide open.Our plan is to to do conducted immunity, ESD, surge/bursts etc inside the chamber and for these types of tests, the lights shouldn't cause a problem.

author: lewis
author_rank: Frequent Contributor
timestamp: April 25, 2014, 05:33:20 pm
content: Quote from: Tabs on April 25, 2014, 03:59:14 pmOf course! Senior moment

author: gregariz
author_rank: Frequent Contributor
timestamp: April 25, 2014, 06:13:56 pm
content: It's interesting to see the varying types of constructions of chambers out there. Is this a common construction method in europe?I've been involved in a couple chamber builds and most in the US that I have seen offered in recent years are constructed simply of about 1" double plated fibreboard. The panels and corners are then clamped together using a steel profile, with the joints stuffed with wool.

author: Tabs
author_rank: Regular Contributor
timestamp: April 25, 2014, 07:12:33 pm
content: Quote from: gregariz on April 25, 2014, 06:13:56 pmAgain, this is probably one for Nick or Global EMC to answer. I've used different types of chambers before but this is the first time I've been involved in building one so I don't really have experience of other methods.I'm assuming the fiberboards are plated with with something good enough to act as a shield and that the wool is what I was calling the EMC gasket?I don't see anything wrong with this type of construction. Using a steel profile may make the chamber stronger than the way this one constructed (especially if using steel profiles on both sides of the plated fibreboard). Because my chamber is small its constructed using only 1 and half panels per row which are arranged like a brick wall pattern. This makes it very strong, especially when combined with the structural steel supports around the outside and not likely to sag/bow in the middle.I've been told the the ceiling can support an additional 1 tonne of weight so this style of chamber construction is very solid. The stage 1 tests done by Nick to measure the shielding effectiveness showed the chamber was achieving between 115-120dB of shielding so the construction style is definitely good enough in terms of EMC performance.I'm guessing the construction style is standardised across all of globals chambers. The chamber installer told me he's done loads of chambers for global across the UK and at least one in Switzerland. Can't say for sure if this style is specific to globals designs or if its normally done this way in Europe.Global can answer that one best.

author: Nick Smith
author_rank: Newbie
timestamp: April 25, 2014, 10:00:57 pm
content: Hi,Firstly regarding the chamber construction using double sided boards - we call this a 'sandwich' system chamber. It's not very popular now in Europe or Asia but still seems to be the preferred system in the US. I've tested many of these and many of the 'pan' type rooms (like this one). I can categorically state that the pan-construction rooms are capable of higher shielding performance, however the sandwich systems are usually good enough for most purposes. I've no idea why the pan system hasn't really caught on in the US yet. The sandwich construction is a bit more flexible because it can be cut on site if required whereas the pan system has to be made correctly in the factory before shipping to site.The 'wool' mentioned is usually bronze wool - this is similar to steel wool - it's commonly used in the leisure marine industry because it doesn't rust, but it's also useful for stuffing the corner joints in sandwich rooms!As far as the chamber testing goes (apologies for the poor photos BTW - I've got loads of better ones but seem to have given Tabs all of the blurry ones) - we have basically performed two tests, one is a 'Normalized Site Attenuation (NSA) test and the other is a Field Uniformity.The NSA test essentially measures the loss between a pair of antennas and compares the measured loss with the theoretically calculated loss for free-space. This is in accordance with the standard CISPR 16-1-4. In this chamber, we treated it like a Fully Anechoic Room (FAR) which has absorber on the floor rather than a semi-anechoic chamber which has a reflective ground-plane and uses a height-scanning antenna mast.In this case, we actually used the 'Reference Site Method', which means that the antenna pair were first measured on an extremely good reference site (the National Physical Laboratory (NPL) in the UK) and we then do a direct comparison with this as the reference site and measure the difference between the two sites.This is the test where you can see a pair of antennas - one antenna is a miniature biconical antenna, which is used as a transmit antenna and we use a biconical or log-periodic antenna as a receive antenna. The measurement is repeated at a number of positions defining a test volume inside the chamber where later on the Equipment Under Test (EUT) (i.e. Tabs' equipment) will be located.The second test is a Field Uniformity test in accordance with the standard IEC 61000-4-3. This uses a transmit antenna at one end of the chamber and an isotropic field probe to measure the field at the EUT end of the chamber. The field is measured at 16 points which describe a test plane of 1.5m x 1.5m (W x H), and we then compute the variation in the field across this plane. To be honest, in this chamber the 1.5m x 1.5m plane is larger than really required, because it is a compact chamber and the EUT is relatively small (i.e. much less than 1.5m^2), but in order to prove the chamber performance we measured over the whole 1.5m plane anyway.As always, I'm happy to answer any questions on the testing process.Regarding the lighting - as far as I am aware, there are no significant emissions from the metal-halide lights that have been installed. Fluorescent lights can definitely be problematic as can some of the newer LED lights, so it's good to be aware of potential problems because they are not all good. There should be no problem with the lights installed in this chamber even if they are left on during testing. It's simple enough to check the emissions anyway if required.the white polystyrene tiles (Styrofoam in US speak??) are really for decoration only - they are pretty much transparent to RF, so have no effect on the chamber performance. They do however help to reflect light and make the chamber a brighter workspace. They also look quite nice and most of the chamber suppliers do offer these nowadays however they are really only for cosmetic purposes.I haven't worked out how to post pictures here yet, but if somebody can explain how, the I have loads of non-blurry images of the testing if anybody is interested...

author: gregariz
author_rank: Frequent Contributor
timestamp: April 25, 2014, 10:12:57 pm
content: Quote from: Nick Smith on April 25, 2014, 10:00:57 pmThanks for that. Not having seen a chamber go up in europe I had not seen that style before. It certainly looks the goods. I suspect though the sandwich construction does go up quicker. I saw a 12x8x12 go up in a day and a 24x12x12 go up in 2 days. However these were standard kit sizes I believe so there was no cutting on site. And that was just the box, no fitout.

author: lewis
author_rank: Frequent Contributor
timestamp: April 25, 2014, 10:13:14 pm
content: Hi Nick, would love to see some more pics. Click reply, then click "Attachments and other options" under where you would usually write your reply. Under 'Attach' -> choose file... etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 25, 2014, 10:17:32 pm
content: Quote from: Nick Smith on April 25, 2014, 10:00:57 pmTransparent? Are you kidding? Those damn things will vastly increase the ambient radiation, especially in the 400THz+ band!...Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 25, 2014, 10:29:13 pm
content: Quote from: lewis on April 25, 2014, 05:33:20 pmI wouldn't have thought halide would be the best choice for lights you want to be turning on & off multiple times - warmup time & restrike issues?

author: Nick Smith
author_rank: Newbie
timestamp: April 25, 2014, 10:34:45 pm
content: Thanks - hopefully these pictures will work:

author: Nick Smith
author_rank: Newbie
timestamp: April 25, 2014, 10:46:58 pm
content: Quote from: T3sl4co1l on April 25, 2014, 10:17:32 pmThey don't increase the ambient radiation, they simply reflect whatever is already there. There are no significant sources of Terahertz emissions in the chamber to be worried about, so the reflections at those frequencies are no great concern.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 26, 2014, 03:07:16 am
content: If that were true at HF as well, all those pricey tiles and foamy things wouldn't be necessary.

author: SeanB
author_rank: Super Contributor
timestamp: April 26, 2014, 06:23:27 am
content: Metal halide lights with a magnetic autotransformer ballast will not have much electric noise over 1kHz and it will mostly be low harmonics of the line frequency, caused by the non linear resistance of the arc in the lamp. There will be a small amount of wideband noise generated by the lamp itself, but most will not make it out of the lamp housing if it is metal. There will be a larger noise generated during the lamp startup as the ignitor generates a very high voltage to start the lamp, but this will stop after about 30s as the lamp warms up. If you want to leave the lamp in the chamber you might want to open it and place a ferrite bead on each lamp lead inside the case, close to the lamp holder. Otherwise you just have the lamp power cord very long and a hook on the outside to hold it, and a power point to connect the lamp so that you can simply carry it in when needed, and take it out still lit and leave it outside during testing, so you do not have the 3-10 minute restrike time of the lamp.

author: Tabs
author_rank: Regular Contributor
timestamp: April 26, 2014, 08:36:40 am
content: Quote from: mikeselectricstuff on April 25, 2014, 10:29:13 pmNick and SeanB are saying its not an issue if the lights were one so I guess its better to leave them on if they've been turned on already.Didn't think about lifetime of the metal halide lights if they were switched in short intervals. Thanks Mike, I would've got through a lot of these if you hadn't asked the question.Would the effect be visible? (similar to the black stuff that develops at the ends of fluorescent tubes? or would I have burnt out the starter?)Would be interesting to do an emissions test with & without the lights on and see how much wideband noise is added.

author: lewis
author_rank: Frequent Contributor
timestamp: April 26, 2014, 09:15:11 am
content: Another dumb question - how is the shield grounded? Did you need to dig in earth rods all around the chamber and use big beefy straps, or is bonding back to the consumer unit with a bit of 10mm2adequate?

author: Nick Smith
author_rank: Newbie
timestamp: April 26, 2014, 09:57:25 am
content: Quote from: lewis on April 26, 2014, 09:15:11 amThere is no need for a special ground for RF purposes. Some people feel the need to put in a special earth rod for their chamber but it makes no difference to it's performance. A faraday cage will still work whether it is grounded or not and a typical ground wire is a pretty high impedance at RF frequencies anyway.It is important to have a good safety ground though - the power filters have big Y-Class capacitors between the lines and ground so there is significant earth leakage - certainly more than enough to take out an earth-leakage trip (RCD).

author: SeanB
author_rank: Super Contributor
timestamp: April 26, 2014, 11:35:15 am
content: Metal halide lamps fail by having a rising internal pressure as they age, and this translates into a rising arc voltage until it eventually is too high for the ballast to maintain the lamp current, so it fails by cycling on and off continuously. You will see a lamp with black and coloured ends near the internal arc tube terminals as normal, it is merely metal halide and metal deposits, and indicates that the lamp has only been run for a short time then turned off. Run it for an hour or more and it will go mostly away, but any lamp that has been used will develop this to some extent. Even a brand new lamp might have some from testing during production.The standard failure mode is cycling for a long time, then bursting of the inner arc tube. Please note that if the lamp is used in a fixture with a missing tempered glass cover, or is used in an open fixture, it must be a blast protected lamp, which has an inner guard of quartz glass surrounding the arc tube to contain the blast of the arc tube failing. If you use a regular unprotected lamp there and the arc tube ruptures then it will blow up the outer envelope as well, showering the area with red hot glass. If you use a fitting with a broken glass front and the lamp gets wet it will also explode.

author: Pat Pending
author_rank: Regular Contributor
timestamp: April 26, 2014, 08:31:36 pm
content: That's a very interesting build - thanks for posting.With a lot of foam in there, what precautions can you take for fire?

author: Nick Smith
author_rank: Newbie
timestamp: April 27, 2014, 10:30:40 am
content: Quote from: Pat Pending on April 26, 2014, 08:31:36 pmThe foam used in the absorbers is treated with fire retardants and meets the requirements of NRL 8093, which is the US Naval Research Laboratories standard for urethane foam absorbers.You do still have pay attention to what you are doing though because it is still possible to burn the absorbers even though they meet the NRL requirements. You need to particularly careful when using very high power RF amplifiers and high gain horn antennas, since this can cause heating inside the absorbers however this shouldn't be a concern in most chambers for commercial EMC measurements.

author: SeanB
author_rank: Super Contributor
timestamp: April 27, 2014, 10:35:59 am
content: The absorbers I worked with for RADAR testing did tend to cook themselves with the first few hours of running them when new, but afterwards they worked well, even though they were rather brittle then, as opposed to them being flexible like foam sponge when new. Those will probably get a little brittle with time as the plasticisers evaporate from them, but they will still work so long as they are mostly intact, a few chipped edges here and there are not going to make much difference.

author: Tabs
author_rank: Regular Contributor
timestamp: May 01, 2014, 09:12:49 pm
content: Been off work the last few days.Here's some pics taken by my colleague:Timber frame going up to separate the corridor from the chamber control area followed by plaster board and skim.Once this work is complete, the electrician will wire the chamber to its proper supply. We'll probably call Nick in again once its all cleaned.

author: Precipice
author_rank: Frequent Contributor
timestamp: June 12, 2014, 09:53:53 pm
content: This is fascinating, thanks for posting.I'm currently reassembling a rather larger third-hand chamber from the same company - the new stuff looks far, far easier to build! (Still, what did I expect for £200 on ebay, buyer collects2 questions - anyone got a source for that dogbone-shaped gasket strip? I was going to reuse the nickel felt gasket material I scavenged from the last build (which looked like it had already been reused), but the crushable dogbone stuff looks like it'll be easier, and might soak up uneven-ness a bit better. (The structure of my chamber is galvanised steel sheets on a wood and ply frame, with gasketted aluminium closure plates covering the joints, so I need the gasket to bite into both galvanised steel and aluminium). I've got all the panels up, so the closure plates are next, and, since it's 2500 self-drilling screws, I'd rather not have to do it twice because I chose the wrong gasket...Second question - those polystyrene cone-covers, are they a GS thing? They're tempting, since some of my cones are much, much bigger (no ferrites, so the cones are trying to work down to a much lower frequency), and I've got cones (and raised walkways) on the floor. Stopping the cones getting damaged, making the chamber less of a black hole for light as well as RF, and reducing the number of things I lose in the cone field on the floor, all sound like good things.Steve